Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session JP11: Poster Session IV: Education and Outreach; High School or Undergraduate Research; C-Mod, MST, & MFE Theoretical Methods |
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Room: Exhibit Hall D |
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JP11.00001: EDUCATION AND OUTREACH |
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JP11.00002: Outreach to Underrepresented Groups in Plasma Physics A. Dominguez, A. Zwicker, D. Ortiz, S.L. Greco Physics, and specifically plasma physics, has a recruitment and retention problem for women and historically underrepresented minorities at all levels of their academic careers[1]. For example, women make up approximately $8\%$ of the APS-DPP membership while making up $13\%$ of APS membership at large. In this presentation, we describe outreach activities we have undertaken targeting retention of these groups after their undergraduate careers. These include: Targeted recruitment visits for undergraduate research internships, as well as plasma physics workshops aimed at undergraduate women in physics, faculty members of minority serving institutions, and underrepresented undergraduates. After the first year of implementation, we have already seen results, including students reached through these programs participating in SULI undergraduate internships at PPPL. This work was support by a grant from the DOE Office of Workforce Development for Teachers and Scientists (WDTS). [1] AIP Physics Trends (www.aip.org/statistics) [Preview Abstract] |
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JP11.00003: Pathway to STEM: Using Outreach Initiatives as a Method of Identifying, Educating and Recruiting the Next Generation of Scientists and Engineers Deedee Ortiz-Arias, Andrew Zwicker, Arturo Dominguez, Shannon Greco The Princeton Plasma Physics Laboratory (PPPL) uses a host of outreach initiatives to inform the general population: the Young Women's Conference, Science Bowl, Science Undergraduate Laboratory Internship, My Brother's Keeper, a variety of workshops for university faculty and undergraduate students, public and scheduled lab tours, school and community interactive plasma science demonstrations. In addition to informing and educating the public about the laboratory's important work in the areas of Plasma and Fusion, these outreach initiatives, are also used as an opportunity to identify/educate/recruit the next generation of the STEM workforce. These programs provide the laboratory with the ability to: engage the next generation at different paths along their development (K-12, undergraduate, graduate, professional), at different levels of scientific content (science demonstrations, remote experiments, lectures, tours), in some instances, targeting underrepresented groups in STEM (women and minorities), and train additional STEM educators to take learned content into their own classrooms. [Preview Abstract] |
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JP11.00004: The ZPIC educational code suite R. Calado, M. Pardal, P. Ninhos, A. Helm, W. B. Mori, V. K. Decyk, J. Vieira, L. O. Silva, R. A. Fonseca Particle-in-Cell (PIC) codes are used in almost all areas of plasma physics, such as fusion energy research, plasma accelerators, space physics, ion propulsion, and plasma processing, and many other areas. In this work, we present the ZPIC educational code suite,~a new initiative to foster training in plasma physics using computer simulations. Leveraging on our expertise and experience from the development and use of the OSIRIS PIC code, we have developed a suite of 1D/2D fully relativistic electromagnetic PIC codes, as well as 1D electrostatic. These codes are self-contained and require only a standard laptop/desktop computer with a C compiler to be run. The output files are written in a new file format called ZDF that can be easily read using the supplied routines in a number of languages, such as Python, and IDL. The code suite also includes a number of example problems that can be used to illustrate several textbook and advanced plasma mechanisms, including instructions for parameter space exploration. We also invite contributions to this repository of test problems that will be made freely available to the community provided the input files comply with the format defined by the ZPIC team. The code suite is freely available and hosted on GitHub at https://github.com/zambzamb/zpic. [Preview Abstract] |
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JP11.00005: Engaging high school students as plasma science outreach ambassadors Amy Wendt, John Boffard Exposure to plasma science among future scientists and engineers is haphazard. In the U.S., plasma science is rare (or absent) in mainstream high school and introductory college physics curricula. As a result, talented students may be drawn to other careers simply due to a lack of awareness of the stimulating science and wide array of fulfilling career opportunities involving plasmas. In the interest of enabling informed decisions about career options, we have initiated an outreach collaboration with the Madison West High School Rocket Club. Rocket Club members regularly exhibit their activities at public venues, including large-scale expos that draw large audiences of all ages. Building on their historical emphasis on small scale rockets with chemical motors, we worked with the group to add a new feature to their exhibit that highlights plasma-based spacecraft propulsion for interplanetary probes. This new exhibit includes a model satellite with a working (low power) plasma thruster. The participating high school students led the development process, to be described, and enthusiastically learned to articulate concepts related to plasma thruster operation and to compare the relative advantages of chemical vs. plasma/electrical propulsion systems for different scenarios. [Preview Abstract] |
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JP11.00006: Terrella for Advanced Undergraduate Physics Laboratory Jim Reardon, Douglass Endrizzi, Cary Forest, Steven Oliva A terrella has been in use in the Advanced Laboratory for undergraduates in the Physics Department at the University of Wisconsin-Madison since spring 2016. Our terrella is a permanent magnet on a pedestal which may be biased in various ways. In the vacuum region B $\leq $ 200 gauss; for typical operation $p$ ~ $10^{-4}$ Torr. Plasma may be created by thermionic emission from a filament or by an S-band magnetron. Students are guided through diagnosis of the terrella plasma using spectroscopy and swept Langmuir probes. A suite of supporting experiments has been developed to introduce basic plasma phenomena, such as the Child-Langmuir law. [Preview Abstract] |
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JP11.00007: HIGH SCHOOL OR UNDERGRADUATE RESEARCH |
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JP11.00008: Development of a Non-Contact, Inductive Depth Sensor for Free-Surface, Liquid-Metal Flows Gerrit Bruhaug, Egemen Kolemen, Adam Fischer, Mike Hvasta This paper details a non-contact based, inductive depth measurement system that can sit behind a layer of steel and measure the depth of the liquid metal flowing over the steel. Free-surface liquid metal depth measurement is usually done with invasive sensors that impact the flow of the liquid metal, or complex external sensors that require lasers and precise alignment. Neither of these methods is suitable for the extreme environment encountered in the diverter region of a nuclear fusion reactor, where liquid metal open channel flows are being investigated for future use. A sensor was developed that used the inductive coupling of a coil to liquid metal to measure the height of the liquid metal present. The sensor was built and tested experimentally, and modeled with finite element modeling software to further understand the physics involved. Future work will attempt to integrate the sensor into the Liquid Metal eXperiment (LMX) at the Princeton Plasma Physics Laboratory for more refined testing. [Preview Abstract] |
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JP11.00009: Detection of an electron beam in a high density plasma via an electrostatic probe Stephen Majeski, Jongsoo Yoo, Stewart Zweben, Masaaki Yamada, Hantao Ji The perturbation in floating potential by an electron beam is detected by a 1D floating potential probe array to evaluate the use of an electron beam for magnetic field line mapping in the Magnetic Reconnection Experiment (MRX) plasma. The MRX plasma is relatively high density (10$^{13}$ cm$^{-3}$) and low temperature (5 eV). Beam electrons are emitted from a tungsten filament and are accelerated by a 200 V potential across the sheath. They stream along the magnetic field lines towards the probe array. The spatial electron beam density profile is assumed to be a Gaussian along the radial axis of MRX and the effective beam width is determined from the radial profile of the floating potential. The magnitude of the perturbation is in agreement with theoretical predictions and the location of the perturbation is also in agreement with field line mapping. In addition, no significant broadening of the electron beam is observed after propagation for tens of centimeters through the high density plasma. These results demonstrate that this method of field line mapping is, in principle, feasible in high density plasmas. [Preview Abstract] |
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JP11.00010: High-Resolution Measurement of the Turbulent Frequency-Wavenumber Power Spectrum in a Laboratory Magnetosphere T.M. Qian, M.E. Mauel In a laboratory magnetosphere, plasma is confined by a strong dipole magnet, where interchange and entropy mode turbulence can be studied and controlled in near steady-state conditions\footnote{Garnier, \textit{et al.}, \textit{Phys Plasmas}, \textbf{24}, 012506 (2017).}. Whole-plasma imaging shows turbulence dominated by long wavelength modes having chaotic amplitudes and phases\footnote {Grierson, \textit{et al.}, \textit{Phys Plasmas}, \textbf{16}, 055902 (2009).}. Here, we report for the first time, high-resolution measurement of the frequency-wavenumber power spectrum by applying the method of Capon\footnote{Capon, \textit{Proc. IEEE}, \textbf{57}, 1408 (1969).} to simultaneous multi-point measurement of electrostatic entropy modes using an array of floating potential probes. Unlike previously reported measurements in which ensemble correlation between two probes detected only the dominant wavenumber, Capon's ``maximum likelihood method'' uses all available probes to produce a frequency-wavenumber spectrum, showing the existence of modes propagating in \emph{both} electron and ion magnetic drift directions. We also discuss the wider application of this technique to laboratory and magnetospheric plasmas with simultaneous multi-point measurements. [Preview Abstract] |
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JP11.00011: Design of a laboratory platform for atmospheric pressure biomedical plasma experiments Sarah Lee, Sara Rutz, Nathaniel Hicks, Brandon Briggs The design of a laboratory set up for atmospheric pressure plasma (APP) experiments with biomedical applications is described. A comparison between various types of cold APP discharges (DC, RF, microwave) is presented, as well as various configurations of electrodes, dielectric materials, and gas feed conditions. Particular attention is paid to designs comprising floating electrode dielectric barrier discharges (FE-DBD) (for example as described in [1]), but atmospheric pressure plasma jets are considered as well. A plan is discussed for initial experiments on the response of bacterial populations of \textit{E. coli} and \textit{Deinococcus radiodurans} to APP treatment as well as to media activated by APP. [1] Dobrynin, D., Fridman, G., Friedman, G., {\&} Fridman, A., \textit{New Journal of Physics}, \textbf{11 }115020 (2009) DOI: 10.1088/1367-2630/11/11/115020 [Preview Abstract] |
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JP11.00012: Towards an understanding of flows in avalanche transport phenomena Suying Jin, Nikolas Ramadan, Bart Van Compernolle, Matt J. Poulos, George J. Morales Recent heat transport experiments[1] conducted in the Large Plasma Device (LAPD) at UCLA, studying avalanche phenomena at steep cross-magnetic field pressure gradients, suggest that flows play a critical role in the evolution of transport phenomena, motivating further characterization. A ring shaped electron beam source injects sub-ionization energy electrons along the strong background magnetic field within a larger quiescent plasma, creating a hollow, high pressure filament. Two distinct regimes are observed as the density decays; the first characterized by multiple small avalanches producing sudden relaxations of the pressure profile which then recovers under continued heating, and the second signaled by a permanent collapse of the density profile after a global avalanche event, then dominated by drift-Alfven waves. The source is modified from previous experiments to gain active control of the flows by controlling the bias between the emitting ring and surrounding carbon masks. The results of flow measurements obtained using a Mach probe and Langmuir/emissive probe are here presented and compared. An analytical model for the behavior of the electron beam source is also in development. [1] B. Van Compernolle et al. Phys Rev. E 91, 031102 (2015) [Preview Abstract] |
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JP11.00013: Microwave Interferometric Density Measurements of a Pulsed Helicon Source Ethan Scime, Earl Scime, Derek Thompson The intense rf environment of a helicon plasma source is problematic for electrostatic probe measurements of plasma density, particularly at low neutral pressures. Here we present measurements of the line-integrated plasma density in a helicon plasma source using a multi-frequency (20-40 GHz) microwave interferometer. The design of the diagnostic and the data acquisition system are presented, as well as a comparison to density profiles obtained with a moveable electrostatic probe. A parametric fit to the probe profile measurements is used to determine the peak density from the microwave density measurements. [Preview Abstract] |
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JP11.00014: Magnet Topology and Homoclinic Tangles in Single-Null Divertor Tokamaks Ayana Crutchfield, Kessiena Ofunreign, Halima Ali, Alkesh Punjabi Divertors are a regular feature of the modern day large tokamaks. Divertors are required for handling the plasma particle and heat exhausts on the walls in fusion plasmas. The single-null divertor can have two distinct magnetic topologies: open unbounded topology and closed compact topology. The simple map (SM) [A. Punjabi, A. Verma, and A. Boozer, \textit{Phys. Rev. Lett}. \textbf{69}, 3322 (1992)] generically represents open unbounded topology; and the symmetric quartic map (SQM) [M. Jones et al, \textit{Phys. Plasmas} \textbf{16}, 042511 (2009)] generically represents the closed compact topology. The parameters in the symmetric quartic map are chosen so that the magnetic geometry of the symmetric quartic map is comparable to the simple map. The new approach for calculation of homoclinic tangles of separatrices in Hamiltonian systems [Punjabi A. and Boozer A., \textit{Phys. Lett. A} \textbf{378}, 2410 (2014)] is used. The map parameters of the SM and the SQM are used to represent the magnetic asymmetries as in the standard map. The homoclinic tangles of the primary separatrix of the single-null divertor tokamaks with the two distinct topologies are calculated, compared, and contrasted. This work is supported by grants DE-FG02-01ER54624, DEFG02-04ER54793, and DE-FG02-07ER54937. This research used resources of the NERSC, supported by the Office of Science, US DOE, under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
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JP11.00015: Online plasma calculator H. Wisniewski, P.-A. Gourdain APOLLO is an online, Linux based plasma calculator. Users can input variables that correspond to their specific plasma, such as ion and electron densities, temperatures, and external magnetic fields. The system is based on a webserver where a FastCGI protocol computes key plasma parameters including frequencies, lengths, velocities, and dimensionless numbers. FastCGI was chosen to overcome security problems caused by JAVA-based plugins. The FastCGI also speeds up calculations over PHP based systems. APOLLO is built upon the WT library, which turns any web browser into a versatile, fast graphic user interface. All values with units are expressed in SI units except temperature, which is in electron-volts. SI units were chosen over cgs units because of the gradual shift to using SI units within the plasma community. APOLLO is intended to be a fast calculator that also provides the user with the proper equations used to calculate the plasma parameters. This system is intended to be used by undergraduates taking plasma courses as well as graduate students and researchers who need a quick reference calculation. [Preview Abstract] |
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JP11.00016: An assessment of surface emissivity variation effects on plasma uniformity analysis using IR cameras Abigail Greenhalgh, Melissa Showers, Theodore Biewer The Prototype-Material Plasma Exposure eXperiment (Proto-MPEX) is a linear plasma device operating at Oak Ridge National Laboratory (ORNL). Its purpose is to test plasma source and heating concepts for the planned Material Plasma Exposure eXperiment (MPEX), which has the mission to test the plasma-material interactions under fusion reactor conditions. In this device material targets will be exposed to high heat fluxes (\textgreater 10 MW/m2). To characterize the heat fluxes to the target a IR thermography system is used taking up to 432 frames per second videos. The data is analyzed to determine the surface temperature on the target in specific regions of interest. The IR analysis has indicated a low level of plasma uniformity; the plasma often deposits more heat to the edge of the plate than the center. An essential parameter for IR temperature calculation is the surface emissivity of the plate (stainless steel). A study has been performed to characterize the variation in the surface emissivity of the plate as its temperature changes and its surface finish is modified by plasma exposure. [Preview Abstract] |
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JP11.00017: Sensitivity of wave propagation in the LHRF to initial poloidal position in finite-aspect-ratio toroidal plasmas J.J. Larson, R.I. Pinsker, P.T. Bonoli, M. Porkolab The important effect of varying the initial poloidal wave-launching location to the core accessibility of lower hybrid slow waves in a torus of finite aspect ratio has been understood for many years [1]. Since the qualitative properties of the wave propagation of the other branch in this regime, known as the `whistler', `helicon' or simply the `fast wave', are similar in some ways to those of the slow wave [2], we expect a dependence on launch position for this wave also. We study this problem for both slow and fast waves, first with simplified analytic models and then using the ray-tracing code GENRAY for realistic plasma equilibria. We assess the prospects of inside, top, bottom or conventional outside launch of waves on each of the two branches. Although the slow wave has been the focus of research for LHRF heating and current drive in the past, the fast wave will play a major role in burning plasmas beyond ITER where T$_{\mathrm{e}}$(0) $=$ 10-20 keV. The stronger electron Landau damping of the slow wave will restrict the power deposition to the outer third of the plasma, while the fast wave's weaker damping allows the wave to penetrate to the hot plasma core before depositing its power. [1] P.T. Bonoli and E. Ott, Phys. Fluids \textbf{25}, 359 (1982) [2] R.I. Pinsker, Phys. Plasmas \textbf{22},090901 (2015) [Preview Abstract] |
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JP11.00018: XPS investigation of depth profiling induced chemistry Quinn Pratt, Charles Skinner, Bruce Koel, Zhu Chen Surface analysis is an important tool for understanding plasma-material interactions. Depth profiles are typically generated by etching with a monatomic argon ion beam, however this can induce unintended chemical changes in the sample. Tantalum pentoxide, a sputtering standard, and PEDOT:PSS, a polymer that was used to mimic the response of amorphous carbon-hydrogen co-deposits, were studied. We compare depth profiles generated with monatomic and gas cluster argon ion beams (GCIB) using X-ray photoelectron spectroscopy (XPS) to quantify chemical changes. In both samples, monatomic ion bombardment led to beam-induced chemical changes. Tantalum pentoxide exhibited preferential sputtering of oxygen and the polymer experienced significant bond modification. Depth profiling with clusters is shown to mitigate these effects. We present sputtering rates for Ta$_2$O$_5$ and PEDOT:PSS as a function of incident energy and flux. [Preview Abstract] |
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JP11.00019: Statistical characterization of surface features from tungsten-coated divertor inserts in the DIII-D Metal Rings Campaign Jacob Adams, Ezekial Unterberg, Christopher Chrobak, Brian Stahl, Tyler Abrams Continuing analysis of tungsten-coated inserts from the recent DIII-D Metal Rings Campaign utilizes a statistical approach to study carbon migration and deposition on W surfaces and to characterize the pre- versus post-exposure surface morphology. A TZM base was coated with W using both CVD and PVD and allowed for comparison between the two coating methods. The W inserts were positioned in the lower DIII-D divertor in both the upper (shelf) region and lower (floor) region and subjected to multiple plasma shots, primarily in H-mode. Currently, the post-exposure W inserts are being characterized using SEM/EDX to qualify the surface morphology and to quantify the surface chemical composition. In addition, profilometry is being used to measure the surface roughness of the inserts both before and after plasma exposure. Preliminary results suggest a correlation between the pre-exposure surface roughness and the level of carbon deposited on the surface. Furthermore, ongoing in-depth analysis may reveal insights into the formation mechanism of nanoscale bumps found in the carbon-rich regions of the W surfaces that have not yet been explained. [Preview Abstract] |
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JP11.00020: Data Analysis of the Gated-LEH X-Ray Imaging Diagnostic at the NIF Matthew Thibodeau, Hui Chen The Gated Laser Entrance Hole (G-LEH) x-ray imaging diagnostic [1, 2] in use at the NIF offers a desirable combination of spatial and temporal resolution. By looking inside of NIF hohlraums with time resolution, G-LEH measures target features including LEH size and capsule size. A framework is presented for automated and systematic analysis of G-LEH images that measures several physical parameters of interest and their evolution over time. The results from these analyses enable comparisons with hohlraum models and allow model validation of LEH closure velocity and the extent of capsule blow-off. \\ 1 H. Chen, N. Palmer, M. Dayton et al., Rev. Sci. Instrum. 87, 11E203 (2016) \\ 2 L. Claus, L. Fang, R. Kay et al., Proc. SPIE 9591, 95910 (2015). [Preview Abstract] |
