Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session JP8: Poster Session IV: Education and Outreach; Undergraduate and High School Research; C-MOD, ITER, MFE Simulation, MST and Other RFPs |
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Room: Hall BC |
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JP8.00001: EDUCATION AND OUTREACH |
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JP8.00002: ABSTRACT WITHDRAWN |
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JP8.00003: Increasing Plasma Science Interest among Underprivileged Students Aliya Merali, Stephanie Wissel, Maureen Quinn, Nicole Hamlet, Andrew Zwicker For the past three years, PPPL has partnered with NASA to study dusty plasmas, convective flows, and other related topics in $\mu$-gravity. During the last year, we worked with a group of highly-motivated but underprivileged high school students to develop an experiment looking at the Rayleigh-Taylor instability in varying gravitational environments. The students were challenged to design, build and test the experiment to be flown aboard NASA's ``Weightless Wonder.'' Students met weekly with PPPL mentors over the course of the 9-month school year. The program provided students with long-term exposure to a scientific experiment, a sense of experimental ownership, and an opportunity to experience the scientific method outside of a classroom setting. Participants used inquiry-based learning techniques to design and build the experiment. We report on impact of the program and the outcome of the students' continued work. [Preview Abstract] |
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JP8.00004: Novel Methods for Communicating Plasma Science to the General Public Andrew Zwicker, Aliya Merali, S.A. Wissel, John DeLooper The broader implications of Plasma Science remains an elusive topic that the general public rarely discusses, regardless of their relevance to energy, the environment, and technology. ~Recently, we have looked beyond print media for methods to reach large numbers of people in creative and informative ways. ~These have included video, art, images, and music. For example, our submission to the ``What is a Flame?'' contest was ranked in the top 15 out of 800 submissions. ~Images of plasmas have won 3 out of 5 of the Princeton University ``Art of Science'' competitions. ~We use a plasma speaker to teach students of all ages about~sound generation and plasma physics.~~We report on the details of each of these and future videos and animations under development. [Preview Abstract] |
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JP8.00005: The 2012 CASPER Physics Circus Jorge Carmona Reyes, Anne Land-Zandstra, Joyce Cheng, Angela Douglass, Brandon Harris, Zhuanhao Zhang, Mudi Chen, Lorin Matthews, Truell Hyde The CASPER Physics Circus is one component of a CASPER ongoing educational outreach initiative known as the CASPER Seamless Pathway. The Physics Circus is funded by the United States Department of Education and is designed to increase interest in, engagement with, and understanding of science, technology, engineering and mathematics (STEM) within students in grades 6 through 12. The program's material and curriculum is aligned with both TEKS (Texas Essentials Knowledge and Skills) and National Science and Mathematics Standards, with its components (theatre, hands-on exhibitions, game show, professional development and curriculum) reinforcing these goals in a creative and entertaining format. Pre- and post-assessments measuring both content understanding and attitude towards science were conducted for a representative sample of the cohort and the analyzed data will be presented. The role the Circus plays within CASPER's Seamless Pathway will also be discussed along with other current CASPER programs including its High School Scholars program, CASPER's Interns program and CASPER NSF funded REU/RET programs for college undergraduates and K-12 teachers. [Preview Abstract] |
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JP8.00006: The CPS Plasma Award at the Intel Science and Engineering Fair Lee Berry For the past eight years, the Coalition for Plasma Science (CPS) has presented an award for a plasma project at the Intel International Science and Engineering Fair (ISEF). We reported on the first five years of this award at the 2009 DPP Symposium. Pulsed neutron-producing experiments are a recurring topic, with the efforts now turning to applications. The most recent award at the Pittsburgh ISEF this past May was given for analysis of data from Brookhaven's Relativistic Heavy Ion Collider. The effort had the goal of understanding the fluid properties of the quark-gluon plasma. All of the CPS award-winning projects so far have been based on experiments, with four awards going to women students and four to men. In 2009 we noted that the number and quality of projects was improving. Since then, as we we predicted (hoped for), that trend has continued. The CPS looks forward to continuing its work with students who are excited about the possibilities of plasma. You too can share this excitement by judging at the 2013 fair in Phoenix on May 12-17. Information may be obtained by emailing cps@plasmacoalition.org. [Preview Abstract] |
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JP8.00007: UNDERGRADUATE AND HIGH SCHOOL RESEARCH |
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JP8.00008: Both Twins Traveling Paradox Florentin Smarandache Two twins T$_{1}$ and T$_{2}$ synchronize their clocks at the same location L, then both of them leave with the same uniform high speed $v$ and on the same large distance $d$ on opposite linear directions to the locations $A$ and respectively $B$ (of course \textit{LA = LB = d}) on that planet. Each twin sees the other twin moving away from him with the relativistic speed $2v$, so each twin considers the other twin younger than him. The time dilation is the same in both twins' inertial reference frames. Here it is a forth symmetry. They stop there at $A$ and respectively at $B$. Afterwards, the twin $T_{1}$ from $A$ travels on a linear route back to $B$ (passing through $L)$ at a uniform high speed $2v$. Again, each twin sees the other twin traveling towards him with a speed $2v$. And again each twin considers the other twin being younger than him, since there is the same time dilation and same space contraction. Again one has a back symmetry. But, when the twin $T_{1}$ from $A$ gets to $B$, he finds out that he is younger than the twin $T_{2}$ in $B$ since he has traveled more that $T_{2}$. [Preview Abstract] |
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JP8.00009: Spectroscopic analysis of light emitted from gyrotron tubes L.S. Riford, M. Cengher, J. Lohr During operation of high power 1 MW class gyrotron tubes, and especially during conditioning to full operational parameters, sparkdowns and normal plasma discharges in the tubes emit light. A spectrometer can be installed on a gyrotron to look back through the diamond output window to measure the spectra of the atoms excited in a discharge. Identifying the emitted lines spectroscopically and inferring the locations from which the light is coming can lead to a better understanding of normal and abnormal gyrotron operation. Data from several gyrotrons will be presented and compared. [Preview Abstract] |
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JP8.00010: Experimental Setup and Characterization of Laser Produced X-Rays P.J. Bonofiglo, M.E. Lowenstern, C. Zerger, C. Arnett, P.A. Keiter, C.C. Kuranz, R.P. Drake Laser produced x-rays are a well-established diagnostic tool for imaging many events in High Energy Density (HED) experiments. However, many of these experiments suffer from low signal-to-noise. We strive to produce both production and detection of the x-ray signal. We will study laser-irradiated foils with a low energy laser. We used a ND:YAG laser at 532 nm with a pulse duration and energy of 8 ns and 160 mJ respectively. We will present our initial design work on the experimental setup, and we will also report the measurement of x-rays from different laser-irradiated metal foils from a photodiode and spectrometer. Our future work will entail characterizing the x-rays through measurements with a streak camera and microchannel plates (MCPs). [Preview Abstract] |
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JP8.00011: Optimization of X-ray Radiography System for Characterizing Micro-scale Targets Molly Flynn, Mariono Lowenstern, Paul Keiter, Heath LeFevre, Martin Di Stefano, Guy Wilson, Donna Marion, R. Drake Characterization of a target before it is shot is crucial for understanding the results obtained in high energy density experiments. We are developing a radiography system using a steady-state Manson x-ray source to better characterize these experimental targets. Due to the micro-scale of these targets, any non-uniformity in the density of target materials -- such as low-density carbon foams or plastics - could have adverse effects on experimental results. These inconsistencies are not necessarily diagnosable through other methods and thus require x-ray imaging for a more accurate analysis. We initially characterized the capabilities of our radiography system using metals of known x-ray opacity and geometric features and later expanded our subjects to include individual common target materials as well as fully fabricated targets. We present findings from a series of exposures varying flux, pinhole size, exposure time, and anode material, with the goal of optimizing resolution and magnification. This work is funded by the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, grant number DE-FG52-09NA29548. [Preview Abstract] |
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JP8.00012: Attempts to apply the ``rotating wall'' technique to a toroidal non-neutral plasma M. Price, A.S. Patterson, M.R. Stoneking We apply ``rotating wall'' electric fields in an attempt to spin up, compress and extend the confinement time of toroidal non-neutral plasma. In cylindrical Penning-Malmberg traps (with magnetic field strengths exceeding 1 tesla), azimuthally propagating electric field perturbations result in arbitrarily long confinement times for non-neutral plasma [X.-P. Huang, F. Anderegg, E.M. Hollmann, C.F. Driscoll and T.M. O'Neil, Phys. Rev. Lett. \textbf{78}, 875 (1997)]. We attempt to apply this technique to the toroidal electron plasma in the Lawrence Non-neutral Torus II device (major radius = 18 cm, minor radius = 3.81 cm, $B \quad \sim $ 550 G). Long confinement times ($>$300 ms) for plasma in a partially toroidal trap (270\r{ } arc) represent nearly steady-state conditions. A rotating electric potential perturbation with $m$=2 symmetry is applied to eight poloidal sectors of the segmented conducting boundary, and attempts are made to ``spin up'' the plasma with a frequency sweep in order to increase the density and confinement time. The plasma is diagnosed by measuring the flow of image charge to and from isolated sectors of the conducting boundary. The frequency of the $m$=2 ($k$=0) diocotron mode is used to measure the density. This work is supported by National Science Foundation Grant Award {\#}1202540. [Preview Abstract] |
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JP8.00013: Exploring higher density regimes using diocotron modes in a toroidal electron plasma A.S. Patterson, M. Price, M.R. Stoneking In the Lawrence Non-neutral Torus II (LNT II), a purely toroidal magnetic field (R$_{o}$=18 cm, B $<$ 550 G) confines an electron plasma at number densities near 10$^{7}$/cm$^{3}$. A recently modified filament design has permitted access to higher density regimes for plasma trapped in a 270$^{\circ}$ toroidal arc. Diocotron mode frequencies and damping rates are contrasted with those typical of cylindrical Penning-Malmberg traps. Exploration of these mode damping rates may provide insight into the relative significance of rotational and magnetic pumping transport mechanisms, an overarching goal of the experiment. We also present an improved computational model which will aid in the interpretation of mode damping measurement in LNT II. This work is supported by the National Science Foundation, Grant PHY-1202540. [Preview Abstract] |
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JP8.00014: Investigation of Electron Emission within Arc Cathodes Mitch Eagles, Yevgeny Raitses Arc discharges have significant applications in the synthesis of carbon nanostructures, especially single-walled carbon nanotubes. Electron emission in arc discharges is currently thought to occur largely due to thermionic emission. Theory suggests, however, that the temperature of the cathode must be over 3000 K in order to produce a high enough electron current to sustain the arc. This clearly conflicts with the use of copper, iron, and many other materials with melting points well below 3000 K as cathodes. Such cathodes do not significantly melt during the arc, indicating that thermionic emission cannot be the only source of electron emission. An investigation of non-thermionic emission methods is conducted, including field emission and secondary electron emission. Results will be presented. [Preview Abstract] |
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JP8.00015: Currents in the DIII-D Tokamak A. Azari, N.W. Eidietis Loss of vertical control of an elongated tokamak plasma results in a vertical displacement event (VDE) which can induce large currents on open field lines and exert high JxB forces on in-vessel components. An array of first-wall tile current monitors on DIII-D provides direct measurement of the poloidal halo currents. These measurements are analyzed to create a database of halo current magnitude and asymmetry, which are found to lie within the ranges seen by numerous other tokamaks in the ITPA Disruption Database. In addition, an analysis of halo asymmetry rotation is presented, as rotation at the resonance frequencies of in-vessel components could lead to significant amplification of the halo forces. Halo current rotation is found to be far more prevalent in old (1997-2002) DIII-D halo current data than recent data (2009), perhaps due to a change in divertor geometry over that time. [Preview Abstract] |
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JP8.00016: Testing the Validity of the Neoclassical Toroidal Viscosity Model of Torque due to 3D Non-Resonant Magnetic Fields A.J. McCubbin, S.P. Smith, N.M. Ferraro, J.D. Callen, O. Meneghini Understanding the torque applied by resonant and non-resonant magnetic perturbations and its effect on rotation is essential to predict confinement and stability in burning plasmas. Non-axisymmetric 3D fields produced in the DIII-D tokamak apply a torque to the plasma, which can be evaluated through its effect on the plasma rotation. One explanation for this torque is Neoclassical Toroidal Viscosity (NTV) acting through non-resonant field components [1]. We have developed a software framework in which magnetic perturbations calculated by the state of the art two fluid MHD code M3D-C1 can be used in NTV calculations. For discharges with applied external magnetic fields in DIII-D, the experimentally determined torques will be analyzed and compared with NTV models.\\[4pt] [1] J.D. Callen, Nucl. Fusion \textbf{51}, 094026 (2011). [Preview Abstract] |
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JP8.00017: Investigating the Validity of the ``Magnetic Flutter'' Model of Tokamak Transport in the Presence of 3D External Magnetic Fields P. Raum, S.P. Smith, N.M. Ferraro, J.D. Callen, O. Meneghini The suppression of edge localized modes (ELMs) is critical for future fusion devices in order to preserve the integrity of plasma facing components. On the DIII-D tokamak, non-axisymmetric coils are used to produce 3D magnetic fields that, under certain conditions, are successful at mitigating and suppressing ELMs. These externally applied resonant magnetic field perturbations (RMPs) help increase plasma transport at the top of the pedestal where we see a reduction in electron density and temperature gradients. A ``magnetic flutter'' model has been proposed to explain this effect, which was originally developed for a cylindrical geometry and has recently been extended to a toroidal geometry. The toroidal model has been evaluated for a single plasma condition using an analytic model of the magnetic field perturbations in the plasma ($\delta B$) with promising results. This has motivated a more systematic test of the validity of this model using $\delta B$ from the M3D-C1 code as a more accurate representation of the perturbations for a variety of discharges in which the 3D fields have been applied. [Preview Abstract] |
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JP8.00018: Benchmarking and Optimizing Techniques for Inverting Images of DIII-D Soft X-Ray Emissions E. Chandler, E.A. Unterberg, M.W. Shafer, A. Wingen A tangential 2-D soft \hbox{x-ray} (SXR) imaging system is installed on DIII-D to directly measure the 3-D magnetic topology at the plasma edge. This diagnostic allows the study of the plasma SXR emissivity at time resolutions $\geq$\,10~ms and spatial resolutions $\sim 1$~cm. Extracting 3-D structure from the 2-D image requires the inversion of large ill-posed matrices $-$ a ubiquitous problem in mathematics. The goal of this work is to reduce the memory usage and computational time of the inversion to a point where image inversions can be processed between shots. We implement the Phillips-Tikohnov and Maximum Entropy regularization techniques on a parallel GPU processor. To optimize the memory demands of computing these matrixes, effects of reducing the inversion grid size and binning images are analyzed and benchmarked. Further benchmarking includes a characterization of the final image quality (with respect to numerical and instrumentation noise). [Preview Abstract] |
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JP8.00019: Effect of Neutral Beam Parameters on Prompt Losses in Tokamaks J. Jurewicz, D.C. Pace, R.K. Fisher, M.A. Van Zeeland, C.T. Holcomb First orbit, or prompt losses occur when an injected neutral atom ionizes such that its first poloidal transit intersects a limiter surface. The heat load from these losses can affect diagnostic components near the first wall. We are developing a code that calculates where prompt losses reach the wall of the DIII-D tokamak as a function of plasma parameters including 3D beam geometry. This code also serves as a synthetic diagnostic for the Fast Ion Loss Detector system. Using magnetic equilibria and beam energies from DIII-D discharges, the pitch angle and density of neutral particles ionized along the beam path are determined and used to calculate ion trajectories, resulting in a map of prompt loss flux to the wall. A modeling study is performed to determine prompt loss versus beam energy. Initial results focus on whether it is possible to shift an existing prompt loss heat load away from a motional Stark effect diagnostic by increasing the energy of the source beam. [Preview Abstract] |
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JP8.00020: Predictive Modeling of DIII-D Tokamak Discharges Victor Genty, Tariq Rafiq, Arnold Kritz, Alexei Pankin This research focuses on validating the Multi-Mode anomalous transport module MMM7.1, recently installed in the PTRANSP code and available in the National Transport Code Collaboration library, w3.pppl.gov/ntcc. The MMM7.1 module is used to compute anomalous thermal, particle and toroidal angular momentum transport. MMM7.1 includes models for electron and ion temperature gradient, trapped electron, MHD, kinetic ballooning and drift resistive inertial ballooning modes. Simulations of DIII-D tokamak discharges are carried out using the PTRANSP predictive integrated modeling code with boundary conditions taken from evolving experimental data. The discharges simulated in the validation study of MMM7.1 include DIII-D Ohmic, L-mode, H-mode plasmas and plasmas with co- and counter-rotations. The time evolution of temperature, toroidal angular frequency and current density profiles predicted using the MMM7.1 transport module are compared with corresponding data from DIII-D tokamak discharges for which analysis is available in the ITPA database. For each class of DIII-D discharges studied in this research, RMS deviations are reported. Differences in predictions of plasma profiles for different plasma parameter regimes are discussed. [Preview Abstract] |
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JP8.00021: Diagnosis of Local Plasma Conditions within an ICRF Antenna on DIII-D W.K. Balunas, R.I. Pinsker, S.J. Diem, M.C. Kaufman, E.A. Unterberg, A. Nagy One of the three ion cyclotron rf antennas on the DIII-D tokamak is equipped with an extensive set of diagnostics. These instruments, including Langmuir probes, light-gathering lenses, fast neutral pressure gauges (``ASDEX gauges''), and rf voltage probes, are used to study the origins of rf breakdown (``arcing'') in the ICRF antenna. For a given plasma condition, the onset of arcing sets the upper power limit that can be coupled to the discharge. Data is obtained both during vacuum conditioning and during high-power operations with plasma. In addition, some of these instruments are tested during plasma operations without rf power being applied to the array. The information is used to search for correlations between the onset of arcing and local conditions in and around the antenna. The ultimate goal of this work is to enable the coupling of increased rf power levels to the discharge. [Preview Abstract] |
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JP8.00022: Modeling of Pellet Ablation and Deposition on Plasma Facing Surfaces A.M. Lietz, G.L. Jackson, W. Wu, L.R. Baylor, N. Commaux The injection of pellets into tokamaks has a variety of uses, such as ELM mitigation, refueling, and the reduction of hydrogenic recycling. We have modeled the ablation rate of pellets as a function of the plasma density and temperature. A computational model is then used to compare pellet ablation, penetration into the plasma and, for non-hydrogenic pellets, the deposition of ablation material on plasma facing surfaces. In the pedestal region, density and temperature profiles are approximated by a tanh fit using DIII-D experimental data. Penetration and ablation profiles of lithium pellets are compared to Be, B, C, and cryogenic deuterium pellets using these profiles. The behaviors at different injection locations, initial velocities, and pellet sizes are also explored, especially to determine the minimum size and velocity necessary to reach just inside of the last closed flux surface. The results of the model can be used to optimize a lithium dropper experiment for DIII-D by determining the placement of the dropper and size of the lithium pellets for maximum penetration. [Preview Abstract] |
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JP8.00023: MHD Spectroscopy of Fusion Plasmas by ECE-Imaging of Reversed-Shear Alfv\'en Eigenmodes C.V. Pieronek, B.J. Tobias, L. Yu, M.A. Van Zeeland The spatial electron temperature data acquired using the electron cyclotron emission imaging (ECEI) diagnostic on \hbox{DIII-D} was used to detect and study the spatial structure of Alfv\'en eigenmodes present in reversed-shear plasmas. The $q$-profile and magnetic field topology of the plasma are deduced from the spatial mode structure and compared with measurements made using the motional Stark effect diagnostic. These results are compared with calculations of the RSAE mode structure obtained using the Nova, TAEFL, Gyro, GTC, and M3D codes with measured plasma profiles as input. The implications for MHD spectroscopy are discussed. [Preview Abstract] |
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JP8.00024: Edge Density Imaging Measurements of DIII-D Tokamak Plasmas using a Lithium Beam Probe and High Resolution Camera M.F. Martin, H. Stoschus, D.M. Thomas, D.C. Pace The Lithium Atomic Beam (LIBEAM) used on DIII-D has shown considerable potential to diagnose the density profile $n_e(r)$ with a radial resolution of $\Delta r = 0.5$~cm within the pedestal region. The LIBEAM parameters are $E < 30$~keV and $\sim$10 mA of equivalent neutral lithium current. Through the use of a filtered high resolution PCO Pixelfly CCD camera, the spectroscopic emission of the 670.8 nm Li[2p$-$2s] transition due to collisional excitation of the neutral lithium atoms is captured and analyzed. By appropriate image analysis, a high resolution profile of the beam intensity $I_b$ can be discerned. Through the use of this beam intensity profile and collisional radiative models (CRM) the fine scale structure of the edge density profile $n_e(r)$ can be observed. [Preview Abstract] |
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JP8.00025: High Resolution Spectral Measurements of Lithium Beam-Induced Emission in Pedestal Region of DIII-D Plasmas C.M. Koch, D.M. Thomas, H. Stoschus A 30 keV neutral lithium beam (LIBEAM) is used on DIII-D to measure the current density the pedestal region and test model predictions. The LIBEAM produces collisionally excited 670.8~nm resonance florescence as well emission at other wavelengths due to plasma ion charge exchange which may be studied using the appropriate spectral analysis. These measurements are being pursued using an $f/3$ transmission grating spectrometer in conjunction with a high efficiency charged coupled device (CCD) camera capable of 8~ms/frame temporal response and a resolving power approaching 20,000 at 670~nm. Surveys are being made at various wavelengths to determine the lithium beam Doppler broadened emission profile as well as to assess the prospects for determining edge ion temperature and impurity profiles based on thermal broadening and integrated spectral line intensity. [Preview Abstract] |
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JP8.00026: Analysis of First Plasma in Madison Plasma Dynamo Experiment Christopher Marchbanks, David Weisberg, John Wallace, Christopher Cooper, Cary Forest The Madison Plasma Dynamo Experiment (MPDX) is designed to investigate the self-generation of magnetic fields in large astrophysical dynamos. Simulations show that an adequately large Magnetic Reynolds number is needed to produce a dynamo; therefore a large, hot, flowing and unmagnetized plasma is necessary. This poster presents an analysis of the first plasma in MPDX using electrodes that both heat and stir the plasma. A lanthanum hexaboride (LaB$_6$) electron source is used within the 3000 Gauss multipole cusp magnetic field and E$\times$B stirring electrodes govern the plasma flow. This poster analyzes the performance of the efficient and fast power supply used in conjunction with the stirring electrodes to create a controllable flow using a feedback loop. In addition, the effectiveness of the LaB$_6$ electron source will be reported. Supported by the DOE and NSF. [Preview Abstract] |
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JP8.00027: Scaling of Magnetic Islands with Applied Guide Fields in MRX H. Lazar, M. Yamada, H. Ji, J. Jara-Almonte, C.E. Myers, J. Yoo Magnetic islands are thought to play a key role in magnetic reconnection. It has been observed that an increase in applied guide field promotes island formation and simultaneously decreases reconnection rate. Recently in the Magnetic Reconnection Experiment (MRX), a series of discharges were obtained to explore island formation under a variety of plasma conditions. In particular, the properties of the applied discharge were varied including (1) the guide field's strength; (2) the guide field's direction relative to the reconnection current; and (3) drive time. Flux plots and vector plots were used to locate and quantify the shape and size of the islands. The size and frequency of island formation is related to the properties of guide fields and reconnection rates. [Preview Abstract] |