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JP11.00021: Experimental analysis of coupling between Compressional Alfv\'{e}n Eigenmodes and Kinetic Alfv\'{e}n Waves in NSTX Z Deng, NA Crocker, Y Ren, EV Belova In NSTX, there exists an unexplained $T_{e}$ profile flattening with increasing toroidal magnetic field, and neutral beam power. Core localized Compressional Alfv\'{e}n Eigenmodes (CAEs) are a candidate cause. Simulations [Belova, PRL 2015] show that CAEs take energy from core energetic ions that would otherwise heat the electrons and deposit it in the edge via coupling with Kinetic Alfv\'{e}n Waves (KAWs) at the Alfv\'{e}n resonance location. However, this theory lacks experimental validation. To provide experimental support, we analyze high-k scattering measurements, which show signatures of CAE-KAW mode conversion. The spectrum of microwaves scattered from $\delta n$ fluctuations with $k_{r}\rho_{s}$ \textasciitilde 1 shows large narrow peaks at typical CAE frequencies ($f $\textgreater \textasciitilde 1 MHz), as expected. Corresponding peaks also appear at negative $f$, but their amplitudes can be much smaller, consistent with expectation for scattering. (Peaks could also result from index of refraction modulation by CAEs, but the peaks at positive and negative $f$ would have equal amplitude.) To further test that the scattering peaks are caused by fluctuations associated with CAEs, we next make sure these peaks are coherent with corresponding CAE peaks in the magnetic fluctuation spectrum, as would be expected for mode conversion. Finally, we compare the location where KAWs are detected with the location of the Alfv\'{e}n resonance. The results of the analysis seem to suggest that peaks in the high-k spectrum do indeed result from KAWs mode converted from CAEs. [Preview Abstract] |
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JP11.00022: Measurement of the thermal effects in the dispersion relation of the dust acoustic wave Joshua Hoyng, Jeremiah Williams A complex (dusty) plasma is a four-component plasma system composed of ions, electrons, neutral particles and charged microparticles. The charged microparticles interact with, and self- consistently modify, the surrounding plasma medium; resulting in a new and unique state of matter that can support a wide range of physical phenomena. Among these is a new wave mode known as the dust acoustic, or dust density, wave (DAW). The DAW is a low- frequency, longitudinal mode that propagates through the microparticle component of the dusty plasma system and is self-excited by the energy from the ions streaming through this component. Over the past twenty years, the dust acoustic wave has been a subject of intense study and recent studies have shown that thermal effects can, in some cases, have a significant role in the measured dispersion relation. A recent theoretical model suggest that the thermal effects are, in part, due to the finite size of the dusty plasma systems that support this wave mode. In this poster, we report the results of an experimental study examining this effect over a range of experimental conditions in a weakly-coupled dusty plasma system in an rf discharge plasma. [Preview Abstract] |
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JP11.00023: Development of a time-resolved stereoscopic PIV system Kristin Irlam, Jeremiah Williams Over the past twenty years, a variety of particle image velocimetry (PIV) techniques have been used to characterize the particle transport and thermal state of dusty plasma systems. While the majority of these techniques required the use of a dedicated PIV system, recent advances in imaging technology have led to the development of a time-resolved two-dimensional (planar) version of this diagnostic technique which allows this diagnostic technique to be applied without the need for a dedicated PIV system. This poster will present recent work developing a relatively inexpensive time-resolved stereoscopic PIV system that can measure the full three-dimensional transport. Preliminary results will be presented. [Preview Abstract] |
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JP11.00024: 2D Measurements of the Balmer Series in Proto-MPEX using a Fast Visible Camera Setup Elizabeth G. Lindquist, Theodore M. Biewer, Holly B. Ray The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear plasma device with densities up to 10$^{\mathrm{20}}$ m$^{\mathrm{-3}}$ and temperatures up to 20 eV. Broadband spectral measurements show the visible emission spectra are solely due to the Balmer lines of deuterium. Monochromatic and RGB color Sanstreak SC1 Edgertronic fast visible cameras capture high speed video of plasmas in Proto-MPEX. The color camera is equipped with a long pass 450 nm filter and an internal Bayer filter to view the D$_{\mathrm{\alpha }}$ line at 656 nm on the red channel and the D$_{\mathrm{\beta }}$ line at 486 nm on the blue channel. The monochromatic camera has a 434 nm narrow bandpass filter to view the D$_{\mathrm{\gamma }}$ intensity. In the setup, a 50/50 beam splitter is used so both cameras image the same region of the plasma discharge. Camera images were aligned to each other by viewing a grid ensuring \textasciitilde 1 pixel registration between the two cameras. A uniform intensity calibrated white light source was used to perform a pixel-to-pixel relative and an absolute intensity calibration for both cameras. Python scripts that combined the dual camera data, rendering the D$_{\mathrm{\alpha }}$, D$_{\mathrm{\beta }}$, and D$_{\mathrm{\gamma }}$ intensity ratios. Observations from Proto-MPEX discharges will be presented. [Preview Abstract] |
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JP11.00025: Design and Control of Small Neutral Beam Arc Chamber for Investigations of DIII-D Neutral Beam Failure During Helium Operation. Carl Fremlin, Jasper Beckers, Brendan Crowley, Joseph Rauch, Jim Scoville The Neutral Beam system on the DIII-D tokamak consists of eight ion sources using the Common Long Pulse Source (CLPS) design. During helium operation, desired for research regarding the ITER pre-nuclear phase, it has been observed that the ion source arc chamber performance steadily deteriorates, eventually failing due to electrical breakdown of the insulation. A significant investment of manpower and time is required for repairs. To study the cause of failure a small analogue of the DIII-D neutral beam arc chamber has been constructed. This poster presents the design and analysis of the arc chamber including the PLC based operational control system for the experiment, analysis of the magnetic confinement and details of the diagnostic suite. [Preview Abstract] |
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JP11.00026: Enhancements to the Low-Energy Ion Facility at SUNY Geneseo Zachariah Barfield, Steven Kostick, Ethan Nagasing, Kurt Fletcher, Stephen Padalino The Low Energy Ion Facility at SUNY Geneseo is used for detector development and characterization for inertial confinement fusion diagnostics. The system has been upgraded to improve the ion beam quality by reducing contaminant ions. In the new configuration, ions produced by the Peabody Scientific duoplasmatron ion source are accelerated through a potential, focused into a new NEC analyzing magnet and directed to an angle of $30^{\circ }$. A new einzel lens on the output of the magnet chamber focuses the beam into a scattering chamber with a water-cooled target mount and rotatable detector mount plates. The analyzing magnet has been calibrated for deuteron, $^{\mathrm{4}}$He$^{\mathrm{+}}$, and $^{\mathrm{4}}$He$^{\mathrm{2+}}$ ion beams at a range of energies, and no significant hysteresis has been observed. The system can accelerate deuterons to energies up to 25 keV to initiate d-d fusion using a deuterated polymer target. Charged particle spectra with protons, tritons, and $^{\mathrm{3}}$He ions from d-d fusion have been measured at scattering angles ranging from $55^{\circ }$ to $135^{\circ }$. A time-of-flight beamline has been designed to measure the energies of ions elastically scattered at $135^{\circ }$. CEM detectors initiate start and stop signals from secondary electrons produced when low energy ions pass through very thin carbon foils. [Preview Abstract] |
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JP11.00027: Study of high field side/low field side asymmetry in the electron temperature profile with electron cyclotron emission V.R. Gugliada, M.E. Austin, M.W. Brookman Electron cyclotron emission (ECE) provides high resolution measurements of electron temperature profiles ($T_{\mathrm{e}}(R,t))$ in tokamaks. Calibration accuracy of this data can be improved using a sawtooth averaging technique. This improved calibration will then be utilized to determine the symmetry of $T_{\mathrm{e\thinspace }}$profiles by comparing low field side (LFS) and high field side (HFS) measurements. Although $T_{\mathrm{e}}$ is considered constant on flux surfaces, cases have been observed in which there are pronounced asymmetries about the magnetic axis, particularly with increased pressure. Trends in LFS/HFS overlap are examined as functions of plasma pressure, MHD mode presence, heating techniques, and other discharge conditions. This research will provide information on the accuracy of the current two-dimensional mapping of flux surfaces in the tokamak. Findings can be used to generate higher quality EFITs and inform ECE calibration. [Preview Abstract] |
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JP11.00028: Building a database for statistical characterization of ELMs on DIII-D B.J. Fritch, A. Marinoni, A. Bortolon Edge localized modes (ELMs) are bursty instabilities which occur in the edge region of H-mode plasmas and have the potential to damage in-vessel components of future fusion machines by exposing the divertor region to large energy and particle fluxes during each ELM event. While most ELM studies focus on average quantities (e.g. energy loss per ELM), this work investigates the statistical distributions of ELM characteristics, as a function of plasma parameters. A semi-automatic algorithm is being used to create a database documenting trigger times of the tens of thousands of ELMs for DIII-D discharges in scenarios relevant to ITER, thus allowing statistically significant analysis. Probability distributions of inter-ELM periods and energy losses will be determined and related to relevant plasma parameters such as density, stored energy, and current in order to constrain models and improve estimates of the expected inter-ELM periods and sizes, both of which must be controlled in future reactors. [Preview Abstract] |
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JP11.00029: Whistler wave generation by electron temperature anisotropy during asymmetric magnetic reconnection in space Josh Swerdlow, Jongsoo Yoo, Eun-Hwa Kim, Masaaki Yamada, Hantao Ji Generation of whistler waves during asymmetric reconnection is studied by analyzing data from a MMS (Magnetospheric Multiscale) event [1]. In particular, the possible role of electron temperature anisotropy in excitation of whistler waves on the magnetosphere side is discussed. The local electron distribution function is fitted into a sum of bi-Maxwellian distribution functions. Then, the dispersion relation solver, WHAMP (waves in homogeneous, anisotropic, multicomponent plasmas [2]), is used to obtain the local dispersion relation and growth rate of the whistler waves. We compare the theoretical calculations with the measured dispersion relation. \\ [][1] Burch et al., Science 352, aaf2939, 2016\\ [][2] Rönnmark, Tech. Rep., Kiruna Geophys. Inst., Kiruna, Sweden, 1982 [Preview Abstract] |
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JP11.00030: Measured vs. Predicted Pedestal Pressure During RMP ELM Control in DIII-D* Bailey Zywicki, Max Fenstermacher, Richard Groebner, Orso Meneghini From database analysis of DIII-D plasmas with Resonant Magnetic Perturbations (RMPs) for ELM control, we will compare the experimental pedestal pressure (p\textunderscore ped) to EPED code predictions and present the dependence of any p\textunderscore ped differences from EPED on RMP parameters not included in the EPED model e.g. RMP field strength, toroidal and poloidal spectrum etc. The EPED code, based on Peeling-Ballooning and Kinetic Ballooning instability constraints, will also be used by ITER to predict the H-mode p\textunderscore ped without RMPs. ITER plans to use RMPs as an effective ELM control method. The need to control ELMs in ITER is of the utmost priority, as it directly correlates to the lifetime of the plasma facing components. An accurate means of determining the impact of RMP ELM control on the p\textunderscore ped is needed, because the device fusion power is strongly dependent on p\textunderscore ped. With this new collection of data, we aim to provide guidance to predictions of the ITER pedestal during RMP ELM control that can be incorporated in a future predictive code. [Preview Abstract] |
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JP11.00031: Origin of Non-Gaussian Spectra Observed via the Charge Exchange Recombination Spectroscopy Diagnostic in the DIII-D Tokamak Alex Sulyman, Colin Chrystal, Shaun Haskey, Keith Burrell, Brian Grierson The possible observation of non-Maxwellian ion distribution functions in the pedestal of DIII-D will be investigated with a synthetic diagnostic that accounts for the effect of finite neutral beam size. Ion distribution functions in tokamak plasmas are typically assumed to be Maxwellian, however non-Gaussian features observed in impurity charge exchange spectra have challenged this concept.$^{\mathrm{2\thinspace }}$Two possible explanations for these observations are spatial averaging over a finite beam size and a local ion distribution that is non-Maxwellian. Non-Maxwellian ion distribution functions could be driven by orbit loss effects in the edge of the plasma,$^{\mathrm{3}}$ and this has implications for momentum transport and intrinsic rotation. To investigate the potential effect of finite beam size on the observed spectra, a synthetic diagnostic has been created that uses FIDAsim to model beam and halo neutral density. Finite beam size effects are investigated for vertical and tangential views in the core and pedestal region with varying gradient scale lengths. [2] C. Chrystal et al., Rev. Sci. Instrum. \textbf{87}, 11E512 (2016) [3] D.J. Battaglia et al., Phys. Plasmas \textbf{21}, 072508 (2014) [Preview Abstract] |
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JP11.00032: Operations Studies of the Gyrotrons on DIII-D Stephen Storment, John Lohr, Mirela Cengher, Yuri Gorelov, Dan Ponce, Antonio Torrezan The gyrotrons are high power vacuum tubes used in fusion research to provide high power density heating and current drive in precisely localized areas of the plasma. Despite the increasing experience with both the manufacture and operation of these devices, individual gyrotrons with similar design and manufacturing processes can exhibit important operational differences in terms of generated rf power, efficiency and lifetime. This report discusses differences in the performance of several gyrotrons in operation at DIII-D and presents the results of a series of measurements that could lead to improved the performance of single units based on a better understanding of the causes of these differences. The rf power generation efficiency can be different from gyrotron to gyrotron. In addition, the power loading of the collector can feature localized hot spots, where the collector can locally be close to the power deposition limits. Measurements of collector power loading provide maps of the power deposition and can provide understanding of the effect of modulation of the output rf beam on the total loading, leading to improved operational rules increasing the safety margins for the gyrotrons under different operational scenarios. [Preview Abstract] |
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JP11.00033: Hydrodynamic instabilities at an oblique interface: Experiments and Simulations E. Douglas-Mann, C. Fiedler Kawaguchi, M. A. Trantham, G. Malamud, W.C. Wan, S. R. Klein, C. C. Kuranz Hydrodynamic instabilities are important phenomena that occur in high-energy-density systems, such as astrophysical systems and inertial confinement fusion experiments, where pressure, density, and velocity gradients are present. Using a \textasciitilde 30 ns laser pulse from the Omega EP laser system, a steady shock wave is driven into a target. A Spherical Crystal Imager provides~high-resolution x-ray radiographs to study the evolution of complex hydrodynamic structures. This experiment has a light-to-heavy interface at an oblique angle with a precision-machined perturbation. The incident shock wave deposits shear and vorticity at the interface causing the perturbation to grow via Richtmyer-Meshkov and Kelvin-Helmholtz processes. We present results from analysis of radiographic data and hydrodynamics simulations showing the evolution of the shock and unstable structure. ~ [Preview Abstract] |
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JP11.00034: An Illustrative Guide to the Minerva Framework Erik Flom, Patrick Leonard, Udo Hoeffel, Sehyun Kwak, Andrea Pavone, Jakob Svensson, Maciej Krychowiak Modern phsyics experiments require tracking and modelling data and their associated uncertainties on a large scale, as well as the combined implementation of multiple independent data streams for sophisticated modelling and analysis. The Minerva Framework offers a centralized, user-friendly method of large-scale physics modelling and scientific inference. Currently used by teams at multiple large-scale fusion experiments including the Joint European Torus (JET) and Wendelstein 7-X (W7-X), the Minerva framework provides a forward-model friendly architecture for developing and implementing models for large-scale experiments. One aspect of the framework involves so-called data sources, which are nodes in the graphical model. These nodes are supplied with engineering and physics parameters. When end-user level code calls a node, it is checked network-wide against its dependent nodes for changes since its last implementation and returns version-specific data. Here, a filterscope data node is used as an illustrative example of the Minerva Framework's data management structure and its further application to Bayesian modelling of complex systems. [Preview Abstract] |
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JP11.00035: Ion Temperature and Velocity Measurements using Fabry-Perot Spectroscopy Megan Tabbutt, Ken Flanagan, Mark D. Nornberg, Jason Milhone, Fred L. Roesler, Cary B. Forest A Fabry-Perot spectrometer system has been used to measure ion temperature and velocity in flowing, astrophysical plasmas at the Wisconsin Plasma Astrophysics Laboratory (WiPAL). WiPAL researches weakly magnetized, fast flowing, plasmas in order to study basic flow-driven MHD instabilities. The Fabry-Perot spectrometer images the ion-velocity distribution function (IVDF) of both Argon (488 nm ion line) and Helium (468.6 nm ion line complex) plasmas. Electron temperatures range from 5 eV to 15 eV; plasma densities range from $10^{11}\sim^{12} cm^{-3}$, ion temperatures range from 0.5 eV to 2 eV and flows can reach 10 km/s. Beyond the increased resolving power compared to grating spectrometers, the Fabry-Perot’s 2D interference pattern can be summed to give a large signal-to-noise increase. [Preview Abstract] |
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JP11.00036: Mode Identification During Biased Rotation on the Large Plasma Device H. K. Johnson, D. A. Schaffner, L. E. Fahim, C. A. Cartagena-Sanchez Previous research on the Large Plasma Device (LAPD) has shown that biased rotation of the plasma can aid confinement. However, the same rotation which aids confinement can also generate waves (and thus turbulence) that can hinder the effectiveness of that confinement. There are multiple wave candidates in the LAPD which may be interacting, including drift, rotational interchange, and Kelvin-Helmholtz waves. Interaction was determined using a bispectral analysis, which scans Fourier spectra for frequencies that exhibit non-linear coupling. Analysis was performed on ion saturation current data at bias voltages ranging from 80-200V. [Preview Abstract] |
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JP11.00037: Compression of an Accelerated Taylor State in SSX J.E. Shrock, E. M. Suen-Lewis, L. J. Barbano, M. Kaur, D.A. Schaffner, M.R. Brown In the Swarthmore Spheromak Experiment (SSX), compact toroidal plasmas are launched from a plasma gun and evolve into minimum energy twisted Taylor states. The plumes initially have a velocity $\sim40$ $km/s$, density $\sim0.4\times10^{16}$ $cm^{-3}$, and proton temperature $\sim 20$ $eV$. After formation, the plumes are accelerated by pulsed pinch coils with rise times $\tau_{1/4}=(\pi/2)\sqrt{LC}$ less than $1$ $\mu s$ and currents $I_{peak}= V_{0}/Z=V_0/\sqrt{L/C}$ on the order of $10^4$ $A$. The accelerated Taylor States are abruptly stagnated in a copper flux conserver, and over the course of $t<10$ $\mu s$, adiabatic compression is observed. The magnetothermodynamics of this compression do not appear to be dictated by the MHD equation of state $d/dt(P/n^\gamma)=0$. Rather, the compression appears to evolve according to the Chew-Goldberger-Low (CGL) double adiabatic model. CGL theory presents two equations of state, one corresponding with particle motion perpendicular to magnetic field in a plasma, the other to particle motion parallel to the field. We observe Taylor state compression most in agreement with the parallel equation of state: $d/dt(P_{\|}B^2/n^3)=0$. [Preview Abstract] |
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JP11.00038: Magnetic Probe Calibration for MHD Turbulence Studies Using a Power Amplifier L. E. Fahim, D.A. Schaffner, H.K. Johnson, C.A. Cartagena-Sanchez Studying magnetic turbulence in the laboratory requires measurements of magnetic fluctuations at high frequencies, but probe diagnostics, such as magnetic pickup coils, can run into inductance issues at such high frequencies. Careful calibration is necessary to be able to report on the the proper scaling of power. B-dot probes and Hall effect sensors are used in the measurement of time varying magnetic fields. In the case of a B dot probe, current is induced in a coil as a result of the varying magnetic field. The coil then produces an output voltage that is proportional to dB/dt. The output voltage of a Hall effect sensor changes in response to the transverse force produced by the magnetic field. For measurements at the Bryn Mawr Plasma Laboratory, both B-dot probes and Hall effect probes are calibrated using an Accel Instruments TS$250$ power amplifier to drive current up to 2A from $100$kHz to 10MHz. [Preview Abstract] |
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JP11.00039: Ross filter development for absolute measurement of Al line radiation on MST N. Lauersdorf, L. M. Reusch, D. J. Den Hartog, J. A. Goetz, P. Franz, P. VanMeter The MST has a two-color soft x-ray tomography (SXT) diagnostic that, using the double-filter technique, measures electron temperature (Te) from the slope of the soft x-ray (SXR) continuum. Because MST has an aluminum plasma-facing surface, bright Al line radiation occurs in the SXR spectrum. In past application of the double-filter technique, these lines have been filtered out using thick Be filters ($\sim$400$\mu$m and $\sim$800$\mu$m), restricting the measurement temperature range to $\geq$1 keV due to the signal strength having a positive correlation with Te. Another way to deal with the line radiation is to explicitly include it into the SXR spectrum analysis from which Te is derived. A Ross filter set has been designed to measure this line radiation, and will enable the absolute intensities of the aluminum lines to be quantified and incorporated into the analysis. The Ross filter will be used to measure Al+11 and Al+12 lines, occurring between 1.59 and 2.04 keV. By using multiple detectors with filters made of varying element concentrations, we create spectral bins in which the dominant transmission is the line radiation. Absolute measurement of Al line intensities will enable use of thinner filters in the SXT diagnostic and accurate measurement of Te $<$ 1 keV. [Preview Abstract] |
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JP11.00040: Precision Electron Density Measurements in the SSX MHD Wind Tunnel Emma M. Suen-Lewis, Luke J. Barbano, Jaron E. Shrock, Manjit Kaur, David A. Schaffner, Michael R. Brown We characterize fluctuations of the line averaged electron density of Taylor states produced by the magnetized coaxial plasma gun of the SSX device using a 632.8 nm HeNe laser interferometer. The analysis method uses the electron density dependence of the refractive index of the plasma to determine the electron density of the Taylor states. Typical magnetic field and density values in the SSX device approach about $B \cong 0.3$ T and $n = 0.4 \times 10^{16}~cm^{-3}$. Analysis is improved from previous density measurement methods by developing a post-processing method to remove relative phase error between interferometer outputs and to account for approximately linear phase drift due to low-frequency mechanical vibrations of the interferometer. Precision density measurements coupled with local measurements of the magnetic field will allow us to characterize the wave composition of SSX plasma via density vs. magnetic field correlation analysis, and compare the wave composition of SSX plasma with that of the solar wind [1]. Preliminary results indicate that density and magnetic field appear negatively correlated. \begin{itemize} \item[{[1]}] G. G Howes et al., \textit{The Astrophysical Journal Letters}, vol. 753, July 2012. \end{itemize} [Preview Abstract] |
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JP11.00041: A Phoswich Detector System to Measure Sub-Second Half-Lives using ICF Reactions Micah Coats, Katelyn Cook, Mark Yuly, Stephen Padalino, Craig Sangster, Sean Regan The $^{\mathrm{3}}$H(t,$\gamma )^{\mathrm{6}}$He cross section has not been measured at any bombarding energy due to the difficulties of simultaneously producing both a tritium beam and target at accelerator labs. ~An alternative technique may be to use an ICF tt implosion at the OMEGA Laser Facility. The $^{\mathrm{3}}$H(t,$\gamma )^{\mathrm{6}}$He cross section could be determined in situ by measuring the beta decay of $^{\mathrm{6}}$He beginning a few milliseconds after the shot along with other ICF diagnostics. A dE-E phoswich system capable of surviving in the OMEGA target chamber was tested using the SUNY Geneseo pelletron to create neutrons via $^{\mathrm{2}}$H(d,n)$^{\mathrm{3}}$He and subsequently $^{\mathrm{6}}$He via $^{\mathrm{9}}$Be(n,$\alpha )^{\mathrm{6}}$He in a beryllium target. The phoswich dE-E detector system was used to select beta decay events and measure the 807 ms half-life of $^{\mathrm{6}}$He. It is composed of a thin, 2 ns decay time dE scintillator optically coupled to a thick, 285 ns E scintillator, with a linear gate to separate the short dE pulse from the longer E tail. Funded in part by a grant from the DOE through the Laboratory for Laser Energetics. [Preview Abstract] |
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JP11.00042: Two-point modeling of SOL losses of HHFW power in NSTX Ayden Kish, Rory Perkins, Joon-Wook Ahn, Ahmed Diallo, Travis Gray, Joel Hosea, Michael Jaworski, Gerrit Kramer, Benoit LeBlanc, Steve Sabbagh High-harmonic fast-wave (HHFW) heating is a heating and current-drive scheme on the National Spherical Torus eXperiment (NSTX) complimentary to neutral beam injection. Previous experiments suggest that a significant fraction, up to 50{\%}, of the HHFW power is promptly lost to the scrape-off layer (SOL). Research indicates that the lost power reaches the divertor via wave propagation and is converted to a heat flux at the divertor through RF rectification rather than heating the SOL plasma at the midplane [1]. This counter-intuitive hypothesis is investigated using a simplified two-point model [2], relating plasma parameters at the divertor to those at the midplane. Taking measurements at the divertor region of NSTX as input, this two-point model is used to predict midplane parameters, using the predicted heat flux as an indicator of power input to the SOL. These predictions are compared to measurements at the midplane to evaluate the extent to which they are consistent with experiment. [1] R. J. Perkins et al., Phys. Plasmas 22 042506 (2015) [2] P. C. Stangeby, The plasma boundary of magnetic fusion devices (Institute of Physics Publishing, Bristol, 2000). [Preview Abstract] |
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JP11.00043: Using Rutherford Backscattering Spectroscopy to Characterize Targets for MTW Gunnar Brown, Barak Stockler, Ryan Ward, Charlie Freeman, Stephen Padalino, Collin Stillman, Steven Ivancic, S.P. Reagan, T.C. Sangster A study is underway to determine the composition and thickness of targets used at the Multiterawatt (MTW) laser facility at the Laboratory for Laser Energetics (LLE) using Rutherford backscattering spectroscopy (RBS). In RBS, an ion beam is incident on a sample and the scattered ions are detected with a surface barrier detector. The resulting energy spectra of the scattered ions can be analyzed to determine important parameters of the target including elemental composition and thickness. Proton, helium and deuterium beams from the 1.7 MV Pelletron accelerator at SUNY Geneseo have been used to characterize several different targets for MTW, including CH and aluminum foils of varying thickness. RBS spectra were also obtained for a cylindrical iron buried-layer target with aluminum dopant which was mounted on a silicon carbide stalk. The computer program SIMNRA is used to analyze the spectra. [Preview Abstract] |
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JP11.00044: Characterizing the volume of a compressed Taylor state object in the SSX plasma L. J. Barbano, E. M. Suen-Lewis, J. E. Shrock, M. Kaur, D. A. Schaffner, M. R. Brown A cookbook of numerical techniques (namely wavelet transforms, smoothing filters, and spline interpolations) is applied to characterize the length of a stagnating Taylor state object in SSX. This length analysis uses magnetic field data from a linear array of 20 evenly spaced 2-D $\dot{B}$ probes positioned along the compression can axis. A 3-D animation of the Taylor state object's magnetic field in the compression volume reveals the object's wavelet-like magnetic structure in space. In order to localize the object in space and characterize its length, a continuous wavelet transform is performed. The most dominant spatial frequency given by the resulting frequency-space spectrogram is taken to be the length of the object in the compression volume. This analysis is performed at every time in the $\dot{B}$ time series to give some measure of the Taylor state object's length as a function of time. This length, in conjunction with the cross-sectional area of the compression can, gives the object's volume. Information about the object's volume as a function of time allows us to identify instances of compressive heating and investigate the magnetothermodynamic (MTD) properties of the SSX plasma. [Preview Abstract] |
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JP11.00045: Opacplot2: Enabling tabulated EoS and opacity compatibility for HEDLP simulations with the FLASH code Jordan Laune, Petros Tzeferacos, Scott Feister, Milad Fatenejad, Roman Yurchak, Norbert Flocke, Klaus Weide, Donald Lamb Thermodynamic and opacity properties of materials are necessary to accurately simulate laser-driven laboratory experiments. Such data are compiled in tabular format since the thermodynamic range that needs to be covered cannot be described with one single theoretical model. Moreover, tabulated data can be made available prior to runtime, reducing both compute cost and code complexity. This approach is employed by the FLASH code. Equation of state (EoS) and opacity data comes in various formats, matrix-layouts, and file-structures. We discuss recent developments on opacplot2, an open-source Python module that manipulates tabulated EoS and opacity data. We present software that builds upon opacplot2 and enables easy-to-use conversion of different table formats into the IONMIX format, the native tabular input used by FLASH. Our work enables FLASH users to take advantage of a wider range of accurate EoS and opacity tables in simulating HELP experiments at the National Laser User Facilities. [Preview Abstract] |
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JP11.00046: Investigating Trapped Particle Asymmetry Modes and Temperature Effects in the Lawrence Non-neutral Torus II R. Nirwan, P. Swanson, M.R. Stoneking Electron plasma is confined in the Lawrence Non-Neutral Torus II using a purely toroidal magnetic field ($R_0 = 18$ cm, $B < 1$ kG) for confinement times exceeding 1 second. The LNT II can be configured for fully toroidal traps or variable-length partial toroidal traps. The behavior of the plasma is observed by monitoring the image charge on isolated wall sectors. The plasma is excited by application of a sinusoidal tone burst to selected wall sectors. Phase-space separatrices are introduced by applying squeeze potentials to toroidally localized, but poloidally continuous sectors and the resulting interaction between trapped and passing particles populations results in asymmetry modes and transport. These experiments provide a comparison with similar experiments in cylindrical traps. We also report on the development of temperature measurement techniques and assess temperature affects on diocotron and asymmetry modes. [Preview Abstract] |
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JP11.00047: Is Bohm's Criterion satisfied in a weakly ionized Kr discharge, in the vicinity of a biased grid that permits counter streaming ion flow? Eugene Wackerbarth, In-Je Kang, In-Sun Park, Kyu-Sun Chung, Noah Hershkowitz, Greg Severn We consider the problem of the sheath near a negatively biased grid (-100V) that permits ion flow in both directions. We show the first laser-induced fluorescence (LIF) measurements of ion velocity distribution functions (IVDFs) in such a system. We worked with a hot filament discharge at the University of San Diego (length = 64 cm, diameter = 32 cm) in which a Kr discharge was operated with a neutral pressure of 0.1mTorr, $n_e \approx 3\times 10^9 cm^{-3}$ and $T_e \approx 3.5 eV$. Sheath potentials were measured with an emissive probe using the inflection point method in the limit of zero emission. The LIF collection optics were recently upgraded to a 4f system with a spatial resolution smaller than 1mm. IVDFs measured near the grid (80mm diam. 40 lines/cm) indicate ion flow from both sides of the grid. Preliminary analysis of the moments of the IVDFs indicate that Bohm's Criterion is satisfied at the sheath edge. [Preview Abstract] |
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JP11.00048: Simulation, Analysis, and Design of the Princeton Adaptable Stellarator for Education and Outreach (PASEO) Jared Carlson, Arturo Dominguez The PPPL Science Education Department, in collaboration with IPP, is currently developing a versatile small scale Stellarator for education and outreach purposes. The Princeton Adaptable Stellarator for Education and Outreach (PASEO) will provide visual demonstrations of Stellarator physics and serve as a lab platform for undergraduate and graduate students. Based off the Columbia Non-Neutral Torus (CNT) (1), and mini-CNTs (2), PASEO will create pure electron plasmas to study magnetic surfaces. PASEO uses similar geometries to these, but has an adjustable coil configuration to increase its versatility and conform to a highly visible vacuum chamber geometry. To simulate the magnetic surfaces in these new configurations, a MATALB code utilizing the Biot Savart law and a Fourth Order Runge--Kutta method was developed, leading to new optimal current ratios. The design for PASEO and its predicted plasma confinement are presented. (1) T.S. Pedersen, et. al, \textit{Fusion Science and Technology} Vol. 46 July 2004 (2) C. Dugan, el. al, \textit{American Physical Society; 48th Annual Meeting of the Division of Plasma Physics}, October 30-November 3, 2006 [Preview Abstract] |
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JP11.00049: Optimization of confocal laser induced fluorescence for long focal length applications Andrew J. Jemiolo, Miguel F. Henriquez, Derek S. Thompson, Earl E. Scime Laser induced fluorescence (LIF) is a non-perturbative diagnostic for measuring ion and neutral particle velocities and temperatures in a plasma. The conventional method for single-photon LIF requires intersecting optical paths for light injection and collection. The multiple vacuum windows needed for such measurements are unavailable in many plasma experiments. Confocal LIF eliminates the need for perpendicular intersecting optical paths by using concentric injection and collection paths through a single window. One of the main challenges with using confocal LIF is achieving high resolution measurements at the longer focal lengths needed for many plasma experiments. We present confocal LIF measurements in HELIX, a helicon plasma experiment at West Virginia University, demonstrating spatial resolution dependence on focal length and spatial filtering. By combining aberration mitigating optics with spatial filtering, our results show high resolution measurements at focal lengths of 0.5 m, long enough to access the interiors of many laboratory plasma experiments. [Preview Abstract] |
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JP11.00050: Optimization of Ion Beam Extraction Jacob McLaughlin, David Caron, Earl Scime Ion beams are used in the fabrication of semiconductor devices to dope silicon structures, etch multilayer memory devices, and to initiate self-assembly of semiconductor nanostructures. To overcome the increasingly small size ratio of semiconductor devices compared to a silicon atom, industry is pursuing a transition from 2D devices to devices with three dimensional features. Using planar laser induced fluorescence and scanning single beam laser induced fluorescence (LIF), we will study the behavior and characteristics of a ribbon ion beam. Here we present a description of the parameters and first results from a new experimental facility designed to provide exceptional optical access for these ion beam experiments. We will describe the vacuum chamber design, the LIF optics, and initial LIF measurements of argon plasma. [Preview Abstract] |
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JP11.00051: The Effect of Background Pressure on Electron Acceleration from Ultra-Intense Laser-Matter Interactions Manh Le, Gregory Ngirmang, Chris Orban, John Morrison, Enam Chowdhury, William Roquemore We present two-dimensional particle-in-cell (PIC) simulations that investigate the role of background pressure on the acceleration of electrons from ultra intense laser interaction at normal incidence with liquid density ethylene glycol targets. The interaction was simulated at ten different pressures varying from 7.8 mTorr to 26 Torr. We calculated conversion efficiencies from the simulation results and plotted the efficiencies with respect to the background pressure. The results revealed that the laser to > 100 keV electron conversion efficiency remained flat around 0.35% from 7.8 mTorr to 1.2 Torr and increased exponentially from 1.2 Torr onward to about 1.47% at 26 Torr. Increasing the background pressure clearly has a dramatic effect on the acceleration of electrons from the target. We explain how electrostatic effects, in particular the neutralization of the target by the background plasma, allows electrons to escape more easily and that this effect is strengthened with higher densities. This work could facilitate the design of future experiments in increasing laser to electron conversion efficiency and generating substantial bursts of electrons with relativistic energies. [Preview Abstract] |
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JP11.00052: Characterizing Scintillator Response with Neutron Time-of-Flight Kevin Palmisano, Hannah Visca, Louis Caves, Corey Wilkinson, Hannah McClow, Stephen Padalino, Chad Forrest, Joe Katz, Craig Sangster, Sean Regan Neutron scintillator diagnostics for ICF can be characterized using the neutron time-of-flight (nTOF) line on Geneseo's 1.7 MV Tandem Pelletron Accelerator. Neutron signals can be differentiated from gamma signals by employing a coincidence method called the associated particle technique (APT). In this measurement, a 2.1 MeV beam of deuterons incident on a deuterated polyethylene target produces neutrons via the d(d,n)3He reaction. A BC-412 plastic scintillator, placed at a scattering angle of 152\textordmasculine , detects 1.76 MeV neutrons in coincidence with the 2.56 MeV 3He ions at an associated angle of 10\textordmasculine . The APT is used to identify the 1.76 MeV neutron while the nTOF line determines its energy. By gating only mono-energetic neutrons, the instrument response function of the scintillator can be determined free from background scattered neutrons and gamma rays. [Preview Abstract] |
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JP11.00053: Mach Probe Measurements in a Large-Scale Helicon Plasma M.W. Hatch, R.F. Kelly, D.M. Fisher, M. Gilmore, R.H. Dwyer A new six-tipped Mach probe, that utilizes a fused-quartz insulator, has been developed and initially tested in the HelCat dual-source plasma device at the University of New Mexico. The new design allows for relatively long duration measurements of parallel and perpendicular flows that suffer less from thermal changes in conductivity and surface build-up seen in previous alumina-insulated designs. Mach probe measurement will be presented in comparison with ongoing laser induced fluorescence (LIF) measurements, previous Mach probe measurements, ExB flow estimates derived from Langmuir probes, and fast-frame CCD camera images, in an effort to better understand previous anomalous ion flow in HelCat. Additionally, Mach probe-LIF comparisons will provide an experimentally obtained Mach probe calibration constant, K, to validate sheath-derived estimates for the weakly magnetized case. [Preview Abstract] |
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JP11.00054: Simulation of an expanding plasma using the Boris algorithm Luke Neal, Evan Aguirre, Thomas Steinberger, Timothy Good, Earl Scime We present a Boris algorithm simulation in a cylindrical geometry of charged particle motion in a helicon plasma confined by a diverging magnetic field. Laboratory measurements of ion velocity distribution functions (ivdfs) provide evidence for acceleration of ions into the divergent field region in the center of the discharge. The increase in ion velocity is inconsistent with expectations for simple magnetic moment conservation given the magnetic field mirror ratio and is therefore attributed to the presence of a double layer in the literature. Using measured electric fields and ivdfs (at different radial locations across the entire plasma column) upstream and downstream of the divergent magnetic field region, we compare predictions for the downstream ivdfs to measurements. We also present predictions for the evolution of the electron velocity distribution function downstream of the divergent magnetic field. [Preview Abstract] |
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JP11.00055: Computational Study of Plasma Response to Higher Order RF Multipoles Amanda Bowman, Katarina Godden, Nathaniel Hicks Initial results are presented from a computational plasma physics study of radio-frequency (RF) multipole structures containing various quasi-neutral plasma conditions. A particle-in-cell code is used to model a 2D cross section of the structure and enclosed plasma. Multipole orders including n $=$ 2 (quadrupole), n $=$ 4 (octupole), n $=$ 8 (hexadecapole) and so on are compared, along with the corresponding effects on shape of the pseudopotential well that traps particles and the interaction between the external field and plasma, including the formation of the RF plasma sheath. The simulation results are guiding the design of experiments to test some of these multipole configurations in the laboratory. [Preview Abstract] |
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JP11.00056: Stability of an axisymmetric, non-paraxial mirror and its applications for a fusion neutron source Roger Waleffe, Ethan Peterson, Vladimir Mirnov, Cary Forest Interchange stability analysis is underway for the design and development of an axisymmetric, non-paraxial (spherical) mirror. Such a system takes advantage of favorable magnetic field line curvature to stabilize the m = 1 flute mode, corresponding to a radial displacement of the plasma as a whole. Our results indicate the presence of a “stability ring” at intermediate values of magnetic flux inside the separatrix. As a result, any pressure distribution inside such a region is also stable. Optimizations for maximum stable volume peak where $B_{Helmholtz}/B_{Mirror}$ = 0.1 at the origin, independent of cylindrical Z location or mirror coil shape. An eigenvalue solver in the ballooning approximation has been developed and applied to the spherical mirror equilibria: it exhibits stability with respect to high-m modes for inner portions of the plasma volume. The stability limit is seen to decrease with increasing plasma beta and is heavily dependent on the pressure profile. Inherent MHD stability, coupled with high-temperature superconducting technology provides a promising path forward for the resurgence of axisymmetric mirrors. Work is ongoing to model a standalone high-field spherical mirror as a fusion neutron source using the GENRAY ray tracing and CQL3D wave deposition codes. [Preview Abstract] |