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JP8.00028: Exploring the variability of ion heating at reconnection events in MST M.S. Cartolano, D. Craig, D.J. Den Hartog, S.T.A. Kumar, M.D. Nornberg The variability of ion heating for individual reconnection events in MST is correlated with key plasma parameters to give insight into the process of ion heating. Global reconnection events in MST convert stored magnetic energy into ion thermal energy. The change in impurity ion temperature during several thousand reconnection events was analyzed for standard plasmas. As expected, the change in ion temperature correlates strongly with the change in magnetic energy. Magnetic fluctuations in MST are thought to be responsible for driving reconnection, and larger heating does correlate with larger increases in mode amplitudes during the event. The strongest correlation is with the rate of change in the m=0 magnetic fluctuation amplitude. Other anomalous behavior appears during reconnection, such as dynamo activity and electron thermal transport. When these activities are stronger, the amount of ion heating is also stronger. Finally, a possible toroidal asymmetry to the ion heating is being investigated. Work supported by U.S.D.O.E. [Preview Abstract] |
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JP8.00029: A table-top demonstration of an exact mechanical analog of a magnetic mirror Patricio Arrangoiz, Paul Bellan Long thought of as a peculiarity of electromagnetism, the well-known phenomenon of magnetic mirroring can now be understood as a property of any 2D Hamiltonian system having fast oscillatory motion in one direction and slow motion in the other. This property has recently been shown to apply to a much wider class of multidimensional systems with a periodic variable [R.J. Perkins and P.M. Bellan, PRL \textbf{105}, 124301 (2010)]. The purpose of this project is to build a table-top system that is an exact mechanical analog of a magnetic mirror. The system involves a small ball that is set in rolling motion on a saddle-like surface. The surface has a downhill parabolic profile (slow direction), with a groove of parabolic cross-section (fast direction) that narrows as one moves away from the center of the hill. The dynamics of the system is such that the adiabatic invariance effectively produces a return force opposing gravity. This return force prevents the ball from falling and makes it oscillate about the top of the hill. This behavior has been verified numerically for a variety of ball masses and surface parameters. Machining possibilities such as CNC microstepping on aluminum and 3D printing techniques are being investigated. [Preview Abstract] |
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JP8.00030: Comparative Study of Radiation Dosage Distribution and Medical Implication of Quasi-monoenergetic Proton Generated from Laser Acceleration of Ultra-thin Foil Temuge Batpurev, Jennifer Cao, Wang Xie, Tung-Chang Liu, Xi Shao, Chuan-Sheng Liu Recently the search for mono-energetic protons has gained great interest, particularly in applications such as proton therapy for cancer treatment. The advantage of proton therapy is that unlike photon radiation, proton beams deposit most of the energy at the tumor, sparing surrounding tissue and vital organs. A compact laser-driven proton accelerator is attractive for proton cancer therapy since the electric fields for particle acceleration can reach the order of tens of GV per cm which allows large reduction of the system size. Recent work by Liu et al. [2012] shows that laser acceleration of an ultra-thin multi-ion foil can generate high quality quasi-monoenergetic proton beams. The proton acceleration is due to the combination of radiation pressure and heavy-ion Coulomb repulsion. To assess the feasibility of laser-proton cancer therapy with such a proton accelerator, we simulated the interaction of protons with water and determine the radiation dosage deposition for particle beams produced from the PIC simulation of laser acceleration of multi-ion targets. We used the SRIM code to calculate the depth and lateral dose distribution of protons. We also compared the dosage map produced from protons generated from laser acceleration of single ion and multi-ion targets. [Preview Abstract] |
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JP8.00031: Simultaneous measurement of proton and electron energy spectra using a Thomson parabola ion spectrometer K.R. Crompton, M.J. Schepis, J.P. Shortino, C.G. Freeman, G. Fiksel, C. Mileham, T.C. Sangster Simultaneous measurements of the energy spectrum of protons and electrons accelerated from the rear side of thin targets illuminated with ultra-intense laser light have been carried out at the Multiterawatt (MTW) laser facility at the Laboratory for Laser Energetics. The particles enter a Thomson parabola ion spectrometer consisting of a permanent magnet and a pair of electrostatic deflector plates. A Fujifilm imaging plate mounted at the rear of the device was used to detect the protons. A thin tantalum foil was placed on top of the imaging plate to prevent all ions other than the protons from reaching the plate. Electrons, with a much smaller magnetic rigidity, are deflected strongly by the Thomson parabola permanent magnet and exit the magnet before entering the electrostatic deflector plates. Another imaging plate placed on top of the permanent magnet was used to detect these electrons. A computer program has been written to analyze the resulting data from the imaging plates. This enables the proton and electron energy spectrum to be determined simultaneously for a particular shot. This work is funded in part by a grant from the DOE through the Laboratory for Laser Energetics. [Preview Abstract] |
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JP8.00032: Measurements of the sensitivity and spatial resolution of radiochromic film using ion beams and X-rays M.J. Schepis, J.P. Shortino, K.R. Crompton, C.R. Stillman, C.G. Freeman, P.M. Nilson, C. Sorce, T.C. Sangster Radiochromic film (RCF) is used to study protons and other ions that are accelerated from the rear side of targets illuminated with ultra-intense laser light. An experiment is underway to measure the sensitivity of RCF to protons and alpha particles of various energies using the 1.7 MV tandem Pelletron accelerator at SUNY Geneseo. An ion beam incident on a gold foil is used to expose the RCF to elastically scattered ions in a 28-inch diameter scattering chamber. The film is positioned in a circular arc in the chamber so the scattered ion fluence on the RCF strip varies as a function of the scattering angle. After exposure, the RCF is scanned in color negative transmission mode using an Epson 10000 XL flatbed scanner. The red channel of the resulting scan is used to determine the optical density of the film. The spatial resolution of the film was measured by blocking part of the film with a tantalum knife edge. The sensitivity of RCF to X-rays was also measured by exposing the film to X-rays produced by a biological irradiator. The response of the new Gafchromic HD-V2 radiochromic film is compared with the discontinued Gafchromic HD radiochromic film. [Preview Abstract] |
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JP8.00033: X-ray Enhancement of Etch Parameters of Nuclear Tracks in CR-39 Micheal Giordano, Krystalyn Sadwick, Kurt Fletcher, Michelle Burke, T. Craig Sangster The nuclear track detector CR-39 is a polymer used to measure charged particles produced in inertial confinement fusion experiments. Alpha particles stopping in CR-39 produce nanometer-scale damage sites. When the CR-39 is etched in 6-N sodium hydroxide at 80\r{ } C for six hours, the difference in etch rates between the damage sites and the bulk material results in the formation of pits 20 to 25 microns in diameter. These can be characterized and counted using optical microscopy. A modest increase in the pit diameter is observed when the CR-39 detector is irradiated by x-rays from a tungsten cathode after exposure to charged particles and before etching. This enhancement of the diameter increases as the total x-ray dose increases, with enhancements of about 1.10 for 1000 Gy doses. Controlled experiments show that the effect is due to the x-ray dose rather than a difference in the handling or the environment. The ratio of the track-etch rate to bulk-etch rate seems to be independent of x-ray dose. The mechanism for this effect is currently under investigation. These results show that modest increases in pit diameters can be obtained through irradiation with x-rays. [Preview Abstract] |
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JP8.00034: The effects of neutral beam injection on momentum transport and rotation resulting from reconnection events in a Reversed Field Pinch T. Dobbins, M.D. Nornberg, J.K. Anderson, D.J. Den Hartog, J.A. Reusch, John Sarff, Scott Eilerman, Darren Craig Magnetic reconnection events are characterized by rapid transport that flattens both the plasma current and parallel flow profiles in a RFP. The tangential neutral beam on the MST is a source of momentum injection into the MST that has also been observed to suppress the core-most mode of the plasma. Ensembles of multiple sawtooth events with and without the NBI were performed over a variety of plasma conditions to observe any effects of the NBI on sawtooth crashes. Observations of both mode rotation and impurity emission Doppler shifts show an increase in toroidal rotation associated with the neutral beam. The suppression of the core-most mode was verified for a broader variety of plasmas then before. The mode data also shows that for some plasma parameters the NBI brings a mode into resonance that is not resonant without the NBI. This is the first evidence of the NBI's effect on the plasma current profile. In addition, Co-injection greatly reduces the mode locking, while counter-injection has been shown to slightly increase mode locking. [Preview Abstract] |
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JP8.00035: The Effects of an External Perturbation on an Existing Dusty Plasma Joseph Shaw, Patrick Donnan, Edward Thomas This project seeks to understand how the confinement and instabilities of an existing dusty plasma cloud are modified by the introduction of external perturbations. This external perturbation can come from one of two sources: the streaming of a secondary population of dust particles or the introduction of a quickly decaying secondary plasma population. In the first case, a streaming population of particles are gravitationally accelerated into the existing dust cloud. In the second case, secondary plasma is generated via additional electrodes that create a plasma shock which perturbs the existing dust cloud. High-speed imaging and Particle Image Velocimetry (PIV) techniques are used to measure and characterize the motion of the particles before and after the applied perturbations over a range of neutral pressures and electrode bias voltages. It is postulated that these processes may be analogous to phenomena that can occur in the space environment. [Preview Abstract] |
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JP8.00036: Magnetic Sense Coil Array for the FAMU Spheromak Robert VanDervort, Amy Keesee, Greg Lusk, Eric Reynolds, Earl Scime First plasmas in the Florida A{\&}M Spheromak are expected in 2012. Here we describe the design, construction, and calibration of a magnetic sense coil array that will provide measurements of the edge poloidal and toroidal magnetic field fluctuations in the spheromak. The electrostatically shielded coils are mounted flush with the inner edge of the vacuum chamber. Custom differential amplifiers, based on designs by the MST group at the University of Wisconsin-Madison, enable fluctuation measurements up to frequencies of 2 MHz. [Preview Abstract] |
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JP8.00037: High frequency umbilical magnetic probe array for SSX wind tunnel A.M. Werth, T. Gray, M.R. Brown The Swarthmore Spheromak Experiment (SSX) wind tunnel consists of a high velocity plume of magnetized plasma injected into a copper flux conserver with dimensions $L = 1~m$ and $R = 0.08~m$ (aspect ratio $10:1$). The plasma spheromaks in this wind tunnel typically have densities on the order of $1 - 5 \times 10^{15}~cm^{-3}$ and flow speeds of $50~km/s$. In the past, fluctuations and turbulence in the SSX plasma wind tunnel during magnetic reconnection have been examined by means of two high resolution (16 position at $0.46~cm$ spacing) radial magnetic probes. Results from the radial probes show high frequency magnetic fluctuations at the site of reconnection. Four more probes have been design to help detect magnetic fluctuations and reconnection activity along the axial direction of the wind tunnel. The four new probes have 8 positions at $0.95~cm$ spacing and have a flexible bellow in vacuum and a quartz jacket. The bellows act as an umbilical giving the probe excellent flexibility and versatility. The flexibility allows the probe to be bent so it lies along the axis of the flux conserver. [Preview Abstract] |
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JP8.00038: Comparison of Monte Carlo simulations with analytic collision model E. Day-Roberts, M.R. Brown, T. Gray, V.S. Lukin We test a Monte Carlo simulation of particle collisions based on a model by Takizuka and Abe [JCP 25, 205, (1977)]. This model is included in the Hamiltonian particle pushing code (PPC) for simulating particle orbits in the Swarthmore Spheromak eXperiment (SSX) MHD wind tunnel. The simulated dynamics, with collisions, are compared with analytical transport equations for slowing down, diffusion, and energy loss. Preliminary results show general agreement with the analytic model. The Takizuka collision model performs binary collisions between the test particle and a field particle drawn from a stationary Maxwellian background distribution of ions. The time difference between collisions is dependent on the current plasma parameters. Realistic particle dynamics in simulated SSX wind tunnel fields will be presented if available. [Preview Abstract] |
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JP8.00039: Incorporation of the Data Acquisition System with a Small Helicon Plasma Experiment (HPX) Stephen Nolan, R.W. James, E.L. Page, J. Zuniga, C. Schlank, M. Lopez, J. Sherman, B.S. Stutzman At the Coast Guard Academy Plasma Lab (CGAPL), a small Helicon Plasma Experiment (HPX) is being developed to utilize the reputed high densities (10$^{13}$ cm$^{-3}$ and higher) at low pressure (.01 T) [1], in high temperature and density diagnostic development for future laboratory investigations. With first plasmas at hand, HPX is constructing triple and mach particle probes, magnetic probes, and a single point Thompson Scattering system for HPX plasma property investigations. A 32-channel National Instruments Data Acquisition (DAQ) Board capable of sampling at 12 bits of precision at 2 MS/s and running multiple simultaneous experiments is currently under construction. This DAQ System with integrated storage and GUI's will gather and digitize plasma data from the associated diagnostics for further analysis. Progress on the current implementation of the DAQ system will be reported. [Preview Abstract] |
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JP8.00040: Design of a combination ion saturation/triple probe for fast plasma measurements Kevin Gilmore, Ivan Arnold, Mark Cianciosa, Edward Thomas, Joseph Johnson III, Kyron Williams, Charles Weatherford This presentation details the design and construction of an in-situ probe diagnostic system for the FAMU Spheromak Turbulent Plasma Experiment (STPX). A spheromak is a confined, magnetized plasma configuration that is created when a ``gun'' injects the plasma from an electrode and into a confinement area - the flux conserver - where it self-organizes into a toroidal shape. This self-organized plasma typically lasts for less than 200 microseconds, and so any diagnostics must be able to operate on very fast time scales. The probe is a linear array that consists of a triple Langmuir probe at the tip and three cylindrical probes along the length of the assembly. This probe is designed to make simultaneous measurements of the ion saturation current, as well as the floating potential, plasma potential, electron temperature, and electron density plasma. This presentation will give a summary of the development process for this probe system: from the initial pulsed plasma experiment to the final probe design for STPX. Preliminary data from STPX may also be presented. [Preview Abstract] |
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JP8.00041: Particle Probe Investigations on the Helicon Plasma Experiment (HPX) Justin Sherman, R.W. James, M. Lopez, S. Nolan, E.L. Page, C. Schlank, B.S. Stutzman, J. Zuniga A small Helicon Plasma Experiment (HPX) has been constructed at the Coast Guard Academy Plasma Lab (CGAPL) to utilize the reputed high densities at low pressure (.01 T) [1], in high temperature and density diagnostic development for future laboratory investigations. With the initial construction phase complete, HPX has produced its first plasmas. Efforts to develop and enhance the high temperature and density (10$^{13}$ cm$^{-3}$ and higher) helicon plasmas at low pressures (.01 T) reported by Toki, Shinohara, et. al. continue. Currently, particle probes to measure plasmas' temperatures and densities, necessary to discern the plasma mode transitions, are in development. Construction of independent mach and triple probes for single point surface investigations are underway and once installed, they will be followed by a triple probe array to produce a more comprehensive density and surface view. Progress on the construction and findings of these probes on HPX will be reported. [Preview Abstract] |
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JP8.00042: Quantitative fitting of transport model parameters to experimental profiles E.O. Hadden, L. Jones, A.S. Ware, M. Gilmore, E. Schuster The results of a quantitative comparison of experimental data from the HELCAT experiment and computational results from a 1D transport code are presented. In this work, we present both recent work on understanding the behavior of the transport model and a new effort to optimize of transport code input parameters in order to fit experimental profiles. This computational work is in collaboration with experimental work on the impact of a set of biased concentric rings on turbulent transport in the HELCAT device. The biased rings are modeled in the transport code as local {\bf E}$\times${\bf B} momentum sources. In our characterization of the transport code, both particle diffusivity and particle flux are shown to decrease with increased ring bias voltages and both are relatively insensitive to which of the rings is biased. On fitting to experimental data, we are developing an optimization routine that selects a set of input parameters to the transport code by minimizing a $\chi^2$ difference between the code results and experimental data. [Preview Abstract] |
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JP8.00043: Are ion acoustic waves supported by high-density plasmas in the Large Plasma Device (LaPD)? Rebecca Roycroft, Seth Dorfman, Troy A. Carter, Walter Gekelman, Shreekrishna Tripathi Ion acoustic waves are a type of longitudinal wave in a plasma, propagating though the motion of the ions. The wave plays a key role in a parametric decay process thought to be responsible for the spectrum of turbulence observed in the solar wind. In recent LaPD experiments aimed at studying this process, modes thought to be ion acoustic waves are strongly damped when the pump Alfven waves are turned off. This observation motivates an experiment focused on directly launching ion acoustic waves under similar conditions. Our first attempt to launch ion acoustic waves using a metal grid in the plasma was unsuccessful at high magnetic fields and densities due to electrons shorting out the bias applied between the grid and the wall. Results from a new device based on [1] to launch ion acoustic waves will be presented; this device will consist of a small chamber with a plasma source separated from the main chamber by two biased grids. The plasma created inside the small device will be held at a different potential from the main plasma; modulation of this difference should affect the ions, allowing ion acoustic waves to be launched and their properties compared to the prior LaPD experiments.\\[4pt] [1] W. Gekelman and R. L. Stenzel, Phys. Fluids 21, 2014 (1978). [Preview Abstract] |
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JP8.00044: Investigation of Turbulence, Intermittent Structures and Driven Flow in Magnetized Plasma Using Visible light Imaging Daniel Guice, David Schaffner, Troy Carter, Giovanni Rossi, Steve Vincena A fast framing camera is used to image plasma in the Large Plasma Device (LAPD) at UCLA. The use of a camera enables high spatial resolution in a single plasma discharge, without perturbing the plasma. Correlation between light fluctuations and plasma density is high, giving a physical link to what the camera records. From the light fluctuations instantaneous velocity fields are calculated using a wavelet based method that gives us the ability to estimate particle flux and Reynolds stress. These quantities are compared with results obtained with probes. Flow and flow shear is driven in the LAPD through applied bias on newly installed limiters; allowing for a detailed study of how shear flow modifies particle flux. Intermittent coherent structures (``blobs'' and ``holes'') have also been observed in the edge turbulence adjacent to the shear flow layer, and there effects on transport are investigated. [Preview Abstract] |
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JP8.00045: Metamaterial Lens for Improved Transverse Resolution of mm-wave Diagnostics, applied to ECE at DIII-D Scott Massidda, William Capecchi, Kenneth Hammond, Francesco Volpe Millimeter wave diagnostics of plasmas typically cover bands of several GHz (reflectometry, scattering), tens of GHz (radiometry) or even hundreds of GHz (Michelson interferometry), but their focus is optimized for a single frequency. For other frequencies, the measuring volume is far from the beam waist. This results in a loss of resolution in the poloidal direction, especially at higher poloidal mode numbers (e.g., Alfv\`{e}n Eigenmodes). Ideally the beam should be focused at different locations for different frequencies. Our recent numerical study suggests that a zoned planar metamaterial lens can achieve this result in the 8-12GHz band [W.J. Capecchi \textit{et al}., \textit{Optics Express} \textbf{20}, 8761 (2012)]. Here we present the design and full-wave simulations of a lens for possible use with Electron Cyclotron Emission (ECE) at DIII-D in the 80-130GHz band, discuss the fabrication challenges due to the required miniaturization, and present a system allowing the adjustment of the focal points to accommodate changes in the magnetic field. Because ECE at DIII-D undergoes one of the largest variations of optimal focal length, similar metamaterial lenses can more easily be designed for other mm-wave diagnostics and/or smaller devices. [Preview Abstract] |
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JP8.00046: Nonlinear Oscillations in a Unijunction Transistor Circuit Steven Christopher, John Zielinski, Mark Koepke Many interesting nonlinear behaviors have been studied in distributed (glow-discharge and Q-machine plasmas) and non-distributed (nonlinear electronic oscillators) nonlinear systems that can be modeled by the van der Pol equation.\footnote{Phys. Plasmas 3, 4421 (1996).}$^,$\footnote{Geophys. Res. Lett., 21, 1011 (1994).}$^,$\footnote{Phys. Rev. A 44, 6877 (1991).} This work describes an experimental, theoretical and computational investigation of two nonlinear electronic oscillators which have a unijunction transistor as a nonlinear element. The circuits that are examined in this paper are a sine wave oscillator and a relaxation oscillator. The functioning of the unijunction transistor is explained in detail. A full derivation of the differential equation describing the sine wave oscillator is made, and the results of numerical simulations based on this differential equation are compared to experimental data. Descriptions and explanations of two types of non-autonomous (driven) phenomena, entrainment and periodic pulling will be given. [1] Present affiliation: Univ. of Notre Dame, [2] Present affiliation: SUNY Oswego [Preview Abstract] |
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JP8.00047: A Faraday rotation diagnostic for Z pinch experiments K.W. Gao, T.P. Intrator, T.E. Weber, C.B. Yoo, J. Klarenbeek The MagLIF experiment is an approach to Magneto Inertial Fusion (MIF) that will compress a laser preheated magnetized plasma inside a small sub cm size beryllium capsule and the magnetic field inside. A good measurement of the compressed magnetic field will help us understand how the compression proceeds, and the time scale over which field diffuses out. We are working on a first step to the direct measurement of vacuum magnetic field (expected to be mostly Bz) compression time history, potentially space-resolved, without a plasma fill. A small magneto-active section of optical fiber can measure magnetic fields in the 1-1000 Tesla range. Directly measured vacuum Bz is an initial but important step towards validating the codes supporting MagLIF. The technology will use a Terbium doped optical fiber as a Faraday rotation medium. The optical path and hardware is simple, inexpensive, and small enough to fit inside a MagLIF capsule, and can be radiation hardened. Low noise, optically coupled magnetic field measurements will be possible for vacuum MagLIF shots. [Preview Abstract] |
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JP8.00048: A Fiber Interferometer for the Magnetized Shock Experiment C.B. Yoo, K.W. Gao, T.E. Weber, T.P. Intrator The Magnetized Shock Experiment (MSX) at Los Alamos National Laboratory requires remote diagnostics of plasma density. Laser interferometry can be used to determine the line-integrated density of the plasma. A multi-chord heterodyne fiber optic Mach-Zehnder interferometer is being assembled and integrated into the experiment. The advantage of the fiber coupling is that many different view chords can be easily obtained by simply moving transmit and receive fiber couplers. Several such fiber sets will be implemented to provide a time history of line-averaged density for several chords at once. The multiple chord data can then be Abel inverted to provide radially resolved spatial profiles of density. We describe the design and execution of this multiple fiber interferometer. [Preview Abstract] |
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JP8.00049: Magnetic Field Measurements Inside a Supersonic Plasma Jet Formed by a Pulsed Railgun D. Martens, S.C. Hsu The Plasma Liner Experiment (PLX) at Los Alamos National Laboratory (LANL) was designed to create a spherically imploding plasma liner via the merging of 30 supersonic plasma jets formed by pulsed plasma railguns. The propagation and merging of two plasma jets is being studied via spectroscopy, a photodiode array, and interferometry to analyze plasma temperature, velocity, density, and ionization fraction of the evolving jets. Although the magnetic field strength is known to be strong ($>1$~T) within the bore of the plasma railgun, no measurements have been made of the field in the jet once it propagates away from the gun. Based on the expected jet radius ($\sim 5$~cm) and temperature ($\sim 1$~eV) near the exit of the gun, the classical diffusion time of the field ($\sim 3$~$\mu$s) is expected to be short enough such that the field will be very small by the time the jets propagate a few tens of centimeters. This work presents the design of a magnetic probe array to measure the magnetic field vectors at 5 and 30 cm from the plasma gun nozzle, and also the initial experimental results from the magnetic probe diagnostic. [Preview Abstract] |
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JP8.00050: Gorgon simulations of Magnetized Liner Inertial Fusion Jonathan Roeltgen, Christopher Jennings, Adam Sefkow, Stephen Slutz Substantial fusion yields are predicted with pulsed power machines driving cylindrical liner implosions with preheated and magnetized deuterium-tritium [S.A. Slutz et al Phys. Plasmas 17, 056303 (2010)]. The Rayleigh-Taylor instability is the most likely mechanism that could degrade the fusion yield of this concept which we call Magnetized Liner Inertial Fusion (MagLIF). Gorgon is a 3D magnetohydrodynamics code that is well suited to simulating the effects of 3D Magneto-Rayleigh-Taylor instabilities. It has successfully simulated wire array z-pinches in 3D. We plan to use Gorgon to simulate the MagLIF concept in 3D, but first we are performing 1D simulations to test the essential physics necessary to simulate the MagLIF concept, for example bremsstrahlung losses from the fuel and the inhibition of transport by the magnetic field. We will present 1D Gorgon results of optimized MagLIF yields as a function of drive current. [Preview Abstract] |
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JP8.00051: Amplification and acceleration of nonlinear plasma waves by magnetic compression Jason Rocks, Paul Schmit, Nathaniel Fisch The effects of magnetic compression on nonlinear electron plasma waves traveling parallel to the background field are studied using novel particle-in-cell simulations. The waves are excited through the application of an amplitude- and frequency-modulated sinusoidal external driver near resonance over a finite period of time. Subsequently, magnetic compression of the plasma results in adiabatic wave amplification and evolution of the nonlinear mode frequency. This effect is observed for adequately slow compression, while faster compression results in particle detrapping and wave degradation. Potential applications and connections to analytic theory will be explored. [Preview Abstract] |
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JP8.00052: Multi-Point Thomson Scattering Diagnostic Michael Lopez, Royce James, Carter Schlank, Stephen Nolan, Nicholas Thayer, Justin Sherman The obtainment and implementation of Thomson Scattering (TS) equipment from Lawrence Livermore National Laboratory will allow for the upgrade of the TS system at HBT-EP from a single spatial-point to a multiple spatial-point system. The additional spatial-points allows for a more developed profile of electron temperature and density via an equilibrium reconstruction process. This allows for multi-mode feedback control and stability calculations over a larger spatial region. Subsequently the multipoint reconstruction should enable the separation of \textit{$\beta $}$_{p}$, the ratio between plasma pressure and poloidal magnetic field pressure, and $l_{i}$, the plasma's internal induction, which cannot otherwise be separated on HBT-EP. Over the summer, work on the installation and alignment of the TS system was performed during a 5-week internship. Progress made over that time window will be reported. [Preview Abstract] |
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JP8.00053: Imaging plasmas with coded aperture methods instead of conventional optics Pakorn Wongwaitayakornkul, Paul Bellan The spheromak and astrophysical jet plasma at Caltech emits localized EUV and X-rays associated with magnetic reconnection. However, conventional optics does not work for EUV or X-rays due to their high energy. Coded aperture imaging is an alternative method that will work at these energies. The technique has been used in spacecraft for high-energy radiation and also in nuclear medicine. Coded aperture imaging works by having patterns of materials opaque to various wavelengths block and unblock radiation in a known pattern. The original image can be determined from a numerical procedure that inverts information from the coded shadow on the detector plane. A one-dimensional coded mask has been designed and constructed for visualization of the evolution of a 1-d cross-section image of the Caltech plasmas. The mask is constructed from Hadamard matrices. Arrays of photo-detectors will be assembled to obtain an image of the plasmas in the visible light range. The experiment will ultimately be re-configured to image X-ray and EUV radiation. [Preview Abstract] |
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JP8.00054: Scaling of spontaneous edge plasma rotation in the Large Plasma Device Milo Taylor, Troy Carter Spontaneous cross-field rotation is measured in the edge plasma of the Large Plasma Device at UCLA. A spatially-broad mean flow in the ion diamagnetic drift direction is observed near the edge of the cathode source region. Shearing associated with this flow appears to modify edge turbulence and create a weak particle transport barrier. Multiple flow measurements have been made in LAPD using different techniques such as Mach probe, $\bf E$$\times$$\bf B$ from plasma potential measurements, and time-delay estimation using both Langmuir probe and fast-framing camera images of visible light emission. A comparison between these measurement techniques and how the cross-field flow in LAPD scales with plasma parameters such as magnetic field, discharge current, and fill pressure is demonstrated. [Preview Abstract] |
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JP8.00055: Commissioning of an ECR Plasma Source for Lithium Surface Science Studies and Initial Results Ryan Norval, Angie Capece, Oluseyi Faroranti, Charles Skinner, Bruce Koel Lithium-conditioned plasma facing components reduce hydrogen recycling in plasmas by readily forming hydride compounds, which results in improved plasma confinement and performance. A new ultrahigh vacuum (UHV)-compatible electron cyclotron resonance (ECR) plasma source, which allows surface spectroscopic studies, was recently commissioned at the Princeton Plasma Physics Laboratory to study the uptake of deuterium by lithium coatings on single crystal molybdenum as a precursor to NSTX experiments. The ECR plasma source is capable of delivering deuterium ions to the surface at energies of 50-2000 eV and current densities between 0.05 and 2.0 mA/cm$^{2}$. Surface science studies will be performed on clean Li films on Mo(100) substrates before and after D+ irradiation under UHV conditions at base pressures of 10$^{-10}$ Torr. X-ray photoelectron spectroscopy will be used to determine the chemical state of lithium, and thermal desorption spectroscopy will be used to measure the amount of deuterium retained in the sample as a function of ion fluence and temperature. [Preview Abstract] |
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JP8.00056: Comparison of M3DC1 Finite-Elements Code to Analytic Asymptotic Matching Solutions of Tearing Modes Andrew Ritchie, Thomas Scaffidi, Stephen Jardin The M3DC1 implicit high-order two-fluid finite-element plasma-modeling code has been benchmarked in the linear resistive regime by comparison to an asymptotic matching solution for the case of a tearing mode instability in a circular cylindrical tokamak. The instability is driven by current and pressure gradients and involves reconnection of the magnetic field. The asymptotic matching technique combines the solution to the resistive magnetohydrodynamic (MHD) equations in the thin region around the mode rational surface with the solution to the ideal MHD equations beyond it. The M3DC1 and asymptotic matching results for the mode growth rate were compared over a wide range of Lundquist numbers $(10^{4} |
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JP8.00057: Application of 3D Stereo Vision to the Reconnection Scaling Experiment J. Klarenbeek, J.A. Sears, K.W. Gao, T.P. Intrator, T.E. Weber The measurement and simulation of the three-dimensional structure of magnetic reconnection in astrophysical and lab plasmas is a challenging problem. At Los Alamos National Laboratory we use the Reconnection Scaling Experiment (RSX) to model 3D magnetohydrodynamic (MHD) relaxation of plasma filled tubes. These magnetic flux tubes are called flux ropes. In RSX, the 3D structure of the flux ropes is explored with insertable probes. Stereo triangulation can be used to compute the 3D position of a probe from point correspondences in images from two calibrated cameras. While common applications of stereo triangulation include 3D scene reconstruction and robotics navigation, we will investigate the novel application of stereo triangulation in plasma physics to aid reconstruction of 3D data for RSX plasmas. Several challenges will be explored and addressed, such as minimizing 3D reconstruction errors in stereo camera systems and dealing with point correspondence problems. [Preview Abstract] |
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JP8.00058: Modeling of Particle Orbits in 3D MHD Equilibria Matthew McMillan, Samuel Lazerson, Eliot Feibush Modern Tokamaks and stellarators have significant auxiliary fast ion heating systems which must be accounted for in equilibrium calculations. Direct measurement of fast ion profiles is difficult, so often in Tokamaks they are provided by forward modeling. The 3D nature of fields from Tokamaks with resonant magnetic perturbations (RMPs) and from stellarators necessitates a fully 3D model. The BEAMS3D code uses guiding center approximations to find particle trajectories in 3D fields, and can include the physical effects of hot ion collisions and scattering, charge-exchange and recombination, pitch angle and energy scattering, and viscous velocity reduction. We benchmark the code for beam deposition, charge exchange, and collisionless particle orbits, with positive results. We apply the code to MHD solutions representing the geometry of DIII-D and NCSX, and extract the hot ion pressure profiles. While currently designed to work with the equilibria produced by VMEC or with vacuum fields, the code easily could be modified to work with other equilibria. [Preview Abstract] |
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JP8.00059: Exploring the accuracy and reliability of plasma fluid models for direct current glow discharges Thomas Underwood, Igor Kaganovich, Alexander Khrabrov Glow discharges are of fundamental importance to a variety of industrial applications such as plasma processing, discharge lighting and plasma chemistry. Therefore, a detailed understanding of the physical phenomena that occur within glow discharges is necessary for further advances in design and optimization of relevant plasma applications. Of the various plasma modeling approaches, most use either a fluid approximation, kinetic (particle) approach, or a hybrid model. Although each method has its own set of unique advantages, recent advances in hybrid techniques have shown unique promise to maintain computational efficiency and accuracy. The validity of assumptions within the fluid and hybrid models will be established by direct comparison with results obtained in an electrostatic direct implicit particle-in-cell code (EDIPIC). The accuracy of these assumptions will also be explored within each region of a typical glow discharge and relevant theory will be discussed to explain these results. [Preview Abstract] |
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JP8.00060: Surface Treatment System Using Dielectric Barrier Discharge Ryan Daniels, Sophia Gershman, Jessica Faust Atmospheric pressure plasmas have been used recently to improve surface properties of materials. For example, plasma treatment improves wettability, activates and functionalizes the surface of polyethylene making it more suitable for biological applications. We have designed and constructed a system that allows the study of the effect of dielectric barrier discharge (DBD) on the surface properties of treated materials. Preliminary results show that 55 second treatment by the DBD in a 1mm gap reduces the contact angle of polyethylene from 78\r{ } +/- 1\r{ } before to 40\r{ } +/- 1\r{ } after the treatment. The DBD is generated using a 15kV, 1kHz pulsed dc power supply a mixture of Ar and O2 as the carrier gas. The study parameters include the ratios of O2 to Ar, the power supply frequency and duty cycle. To perform surface analysis, we have designed a transfer chamber. A bellows drive is used to transport the sample to the mobile transfer chamber and then to a test chamber without exposure to the environment. Plasma treatment improves biological compatibility of polyethylene and makes it suitable for use in implants, prosthetics, and cell cultures. [Preview Abstract] |
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JP8.00061: Transfer Chamber for DBD Surface Treatment and Analysis Jessica Faust, Sophia Gershman, Ryan Daniels Materials that are hydrophobic, smooth, and have low surface tension lead to poor adhesion for printing and coating. Surface modification using atmospheric pressure dielectric barrier discharge (DBD) improves adhesion by activating the surface, increasing its roughness and functionalizing. We have constructed and tested a DBD discharge system that uses a 15kV, 800-1350Hz pulsed dc source and a mixture of gasses at 300-600Torr. Treated surfaces are studied using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and other methods that require high vacuum environment. We designed a portable transfer chamber to move the sample from the atmospheric pressure treatment chamber to a high vacuum surface analysis chamber with minimal interaction with the environment. The transfer chamber was designed to meet the specific requirements of each system; a bellows drive and sample holder to manually move the sample into the transfer chamber; the ability to fill with pure nitrogen gas to prevent contamination of the sample surface; constructed to withstand a range of pressures from 300 to 10$^{-8}$Torr; the ability to connect compatibly with each system. Proper surface characterization of the sample is crucial to designing an effective treatment system. [Preview Abstract] |
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JP8.00062: Development of an electrostatic dust detector for tungsten dust D. Starkey, K. Hammond, L. Roquemore, C.H. Skinner Next-step fusion reactors, such as ITER, are expected to have large quantities of dust that will present hazards that have yet to be encountered in current fusion devices. To manage the amount of dust within the reactors a real-time dust detector must be implemented to ensure that dust does not reach hazardous levels. An electrostatic device that accomplishes this has already been tested on NSTX and Tore Supra [1,2]. We will present modifications of this device to improve its ruggedness to withstand the conditions that will be present in ITER. The detector consists of two tungsten wires wrapped around a macor cylinder that are biased at 100-300 V. Incident dust causes a measurable transient short circuit. Initial results have demonstrated the detection of tungsten particles. We will also present a potential method of electrostatic cleaning of residual dust from the detector.\\[4pt] [1] C. H. Skinner et al., Rev. Sci. Instrum., 81, 10E102 (2010)\\[0pt] [2] H. Roche et al., Phys. Scr., T145, (2011). [Preview Abstract] |
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JP8.00063: Experimental Study of a Pure Ion Plasma in a Linear Paul Trap Subject to Dipole and Quadrupole Perturbations to Study Magnetic Lattice Errors Matthew Talley, Erik Gilson, Ronald Davidson The Paul Trap Simulator Experiment (PTSX) simulates the collective dynamics of a charged particle bunch traveling through a magnetic alternating-gradient particle accelerator. For example, PTSX is used to study the effect of random lattice errors caused by quadrupole magnets that are misaligned or vary in magnetic field strength. A pair of arbitrary function generators was used to apply a trapping voltage waveform for many lattice periods with a range of frequencies and with either a dipole or quadrupole spatial structure. Every 12th period, the amplitude was changed to create a perturbed waveform to stimulate the bunch. The duration of the perturbation was also varied to simulate different numbers of revolutions in the ring. The experimental results demonstrate the growth in the equivalent beam emittance that occurs due to the perturbation amplitude and duration. The data does not demonstrate a strictly monotonic decrease in charge but rather a periodic relationship that depends on the perturbation amplitude and duration. In the dipole and quadrupole experiments the data revealed a power-law relationship between these parameters. To explain this behavior, models based on the individual particle motion and the envelope equation were studied and will be presented with the data. [Preview Abstract] |
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JP8.00064: Lithium Cooling in the Tokamak Scrape-off Layer Zhaonan Qu, Robert Goldston We used collisional radiative model data from the ADAS atomic physics database to investigate the cooling rate of lithium in comparison with beryllium, carbon and nitrogen. The model data incorporate collisional ionization and recombination, collisional excitation, and both radiative and collisional de-excitation. The last is important for Li and Be at the high densities and low temperatures expected close to the divertor surface. The charge state distribution is determined as a function of $n_{e}$ and $T_{e}$ by assuming a steady source of neutrals balanced by losses due to an effective confinement time, $\tau $, the same for all charge states. We then calculate $L_{z}$, the cooling rate, and the total cooling energy per particle injected. For $\tau $ in the range of 10$^{-4}$ -- 10$^{-2}$ sec with $n_{e} \sim $ 6 10$^{19}$/m$^{3}$ and $T_{e } \sim $ 100eV, we find the cooling energy per Li injected to be $\sim $ 500eV. Following Post [1] we have integrated the calculated cooling power along a field line, assuming pressure balance. We corrected the $Z_{eff}$ dependence of $\kappa _{0e} $ and included the local density dependence of the charge-state balance. We find that, at fixed fuel depletion, all four species are similar in reduction of $q_{\vert \vert }$ at upstream temperatures $\sim $ 100 eV, but C and N give greater reductions than Li and Be at lower upstream temperatures. For $\tau \sim $ 1 msec and fuel depletion of $\sim $ 30{\%}, the reduction in $q_{\vert \vert }$ at the divertor plate in cases corresponding to characteristic NSTX experimental conditions can easily equal the expected total $q_{\vert \vert }$. This work supported by the Princeton Environmental Institute and DOE Contract {\#} DE-AC02-09CH11466. \\[4pt] [1] D.E. Post, J. Nucl. Mater. 220-222 (1995) 143. [Preview Abstract] |
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JP8.00065: Neoclassical study of temperature anisotropy in NSTX experiments using the GTC-NEO particle code David Perkins, Stephane Ethier, Weixing Wang It is well-known that the level of ion transport in the National Spherical Torus eXperiment (NSTX) is close to the neoclassical level. This makes self-consistent neoclassical simulations carried out with the GTC-NEO particle code highly relevant for studying transport-related issues in NSTX. GTC-NEO, which now treats multiple species of ion impurities [1], takes as input the experimental profiles from NSTX discharges and calculates the fully non-local, self-consistent neoclassical fluxes and radial electric field. One unanswered question related to NSTX plasmas is that of possible ion temperature anisotropy, which cannot be determined experimentally with the current diagnostics. This work describes new numerical diagnostics and computational improvements that were implemented in GTC-NEO to enable the study of temperature anisotropy.\\[4pt] [1] R.A. Kolesnikov et al., Phy. Plasmas 17, 022506 (2010) [Preview Abstract] |
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JP8.00066: Advanced analysis system and user interface for gyrokinetic simulations of microturbulence Jeff Lestz, Sadik Shahidain, Stephane Ethier, Weixing Wang Fully-global, 5D gyrokinetic simulations of turbulent transport in tokamak devices generate a large amount of time-dependant data that contain a wealth of information about waves, particles, and their self-consistent interactions. To explore these data in spectral space, in both wave numbers and frequencies, the information needs to be written out and analyzed in a post-process stage. This work describes the development of a MATLAB-based system for the extensive analysis of gyrokinetic simulation data, with particular application to the Gyrokinetic Tokamak Simulation code (GTS), which is being used for studying experimental discharges from NSTX, DIIID, and C-MOD. Parallel FORTRAN and C routines are used in some cases to read in the large amount of data and carry out the first stage of post-processing. Advanced MATLAB functions are then used for calculating statistical quantities, correlations, etc. A graphical user interface enhances the user experience and provides advanced plotting capabilities. Examples of microturbulence data analyses are given and discussed. [Preview Abstract] |
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JP8.00067: Shear-Alfv\'en Waves in Gyrokinetic Particle Simulation Thomas D. Dickerson, Edward A. Startsev, W.W. Lee Numerical properties of shear-Alfv\'en waves in slab geometry have been studied using a Particle-in-Cell code implementing the recently developed double split-weight scheme [1]. This scheme separates the non-adiabatic response of the particles from both their adiabatic responses and the field-line bending effects arising from the background density and temperature gradients of both the electrons and the ions. This scheme is an improvement over the original split-weight scheme [2] in the presence of the zeroth-order inhomogeneities. The present studies consist of testing numerical restrictions on temporal resolution in the simulation of these waves in one and two dimensions, and on spatial resolutions on the formation of shear Alfv\'en eigenmodes in two dimensional sheared slab simulations. For example, it is found that the correct behavior of ion temperature gradient modes in terms of frequencies and growth rates can be maintained with time steps larger than the limit imposed by the shear-Alfven waves. Details will be reported.\\[4pt] [1] E. A. Startsev and W. W. Lee, ``Finite-Beta Simulation of Microinstabilities,'' manuscript in preparation (2012). \\[0pt] [2] W. W. Lee, J. L. V. Lewandowski, T. S. Hahm and Z. Lin, ``Shear-Alf\'en Waves in Gyrokinetic Plasmas,'' Phys. Plasmas 10, 4435 (2001). [Preview Abstract] |
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JP8.00068: ABSTRACT WITHDRAWN |
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JP8.00069: C-MOD, ITER, MRF SIMULATION, MST AND OTHER RFPs |
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JP8.00070: Gyrokinetic Simulations of Diluted Plasmas in the LOC regime in Alcator C-Mod M. Porkolab, P. Ennever, M.L. Reinke, J. Rice, C. Rost, N. Tsujii, E. Davis, D. Ernst, C. Fiore, M. Greenwald, A. Hubbard, J. Hughes, E. Marmar, J. Candy, G.M. Staebler, R. Waltz Previous investigations of ITG and TEM/ETG turbulence using the reduced gyro-landau fluid code TGLF, and gyrokinetic code GYRO have predicted that in the linear ohmic confinement (LOC) regime in Alcator C-Mod the dilution of the main D ion species by low-Z impurities reduces the ion transport to experimentally observed levels. This analysis assumed an average impurity ion charge Zi= 8. Recent spectroscopic measurements of the impurity ion species in the LOC regime in C-Mod have shown that the average Zi is approximately 9, which at the measured Zeff values (2-4) results in a significant dilution ($>$10{\%}) of the majority D ion species. By puffing in nitrogen while using a cryopump to keep the density constant, new experiments enabled us to lower Zi to values near 8, thus further increasing dilution. To account for the sensitivity of the turbulent transport on the density (Lne) and temperature (LTe) gradient scale lengths, recently we used TGYRO to improve the agreement between theory and the measurements. The results of such simulations will be presented. [Preview Abstract] |
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JP8.00071: Determining impurity concentrations in plasmas with mixed low-Z and high-Z contamination M. Reinke, M. Chilenski, T. Christensen, P. Ennever, I. Faust, S. Harper, N. Howard, D. Miller, A. James, M. Churchill, C. Fiore, M. Greenwald, J. Hughes, A. Hubbard, B. Lipschultz, E. Marmar, J. Rice, C. Theiler A novel approach is described to find concentrations of impurities in plasmas with a mix of low-Z and high-Z contaminants. In plasmas with high-Z impurities, $\Delta $Z$_{eff}\sim $1 can be reached without meaningful change to the main-ion density, meaning changes in the high-Z concentrations can contribute to collisionality, without playing a role in dilution. When both low-Z and high-Z impurities are present, Z$_{eff}$ measurements have limited utility, requiring an expanded approach in characterizing the contamination. The cumulative effect of low-Z impurities is to reduce the neutron rate, while high-Z impurities dominate the total radiated power. Alcator C-Mod has range of low-Z (B, C, O and F) and high-Z (Fe, Mo) intrinsic impurities, and uses extrinsic seeding of N$_{2}$, Ne (heat flux), He and Ar (diagnostic). Impurities are identified via short-wavelength, $\lambda <$ 30 nm, line emission measured by flat-field spectrometers, which can also track the relative inter- and intra-shot changes in line-brightness. The absolute high-Z impurity density is constrained using resistive bolometry, while the total low-Z impurity density is constrained by comparing the measured and modeled neutron rates using experimental T$_{i}$ profiles. Continuum emission in multiple spectral regions is used to determine Z$_{eff}$. [Preview Abstract] |
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JP8.00072: Stationary Density Profiles in Alcator C-mod J. Kesner, D. Ernst, J.W. Hughes, R. Mumgaard, S. Shiraiwa, D.G. Whyte In the absence of an internal particle source, plasma turbulence will impose an intrinsic relationship between an inwards pinch and an outwards diffusion resulting in a stationary density profile as determined by the turbulent equipartition (TEP) theory. The Alcator C-mod tokamak utilizes RF heating and current drive so that fueling only occurs in the vicinity of the separatrix. Density is determined from Thomson scattering. Discharges that transition from L-mode to I-mode are seen to maintain a stationary profile. For reversed shear discharges maintained by non-inductive current drive ($V_{loop}\approx 0$) a drop of density in the vicinity of the axis is consistent with an observed rise in q, although error in the measurement precludes making this observation definitive. [Preview Abstract] |
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JP8.00073: Collisionality Dependence of Multi-species Density Peaking in Turbulence Simulations of C-Mod Plasmas D.R. Mikkelsen, M. Bitter, L. Delgado-Aparicio, K.W. Hill, M. Greenwald, N. Howard, Y. Podpaly, M. Reinke, J.E. Rice, J.W. Hughes, Y. Ma, J. Candy, R.E. Waltz In nonlinear GYRO simulations of C-Mod plasmas, a turbulently driven pinch produces modest density peaking of all species. The ratio of density at r/a=0.44 and 0.74 is 1.2 for the majority and minority D \& H (and electrons), and increases with impurity Z: 1.1 for helium, 1.15 for boron, 1.29 for neon, 1.36 for argon, 1.47 for molybdenum. Density peaking is only weakly affected when the ion temperature profile is varied to align the predicted heat flux to the experimental transport analysis. New simulations will extend the collisionality to the lower part of the experimentally accessible range in C-Mod to study the collisionality dependence of density peaking, and to establish whether much stronger peaking is predicted for lower collisionalities. Simulations based on measured I-mode ion and electron temperature profiles will also be presented. [Preview Abstract] |