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JP11.00057: Schlieren Technique Applied to Magnetohydrodynamic Generator Plasma Torch Nirbhav Chopra, Jacob Pearcy, Michael Jaworski Magnetohydrodynamic (MHD) generators are a promising augmentation to current hydrocarbon based combustion schemes for creating electrical power. In recent years, interest in MHD generators has been revitalized due to advances in a number of technologies such as superconducting magnets, solid-state power electronics and materials science as well as changing economics associated with carbon capture, utilization, and sequestration. We use a multi-wavelength schlieren imaging system to evaluate electron density independently of gas density in a plasma torch under conditions relevant to MHD generators. The sensitivity and resolution of the optical system are evaluated alongside the development of an automated analysis and calibration program in Python. Preliminary analysis shows spatial resolutions less than 1mm and measures an electron density of $n_e = 1 \times 10^{16}$ cm$^{-3}$ in an atmospheric microwave torch. [Preview Abstract] |
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JP11.00058: Quench and stress coupled analysis of high temperature superconducting coils Jessica Li, Yuhu Zhai High-temperature superconductors (HTS) are promising candidates for compact next step fusion reactor designs due to their low power loss, higher margin and ability to carry extremely high current densities at high magnetic fields. However, unlike their low-temperature counterparts, HTS coils are much more vulnerable to damage during quench events under severe mechanical loading at high field magnet operation. It has been shown that the intensity of quench events may be mitigated by installing inductively coupled inserts around the superconducting coils. To this end, some previously explored designs of force-balanced coils which minimize stress in coil winding packs are reviewed for better stress management in HTS coils for quench mitigation. We use analytic models in FORTRAN and MATLAB to calculate the magnetic fields and resultant forces for various solenoid-like configurations of both high- and low-temperature superconducting coils. We then simulate their thermal, electric, and magnetic behaviors during quench-like events to identify optimal designs for both stability and quench protection. [Preview Abstract] |
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JP11.00059: Simulations of Atmospheric Plasma Arcs Jacob Pearcy, Nirbhav Chopra, Michael Jaworski We present the results of computer simulation of cylindrical plasma arcs with characteristics similar to those predicted to be relevant in magnetohydrodynamic (MHD) power conversion systems. These arcs, with core temperatures on the order of 1 eV, place stringent limitations on the lifetime of conventional electrodes used in such systems, suggesting that a detailed analysis of arc characteristics will be crucial in designing more robust electrode systems. Simulations utilize results from NASA's Chemical Equilibrium with Applications (CEA) program to solve the Elenbaas-Heller equation in a variety of plasma compositions, including approximations of coal-burning plasmas as well as pure gas discharges. The effect of carbon dioxide injection on arc characteristics, emulating discharges from molten carbonate salt electrodes, is also analyzed. Results include radial temperature profiles, composition maps, and current-voltage (IV) characteristics of these arcs. [Preview Abstract] |
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JP11.00060: Predicting high-current disruptions on NSTX with stacked regression trees Nathaniel Barbour, Kornee Kleijwegt, Leonard Lupin-Jimenez, Egemen Kolemen Disruption mitigation and avoidance are critical objectives for the successful operation of ITER. Of particular interest is the prospect of predicting disruptions during its first high-plasma-current experiments, when only low-current data will be available. Toward achieving those objectives, data from an initial sample of 1,000 shots are separated into two groups by plasma current. Four regression tree algorithms are then used as disruption predictors: AdaBoost, random forests, extremely randomized trees, and bootstrap aggregating (``bagging''). Each algorithm is trained using data from low-current shots and used to predict disruptions in the sample of high-current shots. To improve prediction accuracy, multiple methods of scaling the input signal data are examined. The creation of a meta-estimator from the predictions of the four regression tree algorithms is explored. A future extension is to predict high-current disruptions on other devices using a meta-estimator trained with low-current data from NSTX and NSTX-U. [Preview Abstract] |
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JP11.00061: Photon Throughput Calculations for a Spherical Crystal Spectrometer C. J. Gilman, M. Bitter, L. Delgado-Aparicio, P. C. Efthimion, K. Hill, B. Kraus, L. Gao, N. Pablant X-ray imaging crystal spectrometers of the type described in refs. [1, 2] have become a standard diagnostic for Doppler measurements of profiles of the ion temperature and the plasma flow velocities in magnetically confined, hot fusion plasmas. These instruments have by now been implemented on major tokamak and stellarator experiments in Korea, China, Japan, and Germany and are currently also being designed by PPPL for ITER. A still missing part in the present data analysis is an efficient code for photon throughput calculations to evaluate the chord-integrated spectral data. The existing ray tracing codes cannot be used for a data analysis between shots, since they require extensive and time consuming numerical calculations. Here, we present a detailed analysis of the geometrical properties of the ray pattern. This method allows us to minimize the extent of numerical calculations and to create a more efficient code. [1] M. Bitter, \textit{et al., } \textit{Rev.Sci.Instrum. }75, 3660(2004) [2] A. Ince-Cushman, \textit{et al., Rev. Sci. Instrum. }\textbf{79}, 10E302 (2008) [Preview Abstract] |
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JP11.00062: Intrinsic Flow Behavior During Improved Confinement in MST Reversed-field Pinch E. Tan, D. Craig, B. Schott, J. Boguski, Z.A. Xing, M.D. Nornberg, J.K. Anderson We used active charge exchange recombination spectroscopy to measure impurity ion flow velocity in high-current plasmas during periods of improved confinement. Velocity measurements througout the core reveal that ion flow parallel to the magnetic field is dominant compared to the perpendicular flow. The poloidal flow profile reverses at r/a $=$ 0.6, and the flow near the core is larger on outboard positions compared to the inboard positions. A strong shear in the toroidal flow develops near the axis as PPCD proceeds. In the past, the mode velocity has been used to infer the toroidal flow based on the `no-slip' assumption that the mode and local plasma co-rotate. We tested this assumption with direct measurements near the m $=$ 1, n $=$ 6 resonant surface. Inboard flow measurements are consistent with the no-slip condition and exhibit a time dependence where the flow decreases together with the n $=$ 6 mode velocity. The outboard flow is consistent in magnitude with the no-slip condition but the variations in time and across shots do not correlate well with the n $=$ 6 mode velocity. Possible reasons why the inboard and outboard flow exhibit different behavior are discussed. [Preview Abstract] |
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JP11.00063: Accelerating CR-39 Track Detector Processing by Utilizing UV Jonathan Sparling, Stephen Padalino, James McLean, Craig Sangster, Sean Regan The use of CR-39 plastic as a Solid State Nuclear Track Detector is an effective technique for obtaining data in high energy particle experiments including inertial confinement fusion. To reveal particle tracks after irradiation, CR-39 is chemically etched in NaOH at 80$^{\circ}$C, producing micron-scale signal pits at the nuclear track sites. It has been shown that illuminating CR-39 with UV light prior to etching increases bulk and track etch rates, especially when combined with elevated temperature. Spectroscopic analysis for amorphous solids has helped identify which UV wavelengths are most effective at enhancing etch rates. Absorption peaks found in the near infrared range provide for efficient sample heating, and may allow targeting cooperative IR-UV chemistry. Avoiding UV induced noise can be achieved through variations in absorption depths with wavelength. Vacuum drying and water absorption tests allow measurement of the resulting variation of bulk etch rate with depth. [Preview Abstract] |
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JP11.00064: Ballistic dynamics of a relativistic electron beam for mapping of the magnetosphere Michael Greklek-McKeon, Andrew T. Powis, Igor D. Kaganovich, Peter Porazik, Jay Johnson, Jake Willard, Ennio Sanchez Relativistic electrons fired from an orbiting satellite will propagate along the field lines of the magnetosphere. When the electrons impact the Earth’s atmosphere, they produce a characteristic signature that is detectable by ground stations. Such a diagnostic would enable direct validation of magnetospheric models, and assist in answering outstanding questions on auroral arcs. We determine the loss cone of a relativistic electron beam at various injection points within the Earth’s magnetosphere during the stages of a prominent geomagnetic event. We then study the degree of beam spreading during propagation to determine the viability of signal detection at the top of the atmosphere. This verification, using realistic magnetic field data, demonstrates that an electron beam emitted from a satellite can be fired into the loss cone of the Earth’s atmosphere and create an observable signal on the ground. [Preview Abstract] |
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JP11.00065: RF Antenna Design for a Helicon Plasma Source Katarina Godden, Brendan Stassel, Daniel Warta, Isaac Yep, Nathaniel Hicks, Jens Munk A helicon plasma source is under development for the new Plasma Science and Engineering Laboratory at the University of Alaska Anchorage. The helicon source is of a type [1] comprising Pyrex and stainless steel cylindrical sections, joined to an ultrahigh vacuum chamber. A radio frequency (RF) helical antenna surrounds the Pyrex chamber, as well as DC solenoidal magnetic field coils. This presentation focuses on the design of the RF helical antenna and RF matching network, such that helicon wave power is coupled to argon plasma with minimal reflected power to the RF amplifier. The amplifier output is selectable between 2-30 MHz, with forward c.w. power up to 1.5 kW. Details and computer simulation of the antenna geometry, materials, and power matching will be presented, as well as the matching network of RF transmission line, tuning capacitors, and cooling system. An initial computational study of power coupling to the plasma will also be described. [1] Scime, E. E., Keiter, P. A., Balkey, M. M., et al., \textit{J. Plasma Physics}, \textbf{81}, 1--22 (2014), DOI: 10.1017/S0022377814000890 [Preview Abstract] |
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JP11.00066: FLASH Interface; a GUI for managing runtime parameters in FLASH simulations. Christopher Walker, Petros Tzeferacos, Klaus Weide, Donald Lamb, Norbert Flocke, Scott Feister We present FLASH Interface, a novel graphical user interface (GUI) for managing runtime parameters in simulations performed with the FLASH code. FLASH Interface supports full text search of available parameters; provides descriptions of each parameter's role and function; allows for the filtering of parameters based on categories; performs input validation; and maintains all comments and non-parameter information already present in existing parameter files. The GUI can be used to edit existing parameter files or generate new ones. FLASH Interface is open source and was implemented with the Electron framework, making it available on Mac OSX, Windows, and Linux operating systems. The new interface lowers the entry barrier for new FLASH users and provides an easy-to-use tool for experienced FLASH simulators. [Preview Abstract] |
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JP11.00067: Design and Calibration of a Dispersive Imaging Spectrometer Adaptor for a Fast IR Camera on NSTX-U Richard Reksoatmodjo, Travis Gray A dispersive spectrometer adaptor was designed, constructed and calibrated for use on a fast infrared camera employed to measure temperatures on the lower divertor tiles of the NSTX-U tokamak. This adaptor efficiently and evenly filters and distributes long-wavelength infrared photons between 8.0 and 12.0 microns across the 128x128 pixel detector of the fast IR camera. By determining the width of these separated wavelength bands across the camera detector, and then determining the corresponding average photon count for each photon wavelength, a very accurate measurement of the temperature, and thus heat flux, of the divertor tiles can be calculated using Plank's law. This approach of designing an exterior dispersive adaptor for the fast IR camera allows accurate temperature measurements to be made of materials with unknown emissivity. Further, the relative simplicity and affordability of this adaptor design provides an attractive option over more expensive, slower, dispersive IR camera systems. [Preview Abstract] |
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JP11.00068: Analysis Tools for the Ion Cyclotron Emission Diagnostic on DIII-D C. A. del Castillo, K. E. Thome, R. I. Pinsker, O. Meneghini, D. C. Pace Ion cyclotron emission (ICE) waves are excited by suprathermal particles such as neutral beam particles and fusion products. An ICE diagnostic is in consideration for use at ITER, where it could provide important passive measurement of fast ions location and losses, which are otherwise difficult to determine. Simple ICE data analysis codes had previously been developed, but more sophisticated codes are required to facilitate data analysis. Several terabytes of ICE data were collected on DIII-D during the 2015-2017 campaign. The ICE diagnostic consists of antenna straps and dedicated magnetic probes that are both digitized at 200 MHz. A suite of Python spectral analysis tools within the OMFIT framework is under development to perform the memory-intensive analysis of this data. A fast and optimized analysis allows ready access to data visualizations as spectrograms and as plots of both frequency and time cuts of the data. A database of processed ICE data is being constructed to understand the relationship between the frequency and intensity of ICE and a variety of experimental parameters including neutral beam power and geometry, local and global plasma parameters, magnetic fields, and many others. [Preview Abstract] |
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JP11.00069: Identification of a localized core mode in a helicon plasma Daniel A. Green, Saikat Chakraborty Thakur, George R. Tynan, Adam D. Light We present imaging measurements of a newly observed mode in the core of the Controlled Shear Decorrelation Experiment - Upgrade (CSDX-U). CSDX-U is a well-characterized linear machine producing dense plasmas relevant to the tokamak edge ($T_e \sim 3$ eV, $n_e \sim 10^{13}$/cc). Typical fluctuations are dominated by electron drift waves, with evidence for Kelvin-Helmholtz vortices appearing near the plasma edge. A new mode has been observed using high-speed imaging that appears at high magnetic field strengths and is confined to the inner third of the plasma column. A cross-spectral phase technique allows direct visualization of dominant spatial structures as a function of frequency. Experimental dispersion curve estimates are constructed from imaging data alone, and allow direct comparison of theoretical dispersion relations to the observed mode. We present preliminary identification of the mode based on its dispersion curve, and compare the results with electrostatic probe measurements. [Preview Abstract] |
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JP11.00070: Spherical 3-Axis Hall Probe Array Calibration and Implementation for The Big Red Ball Jacob Lynn A 3-axis Helmholtz coil capable of producing 100 G magnetic fields at frequencies ranging from DC to 1 kHz has been built to calibrate an array of 3-axis hall probes. Accurate magnetic field measurements are necessary for diagnosing plasma equilibria and the presence of any MHD instabilities. The array will consist of three single-axis Hall sensors mounted orthogonally, each of which has a frequency response of 100 kHz and a sensitivity of 28$\frac{mV}{G}$. These probes will be placed on the inner surface of the machine to create a spherical array of sensors. Such an array will provide the necessary data to constrain plasma equilibrium parameters, such as current density and plasma pressure from $\nabla{P} = J \times B$. Understanding the plasma equilibrium, and large-scale magnetic fields is critical to understanding the dynamics involved in many phenomena, like the dynamo. Details on the design, calibration, and implementation of the three-axis Helmholtz coil and Hall sensors will be presented. [Preview Abstract] |
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JP11.00071: Einzel lens calculation and design for the ALPHA Experiment B. G. Cole, W. A. Bertsche, M. A. Johnson, T. D. Tharp An Einzel lens can be made from a series of hollow, cylindrical conductors charged to different electric potentials. The electric fields within the cylinders can be arranged to electrostatically focus a beam of non-neutral plasma to a certain focal point. Using electric field simulation tools, we explore possible modes of operation of this configuration to see what kinds of geometry and potentials are most favorable for future deployment in the ALPHA antihydrogen experiment at CERN. [Preview Abstract] |
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JP11.00072: Properties of large amplitude Alfv\'{e}n waves in a magnetized, order-unity $\beta$ laboratory plasma Christina Migliore, Troy Carter, Steve Vincena, Shreekrishna Tripathi, Seth Dorfman, David Hamilton Alfven waves play an important role in magnetized plasmas in the laboratory, e.g. in fusion plasmas, and in space and astrophysical settings, e.g. in the solar wind. The Large Plasma Device (LAPD) at UCLA as been used to study the linear and nonlinear properties of these important waves, however in a low-beta plasma ($\beta \sim 1\times 10^{-4}$). A new LaB$_6$ cathode source has been installed in LAPD, allowing the generation of much higher pressure plasmas; with lowered magnetic field, magnetized plasmas with $\beta \sim 1$ can be generated. New theoretical work by Squire suggests that the firehose instability limits the possible amplitude of Alfv\'{e}n waves in higher $\beta$ plasmas~\footnote{J. Squire, et al., Ap. J. Lett. 830, L25 (2016)}. We will report on experiments with large amplitude waves in $\beta \sim 1$ plasmas in LAPD aiming to test this theory and look for other processes that might limit wave amplitude, such as decay instabilities~\footnote{S. Dorfman & T.A. Carter, Phys. Rev. Lett. 116, 195002 (2016)}. [Preview Abstract] |
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JP11.00073: Nonneutral plasma diagnostic commissioning for the ALPHA Antihydrogen experiment S. Konewko, T. Friesen, T. D. Tharp The ALPHA experiment at CERN creates antihydrogen by mixing antiproton and positron plasmas. Diagnostic measurements of the precursor plasmas are performed using a diagnostic suite, colloquially known as the "stick." This stick has a variety of sensors and is able to move to various heights to align the desired diagnostic with the beamline. A cylindrical electrode, a faraday cup, an electron gun, and a microchannel-plate detector (MCP) are regularly used to control and diagnose plasmas in ALPHA. We have designed, built, and tested a new, upgraded stick which includes measurement capabilities in both beamline directions. [Preview Abstract] |
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JP11.00074: Using Divertor Strike Point Splitting to Study Plasma Response and Its Sensitivity to Equilibrium Uncertainties Abraham Teklu, D.M. Orlov, R.A. Moyer, I. Bykov, T.E. Evans, W. Wu, G.L. Trevisan, B.C. Lyons, T. Abrams, M.A. Makowski, C.S. Lasnier, M.E. Fenstermacher Resonant magnetic perturbations (RMPs) from 3D coils have been varied to modify the splitting of the divertor strike points in DIII-D. This splitting is imaged in filtered visible and infrared emission from the divertor to determine the particle and heat flux patterns on the target plates. The observed splitting is compared to vacuum and plasma response modeling in discharges where a subset of the RMP coils were ramped to shift the divertor footprints from dominantly n $=$ 3 to n $=$ 2 pattern. These results will be used to determine if the plasma response model can be validated with the measured splitting. We will also study the sensitivity of the modeled splitting to details of the 2D equilibrium. This RMP ramp technique could be used in ITER to spread out the heat flux while avoiding excessive forces on the RMP coils. [Preview Abstract] |
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JP11.00075: Direct comparison of neutral velocity distribution measurements and simulations in the vicinity of an absorbing boundary oblique to a magnetic field Miguel F. Henriquez, Derek S. Thompson, Shane Keniley, Davide Curreli, Thomas E. Steinberger, David D. Caron, Andrew J. Jemiolo, Jacob W. McLaughlin, Mikal T. Dufor, Luke A. Neal, Earl E. Scime, M. Umair Siddiqui Plasma-boundary interactions are strongly affected by the sheath and presheath structures that form near the boundary surface. Recent measurements have observed ion transport across magnetic field lines in regions where the surface is oblique to the background magnetic field ( $\psi =74^{\circ})$. In these boundary regions, charge exchange collisions may provide a mechanism through which neutral particles interact with the long distance presheath electric field. We report efforts to directly compare Boltzmann and particle-in-cell simulations with 3D neutral velocity distribution functions (NVDFs) using laser induced fluorescence (LIF) in a magnetized plasma boundary region. We present a novel LIF method for measuring Ar-II metastable velocity distributions, in which we observe the 738.6014 nm fluorescence (2p$_{\mathrm{3}}$ to 1s$_{\mathrm{4}}$ in Paschen's notation), that results from absorption of the 706.9167 nm (1s$_{\mathrm{5}}$ metastable to 2p$_{\mathrm{3}})$ pump laser, providing neutral temperatures and flows. We additionally describe electrostatic probe measurements in the same region. [Preview Abstract] |