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JP8.00074: Transport and Stability in C-Mod ITBs in Diverse Regimes C.L. Fiore, D.R. Ernst, N.T. Howard, C.P. Kasten, D. Mikkelsen, M.L. Reinke, J.E. Rice, A.E. White, W.L. Rowan, I. Bespamyatnov Internal Transport Barriers (ITBs) in C-Mod feature highly peaked density and pressure profiles and are typically induced by the introduction of radio frequency power in the ion cyclotron range of frequencies (ICRF) with the second harmonic of the resonance for minority hydrogen ions positioned off-axis at the plasma half radius on either the low or high field side of the plasma. These ITBs are formed in the absence of particle or momentum injection, and with monotonic q profiles with q$_{min}<$ 1. Thus they allow exploration of ITB dynamics in a reactor relevant regime. Recently, linear and non-linear gyrokinetic simulations have demonstrated that changes in the ion temperature and plasma rotation profiles, coincident with the application of off-axis ICRF heating, contribute to greater stability to ion temperature gradient driven fluctuation in the plasma. This results in reduced turbulent driven outgoing heat flux. To date, ITB formation in C-Mod has only been observed in EDA H-mode plasmas with moderate (2-3 MW) ICRF power. Experiments to explore the formation of ITBs in other operating regimes such as I-mode and also with high ICRF power are being undertaken to understand further the process of ITB formation and sustainment, especially with regard to turbulent driven transport. [Preview Abstract] |
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JP8.00075: Progress in Performance and Understanding of Steady ELM-free I-modes on Alcator C-Mod A.E. Hubbard, D.G. Whyte, A. Dominguez, M.J. Greenwald, N. Howard, J.W. Hughes, B. LaBombard, Y. Lin, B. Lipschultz, E.S. Marmar, M.L. Reinke, J.E. Rice, J.L. Terry, J. Walk, A.E. White, S.J. Wukitch, P. Snyder The I-mode regime of operation has been extended in recent Alcator C-Mod campaigns in duration and robustness, over a wide range of parameters. It features an edge thermal barrier, with L-mode like density profiles and particle transport [1]. I-modes are now routinely maintained in stationary conditions for over 10 $\tau _E $. They are usually ELM free, a key advantage given the concern over divertor heat pulses on ITER. Instead, a continuous pedestal fluctuation, the ``weakly coherent mode,'' appears to enhance selectively particle over thermal transport. High performance I-modes are usually obtained with unfavorable ion drift direction, and extend to low q$_{95}$=2.5and $\nu _{ped}^\ast $=1.3. $\tau _E $ is in the range of H-mode (H$_{98,y2}$ up to 1.2), and exhibits less power degradation (W$\sim $P$^{0.7})$. Power thresholds for I-mode are higher than typical L-H scalings, and increase with I$_{p}$ as well as density. The widest power range for I-mode, nearly a factor of two above the L-I threshold, has been obtained in reversed field, lower null discharges at moderate n$_{e}$. The focus of 2012 experiments is on assessing the density range and dependences, important for extrapolating to ITER, and measuring the pedestal profiles and fluctuations in greater detail.\\[4pt] [1] Whyte D G \textit{et al }2010 \textit{Nucl. Fusion} \textbf{50} 105005 [Preview Abstract] |
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JP8.00076: Connection between the Weakly Coherent Mode and particle transport across the LCFS in the I-mode A. Dominguez, E.S. Marmar, J.L. Terry, B. LaBombard, M. Greenwald, A.E. Hubbard, J.W. Hughes, G.J. Kramer, A.E. White, D.G. Whyte The I-mode regime is characterized by the formation of ion and electron temperature pedestals, while maintaining L-mode-like radial density profiles [1]. Changes in edge density, temperature and magnetic field fluctuations accompany the L-mode to I-mode transition with reduction of fluctuations in the 50-150kHz range as well as the appearance of a Weakly Coherent Mode (WCM) in the 150-300kHz range. Previous work[2] has established a connection between the midrange fluctuation reduction and a decrease in the effective thermal diffusivity in the pedestal region. The mechanism for maintaining sufficient particle transport to avoid impurity accumulation has been unclear. In this work, quantitative comparisons are made between particle transport measurements across the LCFS and the intensity of the WCM in the I-mode. The particle transport has been estimated using D$\alpha$ profiles measured near the outboard midplane and the intensity of the WCM is obtained by spectral analysis of density fluctuations from reflectometry. The measurements show correlations between the particle transport and the WCM intensity, supporting evidence that the WCM causes enhanced transport.\\[4pt] [1] D.G. Whyte, et al., Nucl. Fusion 50 (2010)105005.\\[0pt] [2] A.E. Hubbard, et al., Phys. Plasmas 18(2011)056115 [Preview Abstract] |
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JP8.00077: Recent edge CXRS measurements in I-mode and ELMy H-mode plasmas on C-Mod C. Theiler, R.M. Churchill, A. Hubbard, J.W. Hughes, B. Lipschultz, E. Marmar, M.L. Reinke, J.L. Terry, J.R. Walk, D. Whyte, A. Diallo A high-resolution charge exchange recombination spectroscopy (CXRS) diagnostic is installed on Alcator C-Mod to measure edge profiles of $B^{5+}$ density, temperature, and flows and to deduce radial electric field profiles. As donor particles for CX, both hydrogen from a modulated diagnostic neutral beam and deuterium from a local gas puff are employed. While the former technique often suffers from low signal intensities near the LCFS, the latter can be complicated by contamination of the signal by molecular lines and cross-section effects. After discussing how these challenges are overcome in data analysis, we present recent measurements in regimes which have not extensively been investigated previously with edge CXRS on C-Mod. The first one is the I-mode regime, which is characterized by H-mode like energy confinement and L-mode like particle transport. Focusing on I-modes with unfavorable ion $\mathbf{B} \times \nabla B $ drift, we study profile scalings with plasma parameters such as density, plasma current and heating power. The second regime is ELMy H-mode, where we explore the capabilities of the CXRS diagnostic to measure profile evolutions during and in between ELMs. [Preview Abstract] |
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JP8.00078: Characterization of the Pedestal in I-Mode Plasmas on Alcator C-Mod J.R. Walk, J.W. Hughes, J.L. Terry, A.E. Hubbard, D.G. Whyte, A.E. White, E.S. Marmar, J.E. Rice, R.M. Churchill, C. Theiler, B. LaBombard, D. Brunner, M.L. Reinke, A. DOminguez, C. Sung I-mode is an improved energy confinement regime on C-Mod, notably featuring an edge temperature pedestal without the accompanying density pedestal found in H-mode operation. I-mode has been shown to be capable of stationary operation, with the mode sustained for $\sim 10$ energy confinement times, and to have a pedestal naturally lacking large ELMs. Combined with comparable performance to H-mode operation and weaker degradation with input power, this makes I-modes potentially favorable as a reactor regime. A full characterization using the suite of high-resolution edge diagnostics on C-Mod of the pedestal in reversed-field, lower-single-null I-modes (therefore with the ion $\nabla B$ drift away from the x-point) is presented here. The presented discharges primarily feature detailed scans of plasma current and ICRF heating power at several density values. The pedestal structure found in these I-modes is also contrasted with previous studies of the pedestal in EDA and ELMy H-modes, particularly potential scalings with $\beta_{p,ped}$ consistent with the KBM-limited pedestals found in these H-modes. [Preview Abstract] |
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JP8.00079: Investigation of Impurity Transport in Alcator C-Mod Using Laser Blow-Off Impurity Injection M.A. Chilenski, M. Greenwald, N.T. Howard, L. Delgado-Aparicio, I. Faust, M.L. Reinke, A.E. White Alcator C-Mod is equipped with a multi-pulse laser blow-off impurity injector, capable of injecting a small amount of a given impurity into the plasma as frequently as once every 0.1s. This system is used as an actuator for studies of impurity transport, where calcium is often used as it is non-intrinsic and non-recycling. To observe the effect of the injection, a high-resolution x-ray imaging crystal spectrometer captures temporally-resolved profiles of the emission from helium-like calcium. These emission profiles can then be analyzed to obtain temporally-resolved profiles of the impurity diffusivity and convective pinch velocity. Injections have been performed in a variety of plasmas including L-modes, H-modes, I-modes and plasmas with high fractions of lower hybrid current drive. C-Mod's combination of an impurity injector with a high-resolution x-ray spectrometer provides a powerful system for probing the behavior of impurity transport in these various regimes. [Preview Abstract] |
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JP8.00080: Thermal transport and Plasma Rotation during cold pulse experiments in Ohmic L-mode plasmas C. Gao, J.E. Rice, H.J. Sun, M.L. Reinke, N.T. Howard, Y.A. Podpaly, L. Delgado-Aparicio, M. Chilenski, J.W. Hughes, Y. Ma, A. Hubbard, M. Greenwald, N. Tsujii, P. Ennever, M. Porkolab, W. Rowan Thermal transport is studied by laser blow-off impurity (CaF2) injection. The non-local effect, a cooling of the edge electron temperature with a rapid rise of the central electron temperature, which contradicts the ``local'' assumption of transport, was observed in low density linear Ohmic confinement (LOC) regime plasmas. Simulation shows this phenomenon can be explained either by a fast drop of the core diffusivity, or by a sudden appearance of an inward heat convection. In high density saturated Ohmic confinement (SOC) regime plasmas, the thermal transport becomes ``local'': central electron temperature drops in response to the edge cooling. The transition density is very close to the rotation reversal critical density. This indicates the possible correlation between thermal and momentum transport, which is also linked to the trapped electron mode (TEM) to ion temperature gradient mode (ITG) transition. Cold pulse modulation experiments show in LOC the thermal transport is not purely diffusive, while in SOC the thermal transport is more diffusive like. Linear Gyro-kinetic simulations will be performed to characterize the turbulent transport in these experiments. [Preview Abstract] |
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JP8.00081: Beam-emission spectra in high field, high density plasmas of the Alcator C-Mod tokamak I.O. Bespamyatnov, O. Marchuk, K.T. Liao, W.L. Rowan, Y. Ralchenko, R.T. Mumgaard, S.D. Scott, R.S. Granetz Measurements of relative line intensities within the H-alpha Stark multiplet emitted by a neutral beam have consistently defied predictive attempts. This has impact on impurity density measurements via CXRS and current profile measurements via MSE. New measurements for the relative line intensities on Alcator C-Mod are motivated by new predictions which challenge and replace the usual assumption of statistical distribution among Stark states. The predictions are based on a collisional-radiative model (NOMAD) where magnetic sublevels are described in a parabolic basis, and new proton excitation cross sections are calculated. This model proved to be essential to explain the well-known difference between measured and predicted spectra on JET. C-Mod has the unique ability to cover parameters which test the statistical distribution assumption over the range accessed in contemporary experiments and in ITER. Here we focus on comparison of preditions with measured beam spectra for a selection of C-Mod plasmas. [Preview Abstract] |
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JP8.00082: Transport of light, trace impurities in Alcator C-Mod W.L. Rowan, I.O. Bespamyatnov, K.T. Liao, W. Horton, X.R. Fu, J.W. Hughes Light impurity profiles for boron were measured in ITB, H-mode, L-mode, and I-mode discharges in Alcator C-Mod. Within this wide range of modes, the profiles varied from peaked to hollow to flat. Specifically, hollow profiles are often observed in H-mode, while ITBs produce strong peaking, and L-mode produces moderate peaking. I-mode discharges are characterized by flat impurity profiles. For the study reported here, the profiles were measured with charge exchange recombination spectroscopy. The dependences of Rv/D were sought on dimensionless quantities including ion density scale length, effective charge, collisionality, and temperature scale length. We find that neoclassical transport consistently underestimates the measured transport. The excess measured transport is assumed to be turbulent. The strongest dependence of Rv/D is with temperature scale length. In addition, the measured transport was compared with the prediction of an analytical theory of drift wave turbulence that identifies transport implications for drift waves driven by ion and impurity density gradients. [Preview Abstract] |
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JP8.00083: Transport of helium as an impurity and as a main ion in Alcator C-Mod Ken Liao, William Rowan, Igor Bespamyatnov, Wendell Horton, Xiangrong Fu Local density, temperature, and flow velocity measurements of helium impurity and helium main ion are provided by Charge Exchange Recombination Spectroscopy (CXRS), using a 50keV diagnostic neutral beam. Spectra around the 4686\AA He II line are collected in 30 optical channels which cover the low field side from the core to the edge. Measurements have been taken in L and H modes, and for helium as an impurity and as the main ion. Helium impurity transport experiments have been conducted by puffing helium during the flat top region of diverse plasma conditions. The transport is assumed to take the form $\frac{dn}{dt} = \frac{d}{dr}(-D \frac{d}{dr} + v n)$, with the source term negligible in the core of the plasma. Estimates of the transport coefficients D and v (assumed to be constant with respect to time) are obtained by fitting the temporal evolution of the helium density profile. For typical L-mode plasma, the helium density confinement peaking factor is 0.7 (hollow). The sensitivities of the logarithmic density gradient to magnetic shear and to temperature scale length will be discussed and compared to predictions of neoclassical theory (NCLASS) and analytical treatment of drift wave turbulence. [Preview Abstract] |
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JP8.00084: Turbulence measurements with fast two-color interferometry on Alcator C-Mod Cale Kasten, James Irby, Richard Murray, Anne White, Nathan Howard, Yijun Lin, Paul Bonoli The two-color interferometer diagnostic on Alcator C-Mod has been upgraded to simultaneously measure line-integrated electron density and density gradient fluctuations for turbulence and transport studies. Signals from ten vertical-viewing CO$_2$ laser chords are demodulated relative to a local oscillator with fast analog in-phase/quadrature electronics using a heterodyne technique, giving the line-integrated electron density and fluctuations. The chord signals are also demodulated relative to adjacent chords, providing a measurement of the line-integrated electron density gradient and fluctuations. The diagnostic design, calibration, and analysis methods are presented. Phase and wavenumber sensitivities are determined and a synthetic diagnostic is developed which enables quantitative comparisons between experimental measurements and gyrokinetic simulations using GYRO. The diagnostic is used to study changes in turbulence due possibly to strong electron heating and $E\times B$ shear flow in the ion cyclotron range of frequencies during mode conversion flow drive and heating experiments. [Preview Abstract] |
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JP8.00085: Studies of Electron Temperature Fluctuations in the Core of Alcator C-Mod Plasmas via Correlation ECE C. Oi, C. Sung, N. Howard, A.E. White, J. Irby, R. Leccacorvi, R. Vieira, J. Rice, C. Gao A correlation electron cyclotron emission (CECE) diagnostic has been designed and installed at the Alcator C-Mod tokamak to measure long wavelength fluctuations in electron temperature [C. Sung, this conference, C-Mod oral session]. It is important to characterize turbulent fluctuations in the plasma in order to better understand and predict transport, since the cross-magnetic field transport of particles and energy in fusion plasmas exceeds the values predicted by neoclassical theory. The first electron temperature fluctuation data collected from Alcator C-Mod ohmic plasmas has shown that as the ohmic confinement regime transitions from Linear Ohmic Confinement (LOC) to Saturated Ohmic Confinement (SOC) there is a decrease in the core electron temperature fluctuation level. The edge electron temperature fluctuations are similar between the two modes, suggesting that mainly the core turbulence characteristics change when there is a shift between LOC and SOC regimes. In both ohmic and ICRH plasmas, the measured spectrum is seen to broaden in response to increases in plasma rotation; and fluctuation level increases with radius. A description of the C-Mod CECE diagnostic will be presented, along with the initial measurements of turbulent electron temperature fluctuations. [Preview Abstract] |
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JP8.00086: BOUT++ Simulations of Edge Turbulence in Alcator C-Mod's EDA H-Mode Evan Davis, Miklos Porkolab, Jerry Hughes, Naoto Tsujii, Paul Ennever, Ted Golfinopoulos, Seung Gyou Baek, Jim Terry, Xueqiao Xu Energy confinement in tokamaks is believed to be strongly controlled by plasma transport in the edge region, just inside the last closed magnetic flux surface. The Boundary-plasma Turbulence (BOUT++) code is capable of simulating nonlinear fluid turbulence in this region and is well-suited to Alcator C-Mod's Enhanced D-Alpha (EDA) H-mode ($\nu^{*}>1$). The EDA H-mode is always accompanied by the quasi-coherent mode (QCM), an edge fluctuation believed to reduce impurity confinement and allow steady-state H-mode operation. Using experimentally measured profiles as input, BOUT++ calculations show that typical C-Mod EDA H-modes are ideal MHD stable but become linearly unstable when the pedestal resistivity is included ($\eta>10^{-7}~\Omega$-m). The computed growth rate in these resistive ballooning modes is found to be consistent with theory, while incorporation of experimentally measured flow profiles has allowed the self-consistent temporal evolution of the edge radial electric field. Nonlinear simulations have reached turbulent steady state, and the computed turbulence spectrum will be compared with measurements from relevant C-Mod diagnostics, such as phase contrast imaging (PCI), reflectometry, gas puff imaging (GPI), and magnetic probes. [Preview Abstract] |
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JP8.00087: Alcator C-Mod's Quasi-Coherent Mode Antenna: Experimental Results and Interpretation T. Golfinopoulos, B. LaBombard, R.R. Parker, W. Burke, E.M. Davis, R. Granetz, M. Greenwald, E. Marmar, M. Porkolab, J.L. Terry, S.M. Wolfe, P.P. Woskov, X. Xu A new ``Shoelace'' antenna has been installed on Alcator C-Mod. Its goal is to interact with edge fluctuations, and particularly the quasi-coherent mode (QCM) associated with the steady-state EDA H-mode. With $k_{\perp}=1.5$ cm$^{-1}$ and frequency range, $40 |
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JP8.00088: ABSTRACT WITHDRAWN |
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JP8.00089: Exponential Spectra in Alcator C-Mod Edge Turbulence Victoria Winters, Anne White, David Pace, Jim Terry, Arturo Dominguez, Felix Parra, Jerry Hughes It has been proposed that exponential power spectra seen in edge turbulence of fusion plasmas is the result of deterministic chaos, and is associated with the presence of Lorentzian pulses in the time series data. Using reflectometer and Gas Puff Imaging (GPI) data in the Alcator C-Mod tokamak, we have analyzed exponential power spectra in Ohmic and L-mode plasmas. Both reflectometer homodyne signals and GPI signals measuring density fluctuations just inside or at the Last Closed Flux Surface (LCFS) exhibit exponential power spectra. Theoretically, the characteristic slope of the data on a semi-log plot gives the full width of the underlying Lorentzian pulses. Using a separate fitting routine, individual Lorentzian pulses in the reflectometer time series data have been identified. Preliminary results show that the widths of the Lorentzian pulses match the inverse characteristic frequency of the exponential spectra By using data from variety of different shots we find that the characteristic frequency of the exponential spectrum of the reflectometer homodyne signals varies depending on plasma parameters. Initial results show that characteristic frequency increases with increasing line-averaged plasma density. Other plasma parameter dependencies are being conducted currently. [Preview Abstract] |
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JP8.00090: Edge turbulence flows at two different poloidal angles in Alcator C-Mod Stewart Zweben, James Terry, Matteo Agostini, William Davis, Olaf Grulke, Jerry Hughes, Brian LaBombard, Matt Landreman, Yunxing Ma, David Pace, Bruce Scott High resolution edge turbulence movies have been obtained simultaneously at both the outer midplane and near the lower X-point region of C-Mod, using gas puff imaging (GPI) with two high speed cameras at 400,000 frames/sec. The time-resolved turbulence flow speeds at the outer midplane GPI view was previously estimated using a time-resolved cross-correlation technique [1], but previous results also showed a significantly different turbulence structure in these two regions [2]. Preliminary results indicate that the poloidal turbulence flows are not necessarily the same at these two poloidal angles. For instance, in one shot there is a strong time-averaged poloidal flow near the X-region toward the outer midplane, but mainly poloidally-fluctuating flows at the outer midplane. Examples of these flow measurements will be shown for plasmas with and without ICRH and in L-mode and H-mode plasmas. Evidence for fluctuating zonal flows preceding the L-H transition will be assessed. This work is supported in part by DOE Contracts DE-AC02-09CH11466 and DE-FC02-99ER5412.\\[4pt] [1] S.J. Zweben, J.L. Terry et al, Plasma Phys. Cont. Fusion 54 (2012) 025008\\[0pt] [2] J.L. Terry, S.J. Zweben et al, J. Nucl. Mat. 390-291 (2009) 339 [Preview Abstract] |
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JP8.00091: Upgrade to the Gas Puff Imaging Diagnostic that Views Alcator C-Mod's Inboard Edge J.M. Sierchio, J.L. Terry We describe an upgrade of Alcator C-Mod's Gas Puff Imaging system which views the inboard plasma edge and SOL along lines-of-sight that are approximately parallel to the local magnetic field. The views are arranged in a 2D (R,Z) array with $\sim $2.8 cm radial coverage and $\sim $2.4 cm poloidal coverage. 23 of 54 available views were coupled via fibers to individual interference filters and PIN photodiode detectors. We are in the process of upgrading the system in order to increase the sensitivity of the system by replacing the PIN photodiodes with a 4x8 array of Avalanche Photo-Diodes (APD). Light from 30 views is coupled to the single-chip APD array through a single interference filter. We expect an improvement in signal-to-noise ratio of more than 10x. The frequency response of the system will increase from $\sim $400 kHz to 1MHz. The dynamic range of the new system is manipulated by changing the high-voltages on the APDs. Test results of the detectors' channel-to-channel cross-talk, frequency response, and gain curves will be presented, along with schematics of the experimental setup. The upgraded system allows for more study of inboard edge fluctuations, including whether the quasi-coherent fluctuations observed in the outboard edge also exist inboard. [Preview Abstract] |
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JP8.00092: High Resolution IR Imaging for Alcator C-Mod G.A. Wurden, J.L. Terry We are upgrading the infrared imaging system for divertor heat load studies on Alcator C-Mod by adding a new FLIR SC8303HD camera. The new camera is capable of 1344x784 pixel full frame resolution with 14-bit images at 130 Hz. Faster rates are possible by sub-windowing on the image. The new camera uses full camera link interface over fiber optics from the test cell to the control room, and the resulting data acquisition handles an image stream of 260 Mbytes/second straight to disk. At first we will employ a silicon wafer beam splitter, to enable measurements with both the new and old cameras simultaneously with the existing IR periscope. Initial data from the new camera and the parallel development of real-time imaging software for later use of this hardware on the W7-X stellarator will be discussed. [Preview Abstract] |
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JP8.00093: Design, Engineering, and Testing for the Alcator C-Mod Outer Divertor Upgrade S. Harrison, R. Vieira, B. Lipschultz, R. Ellis, D. Karnes, J. Doody, L. Zhou, P. Titus, H. Zhang, W. Beck, R. Granetz Alcator C-mod's major outer divertor upgrade will enable significant advances in our understanding of reactor relevant physics and operations. Two primary features of the new outer divertor are its toroidally continuous design (electrical and mechanical), and ability to be operated up to or independently heated to 600\r{ }C. Full control of the divertor PFC temperature from ambient vessel temperature to 600\r{ }C, will enable new and important tokamak research into the temperature dependence of fuel retention, PFC deposition and erosion, and divertor recycling. Significant design, analysis, and testing is underway to complete this important and challenging upgrade, which will provide valuable information for ITER and future reactors. Among other aspects of the innovative approach, the divertor plate supports, halo current shunts, and thermal shield assemblies will be discussed. The divertor supports enable pure radial motion of the divertor ring as it expands thermally and robustness to massive disruption induced electro-mechanical loads. Halo current shunts conduct 400kA in an 8T magnetic field and allow for divertor displacement relative to the vessel. Thermal shielding significantly reduces radiation and conduction to surrounding vessel structures. [Preview Abstract] |
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JP8.00094: Lower Hybrid Wave Neutral Excitation, Ionization and SOL Power Loss of the Alcator C-Mod Tokamak I.C. Faust, J.L. Terry, G.M. Wallace, S.G. Baek, C. Lau, M.L. Reinke, R.R. Parker, S. Shiraiwa, R.M. Churchill, B. LaBombard, J.W. Hughes, D.G. Whyte High Density (n$_e >10^{20}$), diverted, Lower Hybrid Current Drive (LHCD) discharges on Alcator C- Mod exhibit little to no current drive, as well as a lack of indicative hard X-ray (HXR) emission. VUV, Visible and infrared light, as well as and measurements of n$_e$,T$_e$ of in the SOL show significant change in the high density regime. Results are presented using experimental, analytical and numerical comparisons to elucidate the overall power loss of LH waves in the SOL. Variations in LH launcher limiter configurations during the FY2012 campaign showed sawtooth-dependent photon emission in near density-limit Lower Hybrid plasmas. Poloidal dependency of LHCD-induced Lyman-alpha emission in in high density plasmas was investigated using a newly-installed camera during the same period. The measurements indicate that Lyman-alpha power is enhanced globally, indicating that LHCD enhanced emission may be widespread. Data from Lyman-alpha is compared to a new HXR analysis code, finding little dependence on the photon temperature of high energy bremsstrahlung. [Preview Abstract] |
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JP8.00095: Observation of current density profile changes during Lower Hybrid Current Drive by the Alcator C-Mod Polarimeter System P. Xu, J.H. Irby, W.F. Bergerson, S. Shiraiwa, G.M. Wallace, J.W. Hughes, R. Mumgaard, S. Scott, W.X. Ding, D.L. Brower, R.R. Parker, S.M. Wolfe, E.S. Marmar A three-chord, poloidally-viewing, polarimetry diagnostic measuring the Faraday effect is now operational on Alcator C-Mod. Faraday effect phase shifts observed during Ohmic discharges for all three chords agree well with calculations using EFIT and Thomson scattering density profiles. During lower hybrid current drive experiments, the Faraday rotation measurements, which are sensitive to the current change, indicate that the current density profile flattens and then relaxes back to an Ohmic current profile within $\sim $150 ms following the RF pulse. A 30{\%} discrepancy between polarimetry measurements and EFIT calculation (without internal constraints) during current drive has been found Using polarimetry data in addition to MSE as constraints on EFIT reconstructions will be discussed. Sources of error, and progress made toward improving the signal-to-noise level of the diagnostic for the lower hybrid current drive experiments will also be described [Preview Abstract] |
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JP8.00096: Multi-chord Faraday-Effect measurements of fluctuations in C-Mod William Bergerson, P. Xu, D.L. Brower, W.X. Ding, J.H. Irby Three chords measuring the Faraday effect are operated routinely across all machine conditions in C-mod and allow for internal measurements of the equilibrium poloidal magnetic field. Absolute error attributed to stray magnetic field effects is below 0.5 degree and noise related to contamination from the lower hybrid and ion cyclotron radio frequency systems are not observed. Tests indicate there is no measurable signal contamination from the toroidal magnetic field due to the Cotton-Mouton effect or misalignment. Polarization sensitivity of the wire mesh beamsplitters requires system calibration which is achieved using a rotating half-wave plate. Individual channels can be modified to measure the Cotton-Mouton effect directly and yield a line integrated density measurement without ``fringe skips.'' Fluctuations on the Faraday signal associated with sawteeth, tearing modes, the quasi-coherent mode, broadband turbulence, and fast particle driven modes are observed at frequencies up to 1 MHz. Efforts are underway to differentiate between density and magnetic fluctuations in the polarimetry measurement via cross correlation techniques and combined density measurements. This work supported by DOE contract DE-FG02-01ER54615 and DE-FC02-99ER54512-CMOD. [Preview Abstract] |
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JP8.00097: Performance assessment of C-Mod MSE Upgrades S.D. Scott, R.M. Mumgaard Two upgrades for the Motional Stark Effect (MSE) diagnostic on Alcator C-Mod have been completed recently to address issues relating to partially polarized background light and drift of the system's calibration over time. The beam-induced MSE signal is contaminated by partially polarized background light generated when unpolarized visible Bremsstrahlung, glowing invessel structures, and quasi-continuum edge emission reflect off the ICRF antennas. The background light varies rapidly in time so the standard approach of interpolating background measurements as the beam is modulated in time does not yield sufficient accuracy. A prototype single-channel, three wavelength polychromator has been installed to measure the polarization properties of the background light in real time at a wavelength close to the MSE spectrum while simultaneously measuring the beam emission. In addition, an invessel calibration system has been installed to provide an MSE calibration with linearly polarized light at four angles within 8 seconds of a C-Mod plasma shot. The performance of these systems will be assessed over the C-Mod 2012 experimental run campaign. Potential application of these techniques to ITER and next step devices will be discussed. [Preview Abstract] |
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JP8.00098: MSE constrained magnetic reconstructions of Lower Hybrid Current Drive on Alcator C-Mod R.T. Mumgaard, S.D. Scott, S. Shiraiwa, G.M. Wallace, R.R. Parker Improved MSE measurements of the magnetic field line pitch angle have been used to constrain equilibrium reconstructions in plasmas with substantial Lower Hybrid (LH) current drive on Alcator C-Mod. The reconstructed driven current profiles were measured over a variety of parameters including LH phasing and LH power. The plasma density was varied from low density where the LH current drive is observed to follow the classical current-drive efficiency scaling proportional to n$_{e}$R I$_{p}$/P$_{LH}$ to n$_{e}$ $\sim $10$^{20}$ m$^{-3}$, beyond which the current drive efficiency is observed to decrease precipitously. Sources of systematic error in the MSE measurement and their influence on the reconstructions are discussed. Results using established MSE intra-shot calibration techniques are compared with calibrations using a new MSE inter-shot calibration system. Future work will include comparing reconstructions of the Lower Hybrid current drive profile with simulations. Supported by USDoE awards DE-FC02-99ER54512 and DE-AC02-09CH11466. [Preview Abstract] |
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JP8.00099: Design of off-midplane launcher (LH3) for Alcator C-Mod S. Shiraiwa, P.T. Bonoli, J. Hillairet, O. Meneghini, R.R. Parker, G.M. Wallace, J.R. Wilson, R.W. Harvey, A.P. Smirnov Improving LHCD efficiency at density above $10^{20}m^{-3}$ is crucial for approaching AT regimes on Alcator C-Mod. In this density regime, an unexpected loss of LHCD efficency has been observed. Modeling and experiments suggest that such loss is intrinsic to the strong multipass absorption regime and can be overcome by improving the wave single pass absorption. For this purpose, an additional launcher (LH3) was designed. Besides doubling total injected LH power, LH3 will be located off-midplane and will enhance the waves single pass absorption by velocity space synergy with the existing launcher (LH2). The poloidal location and launched $N_{//}$ were selected based on a large parameter scan using a ray-tracing code. About 300kA of LH driven current was predicted at 1.4e20m-3 using 1.3MW of forward power. LH3 has a grill of 4x16 active waveguides and is based on a eight-way splitter design, which splits the microwave power into four ways in the poloidal direction and two ways in the toroidal direction. Coupling studies using commercial FEM software and the ALOHA code predict good coupling in a wide range of $N_{//}$ (from 2.2 to 2.7). Details of RF design and progress of launcher fabrication will be reported. [Preview Abstract] |
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JP8.00100: Characterization of neo-classical tearing modes in high-performance I-mode plasmas with ICRF mode conversion flow drive on Alcator C-Mod Yijun Lin, R. Granetz, M.L. Reinke, J.E. Rice, S.M. Wolfe, S.J. Wukitch Neo-classical tearing modes (NTM's) have been observed on Alcator C-Mod in high performance I-mode plasmas (B$_{t0}\sim $5.2 T) that are heated by a combination of ICRF D(H) minority heating at 80 MHz and D(He3) mode conversion (MC) heating at 50 MHz. Due to the stabilizing effect by the energetic minority hydrogen ions, long sawtooth ($\ge $40 ms) and large sawtooth crashes ($\Delta $T$_{e0}\ge $3 keV) are produced in these hot (T$_{e0}\le $9 keV) plasmas. NTMs with (m=4, n=3), (5, 4), (3, 2) and (2, 1) have been observed. The onset criteria of the (3, 2) NTMs, $\beta _{p}\sim $0.4-0.7, $\beta _{N}\sim $1.0-1.4, $\beta _{N}$/$\rho _{i}^{\ast }\sim $200-350, $\nu _{ NTM}$ (q=3/2)$\sim $0.04-0.25, approximately follow the trend that obtained from DIII-D and ASDEX Upgrade. For the (3, 2) mode, the saturated width W$_{sat}\sim $0.8-1 cm, which is 3-4 times the ion banana width. Although the NTMs have a small effect on confinement degradation ($\Delta \beta $/$\beta \sim $ a few percent), they have significant effect on plasma rotation. MC flow drive generates large toroidal rotation above 100 km/sec in L-mode, and when the plasma enters I-mode, plasma rotation is expected to increase significantly due to the additional intrinsic rotation torque from the edge T$_{e}$ pedestal. The appearance of the (3, 2) mode usually rapidly reduces the rotation speed, and the (2, 1) mode, if it occurs, would completely halt the rotation. [Preview Abstract] |
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JP8.00101: Measurement of Mode Converted ICRF Wave Intensity with Phase Contrast Imaging and Comparison with Full-wave Simulations on Alcator C-Mod N. Tsujii, M. Porkolab, P.T. Bonoli, Y. Lin, J.C. Wright, S.J. Wukitch, E.F. Jaeger, D.L. Green, R.W. Harvey Radio frequency (rf) waves in the ion cyclotron range of frequencies (ICRF) are widely used for heating fusion plasmas. In a multi-ion-species plasma, the launched fast waves convert to ion cyclotron waves and ion Bernstein waves around the two-ion hybrid resonances which exist between the cyclotron resonances. The mode converted waves are of interest as actuators to optimize plasma performance through current drive and flow drive. Numerical simulations are essential to describe these processes accurately in a realistic tokamak geometry, and it is important that these simulation codes be validated against experiment. The phase contrast imaging (PCI) technique has been used on Alcator C-Mod to measure directly the rf waves. The measurements were compared to predictions of full-wave simulations using a synthetic diagnostic method. The measured mode converted wave intensity was found to be a factor of 50 weaker than what was expected from the linear wave theory in a strong mode conversion regime. The agreement improved when the wave intensity was weaker, which is a possible indication of nonlinear wave physics being involved. [Preview Abstract] |
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JP8.00102: Comparison of the Impurity Sources from Standard and Field-Aligned ICRF antennas in Alcator C-Mod D.R. Miller, B. Lipschultz, M.L. Garrett, A.N. James, Y. Lin, M.L. Reinke, S.J. Wukitch Impurity contamination associated with ICRF heating can limit antenna performance particularly for experiments with high-Z plasma facing components (PFCs). The previous amelioration techniques involve low Z coatings, field aligned Faraday screens and dipole antenna operation. A new field-aligned antenna (FA), where the antenna straps are aligned perpendicular to the total magnetic field, is installed and currently operating. The change in strap orientation is designed to reduce impurity influxes through symmetry to cancel E-parallel along a field line. We have found marked reduction in the antenna limiter (molybdenum) source rates when ICRF heating is performed with the FA antennas versus the standard antennas. When the FA antenna is operated in monopole phase, the limiter impurity source rate is similar to the ST antenna source rate. We will report the latest results and also present data on comparison of antenna types for other impurity influxes -- boron (coatings on surfaces) and titanium (coatings on Faraday screens). [Preview Abstract] |
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JP8.00103: Field Aligned Antenna Performance Evaluation in Alcator C-Mod Stephen Wukitch, I. Cziegler, M.L. Garrett, C. Lau, Y. Lin, B. Lipschultz, D. Miller, R. Ochoukov, M.L. Reinke, J.L. Terry, A. Tronchin-James, D. Whyte We have installed a new field aligned antenna designed to minimize impurity production by imposing symmetry along the total magnetic field line. Using the standard antennas (ST) as a reference, the impurity contamination and sources in both L and H-mode are significantly lower for the FA-antenna. In both L and H-mode the radiated power is reduced for given injected power for the FA-antenna compared to the ST-antennas. The antenna has achieved voltages up to 45 kV and power density up to 9.6 MW/m2 where the power density is limited by the maximum sustainable voltage. In addition to high voltage limits, we have often observed maximum voltage degradation at high neutral pressure, so-called neutral pressure limit. The FA-antenna's neutral pressure limit is at least twice that of the ST antennas likely due to larger vacuum feedthrus. An unexpected result is that the FA antenna appears to be more load tolerant than ST antennas. We have also observed that low Z impurity seeding reduces antenna faulting in a manner similar to the ST antennas. Using an extensive array of core and boundary plasma diagnostics, the latest assessment of the antenna performance including impurity characteristics, impact on the SOL transport and SOL density profiles will be presented. [Preview Abstract] |
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JP8.00104: Intensified imaging of PFC erosion via Mo I emission in C-mod Alexander James, Vlad Soukhanovskii, Mike Churchill, Bruce Lipschultz, Dan Miller, Matt Reinke, Jim Terry, Christian Theiler Understanding erosion of plasma facing components (PFCs) in high performance discharges is important for minimizing core assimilation of impurities and re-deposition onto optical surfaces. A new diagnostic at C-Mod images PFC erosion via emission from sputtered particles using an intensified charge injection device camera. The system is absolutely calibrated to enable measurement of erosion fluxes. Continuum emission, which enters the camera in addition to the desired Mo I line emission, must be subtracted from the camera image before interpreting the eroded flux. A visible spectrometer with views coincident to the camera is used to separate the continuum and line emission sources in some circumstances, but this technique is only effective when all continuum sources are toroidally symmetric. Localized continuum sources including Planck emission and deuterium molecular emission complicate interpretation. A more robust system is described which is capable of subtracting such localized continuum sources by simultaneously imaging line emission and a spectrally adjacent continuum region. Initial results are also presented comparing C-Mod operation during startup, lower hybrid current drive, ion cyclotron heating, before and after boronization, and various modes of confinement. [Preview Abstract] |
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JP8.00105: Simulations of Validation Platform Experiments by the PSI-Center B.A. Nelson, C. Akcay, A.H. Glasser, C.J. Hansen, T.R. Jarboe, C.C. Kim, G.J. Marklin, R.D. Milroy, U. Shumlak, C.R. Sovinec, J.B. O'Bryan, E. Held, J.-Y. Ji, V.S. Lukin The Plasma Science and Innovation Center (PSI-Center - http://www.psicenter.org) assists collaborating validation platform experiments with extended MHD simulations. Collaborators include the Bellan Plasma Group (Caltech), CTH (Auburn U), FRX-L (Los Alamos National Laboratory), HIT-SI (U Wash - UW), LDX (M.I.T.), MST \& Pegasus (U Wisc-Madison), PHD (UW), PFRC (PPPL), SSX (Swarthmore College), TCS (UW), and ZaP (UW). Modifications have been made to the NIMROD, HiFi, and PSI-Tet codes to specifically model these experiments, including mesh generation/refinement, appropriate boundary conditions (external fields, insulating BCs, etc.), and kinetic and neutral particle interactions. The PSI-Center is planning to add neutrals to NIMROD. When implemented in NIMROD, these results will be compared to the neutral particle physics in the 2D version of HiFi. Coaxial helicity injection BCs will be specified in HiFi to simulate the Caltech co-planar experiment, for verification with previous and ongoing NIMROD simulations. Results from these simulations, as well as an overview of the PSI-Center status will be presented. [Preview Abstract] |
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JP8.00106: NIMROD simulations of the stabilization of the FRC tilt instability with energetic ion beams Richard Milroy, Charlson Kim, Ales Necas The hybrid kinetic-MHD option [Charlson C. Kim, Phys. Plasmas, {\bf15}, 072507 (2008)] in the NIMROD code [C.R. Sovinec, et al., J. Comp. Phys. {\bf195}, 355 (2004)] has been extended to optionally employ a full-orbit, full-f PIC advance. In addition, a current coupling option has been added where the particles are coupled to the fluid through the J x B term in the momentum equation, rather than through the pressure tensor. As an initial test, simulations were performed to compare with previous calculations that investigated the stabilization of the field-reversed configuration (FRC) tilt instability with energetic ion beams [D.C. Barnes and R.D. Milroy, Phys. Fluids B {\bf3}, 2609 (1991)]. FRC parameters were adjusted to duplicate those of the previous study, and the energetic particles were found to stabilize the n=1 mode at an almost identical beam threshold as was found earlier. While the previous study did not include higher order modes, initial results with NIMROD suggest that an n=3 mode may be driven by the energetic particles for these parameters. Further studies investigate scaling to different machine sizes and study FRC profile modifications due to the beams presence. [Preview Abstract] |
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JP8.00107: Implementing a reacting plasma-neutral model in the NIMROD finite element code Peter Norgaard, Uri Shumlak In previous work by E.T. Meier and U. Shumlak, a generalized fluid model was derived from the Boltzmann equation that captures the effects of plasma interacting with a gasdynamic neutral fluid. Our present effort extends this work by beginning implementation in the NIMROD pseudospectral / finite element code, which has been used extensively for simulating MHD and multifluid plasma physics. An extension to NIMROD was developed by V.A. Izzo et al. for modeling injection of impurity gas to quench a tokamak, however it was limited to coupling through the continuity equation. The goal of this work is a more complete implementation of the plasma-neutral interaction, which will allow broader application such as the study of tokamak edge plasmas. As a preliminary step, the neutral fluid is treated as static, and interacts with the single-fluid MHD plasma model as a source term in the MHD continuity, momentum, and energy equations. The background neutral density and temperature is held fixed, so the ionization and recombination rates only vary with the plasma properties. In future work we plan to allow variable neutral density and temperature, and then implement the fully dynamic reacting plasma-neutral model. [Preview Abstract] |
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JP8.00108: Development of a Fast Scalable Parallel Solver for the HiFi 3D Extended MHD Code A.H. Glasser, V.S. Lukin We report on the development of a fast parallel solver for the HiFi 3D extended MHD code. Previous work demonstrated scalable solution of 3D ideal MHD wave propagation on up to 32,768 cores of hopper.nersc.gov, a Cray XE6 parallel computer, using Physics-Based Preconditioning (PBP) and Algebraic Multigrid. We now apply these methods to a more realistic test problem, magnetic reconnection, with the new features of nonuniformity, nonlinearity, and dissipative terms. We have developed a method of Quiet Start for computing the most unstable eigenfunction of the equilibrium, using the SLEPc library to solve an associated complex generalized eigenvalue problem. Using this eigenfunction for initial conditions avoids the excitation of large-amplitude, short-wavelength, weakly damped waves, allowing for cleaner, faster, more understandable evolution. We have developed an improved approximate Schur complement for PBP which substantially improves the rate of Newton convergence. Scaling results of the new solver will be presented. [Preview Abstract] |
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JP8.00109: Implementation of higher-order moment equations in NIMROD Jeong-Young Ji, E.D. Held, C.R. Sovinec A general formulation of higher-order moment equations\footnote{J.-Y. Ji and E. D. Held, Phys. Plasmas 15, 102101 (2008).} is being incorporated into the NIMROD fluid code in conjunction with PSI-Center activities. As an initial implementation, evolution equations for heat flow, heat-weighted heat flow, stress, and heat stress (21 moments) for electrons and ions have been adjoined to NIMROD's existing two-fluid model. Consistent with NIMROD's time staggering scheme, vector moments are centered with the plasma flow advance and rank-2 tensor moments are centered with the plasma density and species temperatures. Results from simulations involving simple wave phenomena are discussed. Although at present brute force truncation is applied, future plans include implementing more rigorous closures for the 21 moment model. [Preview Abstract] |
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JP8.00110: Asymptotic-preserving Lagrangian approach for modeling anisotropic transport in magnetized plasmas for arbitrary magnetic fields Luis Chacon, Diego del-Castillo-Negrete, Cory Hauck Modeling electron transport in magnetized plasmas is extremely challenging due to the extreme anisotropy between parallel (to the magnetic field) and perpendicular directions ($\chi_{\parallel}/\chi_{\perp} \sim 10^{10}$ in fusion plasmas). Recently, a Lagrangian Green's function approach, developed for the purely parallel transport case,\footnote{D. del-Castillo-Negrete, L. Chac\'on, \emph{PRL}, {\bf 106}, 195004 (2011)}$^,$\footnote{D. del-Castillo-Negrete, L. Chac\'on, \emph{Phys. Plasmas}, {\bf 19}, 056112 (2012)} has been extended to the anisotropic transport case in the tokamak-ordering limit with constant density.\footnote{L. Chac\'on, D. del-Castillo-Negrete, C. Hauck, {\em JCP}, submitted (2012)} An operator-split algorithm is proposed that allows one to treat Eulerian and Lagrangian components separately. The approach is shown to feature bounded numerical errors for {\em arbitrary} $\chi_{\parallel}/\chi_{\perp}$ ratios, which renders it asymptotic-preserving. In this poster, we will present the generalization of the Lagrangian approach to arbitrary magnetic fields. We will demonstrate the potential of the approach with various challenging configurations, including the case of transport across a magnetic island in cylindrical geometry. [Preview Abstract] |
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JP8.00111: Three-Dimensional Hybrid Simulations of Magnetized Plasmas Y.A. Omelchenko, H. Karimabadi, H.X. Vu Quasi-neutral hybrid simulations enable macroscopic (confinement, stability, translation) and microscopic (turbulence, reconnection and ion energization) properties of finite-beta plasmas. Target applications, among others, include magnetized plasmas generated by pulsed power systems and laboratory astrophysics experiments (e.g., FRCs, z-pinches and theta-pinches, spheromaks, magnetic dipole configurations). We conduct large-scale hybrid simulations to study ion energy budget, turbulence and flow in laboratory and space plasmas. This is done with a unique, uni-dimensional, event-driven code, HYPERS, which remarkably differs from standard hybrid codes in the way simulation is performed. HYPERS does not step spatially distributed variables synchronously in time but instead performs time integration by executing discrete events: asynchronous updates of particles and fields. Updates are carried out as frequently as dictated by their local physical time scales. This technique results in stable, accurate and fast simulations of strongly inhomogeneous plasmas. We also discuss a new low-frequency plasma simulation capability (X-hybrid) that incorporates transient radiation and finite electron mass effects in low-density plasma and vacuum regions into the hybrid approximation. [Preview Abstract] |
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JP8.00112: Coupled neoclassical-MHD simulations of Ohmic current penetration in an axisymmetric torus B.C. Lyons, S.C. Jardin, J.J. Ramos We self-consistently solve for the current induced by a boundary loop voltage in an axisymmetric, low-collisionality toroidal plasma using a new drift-kinetic equation (DKE) solver coupled to the M3D-C1 magnetohydrodynamics (MHD) time evolution code. A time-dependent DKE is solved for the non-Maxwellian electron distribution function using the full, linearized Fokker-Planck-Landau collision operator. We work in a 4D phase space $(R,Z,v,\chi)$, where $R$ is the major radius, $Z$ is the vertical distance, $v$ is the magnitude of the random velocity, and $\chi$ is the pitch angle in the mean flow reference frame. Coupling with M3D-C1 is achieved primarily through the collisional friction force, represented as a velocity moment of the non-Maxwellian electron distribution function. For the initial work, flat temperature and density profiles are assumed and we use the reduced, two-field MHD equations. Results are compared to theoretical models of the neoclassical conductivity reduction in the banana regime. The algorithms used to solve the DKE are thoroughly discussed. Future extensions of this work will lead to a hybrid neoclassical-MHD solver capable of studying core plasma instabilities, such as sawtooth modes and neoclassical tearing modes. [Preview Abstract] |
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JP8.00113: Analysis of Equilibrium Plasmas in Experimental Device by Virtual-Reality System H. Ohtani, Y. Suzuki, R. Kanno, S. Ishiguro, A. Kageyama, N. Ohno, Y. Tamura, M. Shoji Virtual Reality (VR) technology is a very powerful tool in analysis of simulation data and development of experimental devices, because it is possible to analyze the complex structures in three-dimensional space with a deep absorption into the VR world by scientific visualization technology. In the previous paper, both of simulation results and experimental device data were visualized integrally by the VR system to analyze directly the simulation results in the device. We showed a pressure isosurface, magnetic field line and particle trajectory in the virtual Large Helical Device (LHD). Recently, through the interface code of the data converter, an equilibrium plasma by HINT2 code which can calculate plasmas in the realistic magnetic structure is visualized in the LHD vessel with the objective description by the VR system. It becomes possible to display the magnetic islands and stochastic structure in the periphery and divertor region with Poincare map in the VR space. We will discuss the magnetic structures and drift particle orbits in the periphery and divertor regions. We believe that the buildup in this paper will boost up the research of the fusion plasmas. [Preview Abstract] |