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JP11.00076: Progress On The Thomson Scattering Diagnostic For The Helicon Plasma Experiment (HPX) A. Green, T. Emami, R. Davies, J. Frank, J. Hopson, J. Karama, R.W. James, J. Hopson, R.N. Paolino, E. Sandri, J. Turk, M. Wicke A high-performance spectrometer utilizing volume-phase-holographic (VPH) grating and a charge coupled device (CCD) camera with a range of 380-1090 nm and resolution of 1024x1024 has been assembled on HPX at the Coast Guard Academy Plasma Laboratory (CGAPL). This spectrometer will collect doppler shifted photons, emitted from the plasma by the first harmonic (1064 nm) of a 2.5 J Nd:YAG laser. Direct measurements of the plasma's temperature and density will be determined using HPX's Thomson Scattering (TS) single spatial point diagnostic system. A zero order half wave plate rotates the polarization of the second harmonic TS laser beam when operating at a wavelength of 532 nm. A linear actuated periscope has been constructed to remotely redirect the beam so that 532 and 1064 nm wavelengths can both be used. TS has the capability of determining plasma properties on short time scales and will be used to create a robust picture of the internal plasma parameters. Operating at both 532 and 1064 nm results in a self-consistent measurement and better use our existing spectrometer and soon to be constructed polychrometer. A prototype spectrometer has been constructed to explore the Andor CCD camera's resolution and sensitivity. The current status of the diagnostic development, spectrometer, and collection optics system will be reported. [Preview Abstract] |
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JP11.00077: Progress on the Development of Langmuir Probes on the Helicon Plasma Experiment (HPX)* T. Robledo-Thompson, P. Azzari, T. Emami, J. Frank, A. Green, J. Hopson, R. W. James, J. Karama, R. N. Paolino, E. Sandri, M. Wicke, E. Wright, M. Yepez, J. Turk CGAPL houses four major plasma experiments that span large temperature, density, energy and functionality regimes. Often automation and remote operation of intelligent devices are required in adverse operating environments for digital and analog systems. Plasma data collected by a multitude of diagnostics and sensors (to include Langmuir probes) over long timescales mandates CGAPL’s 40-channel Data Acquisition (DAQ) system that collects and stores data plus controls CGAPL. We have been exploring the use of fully protected Langmuir probe electronics system to track the change in the Ion Saturation current for plasma edge measurements of density and temperature. Probe installation automation, optimization, and data collection obstacles, solutions, and procedures will be reported. [Preview Abstract] |
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JP11.00078: EBT-XD Radiochromic Film Sensitivity Calibrations Using Proton Beams from a Pelletron Accelerator Barak Stockler, Alexander Grun, Gunnar Brown, Matthew Klein, Jacob Wood, Anthony Cooper, Ryan Ward, Charlie Freeman, Stephen Padalino, S.P. Regan, T.C. Sangster Radiochromic film (RCF) is a transparent detector film that permanently changes color following exposure to ionizing radiation. RCF is used frequently in medical applications, but also has been used in a variety of high energy density physics diagnostics. RCF is convenient to use because it requires no chemical processing and can be scanned using commercially available document scanners. In this study, the sensitivity of Gafchromic\texttrademark\ EBT-XD RCF to protons and x-rays was measured.~Proton beams produced by the SUNY Geneseo Pelletron accelerator were directed into an evacuated target chamber where they scattered off a thin gold foil. The scattered protons were incident on a sample of RCF which subtended a range of angles around the scattering center. A new analysis method, which relies on the variation in scattered proton fluence as a function of scattering angle in accordance with the Rutherford scattering law, is currently being developed to speed up the proton calibrations. Samples of RCF were also exposed to x-ray radiation using an X-RAD 160 x-ray irradiator, allowing the sensitivity of RCF to X-rays to be measured. [Preview Abstract] |
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JP11.00079: Pressure Profiles and Pressure-Driven Equilibrium Currents near Small Magnetic Islands and near Divertor Separatrices: Resonance and Symmetry Effects Dhanush Radhakrishnan, Allan Reiman A magnetic island whose width is well below a threshold value, determined by the ratio of perpendicular to parallel transport, has only a small effect on the ambient pressure gradient. We calculate the pressure gradient, and the associated pressure driven current in the neighborhood of such an island, assuming that the pressure is determined by a diffusion equation.We similarly calculate the pressure gradient and pressure driven current in the neighborhood of a divertor separatrix. For the small magnetic island, we consider a cylindrical magnetic field with perturbed circular flux surfaces. The perturbation consists of two components, one that modulates the toroidal magnetic field strength without breaking up the flux surfaces, and a second that introduces a resonant radial component of the magnetic field at the rational surface but has little effect on the toroidal field. The relative phase between the two perturbations is varied. The Pfirsch-Schluter current near the X-line is found to be much larger when both perturbations are present and the relative phase between them breaks the stellarator symmetry than it is when these conditions are not satisfied. The calculations are consistent with previous analytical work predicting a logarithmic singularity at the X-line. [Preview Abstract] |
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JP11.00080: C-MOD |
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JP11.00081: The High Field Path to Practical Fusion Energy Robert Mumgaard, D. Whyte, M. Greenwald, Z. Hartwig, D. Brunner, B. Sorbom, E. Marmar, J. Minervini, P. Bonoli, J. Irby, B. Labombard, J. Terry, R. Vieira, S. Wukitch We propose a faster, lower cost development path for fusion energy enabled by high temperature superconductors, devices at high magnetic field, innovative technologies and modern approaches to technology development. Timeliness, scale, and economic-viability are the drivers for fusion energy to combat climate change and aid economic development. The opportunities provided by high-temperature superconductors, innovative engineering and physics, and new organizational structures identified over the last few years open new possibilities for realizing practical fusion energy that could meet mid-century de-carbonization needs. We discuss re-factoring the fusion energy development path with an emphasis on concrete risk retirement strategies utilizing a modular approach based on the high-field tokamak that leverages the broader tokamak physics understanding of confinement, stability, and operational limits. Elements of this plan include development of high-temperature superconductor magnets, simplified immersion blankets, advanced long-leg divertors, a compact divertor test tokamak, efficient current drive, modular construction, and demountable magnet joints. An R{\&}D plan culminating in the construction of an integrated pilot plant and test facility modeled on the ARC concept is presented. [Preview Abstract] |
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JP11.00082: High Performance Regimes in Alcator C-Mod at High Magnetic Field E.S. Marmar Alcator is the only divertor tokamak in the world capable of operating at magnetic fields up to 8 T, equaling and exceeding that planned for ITER. Using RF and microwave tools for auxiliary heating and current drive, C-Mod accesses high pressure, high density, reactor-relevant regimes with no external torque and equilibrated electrons and ions, with exclusive use of high-Z metal plasma-facing components. The 2016 experimental campaign focused on naturally ELM-suppressed, enhanced energy confinement regimes (including I-mode and EDA H-mode, and approaches to super-H-mode), with emphasis on operation at the highest fields (5\textless B$_{\mathrm{T}}$\textless 8T). Results include extension of pedestal stability studies to 8T, including characterization of edge relaxation mechanisms and comparisons with theory, evaluation of the window for steady-state I-mode access, as well as confirmation of the Eich scaling for SOL power widths (L$_{\mathrm{q}}\propto $1/B$_{\mathrm{p}})$ for B$_{\mathrm{p\thinspace }}$up to 1.3T. On the final operation day of the campaign, a new world record for average confined-plasma pressure (\textgreater 2 atm.) was achieved. Taken together, combined with previous results from C-Mod and the world tokamak database, these results form a strong foundation for the high field, compact approach to achieving fusion energy production. New advances in high temperature, high field superconductors open the possibilities for practical development of this path for commercial fusion. [Preview Abstract] |
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JP11.00083: Conceptual design study for heat exhaust management in the ARC fusion pilot plant C.A. Dennett, N.M. Cao, A.J. Creely, J. Hecla, H. Hoffman, A.Q. Kuang, M. Major, J. Ruiz Ruiz, R.A. Tinguely, E.A. Tolman, D. Brunner, B. LaBombard, B.N. Sorbom, D.G. Whyte, P. Grover, C. Laughman The ARC pilot plant conceptual design study [1] has been extended to explore solutions for managing heat exhaust resulting from \textasciitilde 525 MW of fusion power in a compact (R\textasciitilde 3.3 m) tokamak. Superconducting poloidal field coils are configured to produce double-null equilibria that support X-point target divertors while maintaining the original core plasma shape and toroidal field coil size. Long outer divertor legs are appended to the original vacuum vessel, providing both large surface areas for surface dissipation of radiative heat and significantly reduced neutron damage for divertor components. A molten salt FLiBe blanket adequately shields all superconductors and functions as a tritium breeder, with advanced neutronics calculations indicating a tritium breeding ratio of \textasciitilde 1.08. In addition, FLiBe is used as the active coolant for the entire vessel. A tungsten swirl-tube cooling channel is implemented in the divertor, capable of exhausting 12 MW/m$^{\mathrm{2}}$,$^{\mathrm{\thinspace }}$heat flux while keeping total FliBe pumping power below 1{\%} of fusion power. Finally, three novel diagnostics are explored: Cherenkov radiation emitted in FLiBe to measure fusion reaction rate, microwave interferometry to measure divertor detachment front location, and IR imaging through the FLiBe blanket to monitor selected divertor ``hotspots.'' [1] Sorbom, B. N., et al. \textit{Fusion Engineering and Design} 100 (2015): 378-405. [Preview Abstract] |
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JP11.00084: Integrated, Reactor Relevant Solutions for Lower Hybrid Range of Frequencies Actuators S Shiraiwa, P. T. Bonoli, Y Lin, G. M. Wallace, S. J. Wukitch RF (radiofrequency) actuators with high system efficiency (wall-plug to plasma) and ability for continuous operation have long be recognized as essential tools for realizing a steady state tokamak. A number of physics and technological challenges to utilization remain including current drive efficiency and location, efficient coupling, and impurity contamination. In a reactor environment, plasma material interaction (PMI) issues associated with coupling structures are similar to the first wall and have been identified as a potential show-stopper. High field side (HFS) launch of LHRF power represents an integrated solution that both improves core wave physics and mitigates PMI/coupling issues. For HFS LHRF, wave penetration is vastly improves because wave accessibility scales as 1/B allowing for launching the wave at lower n$_{||}$ (parallel refractive index). The lower n$_{||}$ penetrate to higher electron temperature resulting in higher current drive efficiency (~ 1/n$_{||}^2$). HFS RF launch also provides for a means to dramatically improve launcher robustness in a reactor environment. On the HFS, the SOL is quiescent; local density profile is steep and controlled through magnetic shape; fast particle, neutron, turbulent heat and particle fluxes are eliminated or minim [Preview Abstract] |
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JP11.00085: Separation of particle and energy transport in the I-mode regime A.E. Hubbard, I. Cziegler, T. Happel, J. W. Hughes, P. Manz, J.E. Rice, T. Wilks The I-mode regime is distinct among regimes of improved tokamak energy confinement in that it clearly separates particle and energy transport; I-mode has L-mode particle (main ion and impurity) confinement but high (H-mode-like) energy confinement. The phenomenology of the regime is well measured, and very similar, on both Alcator C-Mod and ASDEX Upgrade. A broad peak in fluctuations at a few hundred kHz, called the Weakly Coherent Mode, is observed. A fluctuating flow at the GAM frequency exchanges energy with the WCM and helps to broaden it. Transport in I-mode is observed to be more intermittent or `bursty' than in L-mode. An Er well exists in I-mode which is deeper than in L-mode but weaker than in some H-modes. The transition from L-mode to I-mode is typically more gradual than the L-H transition and likely not the same type of bifurcation. The reason for the separation of transport channels is less clear. Possible physics mechanisms, and some initial simulations of underlying instabilities, will be discussed. [Preview Abstract] |
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JP11.00086: Impact of magnetic field and heating power on Er profile and fluctuations in I-mode pedestals in C-Mod Jerry Hughes, T.M. Wilks, C. Theiler, A.E. Hubbard, S.-G. Baek, M. Churchill, I. Cziegler, E. Edlund, J. Rice, E. Tolman Cross machine comparisons of I-mode show robust stationary ELM suppressed plasmas over broad operating conditions, suggesting the potential for the regime to be utilized in future reactors. I-modes typically exhibit separation of particle and energy transport channels, often associated with the weakly coherent mode (WCM) coupled to a GAM-like fluctuation in the edge pedestal region. C-Mod I-mode pedestals are analyzed over varied magnetic fields (2.8-5.8T) and auxiliary power (1.5-4.6 MW) to determine trends in the edge radial electric field, ExB shear, rotation, and fluctuations. In a controlled power ramp, the radial electric field well increases with power before reaching its maximum before the I-H transition. With increased input power, preliminary observations show an increase in the fluctuation frequencies, followed by a frequency reversal associated with an increase in mid-spectrum fluctuations. Previous research has explored the L-I and I-H power thresholds dependence on plasma density, surface area and magnetic field, allowing us to examine pedestal ExB shear as a function of proximity to these thresholds. [Preview Abstract] |
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JP11.00087: ABSTRACT WITHDRAWN |
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JP11.00088: Gyrokinetic and experimental investigations of multi-scale turbulence in Alcator C-Mod and DIII-D plasmas N.T. Howard, C. Holland, T.L. Rhodes, A.E. White, J. Candy, A.J. Creely, M. Greenwald, G.R. McKee, P. Rodriguez-Fernandez Extensive comparisons of high physics fidelity, multi-scale gyrokinetic simulations (m\textunderscore i/m\textunderscore e $=$ 60, realistic geometry, collisions, rotation, experimental inputs) with L-mode and ITER-relevant H-mode experiments have been performed on the Alcator C-Mod and DIII-D tokamaks. These simulations suggest that cross-scale interactions of ion and electron-scale turbulence play an important, even dominant, role in setting the experimental levels of both the ion and electron heat fluxes in reactor-relevant conditions. The validation of multi-scale gyrokinetic simulations has been extended further by comparing with turbulence measurements in reactor-relevant scenarios. Experiments on DIII-D in the ITER baseline scenario (H98$=$1, beta\textunderscore N $=$ 1.9, q95$=$3.3, Te\textasciitilde Ti ) documented possible signatures of cross-scale coupling in the wavenumber spectrum of intermediate-k (k*rho\textunderscore s \textasciitilde 2.5-5.0) density fluctuations measured with the Doppler backscattering (DBS) diagnostic. Results from multi-scale simulations of Alcator C-Mod plasmas and progress on analysis and simulation of the DIII-D experiments will be presented. [Preview Abstract] |
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JP11.00089: Flux-driven turbulence GDB simulations of the IWL Alcator C-Mod L-mode edge compared with experiment Manaure Francisquez, Ben Zhu, Barrett Rogers Prior to predicting confinement regime transitions in tokamaks one may need an accurate description of L-mode profiles and turbulence properties. These features determine the heat-flux width upon which wall integrity depends, a topic of major interest for research aid to ITER. To this end our work uses the GDB model to simulate the Alcator C-Mod edge and contributes support for its use in studying critical edge phenomena in current and future tokamaks. We carried out 3D electromagnetic flux-driven two-fluid turbulence simulations of inner wall limited (IWL) C-Mod shots spanning closed and open flux surfaces. These simulations are compared with gas puff imaging (GPI) and mirror Langmuir probe (MLP) data, examining global features and statistical properties of turbulent dynamics. GDB reproduces important qualitative aspects of the C-Mod edge regarding global density and temperature profiles, within reasonable margins, and though the turbulence statistics of the simulated turbulence follow similar quantitative trends questions remain about the code's difficulty in exactly predicting quantities like the autocorrelation time A proposed breakpoint in the near SOL pressure and the posited separation between drift and ballooning dynamics it represents are examined [Preview Abstract] |
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JP11.00090: Validation of TGLF in C-Mod and DIII-D using machine learning and integrated modeling tools P Rodriguez-Fernandez, AE White, NM Cao, AJ Creely, MJ Greenwald, BA Grierson, NT Howard, O Meneghini, CC Petty, JE Rice, F Sciortino, X Yuan Predictive models for steady-state and perturbative transport are necessary to support burning plasma operations. A combination of machine learning algorithms and integrated modeling tools is used to validate TGLF in C-Mod and DIII-D. First, a new code suite, VITALS, is used to compare SAT1 and SAT0 models in C-Mod. VITALS exploits machine learning and optimization algorithms for the validation of transport codes. Unlike SAT0, the SAT1 saturation rule contains a model to capture cross-scale turbulence coupling. Results show that SAT1 agrees better with experiments, further confirming that multi-scale effects are needed to model heat transport in C-Mod L-modes. VITALS will next be used to analyze past data from DIII-D: L-mode ``Shortfall'' plasma and ECH swing experiments. A second code suite, PRIMA, allows for integrated modeling of the plasma response to Laser Blow-Off cold pulses. Preliminary results show that SAT1 qualitatively reproduces the propagation of cold pulses after LBO injections and SAT0 does not, indicating that cross-scale coupling effects play a role in the plasma response. PRIMA will be used to ``predict-first'' cold pulse experiments using the new LBO system at DIII-D, and analyze existing ECH heat pulse data. [Preview Abstract] |
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JP11.00091: Bayesian Inference of H-mode Impurity Transport in Alcator C-Mod F. Sciortino, N. Howard, M.A. Chilenski, E.S. Marmar, P. Rodriguez-Fernandez, N.M. Cao, A.J. Creely, M. Greenwald, J.E. Rice, A.E. White, J.C. Wright, and Alcator C-Mod Team The investigation of impurity transport offers a compelling pathway for the validation of turbulence models, in particular through the computation of radial profiles of impurity transport coefficients. Such validation effort requires a rigorous computation of uncertainties, which we approach in a Bayesian framework using Gaussian Process Regression and the MultiNest algorithm to sample from multi-dimensional, potentially multi-modal posterior distributions. For the first time, we applied such analysis to determine experimental impurity transport in an Alcator C-Mod EDA H-mode plasma, where impurity confinement times are expected to be larger than in previously analyzed L-mode conditions. Using spatially resolved measurements of Ca +18 provided by an X-ray Imaging Crystal Spectrometer (XICS) and the STRAHL impurity transport code, radial profiles of experimental diffusion and convection coefficients were obtained following the injection of impurities via Laser Blow-Off (LBO). These results constitute the first steps towards constructing an impurity transport experimental database that will be used to provide constraints for gyrokinetic model validation. [Preview Abstract] |
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JP11.00092: Electron Profile Stiffness and Critical Gradient Length Studies in the Alcator C-Mod Tokamak Saeid Houshmandyar, David R. Hatch, Kenneth T. Liao, Bingzhe Zhao, Perry E. Phillips, William L. Rowan, Norman Cao, Darin R. Ernst, John E. Rice Electron temperature profile stiffness was investigated at Alcator C-Mod L-mode discharges. Electrons were heated by ion cyclotron range of frequencies (ICRF) through minority heating. The intent of the heating mechanism was to vary the heat flux and simultaneously, gradually change the local gradient. The electron temperature gradient scale length ($L_{Te}^{\mathrm{-1}}=$\textbar $\nabla T_{e}$\textbar /$T_{e})$ was accurately measured through a novel technique, using the high-resolution radiometer ECE diagnostic [Houshmandyar \textit{et al}, Rev. Sci. Instrum. \textbf{87}, 11E101 (2016)]. The TRANSP power balance analysis ($Q$/$Q_{\mathrm{GB}})$ and the measured scale length ($a$/$L_{Te})$ result in critical scale length measurements at all major radius locations. These measurements suggest that the profiles are already at the critical values. Furthermore, the dependence of the stiffness on plasma rotation and magnetic shear will be discussed. In order to understand the underlying mechanism of turbulence for these discharges, simulations using the gyrokinetic code, GENE, were carried out. For linear runs at electron scales, it was found that the largest growth rates are very sensitive to $a$/$L_{Te}$ variation, which suggests the presence of ETG modes, while the sensitivity studies in the ion scales indicate ITG/TEM modes. [Preview Abstract] |