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JP8.00114: 3D Equilibrium Reconstruction in Stellarators and Tokamaks with STELLOPT Samuel Lazerson, Novimir Pablant, David Gates, Hutch Neilson, Raffi Nazikian, Yasuhiro Suzuki, Kiyomasa Watanabe, Katsumi Ida, Satoru Sakakibara The ability to model and predict the behavior of stellarators and tokamaks requires an ability to match simulation parameters with experimental measurements. This process, known as experimental reconstruction, has been used extensively with 2D axisymmetric codes for Tokamaks. These codes, such as EFIT, lack the ability to model the 3D nature of stellarators and the emerging 3D nature of Tokamaks. Phenomena such as, shielding of islands by neoclassical flows and the suppression of edge localized modes through application of 3D fields, highlight the need for such 3D tools. The stellarator optimizer code STELLOPT has been modified to match 3D VMEC equilibria to experimental measurements. This has allowed 3D experimental reconstructions to be preformed on W7-AS, LHD, and DIII-D devices. The free boundary VMEC equilibria are matched to Thomson profiles (ne and Te), charge exchange measurements (Ti), MSE (polarization angle), and magnetic diagnostics (B-probes, flux loops, Rogowski coils). Three dimensional reconstructed equilibria are presented alongside confidence metrics for the reconstruction process. [Preview Abstract] |
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JP8.00115: Upgrades to the ArbiTER edge plasma eigenvalue code D.A. Baver, J.R. Myra, M.V. Umansky The Arbitrary Topology Equation Reader, or ArbiTER, is a flexible eigenvalue code that is under continued development for plasma kinetic problems. The preliminary stage of ArbiTER development has demonstrated its capability in handling complicated geometries (such as multiple X-points) as well as simple kinetic problems. Planned upgrades (such as parallelization and unstructured grids) are expected to expand its range of potential applications. In order to handle large eigenvalue problems produced by realistic kinetic problems, parallelization is necessary. ArbiTER uses the SLEPc [1] eigensolver package, which already has parallel capability, however, early versions of the code lack the structures needed to exploit this capability. Integrating parallel SLEPc into the ArbiTER code is therefore a high priority. In addition, we will also present first physics studies using ArbiTER. This will be analysis of surface-localized phenomena such as coaxial modes, which are relevant to RF heating and current drive in devices such as NSTX. Work supported by the U.S. DOE. \\[4pt] [1] http://www.grycap.upv.es/slepc/ [Preview Abstract] |
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JP8.00116: Variational Integrators for Guiding Center Trajectories C. Leland Ellison, Hong Qin, William M. Tang Geometric integrators exhibit favorable numerical behavior by mimicking properties of the continuous system on a discrete level. For instance, a discrete symplectic structure may be preserved and a discrete Noether's theorem identifies conserved quantities corresponding to symmetry in the system. The long term numerical fidelity of geometric integrators is well suited for studying long term dynamics, in contrast to standard integration methods (such as fourth-order Runge-Kutta) which exhibit non-physical accumulation or dissipation of energy. The non-canonical guiding center Lagrangian serves as a starting point for obtaining variational integrators for guiding center trajectories. Proper treatment of the degenerate Lagrangian is required to avoid numerical instability, however. Methods for avoiding even-odd decoupling and achieving an adaptive time step will be presented in the context of 3-D tokamak geometry. [Preview Abstract] |
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JP8.00117: 2D Simulations of KSTAR edge plasma using SOLPS 4.3 code Seung Bo Shim, Vladislav Kotov, Detelev Reiter, Hyunsun Han, Jin Yong Kim, Yong-Su Na, Hae June Lee Control of plasma density and impurity content is crucial to achieve high performance long pulse operation in tokamaks. In this paper edge plasma scenarios of KSTAR are analyzed numerically. ITER version of the well-known SOLPS code package (SOLPS4.3) which comprises 2D multi-species fluid plasma code B2 and 3D Monte-Carlo particle solver EIRENE coupled self-consistently. This latter allows full kinetic neutral particle transport modeling in realistic geometries including pump ducts and leaks to the main vessel from the sub-divertor volumes. Both connected and disconnected double null configurations are investigated. The main focus is made on studying the sensitivity of the edge plasma parameters with respect to gas puffing rate (density scans), thus, effectiveness of this latter for the plasma control. Preliminary findings show that for the reference KSTAR pumps: total pumping speed 100 m$^{3}$/s, pumping rates up to 13 Pa$\cdot$m$^{3}$/s, and high performance discharges with heating power $\approx $ 8 MW plasma in both divertors is likely to stay attached. At the same time, plasma temperature at the divertor targets as well as the peak incident heat flux density can be effectively reduced with increased gas puff, been especially sensitive in the inner divertors. [Preview Abstract] |
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JP8.00118: Fully three-dimensional time-resolved MHD simulations in wall bounded geometry Wouter Bos, Matthieu Leroy, Jorge Morales, Kai Schneider We present a new method for computing 3D viscous-resistive MHD turbulence in wall bounded geometries of arbitrary shape [1]. The numerical scheme is based on a classical Fourier pseudo-spectral solver combined with a volume penalization method to impose the boundary condition of the velocity and magnetic field. The new code is validated using different test cases, such as three-dimensional Taylor-Couette flow and MHD in cylindrical geometry. Imposing helical magnetic boundary conditions in the latter geometry, the flow shows a self-organization to a chaotic state for elevated Hartmann number. In toroidal geometry, this self-organization leads to toroidal velocity fields. \\[4pt] [1] Morales, Leroy, Bos and Schneider (submitted). [Preview Abstract] |
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JP8.00119: Benchmarking of the Gyrokinetic Microstability Codes GYRO, GS2, and GEM Ronald Bravenec, Yang Chen, Weigang Wan, Scott Parker, Jeff Candy, Michael Barnes, Nathan Howard, Christopher Holland, Eric Wang The physics capabilities of modern gyrokinetic microstability codes are now so extensive that they cannot be verified fully for realistic tokamak plasmas using purely analytic approaches. Instead, verification (demonstrating that the codes correctly solve the gyrokinetic-Maxwell equations) must rely on benchmarking (comparing code results for identical plasmas and physics). Benchmarking exercises for a low-power DIII-D discharge at the mid-radius have been presented recently for the Eulerian codes GYRO and GS2 [R.V. Bravenec, J. Candy, M. Barnes, C. Holland, Phys. Plasmas $\textbf{18}$, 122505 (2011)]. This work omitted \textbf{E}$\times $\textbf{B }flow shear, but we include it here. We also present GYRO/GS2 comparisons for a high-power Alcator C-Mod discharge. To add further confidence to the verification exercises, we have recently added the particle-in-cell (PIC) code GEM to the efforts. We find good agreement of linear frequencies between GEM and GYRO/GS2 for the DIII-D plasma. We also present preliminary nonlinear comparisons. This benchmarking includes electromagnetic effects, plasma shaping, kinetic electrons and one impurity. In addition, we compare linear results among the three codes for the steep-gradient edge region of a DIII-D plasma between edge-localized modes. [Preview Abstract] |
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JP8.00120: Benchmarking Particle-in-Cell drift wave simulations with Eulerian simulations in a flux-tube Yang Chen, scott Parker, Weigang Wan, Ronald Bravenec, Eric Wang, Jeff Candy We present the implementation of a flux-tube option in the global turbulence code GEM.\footnote{Y.~Chen and S.~E.~Parker, J. Comp. Phys. 220, 839 (2007)} This is necessary for benchmarking purposes because of the immense complexity involved in comparing global simulations. The global GEM assumes the magnetic equilibrium to be completely given. Our initial flux-tube implementation simply selects a radial location as the center of the flux-tube and a radial size of the flux-tube, sets all equilibrium quantities ($B$, $\nabla B$, $T$, $\nabla T$, the Jacobian etc.) to be equal to their values at the center of the flux-tube, and retains only a linear radial profile of the safety factor needed for boundary conditions. We found good agreement between GEM and GYRO/GS2 for the mode frequency/growth rate in the case of adiabatic electrons, but a difference of $\sim 15\%$ in the growth rates when kinetic electrons are included. Our goal is to understand the origin of this moderate disagreement. An alternative local geometry model based on a local solution of the Grad-Shafranov equation\footnote{J.~Candy, Plasma Phys. Control. Fusion 51, 105009 (2009)} has been implemented and new benchmarking results from this model will be presented. [Preview Abstract] |
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JP8.00121: NUBEAM developments and 3d halo modeling M.V. Gorelenkova, S.S. Medley, S.M. Kaye Recent developments related to the 3D halo model in NUBEAM code are described. To have a reliable halo neutral source for diagnostic simulation, the TRANSP/NUBEAM code has been enhanced with full implementation of ADAS atomic physic ground state and excited state data for hydrogenic beams and mixed species plasma targets. The ADAS codes and database provide the density and temperature dependence of the atomic data, and the collective nature of the state excitation process. To be able to populate 3D halo output with sufficient statistical resolution, the capability to control the statistics of fast ion CX modeling and for thermal halo launch has been added to NUBEAM. The 3D halo neutral model is based on modification and extension of the ``beam in box'' aligned 3d Cartesian grid that includes the neutral beam itself, 3D fast neutral densities due to CX of partially slowed down fast ions in the beam halo region, 3D thermal neutral densities due to CX deposition and fast neutral recapture source. More details on the 3D halo simulation design will be presented. [Preview Abstract] |
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JP8.00122: Status of RF heating model in NUBEAM B.H. Park, M. Gorelenkova, R.V. Budny, Jae-Min Kwon, S. Jardin, D. Green The ion Monte-Carlo orbit integrator NUBEAM, which is used in TRANSP and also available via the NTCC modules library, has been enhanced to include the interaction of RF fields and fast ions. The standard ion orbit integration uses a magnetic-flux coordinate system. However, in order to implement a RF quasi-linear operator which is localized in space, a second orbit integrator was added to NUBEAM in 2007. It uses cylindrical coordinate fast ion orbit integration. This, RZ orbit integrator incorporates the Kennel-Engelmann quasi-linear RF operator and uses the results of a full wave RF code, TORIC. There are some important issues related to power balance between TORIC and NUBEAM, control of goosing and time step for error control, and convergence characteristics for Monte Carlo particle number and time step which are being investigated. In this presentation we show the specific quantities most affected by this orbit integrator in relation to the above issues. Additional issues concerning the consistent coupling of a RF code and NUBEAM will also be discussed. Comparisons with measurements in plasma discharges in which both neutral beam and ion-cyclotron heating are conducted will be presented. [Preview Abstract] |
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JP8.00123: TRANSP/PTRANSP Isolver Free Boundary Equilibrium Solver Robert Andre The Isolver equilibrium solver runs within TRANSP/PTRANSP to provide a free boundary equilibrium solution inside and outside the plasma boundary which is consistent with the coil and vessel geometry for use by the heating and transport modules. Originally developed in 2005 by J. Menard in IDL, it has been ported to Fortran 95 and used as an alternative to the prescribed boundary equilibrium solvers in TRANSP/PTRANSP. It has been enhanced to enable the use of measured coil currents, the modeling of vessel currents and poloidal field circuit currents and the time evolution of the plasma current through resistive flux diffusion. This poster describes the capabilities of Isolver as implemented in TRANSP/PTRANSP, the basic algorithms used in Isolver and results of verification with TSC. [Preview Abstract] |
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JP8.00124: Eigenmode Tests of Improved Basis Functions for NIMROD C.R. Sovinec The spectral-element basis functions used in the standard NIMROD implementation are appropriate for fluid-based models with dissipation in each equation [JCP 195, p 355]. However, realistic levels of dissipation can be negligible at computationally practical spatial resolution. One- and two-dimensional ideal-MHD eigenmode computations with the CYL\_SPEC and NIMEIG codes demonstrate the effectiveness of different numerical representations for extended-MHD. Keeping all variables that are required for a dissipative, first-order in time system constrains the possible formulations. The eigenmode computations produce favorable results for a vector representation that responds to divergence at all scales. A numerical penalty term that is sensitive to parallel vorticity is adapted from the penalty method described in [Degtyarev and Medvedev, CPC 43, p 29]. The new method retains the first-order in time formulation, responds only to scales near the limit of the spatial representation, and yields convergence on localized interchange from the stable side. [Preview Abstract] |
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JP8.00125: Towards First-principles Control-oriented Modeling of the Magnetic and Kinetic Plasma Profile Evolutions in ITER Justin E. Barton, Eugenio Schuster, Karim Besseghir, Jonathan Lister The ``hybrid'' and ``steady-state'' advanced scenarios are characterized by $q$ profiles higher or equal to one to mitigate plasma instabilities and improve confinement, which are key for ITER to achieve its operational objectives. To achieve these scenarios, active model-based control of the current profile and thermal state of the plasma is required. Towards this goal, two control-oriented, plasma-response models are proposed. First, the poloidal flux diffusion equation is combined with empirical models of the electron density and temperature profiles, plasma resistivity, and non-inductive current drives to obtain a physics-based model of the poloidal flux and stored energy evolutions. Second, the empirical electron temperature model is replaced by the electron heat transport equation, which is combined with empirical models of the electron heat conductivity and heat sources to obtain a physics-based model of the poloidal flux and electron temperature evolutions. Simulation results comparing the evolution of the plasma parameters predicted by the control-oriented, physic-based models and the DINA-CH+CRONOS simulation code are presented for ITER, and the control objectives and challenges are discussed. [Preview Abstract] |
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JP8.00126: Status of the ITER PCS Conceptual Design M.L. Walker Since 2010, an ITER Plasma Control System (PCS) conceptual design activity has been underway involving participants from nearly all ITER partners. The PCS Conceptual Design Review is scheduled for November 2012, at which time functional requirements generated by this activity will be reviewed. The ITER PCS must integrate control of multiple plasma parameters that are controlled separately on present devices, including plasma current, shape, position, stored energy, beta, radiation, impurity fraction, plasma profiles, multiple instabilities, and interactions with plasma facing components, as well as new parameters like fusion power, all while avoiding triggering device protection systems. Plasma control will require technologies which do not yet exist for fusion experiments. Performance and reliability requirements are an order of magnitude greater than present fusion devices due to the nuclear mission and potential consequences of control failure. We will describe experimental physics and operational objectives as defined by the ITER Organization and summarize requirements for the PCS and related systems to satisfy these. [Preview Abstract] |
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JP8.00127: Analysis of Fusion Burn Control Approaches Using METIS Mark Boyer, Eugenio Schuster, Sylvain Bremond, Remy Nouailletas, Jean-Francois Artaud Controlling the fusion power through regulation of the plasma density and temperature is one of the most fundamental problems in fusion reactors and will be critical to the success of burning plasma experiments like ITER. In this work, a volume averaged model for the evolution of the density of energy, deuterium and tritium fuel ions, alpha-particles, and impurity ions is used to synthesize nonlinear feedback controllers for stabilizing and modulating the burn condition. Adaptive control techniques are used to account for modeling uncertainty. The control approaches make use of the different possible methods for altering the fusion power, including adjusting the temperature through auxiliary heating, modulating the density or isotopic mix through fueling, and altering the impurity density through impurity injection. The different methods are simulated and compared using a simulation framework based around METIS, a module of the CRONOS code. The simulations include the effects of 1D plasma profiles and particle recycling, and the framework allows for testing the robustness and performance of the controllers in various scenarios, including confinement changes, impurity content changes, and actuators faults. [Preview Abstract] |
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JP8.00128: ECCD Simulations of ITER Steady State Scenarions M. Murakami, J.M. Park, D.A. Rasmussen, R. Prater, T.C. Luce, P.B. Snyder, M. Henderson The ability of ITER electron cyclotron (EC) launchers to achieve current profile control is analyzed and the implications of existing models in the ITER steady-state regime are examined. EC current drive (CD) deposited at mid radius is needed to form a weak reverse shear profile, supplementing off-axis neutral beam injection. Modification of the equatorial and upper launcher design with steering poloidal angles at larger toroidal angles could drive substantial current at mid to outer radii with good efficiency. Implications of the modified ECCD on ITER steady-state scenarios are examined using an iterative steady-state solution procedure using FASTRAN solver self-consistently with heating and CD, MHD equilibrium, and transport models. The modeling uses a range of theory-based transport models (GLF23, TGLF, CDBM, etc.); edge pedestal/boundary conditions (experimental, EPED); plasma current, and plasma density. The objective of the exercise is to understand the range of steady state solutions that the ITER heating and current drive systems may yield, or conditions under which they may be applied. [Preview Abstract] |
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JP8.00129: Characterization of Mode Content and Losses in the ITER ECH Transmission Lines Sudheer Jawla, Elizabeth Kowalski, Emilio Nanni, Michael Shapiro, Richard Temkin, Timothy Bigelow, David Rasmussen Estimation of overall losses in the transmission line (TL) due to ohmic loss, inaccurate coupling of the quasi-Gaussian beam to the overmoded waveguide and the mode conversion becomes vital to characterize the ITER ECH system which uses 24 MW RF power at 170 GHz. Components in the TL such as 63.5-mm diameter corrugated waveguides, plane mirrors and polarizers at the miter-bends (MBs) must be characterized for these losses. Tilt and offset of the gyrotron output beam w. r. t. the TL result in excitation of higher order modes (particularly LP11 mode) and therefore additional losses. We calculate that tilting of the mirrors in two consecutive MBs in the TL can convert a significant fraction of the unwanted LP11 mode into the HE11 mode. We have observed that the estimation of mode contents in such systems, using the radiated field measurements in several planes after the waveguide end, requires an extremely high precision alignment of the scanner when measuring the field patterns. Characterizing the plane mirrors and polarizers at miter bends in cold test at low power for small length of TL becomes difficult because the losses are very small. We measured the loss of (0.022 $\pm $ 0.008) dB/ miter bend by an S11 technique for a MB with a flat mirror using a vector network analyzer. The same technique is currently being applied to measure the loss of the polarizer miter bends. [Preview Abstract] |
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JP8.00130: Progress on an ITER ECH Transmission system development and testing Tim Bigelow, Greg Hanson, Dave Rasmussen, Alan Barker, Carl Dukes, Stephen Killough, Brian Peters, Robin Rumbolt, Chuck Schaich, Roberto Sanabria, Karen McElehaney, John White, Stephen Allison Progress on further development of the ITER ECH Transmission system design and testing of waveguide components will be presented. Work on the preliminary design of the system configuration is proceeding based on the conceptual design from the ITER Organization. Requirements for precision of waveguide supports and components are being analyzed and thermal and mechanical modeling of prototype components is being performed. Several prototype components have been procured from industry and some have been tested to nearly 1 MW at 170 GHz for long pulses at JAEA in Japan. A high power test stand is being developed at ORNL to provide component, system, and instrumentation tests at 1 MW cw or higher power conditions. The high voltage power supply has been tested and 140 GHz and 170 GHz gyrotrons are expected to be operational in the near future. [Preview Abstract] |
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JP8.00131: Improvement of Current Drive Efficiency in Projected FNSF Discharges R. Prater, V. Chan, A. Garofalo The Fusion Nuclear Science Facility - Advanced Tokamak (FNSF-AT) is envisioned as a facility that uses the tokamak approach to address the development of the AT path to fusion and fusion's energy objectives. It uses copper coils for a compact device with high $\beta_N$ and moderate power gain. The major radius is 2.7 m and central toroidal field is \hbox {5.44 T}. Achieving the required confinement and stability at $\beta_N \sim {3.7}$ requires a current profile with negative central shear and $q_{min} >1$. Off-axis Electron Cyclotron Current Drive (ECCD), in addition to high bootstrap current fraction, can help support this current profile. Using the applied EC frequency and launch location as free parameters, a systematic study has been carried out to optimize the ECCD in the range $\rho {= 0.5-0.7}$. Using a top launch, making use of a large toroidal component to the launch direction, adjusting the vertical launch angle so that the rays propagate nearly parallel to the resonance, and adjusting the frequency for optimum total current give a high dimensionless efficiency of 0.44 for a broad ECCD profile peaked at $\rho =0.7$, and the driven current is 17 kA/MW for $n_{20} = 2.1$ and $T_{e} = 10.3$ keV locally. [Preview Abstract] |
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JP8.00132: World spherical torus (ST) research as coordinated by the IEA ST Implementing Agreement (IA) Y.-K.M. Peng, Y. Takase, B. Lloyd Since the establishment of the IEA ST IA in 2007, the world ST research community has grown to 24 ST experiments in seven countries. For the second 5 years of the IA, major upgrades of the NSTX (USA) and MAST (UK) are or will soon be underway aiming to start enhanced experimental research in 2015. Major tests of solenoid-free start-up are addressed by experiments in Japan including QUEST (Kyushu U.), TST-2 (U. Tokyo), and LATE (Kyoto U.), Pegasus (U. Wisconsin), and SUNIST (Tsinghua U., PRC). These aim to develop database to minimize/remove the central solenoid and enable truly compact fusion energy devices. Lithium-only PFCs are tested in LTX (PPPL). The community is considering low to modest-Q applications to fusion energy R{\&}D. These range from small size (R$\sim $0.5m) volume neutron sources delivering several MW fusion neutrons, to medium size (R$\sim $1.0m, 50-MW) fusion science and technology experimental facilities to enable critical R{\&}D needed to develop database for a fusion DEMO and other energy applications. Progress and remaining ST R{\&}D will be summarized. [Preview Abstract] |
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JP8.00133: Ignitor-like Toroidal Devices for Neutron Production Francesca Bombarda, G. Ramogida, M. Zucchetti, B. Coppi Compact fusion toroidal machines operating in DT have the potential to become efficient sources of neutrons for material testing. An Ignitor-like device could be envisaged for this purpose, making full use of the intense neutron flux that it can generate without reaching ignition. Preliminary radiation damage estimates for some fusion-relevant materials\footnote{F. Bombarda, B. Coppi, et al., \textit{Fus. Eng. Des.} \textbf{86},2632 (2011)} have shown that few full-power months of operation would provide adequate dpa levels. The main features and technological issues of a High Field Neutron Source Facility based on the Columbus concept,\footnote{B. Coppi and M.F. Salvetti, \textit{MIT Report PTP02/06}, (2002)} with about 50\% more volume than Ignitor, are illustrated and discussed. Optimization of the plasma temperature and density relative to the reference ignition scenario (with the assistance of auxiliary heating power) can achieve considerable reductions of duty cycle requirements. The constraints imposed by flux availability, magnet heating and wall loading will inevitably impose a complete redesign of the machine, with the adoption of novel materials (such as MgB$_2$ superconductor already adopted for Ignitor), and new modes of operation will need be investigated. [Preview Abstract] |
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JP8.00134: Overview of MST Research J.S. Sarff MST progress in advancing the RFP for (1) fusion plasma confinement with minimal external magnetization, (2) toroidal confinement physics, and (3) basic plasma physics is summarized. Stellarator-like helical equilibria (QSH) appear spontaneously at high current. The Lundquist number appears a key parameter, unifying QSH onset versus current in both MST and RFX-Mod. Equilibrium modeling for the 3D state is corroborated by several diagnostics, including new 2-color SXR tomography. A 1 MW neutral beam injector stimulates beam-driven instabilities for the first time in the RFP. The observed modes are nonlinearly coupled, causing redistribution of the fast ions. Interestingly, beam-generated ions are accelerated to higher energy during sawtooth events, likely related to non-collisional ion heating. The tearing mode relaxation process is a rich story where single-fluid MHD appears to capture the sawtooth period dynamics, but extended MHD is needed to explain coupled momentum and dynamo processes. A kinetic momentum stress associated with magnetic turbulence has also been discovered. Theoretical analysis of micro-instability indicates micro-tearing will be dominant at high beta. MST diagnostics will assess such instabilities in high-performance plasmas with large Te gradients. [Preview Abstract] |
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JP8.00135: Ion acceleration and time-resolved neutral particle analysis on MST S. Eilerman, J.K. Anderson, J.A. Reusch, J.J. Koliner, M.D. Nornberg, J. Waksman, D. Liu, L. Lin, S. Polosatkin, V. Belykh, J. Titus, G. Fiksel An advanced neutral particle analyzer (ANPA) has enabled new time-resolved analysis of the energetic ion distribution function on MST. The ANPA separates majority deuterons from beam-sourced protons and can be moved between a radial view that samples edge-localized ions with high $v_\bot/|v|$ and a tangential view that is weighted toward core-localized ions with high $v_{||}/|v|$. Interpretation of ANPA signals requires careful consideration of the plasma conditions along each line of sight. The NENE neutral particle tracing code is used with measurements from a 16-chord D$_\alpha$ array to calculate the background neutral density profile, which heavily influences ANPA signal levels and must be accounted for. The ANPA is also very sensitive to the ion velocity-space distribution, which must be considered due to the high pitch of MST's neutral beam ions and the anisotropy observed in reconnection-heated ions. A model of these effects along the ANPA line of sight is presented, as well as studies of ion acceleration due to magnetic mode activity. Energization of beam and bulk ions due to sawtooth crashes is compared to observed acceleration during predominately m=0 activity and quasi-single-helicity (QSH) states. [Preview Abstract] |
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JP8.00136: Characterization of Beam-Driven Instabilities in Several RFP Equilibria J.J. Koliner, C.B. Forest, J.S. Sarff, J.K. Anderson, B.E. Chapman, L. Lin, W.X. Ding, D.L. Brower, D. Liu, J.D. Hanson, D.A. Spong Short-lived, coherent bursts are observed in MST plasmas during 1 MW neutral beam injection (NBI). Fast hydrogen ionizes with v$_{\vert \vert }$/v $\approx $ 0.9, creating a spatially localized beam-like population of fast ions. Primary bursts have mode numbers $n$=5, $m$=1 for magnetic equilibria with edge safety q(a) = 0, and $n=$6$, m$=1 for q(a) $<$ 0 discharges. The frequencies of these modes scale with beam velocity. Secondary bursts with $n$'=$n$-1 exhibit an Alfv\'{e}nic scaling of frequencies, as well as fast down-chirping under varying plasma conditions, including low-current ($\le $ 200 kA) discharges and inductive current profile control (PPCD) cases. Burst prevalence and frequencies exhibit sensitivity to NBI parameters, the q-profile, Alfv\'{e}n speed, and to 3D effects such as the onset of a Single Helical Axis (SHAx) state. Reduced-MHD calculations for Alfv\'{e}n continuum frequencies have been performed for q(a)=0, standard q(a)$<$0, and PPCD cases. ~~The VMEC 3D equilibrium code computes both axisymmetric and 3D SHAx equilibria. We plan to use the V3FIT 3D equilibrium reconstruction~code to generate equilibria consistent with observed diagnostics. Work supported by US DoE. [Preview Abstract] |
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JP8.00137: Energetic-Particle-Driven Instabilities and Their Effect on Fast Ions in a Reversed Field Pinch L. Lin, W.X. Ding, D.L. Brower, J.J. Koliner, S. Eilerman, J. Reusch, J.K. Anderson, A.F. Almagri, B.E. Chapman, M.D. Nornberg, J.S. Sarff, J. Waksman, D. Liu During 1 MW tangential neutral-beam injection (NBI) into the MST reversed field pinch, multiple, bursty instabilities (n=5, 4 and -1) are detected by various fluctuation diagnostics. The spatial structure of associated density fluctuations peaks near the core where fast ions reside. Significant bicoherence among them is measured, indicating nonlinear three-wave coupling. These instabilities are also observed by a laser-based Faraday-rotation diagnostic, containing critical information on the internal magnetic field fluctuations. A tangential-view high-energy neutral particle analyzer (NPA) is used to study the fast-ion population. The measured NPA signal decreases by 15{\%} following NBI-driven instabilities, indicating fluctuation-induced fast-ion transport. The NBI also reduces the amplitude of the innermost-resonant tearing mode by up to 65{\%}. This mode-suppression is lessened following the NBI-driven bursts, consistent with fast ion loss/redistribution weakening the suppression effect. [Preview Abstract] |
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JP8.00138: Neutral Beam Heating Of Reversed Field Pinch Plasmas In MST J. Waksman, J.K. Anderson, M.D. Nornberg, D. Liu, G. Fiksel, H. Sakakita, V.I. Davydenko, A.A. Ivanov, N. Stupishin Thomson scattering measurements detect statistically significant heating of electrons due to neutral beam injection (NBI) in enhanced confinement plasmas in the MST. Heating is observed to be approximately 100$\pm 50$eV in the core of 200 kA plasmas with a line-averaged electron density of ~0.5 x 10$^{13}$ cm$^{-3}$. This is the first definitive measurement of auxiliary (non-Ohmic) heating of an RFP. This heating is consistent with a 1-D classical model that calculates changes in thermal conductivity and Ohmic input power profiles during enhanced confinement (PPCD), and can calculate NBI deposition and classical fast ion diffusion and slowing down. The model is consistent with measured beam heating both during enhanced confinement and after. This is consistent with previous research, which found that fast ions were well-confined on MST and dominated by classical dynamics. Work supported by the USDOE [Preview Abstract] |
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JP8.00139: Momentum transport experiments using NBI in an RFP M.D. Nornberg, J.S. Sarff, D.J. Den Hartog, S. Kumar, J.K. Anderson, J. Waksman, T. Dobbins, D. Craig, W.X. Ding, L. Lin, D.L. Brower The self-organization process that shapes the current density profile in an RFP discharge gives rise to large turbulent stresses that also shape the parallel flow profile. These stresses drive rapid transport during relaxation events flattening both the plasma current and parallel flow profiles. Experiments using tangential neutral beam injection to create a core-localized torque are presented for a range of equilibrium conditions in MST plasmas: from standard RFP discharges where tearing modes give rise to stochastic transport to discharges with inductive profile control (PPCD) which greatly suppress the tearing modes. Measurements of plasma spin-down after NBI turn-off are used to gauge momentum transport in plasmas with varying levels of tearing mode activity. Plasmas tending toward the quasi-single-helicity state have a dominant core mode that induces a braking torque on the plasma. This core mode is suppressed by NBI thereby reducing the braking torque on the plasma. Variation of the magnetic fluctuation level through inductive profile control shows a reduction in momentum transport consistent with stochastic transport theory. [Preview Abstract] |
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JP8.00140: Physics and optimization of plasma startup in the MST RFP W. Mao, B.E. Chapman, A.F. Almagri, J.K. Anderson, D.J. Den Hartog, J. Ko, S.T.A. Kumar, L. Morton, E. Parke, J.A. Reusch, J. Waksman, D.L. Brower, W.X. Ding, L. Lin MST's Bp circuit relies on an iron core transformer, and the 2 V-s flux swing of the iron sets limits on the peak Ip and discharge duration. A substantial fraction of this flux is consumed during startup of each discharge. To some extent, this flux consumption can be reduced by applying a larger vacuum Bt at discharge initiation, a fact long known in RFP research. However, the detailed physics of this Bt dependence is not completely understood. Toward better understanding, MST's profile diagnostics are being employed to try to measure the temporal evolution of, e.g., the magnetic equilibrium and plasma resistance. Initial target plasmas have a peak Ip of about 600 kA but with different vacuum Bt. Initial results include the observation of m = 1 modes, with n = 1, 2, 3... growing and decaying in succession. This occurs as the Bt profile evolves rapidly from that of a tokamak to that of an RFP. Preliminary reconstructions of the toroidal current profile suggest that it is initially quite hollow. This work should help optimize startup with MST's new Bt programmable power supply. [Preview Abstract] |
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JP8.00141: Spectroscopic measurements and impurity transport studies on MST Reversed Field Pinch Tullio Barbui, Lorella Carraro, Santhosh Kumar, Daniel Den Hartog, Mark Nornberg The results of impurity transport studies in MST improved confinement pulsed poloidal current drive (PPCD) and standard regimes are presented and discussed. The impurity diffusion coefficient and pinch velocity are obtained through comparing experimental impurity density time evolution and radial profiles with the results of a 1-D impurity transport code. Experimental measurements have been obtained in the past with Charge Exchange Recombination Spectroscopy for several impurity species (C, O, B, Al). In particular previous analysis of fully stripped carbon measurements showed that carbon density decays at the core and radial profile evolves into a stationary hollow shape in PPCD discharges [S Kumar, Plasma Phys. Control. Fusion, 53 (2011) 032001]. Transport code has been applied to carbon, oxygen and boron impurities in order to deduce transport coefficients profiles which lead to the experimental impurities behaviour. Finally these coefficients have been compared with those obtained for RFX-mod Reversed Field Pinch in Quasi-Single Helicity (QSH) and Multiple Helicity (MH) regimes. In RFX-mod the pinch velocity, always directed outwards, features a transport barrier preventing impurities from penetrating into the plasma, especially in QSH regime. [Preview Abstract] |
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JP8.00142: Parametric dependence and inductive control of 3D helical equilibria in the MST RFP B.E. Chapman, D.J. Den Hartog, S.T.A. Kumar, M. Nornberg, E. Parke, J.A. Reusch, S. Cappello, P. Franz, P. Piovesan, M. Puiatti, M. Spolaore, D.L. Brower, W.X. Ding, L. Lin A stellarator-like equilibrium emerges in the core of RFP plasmas when the innermost resonant m = 1 tearing mode grows to large amplitude and the other, secondary m = 1 mode amplitudes are reduced. In MST, the likelihood and duration of these quasi-single-helicity (QSH) spectra increase strongly with Ip, similar to the trend observed in RFX-mod RFP plasmas which also develop a helical equilibrium. However, the Ip at which these spectra emerge in MST is nearly three times smaller than in RFX-mod. But due to additional differences in Te, Zeff, majority ion mass, and density, the two devices share a similar range of Lundquist number, S $>$ 6 x 10$^5$, an important dimensionless parameter in resistive MHD, suggesting that S may be a predictive parameter for the onset of the helical equilibrium. At the largest S in MST, the amplitude of the dominant mode reaches 8{\%} of the equilibrium field strength. The simultaneous reduction in secondary mode amplitudes leads to an increase in energy confinement time, reaching $>$ 1 ms, about 50{\%} larger than lower-S plasmas lacking a QSH spectrum. The secondary modes are reduced even further when the surface parallel electric field is inductively increased. This leads to a central Te $>$ 1 keV and an energy confinement time $\sim $ 3 ms. There is also a 6 keV/m Te gradient in the core. Supported by USDOE. [Preview Abstract] |
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JP8.00143: MST Pellet Injector Upgrades to Probe Beta and Density Limits and Impurity Particle Transport K.J. Caspary, B.E. Chapman, J.K. Anderson, S.T.A. Kumar, S.T. Limbach, S.P. Oliva, J.S. Sarff, J. Waksman, S.K. Combs, C.R. Foust Upgrades to the pellet injector on MST will allow for significantly increased fueling capability enabling density limit studies for previously unavailable density regimes. Thus far, Greenwald fractions of 1.2 and 1.5 have been achieved in 500 kA and 200 kA improved confinement plasmas, respectively. The size of the pellet guide tubes, which constrain the lateral motion of the pellet in flight, was increased to accommodate pellets of up to 4.0 mm in diameter, capable of fueling to Greenwald fractions $>$ 2.0 for MST's peak current of 600 kA. Exploring the effect of increased density on NBI deposition shows that for MST's NBI, core deposition of 25 keV neutrals is optimized for densities of 2 -- 3 x 10$^{19}$ m$^{-3}$. This is key for beta limit studies in pellet fueled discharges with improved confinement where maximum NBI heating is desired. In addition, a modification to the injector has allowed operation using alternative pellet fuels with triple points significantly higher than that of deuterium (18.7 K). A small flow of helium into the pellet formation vacuum chamber introduces a controllable heat source capable of elevating the operating temperature of the injector. Injection of methane pellets with a triple point of 90.7 K results in a 12-fold increase in the core carbon impurity density. The flow rate is easily adjusted to optimize injector operating temperature for other fuel gases as well. Work supported by US DoE. [Preview Abstract] |
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JP8.00144: Simulating Global Sawtooth Magnetic Reconnection Events in the MST RFP J.A. Reusch, J.K. Anderson, C.B. Forest, J.S. Sarff, D.D. Schnack Advances in computational power have now made possible nonlinear resistive MHD simulations of the reversed field pinch (RFP) at experimentally relevant parameters. In particular, global magnetic reconnection events known as sawtooth crashes have now been simulated at parameters matching those of 400kA discharges in MST (S$\sim $4x10$^{6})$ with the D\textsc{ebs} code. At these parameters, the simulated sawtooth event is not only similar in character, but also in duration to the events observed in MST. This implies that a single fluid MHD model is able to reproduce the dynamics leading to reconnection times that are significantly faster than Sweet-Parker. One possible mechanism for reducing the reconnection time is a plasmoid-like structure forming along the reversal surface during a sawtooth crash. To explore this, several simulations with an artificially truncated m=0 mode spectrum were performed. As the number of allowed toroidal mode harmonics is reduced, the sawtooth crash duration increases and the magnitude of the dynamo electric field decreases. Interestingly, while it has long been known that the m=0, n=1 mode plays a critical role in the sawtooth dynamics in MST, without the higher n, m=0 modes the simulations do not produce well defined sawteeth. The effects of limiting the number of m=0 modes on the character and duration of the sawtooth crash as well as initial work using the magnetic field information from these simulations to model the evolution of both the thermal and beam generated fast ion populations will be presented. This work supported by the US DOE and NSF. [Preview Abstract] |
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JP8.00145: Measurement of high-frequency density fluctuations using far-forward collective scattering and interferometric techniques in improved-confinement RFP plasmas James Duff, Brett Chapman, John Sarff, Weixing Ding, David Brower, Liang Lin In standard RFP plasmas, transport is governed by magnetic fluctuations associated with global tearing modes. For improved-confinement plasmas using inductive current profile control (PPCD), smaller-scale fluctuations at higher frequencies might become important for transport, especially drift-wave-like instabilities which may be theoretically unstable for the larger temperature gradients achieved. On the MST-RFP, an 11-chord laser-based diagnostic with $\sim $8 cm chord spacing is and frequency 694 GHz used to measure electron density fluctuations both interferometrically and by far-forward collective scattering. The existing diagnostic configuration measures the line-integrated fluctuations within the divergence of the probe beam covering a wavenumber range k$_{\perp}<$1.3 cm$^{-1}$, corresponding to k$_{\perp}\rho _{s}$ $<$1.3 ($\rho _{s}$ is the ion-sound Larmor radius). Of particular interest is comparing fluctuations in standard and PPCD plasmas. Relative to standard plasmas, tearing mode and higher frequency broadband fluctuations (up to 600 kHz) are suppressed with PPCD. This suppression in PPCD plasmas corresponds to the improved confinement. A diagnostic upgrade, in progress, will improve sensitivity and cover shorter wavelengths. Work supported by U.S.D.O.E. [Preview Abstract] |
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JP8.00146: Magnetic Fluctuation-Driven Intrinsic Flow in a Toroidal Plasma D.L. Brower, W.X. Ding, L. Lin, A.F. Almagri, D.J. Den Hartog, J.S. Sarff Magnetic fluctuations have been long observed in various magnetic confinement configurations. These perturbations may arise naturally from plasma instabilities such as tearing modes and energetic particle driven modes, but they can also be externally imposed by error fields or external magnetic coils. It is commonly observed that large MHD modes lead to plasma locking (no rotation) due to torque produced by eddy currents on the wall, and it is predicted that stochastic field induces flow damping where the radial electric field is reduced. Flow generation is of great importance to fusion plasma research, especially low-torque devices like ITER, as it can act to improve performance. Here we describe new measurements in the MST reversed field pinch (RFP) showing that the coherent interaction of magnetic and particle density fluctuations can produce a turbulent fluctuation-induced kinetic force, which acts to drive intrinsic plasma rotation. Key observations include; (1) the average kinetic force resulting from density fluctuations, $\sim $ 0.5~N/m$^{3}$, is comparable to the intrinsic flow acceleration, and (2) between sawtooth crashes, the spatial distribution of the kinetic force is directed to create a sheared parallel flow profile that is consistent with the measured flow profile in direction and amplitude, suggesting the kinetic force is responsible for intrinsic plasma rotation. [Preview Abstract] |
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JP8.00147: Two-Fluid Modeling of Current and Flow Relaxation in the Reversed-Field Pinch Joshua Sauppe, Carl Sovinec Reversed-field pinch devices typically exhibit periodic relaxation events that flatten the parallel current profile. These can be explained within the context of resistive magnetohydrodynamics by the MHD dynamo. Including the Hall term in the generalized Ohm's Law gives rise to the Hall dynamo through the correlation of fluctuating magnetic field and current density. This correlation also appears in the ion momentum equation as a Maxwell stress indicating a natural coupling between current and flow relaxation. Previous NIMROD simulations of two-fluid dynamics without background flow demonstrate a relaxation-induced change in flow consistent with experimental observations on the Madison Symmetric Torus [King et. al., Physics of Plasmas Vol. 19 No. 5, 2012]. However the current was parallel to the magnetic field which is opposite to that of the experiment. Simulations with current oriented anti-parallel to the field show a change in flow opposite to that of the previous simulations as the Hall dynamo and Maxwell stress change directions across the plasma. Cases initialized with a background flow profile comparable to MST do not display the same symmetry with respect to the orientation of current, but the net flow relaxation is similar to the cases without background flow. [Preview Abstract] |
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JP8.00148: Deep Insertion Probe Measurements of the Hall Dynamo on MST J.C. Triana, A.F. Almagri, J.S. Sarff, J.P. Sauppe, C.R. Sovinec Fluctuation-induced forces and stresses have been shown to be of importance in the self-organization processes of the RFP. These forces have been extensively studied in the edge of MST, $(\frac{r}{a}>0.8)$, and have shown that the MHD and Hall dynamo terms, $( \langle\tilde{\bf{v}} \times \tilde{\bf{b}}\rangle_{||}\mbox{and}\langle\tilde{\bf{j}} \times \tilde{\bf{b}}\rangle_{||})$, are large but have opposite trends in their radial profiles. Our newest probe, designed to study the Hall term only, allows for deep insertion on MST and measures an extensive profile of the Hall dynamo. It's predecessor had obtained insertion up to $\frac{r}{a} \sim 0.6$ and found the radial profile of the Hall dynamo term had rich structure beyond the previously probed edge region. The new probe can access a minor radius of $0.5$ and plasma parameters of the studied discharge are similar to those used in the two-fluid NIMROD simulations, making direct comparison with simulation results more straightforward. [Preview Abstract] |
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JP8.00149: Magnetic Relaxation with Oscillating Field Current Drive on MST D.R. Stone, A.F. Almagri, K.J. McCollam, J.S. Sarff In oscillating field current drive (OFCD), poloidal and toroidal ac magnetic fields are inductively applied to the plasma to drive dc plasma current through magnetic relaxation. Measurements of the dynamo mechanisms associated with magnetic relaxation are conducted during OFCD to better understand the relaxation dynamics and to possibly aid in optimizing OFCD performance. The fluctuation-induced dynamo ${\left\langle {\tilde {E}\cdot \tilde {B}} \right\rangle } \mathord{\left/ {\vphantom {{\left\langle {\tilde {E}\cdot \tilde {B}} \right\rangle } B}} \right. \kern-\nulldelimiterspace} B\cong {\left\langle {\tilde {V}\times \tilde {B}} \right\rangle -\left\langle {\tilde {J}\times \tilde {B}} \right\rangle } \mathord{\left/ {\vphantom {{\left\langle {\tilde {V}\times \tilde {B}} \right\rangle -\left\langle {\tilde {J}\times \tilde {B}} \right\rangle } {en}}} \right. \kern-\nulldelimiterspace} {en}$ and its constituent Hall dynamo are measured in the edge using inserted probes, and compared to $\eta J-E$. A passive secondary-emission capacitive probe for relatively high temperature edge plasmas was developed to measure the electric field fluctuations. During sawtooth relaxation events with OFCD the edge dynamo is enhanced compared to events without OFCD. Between events with OFCD the edge dynamo is a few V/m and opposes the edge current. This opposition is expected since the OFCD electric field adds current at the plasma edge and the dynamo tends to flatten the current profile. The linear stability of individual modes and their contribution to the dynamo is determined using pseudospectral techniques. This work is supported by the US DOE. [Preview Abstract] |
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JP8.00150: AC loop voltages and MHD stability in RFP plasmas K.J. McCollam, D.J. Holly, V.V. Mirnov, J.S. Sarff, D.R. Stone Applied AC loop voltages provide a means to study and control the dynamics of MHD activity in RFP plasmas. In MST experiments with a new programmable power supply, applying a poloidal loop voltage oscillation of sufficient amplitude is observed to tightly entrain the ambient quasiperiodic sawtooth magnetic-relaxation cycle in the RFP, making it almost strictly periodic. The RFP's limit-cycle trajectory in (F, $\Theta )$ space, where F and $\Theta $ are the equilibrium reversal and pinch parameters, is drastically modified and suggests a fundamentally different relaxation regime. Applying both poloidal and toroidal AC loop voltages, as in oscillating-field current drive (OFCD), changes the limit cycle and can reduce MHD fluctuation amplitudes. The MHD response in OFCD experiments with varying source amplitudes and phase lags is examined in terms of linear stability and nonlinear mode coupling. Linear stability for MHD current-driven modes is calculated in cylindrical geometry, including the effect of conducting-wall proximity, and preliminary results indicate the presence of a stable region in (F, $\Theta )$ space, consistent with past results for the RFP. By using OFCD to control the RFP's positioning in (F, $\Theta )$ space, it might be possible to control or suppress MHD activity while driving steady-state plasma current. [Preview Abstract] |
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JP8.00151: Analysis of core density and potential fluctuations and tearing modes in MST improved confinement plasmas P.J. Fimognari, D.R. Demers A heavy ion beam probe is used to concurrently study spatially localized density and potential fluctuations in the interior of MST. Measurements are acquired simultaneously from two sample volumes during periods of improved confinement. These are strongly influenced by characteristics of the MST reversed field pinch device and plasma. Various analysis techniques (including wavelet transforms and short-time FFTs) are used to unfold the dynamics of $\tilde{n}_e$ and $\tilde{\phi}$ along with tearing mode data measured by magnetic pick-up coils at the plasma edge. The complex relationship between the fluctuations and the modes resonant near the sample locations and those present at other points along the beam trajectory are investigated. [Preview Abstract] |
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JP8.00152: Charge-to-mass-ratio-dependent ion heating during magnetic reconnection in the MST RFP S.T.A. Kumar, D.J. Den Hartog, M.D. Nornberg, A.F. Almagri, J.S. Sarff, D. Craig Global magnetic reconnection events in the MST reversed field pinch (RFP) result in significant ion heating and acceleration. It is well understood that the ultimate energy source is the equilibrium magnetic energy, but the exact mechanism of the conversion of magnetic energy to particle kinetic energy is a topic of continuing research. In a recent experiment, we measured the temperature evolution of various charge states of dominant impurity ions in MST (Al, C, N and O), using passive spectroscopy. Measurements are made in standard RFP discharges with plasma current, $I_p \sim$400 kA and central line averaged electron density, $n_e \sim1.0\times10^{19}\ \rm{m}^{-3}$. Temperatures of these impurity ions are measured at the edge of the plasma, predominantly in the direction parallel to the equilibrium magnetic field. It is found that the increase in ion temperature during magnetic reconnection events is proportional to the charge-to-mass ratio of the ion species. Consistency with existing theoretical models for ion heating is investigated. [Preview Abstract] |
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JP8.00153: Changes in core-resonant m=1 tearing mode activity with and without edge-resonant m=0 tearing modes in MST RFP D. Craig, D.J. Den Hartog, S.T.A. Kumar, M.D. Nornberg, J.S. Sarff Dynamo activity, momentum transport, ion heating, and electron thermal transport result from core-resonant m=1 magnetic fluctuations in the Madison Symmetric Torus. However, these effects are severely suppressed when edge-resonant m=0 fluctuations are absent. We investigate changes in the m=1 mode structure and temporal dynamics with and without m=0 modes present. Passive Doppler spectroscopy measurements of m=1 ion velocity fluctuations show a clear change in phase with and without m=0 present. Active charge exchange recombination spectroscopy measurements are underway to confirm this trend and to yield spatially localized information regarding the changes in velocity fluctuations. This change in phase alters the degree to which the modes produce dynamo terms even though the mode amplitudes remain similar with and without m=0 modes. Despite the lack of m=0 fluctuations, it appears that m=1 fluctuations with different toroidal mode numbers still couple to one another, though perhaps less strongly. Work supported by U.S.D.O.E. [Preview Abstract] |
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JP8.00154: Statistical analysis of broadband electrostatic and magnetic turbulence in the MST reversed-field pinch Derek Thuecks, Abdulgader Almagri, John Sarff, Paul Terry The dominant magnetic fluctuations in the reversed field pinch arise from large scale tearing instabilities, but a broadband spectrum is observed. Recent measurements in MST suggest that the shorter wavelength fluctuations (spatially resolved to the ion gyro-radius scale, $\sim$1cm in our experiment) in both magnetic and electric fields arise via a nonlinear cascade driven by the tearing modes, but the source of the constituent fluctuations is poorly understood. The present work presents results from an insertable multi-tip probe capable of measuring electrostatic and magnetic fluctuations simultaneously in the edge plasma region. While magnetic fluctuations dominate the power spectrum in the plasma edge at low frequencies, electric field fluctuations become dominant at high frequencies. The cross-coherence between electric and magnetic fluctuations peak near the frequency where the fluctuation powers are found to be in equipartition. Bi-spectral analysis techniques are used to identify nonlinear interactions among electrostatic fluctuations. Additionally, the parabolic relationship between the skewness and kurtosis of the fluctuation PDFs may help illuminate the nature of these plasma fluctuations. NSF and DOE support this work. [Preview Abstract] |