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JP11.00093: The Multi-Spectral Imaging Diagnostic on Alcator C-MOD and TCV B. L. Linehan, R. T. Mumgaard, B. P. Duval, C. G. Theiler The Multi-Spectral Imaging (MSI) diagnostic is a new instrument that captures simultaneous spectrally filtered images from a common sight view while maintaining a large étendue and high spatial resolution. The system uses a polychromator layout where each image is sequentially filtered. This procedure yields a high transmission for each spectral channel with minimal vignetting and aberrations. A four-wavelength system was installed on Alcator C-Mod and then moved to TCV. The system uses industrial cameras to simultaneously image the divertor region at 95 frames per second at f/# 2.8 via a coherent fiber bundle (C-Mod) or a lens-based relay optic (TCV). The images are absolutely calibrated and spatially registered enabling accurate measurement of atomic line ratios and absolute line intensities. The images will be used to study divertor detachment by imaging impurities and Balmer series emissions. Furthermore, the large field of view and an ability to support many types of detectors opens the door for other novel approaches to optically measuring plasma with high temporal, spatial, and spectral resolution. Such measurements will allow for the study of Stark broadening and divertor turbulence. Here, we present the first measurements taken with this cavity imaging system [Preview Abstract] |
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JP11.00094: Universality in scrape-off layer plasma fluctuations: Comparison of experiment to numerical simulations Ralph Kube, Odd Erik Garcia, Audun Theodorsen, Dan Brunner, Brian LaBombard, James Terry, Matthias Wiesenberger Particle density time series, sampled in the outboard mid-plane scrape-off layer, are interspersed by large amplitude bursts due to radial propagation of plasma blobs. GPI and Langmuir probe time series measured in the Alcator C-Mod tokamak suggest that conditionally averaged wave forms of large amplitude bursts are well described by a double exponential function. Furthermore remains the ratio of the rise and fall e-folding time of the conditionally averaged wave form constant over a range of line-averaged plasma densities. In this contribution we compare this finding to results from numerical simulations. A two-dimensional drift-fluid model has been used to simulate the propagation of seeded plasma blobs in scrape-off layer plasmas for various initial amplitudes and cross-field sizes. Time traces of the particle density, sampled at a single point, are compared to the conditionally averaged waveform of the experimental data time series. The results are interpreted in the framework of a stochastic model which relates the statistical properties of the SOL fluctuations to the profile scale length. [Preview Abstract] |
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JP11.00095: Evidence for Chaotic Edge Turbulence in the Alcator C-Mod Tokamak Ziyan Zhu, Anne White, Troy Carter, Jim Terry, Seung Gyou Baek Turbulence greatly reduces the confinement time of magnetic-confined plasmas; understanding the nature of this turbulence and the associated transport is therefore of great importance. This research seeks to establish whether turbulent fluctuations in Alcator C-Mod are chaotic or stochastic. Stochastic fluctuations may lead to a random walk diffusive transport, whereas a diffusive description is unlikely to be valid for chaotic fluctuations since it lives in restricted areas of phase space (e.g., on attractors). Analysis of the time series obtained with the O-mode reflectometer and the gas puff imaging (GPI) systems reveals that the turbulent density fluctuations in C-Mod are chaotic. Supporting evidence for this conclusion includes the observation of an exponential power spectra (which is associated with Lorentzian-shaped pulses in the time series) [1], the population of the corresponding Bandt-Pompe (BP) probability distribution [2], and the location of the signal on the Complexity-Entropy plane (C-H plane) [3]. These analysis techniques will be briefly introduced along with a discussion of the analysis results. The classification of edge turbulence as chaotic opens the door for further work to understand the underlying process and the impact on turbulent transport. [1] Maggs and Morales, Phys. Rev. E 86, 015401 (2012). [2] Bandt and Pompe, Phys. Rev. Lett. 88 , 174102 (2002) [3] Rosso \textit{et al}. , Phys. Rev. Lett. 99 , 154102 (2007) [Preview Abstract] |
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JP11.00096: Synchrotron emission in Alcator C-Mod: spectra at three magnetic fields, visible camera images, and polarization data R Alex Tinguely, Robert Granetz, Mathias Hoppe, Ola Embr{\'e}us, Adam Stahl, T{\"u}nde F{\"u}l{\"o}p Alcator C-Mod's high magnetic field allows runaway electron synchrotron emission to be observed in the visible wavelength range. Visible spectrometers were used to measure synchrotron spectra at three magnetic fields: 2.7, 5.4, and 7.8 T. Both a test-particle model [1] and kinetic solver CODE (COllisional Distribution of Electrons) [2,3] explore the energy evolution of the runaway population and the impact of magnetic-field-dependent synchrotron radiation as a power loss mechanism. Additionally, distortion-corrected visible camera images capture the spatial distribution and evolution of synchrotron emission in C-Mod. Initial results show good agreement between experiment and the new synthetic diagnostic SOFT (Synchrotron-detecting Orbit-Following Toolkit) [4]. Finally, a first look at synchrotron polarization data is presented.\newline \newline \noindent[1] JR Mart{\'i}n-Sol{\'i}s, et al. PoP 5 (1998)\newline \noindent[2] M Landreman, et al. CPC 185 (2014)\newline \noindent[3] A Stahl, et al. NF 56 (2016)\newline \noindent[4] M Hoppe, et al. Synthetic synchrotron diagnostic for runaway electrons in tokamaks. In progress. [Preview Abstract] |
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JP11.00097: An assessment of methods to compute secondary electron emission for tungsten and molybdenum and implications for plasma potential measurements by Langmuir probes in Alcator C-Mod William McCarthy, Brian LaBombard, Dan Brunner, Adam Kuang Plasma potentials measured by Langmuir probes rely on a method to compute secondary electron emission (SEE) yield. However, significant variations exist among published models for SEE and the datasets used to evaluate them. As a means to critically assess SEE evaluation methods, two of four tungsten electrodes on a Langmuir-Mach probe head were replaced with molybdenum and exposed to high temperature (\textgreater 50 eV) plasmas. In this situation, plasma potentials computed for either material should be identical, with the SEE evaluation method properly accounting for significant differences in SEE yields. Of the six methods to compute SEE examined, two are found to produce consistent results (Sternglass-Bronstein and Young-Dekker-Bronstein). In contrast, the method previously used for C-Mod data analysis (Sternglass-Kollath) was found to be inconsistent. We have since adopted Young-Dekker-Bronstein. An important consequence is that values for plasma potential, electric field and ExB flow shear near the LCFS in Alcator C-Mod has substantially increased compared to what had been reported previously. This work was supported by DoE Contract DE-FC02-99ER54512 on Alcator C-Mod, a DoE Office of Science user facility. [Preview Abstract] |
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JP11.00098: Fluctuations measured by flush mounted versus proud divertor Langmuir probes -- why are they different? O.E. Garcia, A.Q. Kuang, D. Brunner, B. LaBombard, R. Kube A flush-mounted, toroidally-elongated, and field-aligned divertor `rail' Langmuir probe array was installed in Alcator C-Mod in 2015. This geometry is heat flux tolerant and effectively mitigates sheath expansion effects down to incident field line angles of 0.5 degree [1]. Further complications have arisen that cannot be explained by sheath-expansion. In particular, the 'rail' probe geometry measures significantly higher plasma fluctuation levels in the common flux region compared to traditional proud probes, whereas they are similar in the private flux zone. In some instances, the amplitudes of ion saturation current fluctuations normalized to the mean are a factor of 2 higher; probability distribution functions correspondingly record large amplitude events that are not seen by the proud probes. This discrepancy also appears to depend on divertor plasma regime. For example, fluctuations become similar near the strikepoint when the electron temperature is low. To ensure that these discrepancies were not due to perturbations caused by the voltage bias or currents collected by the probes, the two Langmuir probe systems were left to 'float' and the fluctuation statistics analyzed. Yet, even in this non-perturbative situation, there exist clear differences in the fluctuation characteristics. The raises two questions: how does the probe geometry affect plasma fluctuations measurements and what are the true plasma fluctuations experienced by the divertor surface? [1] A.Q., Kuang, et. al. (2016). \textit{Nucl. Mat.and Energy. }Supported by USDoE awards DE-FC02-99ER54512. [Preview Abstract] |
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JP11.00099: Characterization and Mitigation of ICRF Antenna – Plasma Edge Interaction Rongjie Hong, George Tynan, Steve Wukitch, Yijun Lin, Jim Terry, M. Chilenski, T. Golfinopoulos, A. Hubbard, R.T. Mumgaard, R. Perkins, M.L. Reinke Recent experiments reveal that RF-induced potentials ($V_{RF}$) in the SOL and impurity source at the antenna can be reduced to background levels via optimizing the power ratio between the inner and outer current straps, $P_{cent}/P_{out}$. Experiments indicate the antenna impurity source reduction for the field aligned antenna is due to geometrical alignment rather than electrical symmetry. Additional experiments performed without an optimized $P_{cent}/P_{out}$ showed that $V_{RF}$ and the associated convection cells do not influence the impurity penetration or core impurity confinement. These results suggest the core impurity contamination associated with ICRF heating is dominated by an increased impurity source rather than a change in impurity transport. Further, the convective cell strength was expected to scale inversely with B-field. The observed poloidal velocity (measure of convective cell strength), however, decreased less than expected. In addition, the measured maximum $V_{RF}$ increased and penetrated farther into the SOL at higher B-field and plasma current. Results also suggest $V_{RF}$ is strongly influenced by the SOL plasma parameters rather than by RF parameters. [Preview Abstract] |
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JP11.00100: Mode conversion in ICRF experiments on Alcator C-Mod Y. Lin, S.J. Wukitch, E. Edlund, P. Ennever, A.E. Hubbard, M. Porkolab, J. Rice, J. Wright In recent three-ion species (majority D and H plus a trace level of $^{\mathrm{3}}$He) ICRF heating experiment on Alcator C-Mod [1], double mode conversion on both sides of the $^{\mathrm{3}}$He cyclotron resonance has been observed using the phase contrast imaging (PCI) system. The MC locations are used to estimate the species concentrations in the plasma. Simulation using TORIC shows that with the $^{\mathrm{3}}$He level \textless 1{\%}, most RF power is absorbed by the $^{\mathrm{3}}$He ions and the process can generate energetic $^{\mathrm{3}}$He ions. In recent mode conversion flow drive experiment in D($^{\mathrm{3}}$He) plasma at 8 T, MC waves were also monitored by PCI. The MC ion cyclotron wave (ICW) amplitude and wavenumber k$_{\mathrm{R}}$ have been found to correlate with the flow drive force. The MC efficiency, wave-number k of the MC ICW and their dependence on plasma parameters like T$_{\mathrm{e0}}$ are shown to play important roles. Based on the experimental observation and numerical study of the dispersion solutions, a hypothesis of the flow drive mechanism has been proposed. [1] Y.O. Kazakov et al, Nature Phys. (19 June 2017) doi: 10.1039/nphys4167. [Preview Abstract] |
(Author Not Attending)
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JP11.00101: A Lithium Vapor Box Divertor Similarity Experiment Robert A. Cohen, Eric D. Emdee, Robert J. Goldston, Michael A. Jaworski, Jacob A. Schwartz A lithium vapor box divertor offers an alternate means of managing the extreme power density of divertor plasmas by leveraging gaseous lithium to volumetrically extract power. The vapor box divertor is a baffled slot with liquid lithium coated walls held at temperatures which increase toward the divertor floor. The resulting vapor pressure differential drives gaseous lithium from hotter chambers into cooler ones, where the lithium condenses and returns. A similarity experiment was devised to investigate the advantages offered by a vapor box divertor design. We discuss the design, construction, and early findings of the vapor box divertor experiment including vapor can construction, power transfer calculations, joint integrity tests, and thermocouple data logging. Heat redistribution of an incident plasma-based heat flux from a typical linear plasma device is also presented. [Preview Abstract] |
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JP11.00102: MST |
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JP11.00103: Overview of MST Research B.E. Chapman MST progress in advancing the RFP for (1) fusion plasma confinement with ohmic heating and minimal external magnetization, (2) predictive capability in toroidal confinement physics, and (3) basic plasma physics is summarized. Validation of key plasma models is a program priority, which is enhanced by programmable power supplies (PPS) to maximize inductive capability. The existing PPS enables access to very low plasma current, down to Ip$=$0.02 MA. This greatly expands the Lundquist number range S$=$10\textasciicircum 4-10\textasciicircum 8 and allows nonlinear, 3D MHD computation using NIMROD and DEBS with dimensionless parameters that overlap those of MST plasmas. A new, second PPS will allow simultaneous PPS control of the Bp and Bt circuits. The PPS also enables MST tokamak operation, thus far focused on disruptions and RMP suppression of runaway electrons. Gyrokinetic modeling with GENE predicts unstable TEM in improved-confinement RFP plasmas. Measured fluctuations have TEM properties including a density-gradient threshold larger than for tokamak plasmas. Turbulent energization of an electron tail occurs during sawtooth reconnection. Probe measurements hint that drift waves are also excited via the turbulent cascade in standard RFP plasmas. Exploration of basic plasma science frontiers in MST RFP and tokamak plasmas is proposed as part of WiPPL, a basic science user facility. Work supported by USDoE. [Preview Abstract] |
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JP11.00104: Intrinsic Flow and Momentum Transport during Improved Confinement in MST D. Craig, E. Tan, B. Schott, J.K. Anderson, J. Boguski, M.D. Nornberg, Z.A. Xing Progress in absolute wavelength calibration of the Charge Exchange Recombination Spectroscopy (CHERS) system on MST has enabled new observations and analysis of intrinsic flow and momentum transport. Localized toroidal and poloidal flow measurements with systematic accuracy of $+$/- 3 km/s have been obtained during improved confinement Pulsed Parallel Current Drive (PPCD) plasmas at high plasma current (400-500 kA). The magnetic activity prior to and during the transition to improved confinement tends to increase the flow and sets the initial condition for the momentum profile evolution during improved confinement where intrinsic flow drive appears to weaken. Inboard flows change in time during PPCD, consistent with changes in the core-resonant m$=$1, n$=$6 tearing mode phase velocity. Outboard flows near the magnetic axis are time-independent, resulting in the development of a strongly sheared toroidal flow in the core and asymmetry in the poloidal flow profile. The deceleration of the n$=$6 mode during the period of improved confinement correlates well with the n$=$6 mode amplitude and is roughly consistent with the expected torque from eddy currents in the conducting shell. The level of D$_{\mathrm{\alpha }}$ emission and secondary mode amplitudes (n$=$7-10) do not correlate with the mode deceleration suggesting that the momentum loss from charge exchange with neutrals and diffusion due to residual magnetic stochasticity are not significant in PPCD. This work has been supported by the U.S.D.O.E. [Preview Abstract] |
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JP11.00105: Ion energy balance in enhanced-confinement reversed-field pinch plasmas Z. A. Xing, M.D. Nornberg, J. Boguski, D. Craig, D.J. Den Hartog, K. McCollam Testing the applicability of collisional ion transport theory using tearing suppressed RFP plasma in MST achieved through Pulsed Poloidal Current Drive (PPCD), we find that the ion temperature dynamics in the core to be well-predicted by classical and collisional terms. Prior work demonstrated that impurity ion particle transport in PPCD plasmas is classical. Neoclassical effects on ions in the RFP are small and the stochastic transport is greatly suppressed during PPCD. Recent neutral modelling with DEGAS2 suggests higher core neutral temperatures than expected due to the preferential penetration of higher temperature neutrals generated by charge exchange. Further, investigations through equilibrium reconstruction point to the existence of an inward pinch flow associated with ExB drift. The heat balance model pulls together a wide range of diagnostic data to forward model T$_{i}$ evolution in PPCD, which is then compared to charge exchange spectroscopy measurements of T$_{i}$. Ion power balance is mostly driven by classical effects including compressional heating, electron collisional heating, and charge exchange transport. This understanding provides a good baseline for investigations of anomalous heating in plasmas with tearing mode activity. This work is supported by US DOE. [Preview Abstract] |
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JP11.00106: Measurements of Impurity Particle Transport Associated with Drift-Wave Turbulence in MST Takashi Nishizawa, Mark Nornberg, John Boguski, Darren Craig, Daniel Den Hartog, M.J. Pueschel, John Sarff, Paul Terry, Zach Williams, Zichuan Xing Understanding and controlling impurity transport in a toroidal magnetized plasma is one of the critical issues that need to be addressed in order to achieve controlled fusion. Gyrokinetic modeling shows turbulence can drive impurity transport, but direct measurements of the turbulent flux have not been made. Particle balance is typically used to infer the presence of turbulent impurity transport. We report, for the first time in a toroidal plasma, direct measurements of turbulence-driven impurity transport. Trapped electron mode (TEM) turbulence appears in MST plasmas when MHD tearing fluctuations are suppressed. Impurity ion-Doppler spectroscopy is used to correlate impurity density and radial velocity fluctuations associated with TEM. Small Doppler shifts associated with the radial velocity fluctuations (rms~1km/s) are resolved with the use of a new linearized spectrum correlation analysis method, which improves the rejection of Poisson noise. The method employs frequency-domain correlation analysis to expose the fluctuation and transport spectrum. The C${}^+2$ impurity transport velocity driven by turbulence is found to be 48m/s (inward), which is sufficiently large to impact an impurity flux balance in MST improved-confinement plasmas. This work is supported by the US DOE. [Preview Abstract] |
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JP11.00107: Measurement of ion velocities in the locked Single Helical Axis state in MST RFP plasmas J. Boguski, M. D. Nornberg, B. E. Chapman, M. Cianciosa, D. J. Den Hartog, D. Craig, K. J. McCollam, T. Nishizawa, Z. A. Xing Charge Exchange Recombination Spectroscopy (CHERS) provides the first core-localized measurements of the 3D ion flow structure in Single Helical Axis (SHAx) plasmas. In high-current and low-density (large Lundquist number) RFP plasmas, the island associated with the innermost resonant tearing mode can grow to large amplitude and envelop the magnetic axis creating a 3D equilibrium. Measurements of the flow profile with various orientations (phases) of the helical structure relative to the CHERS diagnostic were achieved by locking the plasma with resonant magnetic perturbations. The flows persist despite mode locking, and are correlated with the amplitude and phase of the innermost resonant tearing mode. At mid-radius, a dominantly m$=$2 poloidal flow structure appears relative to the phase of the helical core. Near the core, non-axisymmetric flows become less pronounced, and cannot be distinguished at the innermost radii. These results place more significant constraints on the nature of the flow structure than previous line-integrated spectroscopy measurements and challenge predictions of visco-resistive MHD models of these helical RFP plasmas. [Preview Abstract] |
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JP11.00108: Validation of MHD Models using MST RFP Plasmas C.M. Jacobson, B.E. Chapman, D.J. Den Hartog, K.J. McCollam, J.S. Sarff, C.R. Sovinec Rigorous validation of computational models used in fusion energy sciences over a large parameter space and across multiple magnetic configurations can increase confidence in their ability to predict the performance of future devices. MST is a well diagnosed reversed-field pinch (RFP) capable of operation with plasma current ranging from 60~kA to 500~kA. The resulting Lundquist number $S$, a key parameter in resistive magnetohydrodynamics (MHD), ranges from $4\times10^4$ to $8\times10^6$ for standard RFP plasmas and provides substantial overlap with MHD RFP simulations. MST RFP plasmas are simulated using both DEBS, a nonlinear single-fluid visco-resistive MHD code, and NIMROD, a nonlinear extended MHD code, with $S$ ranging from $10^4$ to $10^5$ for single-fluid runs, and the magnetic Prandtl number $Pm=1$. Validation metric comparisons are presented, focusing on how normalized magnetic fluctuations at the edge $\tilde{b}$ scale with $S$. Preliminary results for the dominant $n=6$ mode are $\tilde{b}\sim S^{-0.20\pm0.02}$ for single-fluid NIMROD, $\tilde{b}\sim S^{-0.25\pm0.05}$ for DEBS, and $\tilde{b}\sim S^{-0.20\pm0.02}$ for experimental measurements, however there is a significant discrepancy in mode amplitudes. Preliminary two-fluid NIMROD results are also presented. [Preview Abstract] |