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JP8.00155: Microtearing Simulations in the Madison Symmetric Torus Daniel Carmody, M.J. Pueschel, P.W. Terry Recent studies of improved confinement discharges in reversed field pinch (RFP) plasmas have revealed the need for greater understanding of the role that microinstabilities may be playing in these devices. In this work, the gyrokinetic codes GYRO and GENE are used to investigate the characteristics of these instabilities in an RFP-type equilibrium. Microtearing is found to have large growth rates across a range of scales for the parameter regime of the Madison Symmetric Torus and may be especially relevant to high-beta PPCD discharges, where large scale tearing mode activity is suppressed and microturbulence is believed to play a greater role in transport. Important features of the mode, such as critical values for beta and electron temperature gradient, are studied for a number of geometric parameters, including flux surface radius and RFP $\Theta$. The dependence on collisionality, which plays an important role in this instability, is explored using a Lorentz operator. We find that the RFP microtearing mode is unstable across a range of collisional frequency, and parameter scans show different responses at low versus moderate/high collisionality, possible evidence for different physical drive mechanisms. The source of these drives is explored and compared with existing theory. [Preview Abstract] |
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JP8.00156: High-Frequency Electron Temperature Fluctuations in MST E. Parke, D.J. Den Hartog, L.A. Morton, J.A. Reusch The behavior of electron temperature fluctuations at frequencies above tearing mode frequencies (20 kHz) is of interest for a wide range of plasma conditions in the MST reversed field pinch. The MST Thomson scattering system is capable of measurements at high effective frequencies, and we present preliminary measurements of correlated and uncorrelated fluctuations. During improved confinement plasmas, magnetic-fluctuation-driven transport is reduced. To determine the importance of electrostatic or other transport mechanisms, a large ensemble of two-time-point T$_{e}$ measurements has been collected. Time separations of the measurements vary (1-10 $\mu$s), allowing access to fluctuations over a wide range of frequencies. Observations of TAE-like and EPM-like modes during neutral beam injection show correlated density fluctuations at high frequencies. We plan to correlate an ensemble of electron temperature measurements with magnetic signals for similar discharges, and compare the spatial structure and fluctuation amplitudes with those obtained from density fluctuation measurements. This work is supported by the U. S. DOE and NSF. [Preview Abstract] |
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JP8.00157: Improved Calibration and Uncertainty Analysis of the MST Thomson Scattering System L.A. Morton, E. Parke, D.J. Den Hartog, H.D. Stephens, M.A. Thomas, J. Goglio, J. Robinson Thomson Scattering on MST is an established and reliable diagnostic of electron temperature profiles and fluctuations with high spatial (2cm) and temporal (40$\mu $s) resolution. In order to study small-amplitude fluctuations and correlations at even higher frequencies, the new `Fast Laser' (4$\mu $s pulse period) is being commissioned. Uncorrelated fluctuation studies necessitate a thorough accounting of uncertainties in the measured results to establish the noise floor, since expected temperature fluctuations are only a few percent of the equilibrium values. To this end, an improved calibration procedure has been implemented. Three avalanche photodiode detector modules were absolutely calibrated over a wide range of wavelengths at Canada's Institute of National Measurement Standards. The remaining detectors have been calibrated against these transfer standards at four wavelengths. The nonuniformity of the gain over the detector diode surface has been characterized, as well as the dependence of the gain on operating temperature and signal pulse width. A thorough statistical analysis has been performed to determine the final uncertainties of the derived electron temperature. [Preview Abstract] |
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JP8.00158: Electron Temperature Measurements of PPCD and Non-reversed Plasmas on MST Using SXR Double-foil Brightness J.A. Goetz, M.B. McGarry, P. Franz, D.J. Den Hartog, M.A. Thomas A new diagnostic has been developed that uses soft-x-ray (SXR) emission to provide tomographically reconstructed x-ray emissivity, double-foil electron temperature from either brightness or tomographic emissivity, and bulk plasma electron energy spectra on MST. Full, double-foil, radial profiles of electron temperature have been compared with Thomson scattering measurements, confirming that plasmas typically reach electron temperatures of $\sim$1keV in 600kA non-reversed (F=0) conditions and $\sim$1.2-1.4keV during non-crash-heated high current pulsed parallel current drive (PPCD). The diagnostic has also confirmed, during F=0 plasmas, enhanced SXR emission from a helical structure whose spatial location is consistent with that determined by external magnetic measurements. Additionally, rotating SXR emissivity structures correlated to remnant magnetic islands during PPCD have been identified. Ongoing work will determine whether or not there are also temperature structures that correspond to these remnant islands. Investigation of potential temperature structures during F=0 plasmas will also be pursued as the limits of the double-foil diagnostic are quantified. [Preview Abstract] |
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JP8.00159: Fast electron studies using a multichord x-ray spectrometer on MST J.D. Lee The X-ray spectroscopy diagnostic on MST consists of six SXR detectors and six HXR detectors capable of measuring photons in the energy range $3-25\ \mathrm{keV}$ and $10-60\ \mathrm{keV}$, respectively. The detectors can be installed on any of $17$ ports viewing a poloidal cross-section, with impact parameters from $\frac{r}{a}=0.87$ inboard to $\frac{r}{a}=0.84$ outboard. An updated code processes the digitized signals, providing the time and energy of photons incident on each detector, and is capable of resolving individual photons arriving at the detector less than $50\ \mathrm{ns}$ apart. The spatial resolution of the system may allow improved diagnosis of the QSH tearing mode on MST, including the x-ray enhancement and spatial structure. The system may be able to probe electron heating during magnetic reconnection, the counterpart to noncollisional ion heating observed during magnetic reconnection on MST. X-ray flux from multiple view chords are compared with prediction from the kinetic code CQL3D. X-ray measurements can be used to constrain CQL3D in order to estimate $Z_{\mathrm{eff}}$ and radial diffusion profiles. Additionally, radially-localized x-ray measurements from lower hybrid current drive experiments are shown. [Preview Abstract] |
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JP8.00160: Electron Bernstein Wave Studies in MST Andrew Seltzman, Jay Anderson, Cary Forest, Paul Nonn, Mark Thomas, Eric Hendries, Stephanie Diem The overdense condition in an RFP prevents electron cyclotron waves from propagating past the edge, however use of the electron Bernstein wave (EBW) has the potential to heat and drive current in the plasma. MHD simulations have demonstrated that resistive tearing mode stability is very sensitive to the gradient in the edge current density profile, allowing EBW current drive to potentially stabilize mode growth. Single fluid MHD simulation at experimental Lundquist numbers predicts the effect of RF current drive on mode activity in the RFP to determine the suitability of EBW heating for sustained current drive and mode stabilization. Preliminary experiments that verify coupling and heating via observed x-ray emission are compared to simulation. Numerical modeling of an RF heating experiment with an experimentally realizable power level of 1MW has been conducted. Genray predictions of RF propagation path and Fokker-Plank modeling in CQL3D has determined optimum coupling, launch and target plasma conditions. Experimental verification of coupling from a cylindrical waveguide has been performed to determine optimal target discharge conditions, antenna position, and end cap geometry. Preliminary testing and simulation indicates that optimal target plasmas are low density (Ne=5E13/cm$^3$), 300kA discharges with the antenna positioned flush with the RFP edge and shielded with a boron nitride limiter. Work supported by USDOE. [Preview Abstract] |
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JP8.00161: Multi-frequency reflectometer for edge density profile and fluctuation measurement on MST William Capecchi, Jay Anderson, Andrew Seltzman, Mark Thomas, Paul Nonn, Dinh Truong An accurate temporally and spatially resolved measurement of the electron density profile is an important diagnostic for a confined plasma device. An understanding of particle transport during events like a sawtooth crash can be obtained from the changes in the density profile, and resolving density fluctuations can help in the understanding of plasma instabilities. Proposed here is a multi-frequency phase-measuring reflectometer which uses O-mode launch to measure the signal reflected from the plasma cutoff layer. Simultaneous measurements of phase change for four frequencies between 4-8 GHz are used to reconstruct the edge density profile. Each phase shift is measured via digital complex demodulation of a 455 kHz signal created by mixing each drive signal with an appropriately tuned local oscillator which can in principle deliver fluctuation information up to around 200 kHz. Included in this design is a phase and amplitude control to zero out the signal from the vacuum interface reflection. Far-edge density profiles and fluctuations are noninvasively probed for a variety of MST operating conditions. [Preview Abstract] |
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JP8.00162: Analyzing the cooperation between field shaping coils and conductive shell of the plasma equilibrium W. You, W. Mao, Chenguang Li, M. Tan, Tao Lan, Jinlin Xie, Adi Liu, Hong Li, Wangdong Liu, Weixing Ding, C.J. Xiao The copper shell in KTX reversed field pinch plays an important role in plasma equilibrium. By using simulation software, we set up a KTX electromagnetic model and calculate the eddy current distribution on conductive shell with the existence of field shaping coils. According to the current distribution, we have analyzed how both the field shaping coils and conductive shell effect on plasma equilibrium when plasma ramps up. Finally, this paper proposes a method for future design of field shaping coils, which can help to eliminate the eddy current on conductive shell and stray fields can be reduced. [Preview Abstract] |
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JP8.00163: Magnetohydrodyamic mode feedback control using all components of $\tilde{B}$ in the presence of two resistive walls for reversed field pinches K. Sassenberg, A.S. Richardson, D.P. Brennan, J.M. Finn A theoretical and computational analysis is presented of magnetohydrodynamic instability control through sensing and proportional feedback in Reversed Field Pinches (RFPs) with two resistive walls. The feedback signal incorporates all three components of the magnetic field perturbation at variable locations, each with its own gain factor and important features of the RFX-mod experiment, namely two resistive walls with measurements taken in between. Depending on the nominal magnetic flux diffusion times of the walls the two wall scenario reflects the physics of the one wall scenarios with measurements taken inside and outside. Therefore studies with a single wall provides the key to understanding the more physically relevant two wall results. It is found that feedback control can stabilise the plasma in the presence of two walls and at currents much larger than the ideal wall tearing limit, but less than the upper bound set by the one wall scenario with the derivative of the helical flux measured inside. Furthermore, when a thick wall formulation is considered high frequency magnetosonic modes were found to be stable, demonstrating the thin wall approximation is sufficient when selectively treating the comparatively low frequency tearing mode and ignoring the magnetosonic mode. [Preview Abstract] |
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JP8.00164: Overview of RFX-MOD Fusion Science Program Piero Martin, Maria Ester Puiatti The European RFX-mod reversed field pinch experiment is realizing a research plan, which aims at exploring the fusion capabilities of the RFP configuration in the 2 MA plasma current regime and at contributing to fusion physics development. This paper will report on the most recent developments in these areas. Key results have been obtained in lithization of the first wall through a number of techniques and in general on the issue of plasma-wall interaction, on active control of MHD stability both in RFP and tokamak configurations, on three-dimensional physics and on turbulence studies. New projects on upgrades of the feedback control system will be illustrated. [Preview Abstract] |
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JP8.00165: Thermal characterization of QSH crashes in RFX-mod Alessandro Fassina, Marco Gobbin, Paolo Franz, Lionello Marrelli, Alberto Ruzzon QSH (Quasi Single Helicity) states have gained a growing interest in RFP research since they show improved confinement and transport features with respect to standard discharges. However, ITBs associated with QSH states can be obtained only in a transient way, and in general with a shorter lifetime with respect to that of the QSH phase [1]. In this work the analysis has essentially the purpose of confirming, with TS data, the Te dynamics seen with the double filter, multichord SXR spectrometer in [1]: TS data allow a better spatial definition of temperature profile and a more reliable description of plasma edge. Te profile features in rising and crashing phases are determined via ensemble averaging, possible precursors of thermal crashes are identified, while q(r) behavior is studied identifying the thermal structures associated with rational surfaces. \\[4pt] [1] Ruzzon et al, 39th EPS Conference, P2.023 [Preview Abstract] |
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JP8.00166: Thermal Dynamics of QSH Regimes in RFX-Mod Paolo Franz, Marco Gobbin, Lionello Marrelli, Alberto Ruzzon, Alessandro Fassina A statistical analysis of the electron temperature gradient Te in helical plasmas from the reversed field pinch RFX-mod will be presented. Self-organization of the plasma when a magnetic tearing mode dominates the m=1 spectrum (QSH regimes) is observed when the plasma current is increased above 1 MA. When the dominant mode amplitude is sufficiently large the plasma becomes helical (Single Helical Axis state, or SHAx) and an internal transport barrier appears. The evolution of electron temperature profiles from the soft-x-ray (SXR) tomography of RFX-Mod has been analyzed in time. Te is calculated along 19 sight lines using the two-foil technique. In order to locate the Te data point in the plasma cross-section the temperature profiles have been remapped on reconstructed magnetic surfaces. QSH cycles have been divided in two phases, rising (dominant mode increasing) and flattop (dominant mode amplitude saturated). The Te gradient exhibits different behavior in these two intervals: during the rising phase it reaches higher values, and the size of the barrier tends to increase with the amplitude of the dominant mode; smaller secondary modes (residual m=1 tearing modes) also helps to achieve high values of the gradient. During the flattop phase the Te gradient shows an oscillating pattern and the barrier in the profile often disappear, so that the thermal helical configuration in general does not last as the magnetic one. [Preview Abstract] |
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JP8.00167: Effects of 3D magnetic fields on plasma rotation in RFX-mod tokamak experiments Lidia Piron, Paolo Piovesan, Fulvio Auriemma, Daniele Bonfiglio, Lorella Carraro, Marco Gobbin, Lionello Marrelli, Marco Valisa, Marco Veranda, Barbara Zaniol, Luis Chacon In tokamaks, plasma rotation is important to stabilize deleterious MHD instabilities, to screen resonant magnetic perturbations, but also to improve energy confinement through turbulence suppression. Rotation can be affected by external 3D magnetic fields, e.g. due to error fields or active coils. The RFX-mod device has been recently run as an Ohmic tokamak with q(a)$<$2 thanks to the suppression of the m=2, n=1 external kink through active control. In this work, we study plasma rotation in these plasmas in the presence of 3D magnetic fields, applied through magnetic feedback or due to MHD instabilities. In particular, in plasmas with q(a)$>$2 toroidal rotation braking has been observed as the 2/1 tearing mode locks. However with q(a)$<$2, the rotation at first decelerates as the 2/1 RWM grows in amplitude and then reverses its sign from counter to co-Ip direction. The results are interpreted by considering the various physical mechanisms playing a role on momentum transport, such as NTV, ambipolar electric fields due to residual stochasticity and the friction force due to neutrals. [Preview Abstract] |
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JP8.00168: 3D equilibrum reconstruction for the RFP with V3FIT David Terranova, Lionello Marrelli, James Hanson, Steven Hirshman, Gobbin Marco, Gregorio Trevisan Helical states are routinely found in all Reversed Field Pinch experiments and their description requires a 3D equilibrium reconstruction. We present the application of the V3FIT code for the RFX-mod experiment. Magnetic and kinetic diagnostics ($T_e$ from Thomson scattering, SXR emissivity, $N_e$ from interferometer) are used in order to properly deal with the problem of degeneracy when only external magnetic measurements are used. A sensitivity study of external measurements on the internal topological structure is also presented, showing a link between external measurements and internal profiles. Fixed-boundary equilibria can be computed by independently calculating vacuum fields. The results provide a good match with experimental data and the obtained equilibria are suitable for both transport and stability analysis. [Preview Abstract] |
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JP8.00169: Ambipolar edge electric field with energy dependence G. Spizzo, R.B. White, M. Agostini, P. Scarin, N. Vianello In a recent work,\footnote{G.Spizzo \textit{et al.}, Nucl. Fusion \textbf{52} (2012) 054015} it was shown that in the edge of toroidal confinement devices a sheath potential can arise when the ambipolarity constraint is applied to ion and electron motion in proximity of edge islands: in this way, the symmetry of the electric field is the same as the generating island. This was shown by analyzing ion and electron motion in presence of an electrostatic potential in a $\sim 8$ ion Larmor radius layer next the wall, by means of the guiding-center code \textsc{Orbit}. Simulations show that there is a phase shift between the potential ``island'' and the magnetic island, with this difference $\phi_{pot}-\phi_{island} \approx \pi/2$ for monoenergetic ions and electrons with bulk energy. Measurements in the RFX reversed field pinch show that in experiment the phase shift depends on collision frequency, the value $\pi/2$ being its collisionless extrapolation. This suggests that there is an energy dependence during the potential formation. In this paper we will show initial results with the implementation of a full profile Monte Carlo operator, based on the energy scattering formula of Boozer-Kuo.\footnote{A. H. Boozer A.H. and G. Kuo-Petravic, Phys. Fluids \textbf{24} (1981) 851} [Preview Abstract] |
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JP8.00170: MHD instabilities in non-circular Reversed Field Pinch plasmas S.C. Guo, Z.R. Wang, X.Y. Xu, Y.Q. Liu The toroidal hybrid MHD-kinetic code MARS-K has been modified and applied to non-circular Reversed Field Pinch (RFP) plasmas where both the plasma elongation and the triangularity are taken into account. For the Resistive Wall Mode (RWM), in comparison with the circular RFP, the elongation leads to a rather strong poloidal mode coupling, and causing many growing higher m harmonics. As a consequence, the critical ideal wall position for the ideal kink instability becomes closer to the plasma surface than the circular case. On the other hand, kinetic effects become more significant and result in a lower $\beta _{p}$ value for the kinetic stabilization by transit thermal ions. Due to the strong poloidal magnetic field in RFPs, the shaping effect causes a significant variation of the field strength along the poloidal angle, resulting in an occurrence of multiple trapping regions. The kinetic effects of these multiple trapping fractions will be discussed. The shaping effect on the resistive tearing mode in RFP will also be discussed and compared with the circular case. [Preview Abstract] |
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JP8.00171: Trapped electron effects on ITG modes and TEM in the reversed-field pinch Songfen Liu, Shichong Guo, Jiaqi Dong The trapped electron effects on the ion temperature gradient (ITG) driven mode and the trapped electron modes (TEM) are studied in the reversed-field pinch (RFP) plasmas by solving the gyrokinetic integral eigenmode equation. Both collisionless and collisional plasmas are investigated. It is found that, the trapped electron effects usually give a little influence on ITG mode except in very steep density gradient region, where the TEM can couple with ITG and induces the instability even below ITG stability threshold $\eta_{ic}$. TEM instability in RFP configuration has been studied extensively in various parameter regions, and compared with the circular tokamak cases. The characteristics of electron temperature gradient (ETG) instability in RFP plasmas are similar with that of the tokamak plasmas in some aspects. However, the excitation of the TEM instability in RFPs requires much steeper density gradient than that in tokamaks. This could be due to the stronger ion Landau damping in RFPs, which has been found to play an important role in the ITG mode physics.\footnote{S. C. Guo, \emph{Phys. Plasmas}, \textbf{15}, 122510(2008). }$^,$\footnote{Songfen Liu, et al, \emph{Phys. Plasmas}, \textbf{17}, 052505(2010).} The detail discussion will be presented. [Preview Abstract] |
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JP8.00172: Progress in low-aspect-ratio RFP research in RELAX Sadao Masamune, A. Sanpei, S. Nakaki, K. Deguchi, K. Nishimura, H. Tanaka, H. Himura, A. Ejiri, N. Mizuguchi, K. Kawahata, D.J. Den Hartog, R. Paccagnella RELAX (A=R/a=0.5m/0.25m) is a low-A RFP machine to study optimization of the RFP configuration, with exploration of the potential advantages of the low-A RFP. The discharge and plasma parameters in RELAX to date are as follows: plasma current of up to 125kA, discharge duration of up to 2-2.5ms, electron density in the range from 0.2 to 2.0 $\times$ 10$^{19 }$m$^{-3}$, and central electron temperature of 100-150eV (at Ip $\sim $ 100kA) from Thomson scattering. Recent modification of the insulated gap flanges has improved axisymmetry of the average toroidal field. The QSH duration has also been improved as a result of the improved axisymmetry of the toroidal field. The vacuum vessel has been covered with 64 saddle coils (4 polidally $\times$ 16 toroidally) for the RWM stabilization, and initial experiments have been started to stabilize a specific mode by using limited number of the power supplies. The experimental results will be compared with a non-linear 3-D MHD simulation result which has revealed that the QSH state with experimentally observed m/n=1/4 structure can be realized for A=2 configuration. Some scenarios for high current (Ip$>$100kA) discharges with sufficient controllability of the density will also be discussed. [Preview Abstract] |
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JP8.00173: Evaluation of 3D Structure in RELAX RFP with SXR Imaging Technique Akio Sanpei, Sadao Masamune, Kazuaki Deguchi, Seiya Nakaki, Hiroyuki Tanaka, Kanae Nishimura, Haruhiko Himura, Satoshi Ohdachi, Naoki Mizuguchi In a low-A RFP machine RELAX ($R$ = 0.51 m/$a$ = 0.25 m ($A$ = 2)), a quasi-periodic transition to quasi-single helicity (QSH) state has been observed. During the QSH state, the fluctuation power concentrates in the dominant $m$ = 1/$n$ = 4 mode, and a (toroidally rotating) 3-D helical structure has been observed with radial array of magnetic probes [1]. We applied a high-speed (10-microsecond time resolution) soft-X ray (SXR) imaging diagnostic system to take SXR images during the QSH state, identifying the characteristic helical SXR structures which suggest hot or dense helical core [2]. The high-speed SXR imaging system has been extended to take the images from tangential and vertical directions simultaneously to observe 3-D dynamic structures of the SXR emissivity. The time evolution of the 3-D helical structures associated with the QSH state will be reported, together with some discussion on 3-D reconstruction techniques.\\[4pt] [1] Oki et al., Plasma Fusion Res. vol.7, 1402028 (2012).\\[0pt] [2] Sanpei et al., IEEE Transaction Plasma Science, vol.39, 2410 (2011). [Preview Abstract] |
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JP8.00174: A Technique for the Estimation of the Wall Diffusion Time Lorenzo Frassinetti, Richard Fridstrom, Erik Olofsson, Agung Chris Setiadi, Per R. Brunsell, Francesco Volpe, James R. Drake Feedback systems are important tools for an advanced control of the MHD instabilities in fusion plasmas, both for the suppression of undesired modes, such as RWMs, and for the generation of external perturbations for ELM suppression. A good knowledge of the diffusion time through the machine wall of each external harmonics is necessary for reaching optimal performances of the feedback algorithms. A correct theoretical estimation is not easy due the presence of three-dimensional mechanical structures in the devices, such as shell cuts and external conductive structures that need to be considered. Identification of differences in the vertical and horizontal diffusion time are not simple from a theoretical point of view. This work will present a relatively simple technique to experimentally estimate the diffusion time for each harmonic. The technique is based on the generation of rotating external magnetic perturbations in vacuum and on the quantification of the wall screening from the measured field inside the wall. The technique will be able to quantify possible differences among the horizontal and vertical diffusion time. In the final part of the work, the comparison with the results obtained with a closed-loop identification algorithm of the machine plant will be discussed. [Preview Abstract] |
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