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JP11.00109: Synthetic Camera Diagnostics for Edge Modeling in MST Ryan Norval, Heinke Frerichs, John Goetz, Oliver Schmitz A nearly full coverage camera system on MST aids power balance studies by measuring of spatially resolved $D_\alpha$, other Balmar lines, or broadband visible emission. Camera measurements have found the wall recycling in MST to be asymmetric both in standard RFP mode and Quasi-Single Helically (QSH) mode operation. The EIRENE code is used to interpret the measured light and estimate the neutral density from the Balmar lines and a background plasma. A synthetic camera module for EIRENE is currently being implemented which allows comparison between the images that would result from the simulation to the actual images observed by the cameras. Additional diagnostic data from line integrated $D_\alpha$ detectors, and an edge Langmuir probe will help constrain the model. It is expected that an iterative technique to match simulated images to real images will result in more accurate neutral density estimates as well as constraining the relatively unknown edge parameters. This increased accuracy of the neutral profile and edge parameters of MST will allow for better comparison between the standard RFP and the QSH mode with respect to neutral particles acting as an energy loss pathway via charge exchange and radiative losses. Supported by US DOE [Preview Abstract] |
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JP11.00110: Measurements of fast ion spatial dynamics during magnetic activity in the RFP J.A. Goetz, J.K. Anderson, P. Bonofiglo, J. Kim, R. McConnell, R.M. Magee Fast ions in the RFP are only weakly affected by a stochastic magnetic field and behave nearly classically in concentration too low to excite Alfvenic activity. At high fast ion concentration sourced by H-NBI in 300kA RFP discharges, a substantial drop in core-localized high pitch fast ions is observed during bursts of coupled EPM and IAE (magnetic island-induced Alfven eigenmode) activity (100-200kHz) through neutral particle analysis. Sourcing instead fast deuterium with NBI, the DD fusion products can measure the dynamics of the fast ion density profile. Both a collimated neutron detector and a new 3MeV fusion proton detector loaned by TriAlpha Energy measure the fast ion density profile with \textasciitilde 5cm spatial resolution and 100$\mu $s temporal resolution. In D-NBI, the bursting EPM is excited at slightly lower frequency and the IAE activity is nearly absent, likely due to an isotope effect and loss of wave-particle interaction. In these cases, neutral particle analysis shows little change in the core-localized high pitch fast ion content, and the fusion product profile indicates little change in the fast ion density profile, leaving unexplained the mechanism removing EPM drive. We measure a substantial redistribution of the fast ion profile due to strong lower-frequency (\textasciitilde 30kHz) MHD activity that accompanies the current profile relaxation in the RFP. Profile flattening is strongest in low bulk density discharges, which often occur with a total increase in global neutron flux from acceleration of the beam ions. [Preview Abstract] |
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JP11.00111: Anisotropic and asymmetric fast ion distribution generated by magnetic reconnection in MST plasmas Jungha Kim, Jay Anderson, Phillip Bonofiglo, Robert Harvey, John Sarff Magnetic reconnection is important in particle transport and energization in both astrophysical and laboratory plasmas. Global reconnection events in MST spontaneously generate an anisotropic ion distribution with a high energy tail extending up to 30x thermal energy, likely through a multi-step process that involves multiple physical scale lengths. First, thermal ions are heated by a mechanism that operates preferentially perpendicular to the magnetic field. Second, the higher energy portion of the thermal ion distribution moves into orbits that drift off the stochastic background magnetic field. In the reversed field pinch (RFP) configuration, these drift velocities contribute to stable fast ion orbits that are low in diffusivity and favorable to confinement. These fast ions, separated from the background magnetic field, are unaffected by fluctuation-based, dynamo-like emfs that reduce the total electric field to 0.5 V/m. Finally, a parallel electric field (\textasciitilde 80 V/m), induced by the fast change in the equilibrium during magnetic relaxation, accelerates these fast ions, resulting in an ion distribution that favors high energy, parallel-streaming ions. Work is underway to model the time evolution of the fast ion distribution using CQL3D (Fokker-Planck equation solver) and RIO (full orbit tracer). Work supported by US DOE. [Preview Abstract] |
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JP11.00112: Fast Ion Transport in the Three-Dimensional Reversed Field Pinch P. J. Bonofiglo, J. K. Anderson, E. Parke, M. Gobbin, J. Kim, J. Egedal The reversed field pinch (RFP) provides a unique environment to study fast ion confinement and transport in both 2D and 3D geometries. In the axisymmetric RFP, guiding center drifts are along flux surfaces, resulting in naturally well-confined fast ions. At sufficiently high Lundquist number, the innermost tearing mode can grow and envelop the magnetic axis, creating a helical axis and 3D equilibrium. Experiments on MST reveal reduced confinement of fast ions with the transition to this quasi-single helicity (QSH) state. Current work aims to probe the dynamics of fast ion transport during QSH. Energetic particle modes (EPMs) upshift in frequency with increasing core tearing mode amplitude, disappear in high current QSH plasmas, and depend on NBI isotope. Additionally, FIR interferometry has resolved electron density perturbations associated with EPMs. The FIR measurements show the upshifting EPMs moving radially outward as they grow in frequency, indicating transport associated with the transition to QSH. The Hamiltonian guiding center code ORBIT corroborates rapid fast ion loss times in QSH and is being actively used to simulate diffusion coefficients and particle orbits for examining neoclassical transport. This research is supported by US DOE. [Preview Abstract] |
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JP11.00113: Measurement of beam-driven Alfv\'{e}nic instabilities in MST and comparison to predictions E. Parke, J. K. Anderson, J. Boguski, P. J. Bonofiglo, D. L. Brower, W. X. Ding Neutral beam injection in MST drives a variety of energetic particle modes (EPMs) and Alfv\'{e}nic modes (AEs). These instabilities can lead to avalanche processes with enhanced fast ion transport, the most commonly observed avalanche involving coupling between modes with toroidal number n = 5, 4, and -1. Density fluctuations correlated with these avalanches, as well as internal magnetic fluctuations associated with the dominant, n = 5 EPM, have previously been characterized with the FIR interferometer-polarimeter. However, the n = 4 and -1 AEs were too weak to observe with polarimetry. Recent upgrades to the interferometer-polarimeter have further reduced the noise floor for fluctuation measurements. The upgraded system allows clear identification of Faraday rotation fluctuations associated with the n = -1 AE. These fluctuations are highly localized, with narrow structure peaking at the same location as the density fluctuations. Previous work has established that the n = 4 and -1 fluctuations are consistent with island-induced AEs (IAEs). Measured structures are compared to predictions for IAEs as well as helicity-induced AEs, and ongoing work to resolve the dependence of observed structures on tearing mode island phase and identify n = 4 correlated fluctuations is presented. [Preview Abstract] |
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JP11.00114: Core Localized Anisotropic Electron Energization During Magnetic Reconnection in RFP Plasma Alexander Scherer, Abdulgader Almagri, Mihir Pandya, Ami DuBois We investigate the generation of an anisotropic electron tail during magnetic reconnection events in the MST RFP device. We have observed a strong, high-energy tail in the electron bremsstrahlung x-ray spectrum for a view perpendicular to the toroidal magnetic field and a relatively weak tail in the x-ray spectra for parallel and antiparallel views$^{[1]}$. Runaway mechanisms are ruled out for this energization due to symmetry between the parallel and antiparallel spectra. These experimental observations have been reproduced by a bremsstrahlung calculation using the CQL3D code in which a distribution function with a large $v_\perp$ tail is input and localized to a core with a 9 cm radius around the magnetic axis$^{[1]}$. An experiment to isolate the spatial extent of this core region is performed by scanning a fast response x-ray detector, with an output pulse width of 20 ns, along a series of radial, chord-like views. The presence of a strong tail in the chord spectra determines the extent of this core region. Spectra viewing the poloidal magnetic field near the reversal surface are also investigated for runaway energization and compared with toroidal results. Work supported by the US DOE.\\ $^{[1]}$A.M. DuBois, et al., \textit{Phys. Rev. Lett.} 118, 075001 [Preview Abstract] |
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JP11.00115: Dissipation Range of Anisotropic Magnetic Fluctuations in MST plasmas James Titus, Abdul Almagri, John Sarff, Paul Terry, Ephrem Mezonlin Previous measurements of broadband magnetic fluctuations in the MST reversed field pinch (RFP) revealed a turbulent cascade that is anisotropic with respect to the large-scale (equilibrium) magnetic field and characterized by a power spectrum with exponential falloff at scales larger than expected for classical processes. The cascade is supported by tearing instabilities at the global scale that undergo strong nonlinear coupling, especially through poloidal mode m$=$1 and m$=$0 fluctuations. The non-classical dissipation feature may be indicative of the powerful non-collisional ion heating observed in MST plasmas. The previous measurements were done with pickup coils separated in both the toroidal and poloidal directions that allowed a resolution of \textbar k\textbar \textless 1.5 cm$^{\mathrm{-1}}$. We report new measurements with increased spatial resolution, from increasing the number of coil sets (from 2 to 7). This enables an increase in the amount of two-point correlated spectra to be ensemble. Calibration analysis show the new probe measurements agree with the previous probe measurements at the same insertion depth. As the new probe is inserted deeper into the plasma, towards the reversal surface, the exponential component dominates as the power law component goes to zero. This is either due to stronger dissipation or the change in wavenumber resistivity. [Preview Abstract] |
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JP11.00116: Observation of Electron Bernstein Wave Heating in the RFP Andrew Seltzman, Jay Anderson, John Goetz, Cary Forest The first observation of RF heating in a reversed field pinch (RFP) using the electron Bernstein wave (EBW) has been demonstrated on MST. Efficient mode conversion of an outboard-launched X mode wave at 5.5 GHz leads to Doppler-shifted resonant absorption ($\omega_{\mathrm{rf}}=$n$\omega _{\mathrm{ce}}$--k$_{\mathrm{\vert \vert }}$v$_{\mathrm{\vert \vert }})$ for a broad range (n$=$1-7) of harmonics. The dynamics of EBW-heated electrons are measured using a spatial distribution of solid targets with diametrically opposed x-ray detectors. EBW heating produces a clear supra-thermal electron tail in MST. Radial deposition of the EBW is controlled with \textbar B\textbar and is measured using the HXR flux emitted from an insertable probe. In the thick-shelled MST RFP, the radial accessibility of EBW is limited to r/a \textgreater 0.8 (\textasciitilde 10cm) by magnetic field error induced by the porthole necessary for the antenna. Experimental measurements show EBW propagation inward through a stochastic magnetic field. EBW-heated test electrons are used as a direct probe of edge (r/a \textgreater 0.9) radial transport, showing a modest transition from `standard' to reduced-tearing RFP operation. Electron loss is too fast for collisional effects and implies a large non-collisional radial diffusivity. EBW heating has been demonstrated in reduced magnetic stochasticity plasmas with $\beta =$15-20{\%}. Work supported by USDOE. [Preview Abstract] |
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JP11.00117: Advanced control for inductive programming of MST plasmas I. R. Goumiri, K. J. McCollam, A. Squitieri, D. J. Holly, J. S. sarff, C. M. Jacobson MST is a reversed field pinch whose poloidal and toroidal magnetic fields (Bp and Bt) can be sourced by IGBT-based programmable power supplies. In order to provide real-time simultaneous control of both Bp and Bt circuits, a time-dependent integrated modeling code is developed. Relaxed-state RFP physics simulations provide prediction and interpretive analysis of MST experimental data. The actuators considered for the control are the Bp and Bt primary currents. However, the physical quantities which MST operators want to demand can vary for different experiments and can have complicated dependences on the two actuator quantities as well as time. To develop our advanced control system, we choose to focus on two demand quantities, the plasma current Ip, directly related to Bp, and the reversal parameter F, closely related to Bt. To understand the response of Ip and F to the actuators and to enable systematic design of control algorithms, a linearized dynamic response model is generated using a system identification method. A multi-variable model based control scheme that accounts for the coupled dynamics of the system while mitigating the effect of actuator limitations is designed. A series of experiments are planned to test our controllers and validate our modeling. [Preview Abstract] |
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JP11.00118: Ohmic ignition with high engineering beta based on the RFP J.S. Sarff, J.K. Anderson, B.E. Chapman, K.J. McCollam The RFP configuration allows the possibility of ohmic ignition for fusion energy, eliminating the need for auxiliary heating~by rf or neutral beam injection. Complex plasma-facing antennas and NBI sources are therefore not required, simplifying the~difficult fusion materials challenge. While all toroidal configurations require a volume-average $\langle B\rangle \ge 5$T, the field strength~at the magnet in the RFP is only B$_{\mathrm{coil}}\approx $3T since plasma current generates almost all of the field. Engineering beta is~therefore maximized. We summarize access to ohmic ignition by examining a Lawson-like power balance for an RFP fusion plasma~comparable to the ARIES-AT advanced tokamak, which generates neutron wall loading $P_{n} /A\approx 5$MW/m$^{\mathrm{2}}$. The required energy confinement~for ohmic ignition in an RFP is similar to that for a tokamak. Confinement in MST is comparable to a same-size, same-field~tokamak plasma, but $\langle B\rangle $ in MST is only 1/20th that required for fusion. While transport could ultimately be dominated by micro~turbulence, extrapolation of stochastic transport using Lundquist number scaling for MHD tearing indicates standard RFP~confinement (not enhanced by current profile control) could be sufficient to access ohmic ignition. This bolsters the~possibility for steady-state inductive sustainment using oscillating field current drive. The high beta and classical energetic ion confinement measured in MST also bolster the RFP's fusion potential. Work supported by U.S. DoE. [Preview Abstract] |
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JP11.00119: Density-Gradient-Driven trapped-electron-modes in improved-confinement RFP plasmas James Duff, John Sarff, Weixing Ding, David Brower, Eli Parke, Brett Chapman, Paul Terry, M.J. Pueschel, Zach Williams Short wavelength density fluctuations in improved-confinement MST plasmas exhibit multiple features characteristic of the trapped-electron-mode (TEM). Core transport in the RFP is normally governed by magnetic stochasticity stemming from long wavelength tearing modes that arise from current profile peaking, which are suppressed via inductive control for this work. The improved confinement is associated with an increase in the pressure gradient that can destabilize drift waves. The measured density fluctuations have $f\sim50$ kHz, $k_\phi~\rho_s<0.14$, and propagate in the electron drift direction. Their spectral emergence coincides with a sharp decrease in global tearing mode associated fluctuations, their amplitude increases with local density gradient, and they exhibit a density-gradient threshold at $R/L_n\sim15$. The GENE code, modified for the RFP, predicts the onset of density-gradient-driven TEM for these strong-gradient plasma conditions. While nonlinear analysis shows a large Dimits shift associated with predicted strong zonal flows, the inclusion of residual magnetic fluctuations, comparable to experimental magnetic fluctuations, causes a collapse of the zonal flows and an increase in the predicted transport to a level close to the experimentally measured heat flux. [Preview Abstract] |
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JP11.00120: Soft X-ray studies on MST: Measuring the effects of toroidicity on tearing mode phase and installation of a multi-energy camera Patrick VanMeter, Lisa Reusch, Paolo Franz, John Sarff, John Goetz, Louis Delgado-Aparicio, Daniel Den Hartog The soft X-ray tomography (SXT) system on MST uses four cameras in a double-filter configuration to measure the emitted brightness along forty distinct lines of sight. These measurements can then be inverted to determine the emissivity, which depends on physical properties such as temperature, density, and impurity content. The SXR emissivity should correspond to the structure of the magnetic field; however, there is a discrepancy between the phase of the emissivity inversions and magnetic field reconstructions when using the typical cylindrical approximation to interpret the signal from the toroidal magnetics array. This discrepancy was measured for two distinct plasma conditions using all four SXT cameras, with results supporting the interpretation that it emerges from physical effects of the toroidal geometry. In addition, a new soft x-ray measurement system based on the PILATUS3 photon counting detector will be installed on MST. Emitted photons are counted by an array of pixels with individually adjustable energy cutoffs giving the device more spectral information than the double-filter system. [Preview Abstract] |
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JP11.00121: Improvements to the MST Thomson Scattering Diagnostic D.T. Adams, M.T. Borchardt, D.J. Den Hartog, D.J. Holly, T. Kile, S.Z. Kubala, C.M. Jacobson, M.A. Thomas, J.P. Wallace, W.C. Young Multiple upgrades to the MST Thomson Scattering diagnostic have been implemented to expand capabilities of the system. In the past, stray laser light prevented electron density measurements everywhere and temperature measurements for -z/a \textgreater 0.75. To mitigate stray light, a new laser beamline is being commissioned that includes a longer entrance flight tube, close-fitting apertures, and baffles. A polarizer has been added to the collection optics to further reduce stray light. An absolute density calibration using Rayleigh scattering in argon will be performed. An insertable integrating sphere will provide a full-system spectral calibration as well as maps optical fibers to machine coordinates. Reduced transmission of the collection optics due to coatings from plasma-surface interactions is regularly monitored to inform timely replacements of the first lens. Long-wavelength filters have been installed to better characterize non-Maxwellian electron distribution features. Previous work has identified residual photons not described by a Maxwellian distribution during m$=$0 magnetic bursts. Further effort to characterize the distribution function will be described. [Preview Abstract] |
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JP11.00122: Absolute brightness modeling for improved measurement of electron temperature from soft x-rays on MST L. M. Reusch, P. Franz, J. A. Goetz, D. J. Den Hartog, M. D. Nornberg, P. Van Meter The two-color soft x-ray tomography (SXT) diagnostic on MST is now capable of Te measurement down to ~500 eV. The previous lower limit was ~1 keV, due to the presence of SXR emission lines from Al sputtered from the MST wall. The two-color technique uses two filters of different thickness to form a coarse spectrometer to estimate the slope of the continuum x-ray spectrum, which depends on Te. The 1.6 - 2.0 keV Al emission lines were previously filtered out by using thick Be filters (~400µm and ~800µm), thus restricting the range of the SXT diagnostic to Te ≥ 1 keV. Absolute brightness modeling explicitly includes several sources of radiation in the analysis model, enabling the use of thinner filters and measurement of much lower Te. Models based on the atomic database and analysis structure (ADAS) agree very well with our experimental SXR measurements. We used ADAS to assess the effect of bremsstrahlung, recombination, dielectronic recombination, and line emission on the inferred Te. This assessment informed the choice of the optimum filter pair to extend the Te range of the SXT diagnostic. [Preview Abstract] |
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JP11.00123: An Integrated Data Analysis model to determine ion effective charge from beam attenuation and charge exchange emission measurements M.D. Nornberg, D.J. Den Hartog, L.M. Reusch We have created a forward model for charge-exchange impurity density measurements that incorporates neutral beam attenuation measurements self-consistently for determining the ion effective charge $Z_{\rm eff}$ in MST PPCD plasmas. Detailed knowledge of $Z_{\rm eff}$ is critical to determining the resistive dissipation of hot plasmas and requires knowledge of the impurity content and dynamics. Previously, $Z_{\rm eff}$ profiles were determined from soft-x-ray brightness measurements by using charge-exchange impurity density measurements as prior information using an Integrated Data Analysis (IDA) method. The model is extended to include a self-consistent calculation of the neutral beam attenuation and includes measurements of the beam Doppler-shift spectrum and shine-through particle flux. Methods of experimental design are employed to calculate the information gained from different diagnostic combinations. The analysis shows that while attenuation measurements alone do not provide a unique impurity density measurement in the case of a multi-species inhomogeneous plasmas, they do provide a valuable measurement of the $Z_{\rm eff}$ profile and constrain the range of contributing impurity densities. [Preview Abstract] |
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JP11.00124: Development of Poloidal Magnetic Flux Measurements and Ion Beam Velocity Detectors P.J. Fimognari, T.P. Crowley, D.R. Demers, T.D. Kile Local, non-perturbative measurements of the current density profile and magnetic fluctuations in fusion plasmas will provide information capable of advancing equilibrium, transport, and stability studies. We are developing beam-based measurement techniques and a detector to determine the poloidal flux function, poloidal magnetic field, current density profile, and poloidal flux fluctuations in axisymmetric toroidal plasmas. We have injected a K$^{\mathrm{+}}$ beam into the MST reversed field pinch plasma and successfully measured the intensity and toroidal velocity of K$^{\mathrm{2+}}$ ions created at a sample location along its trajectory. Because angular momentum of ions is conserved in an axisymmetric system, the toroidal velocity of the K$^{\mathrm{+\thinspace }}$and K$^{\mathrm{2+}}$ beams can be used to determine the poloidal flux function $\psi \quad =$ RA$_{\mathrm{\varphi }}$ at the sample location. We have begun developing simulation tools needed to unfold the poloidal flux from the beam velocity measurement and confirm accuracy of this diagnostic technique. Variations with time and initial injection angle of measured signals are consistent with simulations of the experiment. The detectors are operated with a direct view of the plasma and therefore subject to much higher photon-induced and plasma particle noise currents than a traditional heavy ion beam probe (HIBP) detector. The detector has features that reduce this noise and facilitate the broader goal of developing diagnostic tools that have HIBP attributes, but use a lower energy beam and a much smaller detector. This work is supported by US DoE award DE-SC0006077. [Preview Abstract] |
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JP11.00125: Effects of oscillating poloidal current drive on magnetic relaxation in the MST reversed field pinch. Zichao Li, K. J. McCollam, John S. Sarff, Hong Li, Wandong Liu, Weixing Ding Magnetic relaxation behavior in the reversed field pinch (RFP) is modified by oscillating poloidal current drive (OPCD) in the Madison Symmetric Torus (MST). OPCD is the application of a sinusoidal poloidal inductive loop voltage. OPCD amplitude and frequency are varied to investigate the entrainment of the RFP sawtooth cycle. In the standard RFP without OPCD, the sawtooth magnetic relaxation cycle is quasi-periodic, but it can be entrained by the OPCD cycle to be strictly periodic. With increasing OPCD amplitude at the same frequency, the number of entrained sawtooth crashes increases gradually; these crashes are bursts of magnetic tearing modes that nonlinearly interact to relax the current profile. The direction of cyclic trajectories in a 2D (Theta, F) space of RFP equilibrium parameters changes from clockwise in the standard case to counterclockwise with OPCD. At low frequency (\textless 200Hz), the OPCD shows a clear PPCD-like effect --- no sawteeth occur during the decreasing phase of the edge toroidal magnetic field. At high frequency(\textgreater 500Hz), the (Theta, F) trajectory seems to show a bifurcation effect not present at lower frequencies. We hypothesize that the OPCD modifies the behavior of the core tearing modes by affecting their linear stability as it moves the plasma equilibrium in its (Theta, F) trajectory. Future work will focus on determining the current profiles and linear stability analysis. This work is supported by the US DOE and the CSC. [Preview Abstract] |
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JP11.00126: Comparing magnetic fluctuation dynamics in nonlinear MHD simulations of low-aspect-ratio RFPs to RELAX experiments K.J. McCollam, D.J. Den Hartog, C.M. Jacobson, C.R. Sovinec, S. Masamune, A. Sanpei We present comparisons of magnetic tearing fluctuation activity between RFP experiments on the low-aspect-ratio RELAX device ($R/a\approx2$) and nonlinear simulations of zero-beta, single-fluid MHD using the NIMROD code in both cylindrical and toroidal geometries at a Lundquist number of $S=10^4$, nearly as high as experimental values. Time-average fluctuation amplitudes observed in the simulations are similar to those from the experiments, but more rigorous comparisons versus spectral mode numbers are in progress. We also focus on how the spatiotemporal dynamics of the fluctuations vary with RFP equilibrium parameters. Interestingly, at shallow reversal, cylindrical simulations show a relatively uncoupled spectrum of nearly quiescent modes periodically varying in time, whereas the corresponding toroidal cases show a fully chaotic spectrum of strongly nonlinearly interacting modes. We ascribe this to the geometric $m=1$ coupling present in the toroidal but not the cylindrical case. We present initial results from convergence studies with increased spatial resolution for both geometries. Simulations at higher $S$ are planned. This work is supported by the U.S.\ DOE and by the Japan Society for the Promotion of Science. [Preview Abstract] |
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JP11.00127: THEORETICAL METHODS |
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JP11.00128: Eulearian approach to bounce-transit and drift resonance with magnetic drifts in tokamaks Ker Chung Shaing, J. Seol, M. S. Chu, S. A. Sabbagh Bounce-transit and drift resonance can be important to plasma confinement in tokamaks with broken symmetry, and can have implications on the wave-particle resonance. Usually, the resonance is either treated by integrating along the unperturbed orbits or calculated using an action-angle approach. An Eulerian approach has been developed so that momentum conservation property of the Coulomb collision operator can be taken into account. The parallel flows appear in the thermodynamic forces in the Eulerian approach. However, in the existing theory, only \textbf{\textit{E }}x \textbf{\textit{B}} drift is kept; the magnetic drifts are neglected by adopting the large aspect ratio assumption. Here, \textbf{\textit{E}} is the electric field, and \textbf{\textit{B}} is the magnetic field. The importance of the magnetic drifts in finite aspect ratio tokamaks is demonstrated in [C. G. Albert, et al., Phys. Plasmas \textbf{23}, 082515 (2016)]. Here, the Eulerian approach is extended to include the magnetic drifts to calculate neoclassical toroidal plasma viscosity in finite aspect ratio tokamaks. The relation to the nonlinear plasma viscosity in the plateau regime [K. C. Shaing, K. Ida, and S. A. Sabbagh, Nucl. Fusion \textbf{55}, 125001 (2015) ] will also be discussed. [Preview Abstract] |
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JP11.00129: A multigrid method for drift-kinetic calculations in stellarators and rippled tokamaks Matt Landreman, H{\aa}kan Smith Important phenomena such as the bootstrap current and collisional transport in stellarators, and neoclassical toroidal viscosity in tokamaks, must be computed by numerical solution of the drift-kinetic equation in nonaxisymmetric geometry. This equation has the form of a linear, inhomogeneous, advection-dominated advection-diffusion PDE with recirculating flow, internal boundary layers, and a null space, with typically five coupled dimensions (poloidal and toroidal angle, speed, velocity pitch angle, and species.) While multigrid algorithms are a preferred method for efficient solution of some PDEs, multigrid smoothers are typically unstable for accurate discretizations of the drift-kinetic equation due to the absence of any physical diffusion in the spatial dimensions, and the dominance of advection over diffusion in the velocity dimensions. In this work we demonstrate a high-order-accurate multigrid solution of the drift-kinetic equation in nonaxisymmetric geometry. A defect correction approach is used: solution of a high-order discretized problem is preconditioned by a multigrid cycle in which a low-order upwinded discretization is used for smoothing. Compared to a direct solver, the multigrid method can reduce the memory requirement by several orders of magnitude. [Preview Abstract] |
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JP11.00130: A New Parallel Boundary Condition for Turbulence Simulations in Stellarators Mike F Martin, Matt Landreman, William Dorland, Pavlos Xanthopoulos For gyrokinetic simulations of core turbulence, the ``twist-and-shift'' parallel boundary condition (Beer et al, PoP, 1995), which involves a shift in radial wavenumber proportional to the global shear and a quantization of the simulation domain's aspect ratio, is the standard choice. But as this condition was derived under the assumption of axisymmetry, ``twist-and-shift'' as it stands is formally incorrect for turbulence simulations in stellarators. Moreover, for low-shear stellarators like W7X and HSX, the use of a global shear in the traditional boundary condition places an inflexible constraint on the aspect ratio of the domain, requiring more grid points to fully resolve its extent. Here, we present a parallel boundary condition for ``stellarator-symmetric'' simulations that relies on the \textit{local shear} along a field line. This boundary condition is similar to ``twist-and-shift'', but has an added flexibility in choosing the parallel length of the domain based on local shear consideration in order to optimize certain parameters such as the aspect ratio of the simulation domain. [Preview Abstract] |
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JP11.00131: A conservative, relativistic Fokker-Planck solver for runaway electrons Luis Chacon, W. Taitano, X. Tang, Z. Guo, C. McDevitt Relativistic runaway electrons develop when electric fields surpass a critical electric field,% \footnote{J. W. Connor and R. J. Hastie, \emph{Nuc. Fusion}, \textbf{15} (1975)% } $E_{c}=E_{D}\left(\frac{v_{th}}{c}\right)^{2}$, with $E_{D}$ the Dreicer field (which is the electric field at which the whole thermal electron population runs away). Above this critical field, electron tails accelerate relativistically until they are arrested by radiative processes.% \footnote{Z. Guo et al., Plasma Phys. Control Fusion, \textbf{59} (2017)% } In regimes above this critical electric field (but below the Dreicer field), correctly capturing the interplay between the electron thermal population and the runaway tail is key, and demands a full nonlinear relativistic Fokker-Planck treatment. In this presentation, we report on progress towards a fully conservative, implicit, adaptive implementation of the relativistic electron Fokker-Planck equation. Strict conservation properties, as well as positivity preservation, are a must to avoid spurious numerical effects, and to be able to capture tenuous electron runaway tails for fields just above $E_{c}$. [Preview Abstract] |
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JP11.00132: A backward Monte Carlo method for efficient computation of runaway probabilities in runaway electron simulation Guannan Zhang, Diego Del-Castillo-Negrete Kinetic descriptions of RE are usually based on the bounced-averaged Fokker-Planck model that determines the PDFs of RE. Despite of the simplification involved, the Fokker-Planck equation can rarely be solved analytically and direct numerical approaches (e.g., continuum and particle-based Monte Carlo (MC)) can be time consuming specially in the computation of asymptotic-type observable including the runaway probability, the slowing-down and runaway mean times, and the energy limit probability. Here we present a novel backward MC approach to these problems based on backward stochastic differential equations (BSDEs). The BSDE model can simultaneously describe the PDF of RE and the runaway probabilities by means of the well-known Feynman-Kac theory. The key ingredient of the backward MC algorithm is to place all the particles in a runaway state and simulate them backward from the terminal time to the initial time. As such, our approach can provide much faster convergence than the brute-force MC methods, which can significantly reduce the number of particles required to achieve a prescribed accuracy. Moreover, our algorithm can be parallelized as easy as the direct MC code, which paves the way for conducting large-scale RE simulation. [Preview Abstract] |
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JP11.00133: Application of electron closures in extended MHD Eric Held, Brett Adair, Trevor Taylor Rigorous closure of the extended MHD equations in plasma fluid codes includes the effects of electron heat conduction along perturbed magnetic fields and contributions of the electron collisional friction and stress to the extended Ohms law. In this work we discuss application of a continuum numerical solution to the Chapman-Enskog-like electron drift kinetic equation \footnote{J. J. Ramos, Phys Plasmas 17, 082502 (2010).} using the NIMROD code. The implementation is a tightly-coupled fluid/kinetic system that carefully addresses time-centering in the advance of the fluid variables with their kinetically-computed closures. Comparisons of spatial accuracy, computational efficiency and required velocity space resolution are presented for applications involving growing magnetic islands in cylindrical and toroidal geometry. The reduction in parallel heat conduction due to particle trapping in toroidal geometry is emphasized. [Preview Abstract] |
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JP11.00134: NIMROD modeling of poloidal flow damping in tokamaks using kinetic closures J. R. Jepson, C. C. Hegna, E. D. Held Calculations of poloidal flow damping in a tokamak are undertaken using two different implementations of the ion drift kinetic equation (DKE) in the extended MHD code NIMROD. The first approach is hybrid fluid/kinetic and uses a Chapman Enskog-like (CEL) Ansatz. Closure of the evolving lower-order fluid moment equations for $n$, $\bf {V}$, and $T$ is provided by solutions to the ion CEL-DKE written in the macroscopic flow reference frame [1]. The second implementation solves the DKE using a delta-f approach. Here, the delta-f distribution describes all of the information beyond a static, lowest-order Maxwellian. We compare the efficiency and accuracy of these two approaches for a simple initial value problem that monitors the relaxation of the poloidal flow profile in high- and low-aspect-ratio tokamak geometry. The computation results are compared against analytic predictions of time dependent closures for the parallel viscous force [2,3]. [1] J. J. Ramos, Phys. Plasmas \textbf {18}, 102506 (2011). [2] A. L. Garcia-Perciante et al, Phys. Plasmas \textbf {12}, 052516 (2005). [3] R. C. Morris et al, Phys. Plasmas \textbf {3}, 4513-4520 (1996). [Preview Abstract] |
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JP11.00135: Parallel closure theory for toroidally confined plasmas Jeong-Young Ji, Eric D. Held We solve a system of general moment equations\footnote{J.-Y. Ji and E. D. Held, Phys. Plasmas 15, 102101 (2008).} to obtain parallel closures for electrons and ions in an axisymmetric toroidal magnetic field. Magnetic field gradient terms are kept and treated using the Fourier series method. Assuming lowest order density (pressure) and temperature to be flux labels, the parallel heat flow, friction, and viscosity are expressed in terms of radial gradients of the lowest-order temperature and pressure, parallel gradients of temperature and parallel flow, and the relative electron-ion parallel flow velocity. Convergence of closure quantities is demonstrated as the number of moments and Fourier modes are increased. Properties of the moment equations in the collisionless limit are also discussed. Combining closures with fluid equations parallel mass flow and electric current are also obtained. [Preview Abstract] |
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JP11.00136: Abstract Withdrawn
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JP11.00137: Confinement properties of tokamak plasmas with extended regions of low magnetic shear J. P. Graves, W. A. Cooper, A. Kleiner, M. Raghunathan, E. Neto, T. Nicolas, S Lanthaler, H. Patten, D. Pfefferle, D. Brunetti, H. Lutjens Extended regions of low magnetic shear can be advantageous to tokamak plasmas. But the core and edge can be susceptible to non-resonant ideal fluctuations due to the weakened restoring force associated with magnetic field line bending. This contribution shows how saturated non-linear phenomenology, such as $1/1$ Long Lived Modes, and Edge Harmonic Oscillations associated with QH-modes, can be modelled accurately using the non-linear stability code XTOR, the free boundary 3D equilibrium code VMEC, and non-linear analytic theory. That the equilibrium approach is valid is particularly valuable because it enables advanced particle confinement studies to be undertaken in the ordinarily difficult environment of strongly 3D magnetic fields. The VENUS-LEVIS code exploits the Fourier description of the VMEC equilibrium fields, such that full Lorenzian and guiding centre approximated differential operators in curvilinear angular coordinates can be evaluated analytically. Consequently, the confinement properties of minority ions such as energetic particles and high $Z$ impurities can be calculated accurately over slowing down timescales in experimentally relevant 3D plasmas. [Preview Abstract] |
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JP11.00138: Investigation of flow-induced numerical instability in a mixed semi-implicit, implicit leapfrog time discretization Jacob King, Scott Kruger Flow can impact the stability and nonlinear evolution of range of instabilities (e.g. RWMs, NTMs, sawteeth, locked modes, PBMs, and high-k turbulence) and thus robust numerical algorithms for simulations with flow are essential. Recent simulations of DIII-D QH-mode [King et al., Phys. Plasmas and Nucl. Fus. 2017] with flow have been restricted to smaller time-step sizes than corresponding computations without flow. These computations use a mixed semi-implicit, implicit leapfrog time discretization as implemented in the NIMROD code [Sovinec et al., JCP 2004]. While prior analysis has shown that this algorithm is unconditionally stable with respect to the effect of large flows on the MHD waves in slab geometry [Sovinec et al., JCP 2010], our present Von Neumann stability analysis shows that a flow-induced numerical instability may arise when ad-hoc cylindrical curvature is included. Computations with the NIMROD code in cylindrical geometry with rigid rotation and without free-energy drive from current or pressure gradients qualitatively confirm this analysis. We explore potential methods to circumvent this flow-induced numerical instability such as using a semi-Lagrangian formulation instead of time-centered implicit advection and/or modification to the semi-implicit operator. [Preview Abstract] |
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JP11.00139: Non-symmetric steady ideal magnetohydrodynamic flows in a non-symmetric topological torus Harold Weitzner, Wrick Sengupta Previous work, Phys. Plasmas 21,022515 (2014) on expansions of ideal magnetohydrodynamic equilibria in a topological torus is extended and modified to allow the addition of steady flows of leading order in the formal expansion parameter. The leading order flow and magnetic field depend on one coordinate direction only and have components in the other two coordinate directions. It is shown that an expansion can be carried out to all orders in a parameter which measures the amplitude of the ``helical'' flows and fields. Resonances appear, but can be resolved exactly as in the previous work by the addition of appropriate lower order flows and fields. The resonance conditions involve two different linear combination of the lowest order flows and fields. As in the earlier work the elimination of the resonances requires that the boundaries of the domain of the steady flow be chosen correctly. Although the the expansion may be carried to all orders, convergence is not proven. [Preview Abstract] |
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JP11.00140: Numerical optimization of the ramp-down phase with the RAPTOR code Anna Teplukhina, Olivier Sauter, Federico Felici The ramp-down optimization goal in this work is defined as the fastest possible decrease of a plasma current while avoiding any disruptions caused by reaching physical or technical limits. Numerical simulations and preliminary experiments on TCV and AUG have shown that a fast decrease of plasma elongation and an adequate timing of the H-L transition during current ramp-down can help to avoid reaching high values of the plasma internal inductance. The RAPTOR code (F. Felici et al, 2012 PPCF 54; F. Felici, 2011 EPFL PhD thesis), developed for real-time plasma control, has been used for an optimization problem solving. Recently the transport model has been extended to include the ion temperature and electron density transport equations in addition to the electron temperature and current density transport equations, increasing the physical applications of the code. The gradient-based models for the transport coefficients (O. Sauter et al, 2014 PPCF 21; D. Kim et al, 2016 PPCF 58) have been implemented to RAPTOR and tested during this work. Simulations of the AUG and TCV entire plasma discharges will be presented. [Preview Abstract] |
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JP11.00141: Validation and Continued Development of Methods for Spheromak Simulation Thomas Benedett The HIT-SI experiment has demonstrated stable sustainment of spheromaks. Determining how the underlying physics extrapolate to larger, higher-temperature regimes is of prime importance in determining the viability of the inductively-driven spheromak. It is thus prudent to develop and validate a computational model that can be used to study current results and study the effect of possible design choices on plasma behavior. An extended MHD model has shown good agreement with experimental data at 14 kHz injector operation. Efforts to extend the existing validation to a range of higher frequencies (36, 53, 68 kHz) using the PSI-Tet 3D extended MHD code will be presented, along with simulations of potential combinations of flux conserver features and helicity injector configurations and their impact on current drive performance, density control, and temperature for future SIHI experiments. Work supported by USDoE [Preview Abstract] |
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JP11.00142: Modeling MHD Equilibrium and Dynamics with Non-Axisymmetric Resistive Walls in LTX and HBT-EP C. Hansen, J. Levesque, D. P. Boyle, P. Hughes In experimental magnetized plasmas, currents in the first wall, vacuum vessel, and other conducting structures can have a strong influence on plasma shape and dynamics. These effects are complicated by the 3D nature of these structures, which dictate available current paths. Results from simulations to study the effect of external currents on plasmas in two different experiments will be presented: 1) The arbitrary geometry, 3D extended MHD code PSI-Tet is applied to study linear and non-linear plasma dynamics in the High Beta Tokamak (HBT-EP) focusing on toroidal asymmetries in the adjustable conducting wall. 2) Equilibrium reconstructions of the Lithium Tokamak eXperiment (LTX) in the presence of non-axisymmetric eddy currents. An axisymmetric model is used to reconstruct the plasma equilibrium, using the PSI-Tri code, along with a set of fixed 3D eddy current distributions in the first wall and vacuum vessel [C. Hansen et al. PoP Apr. 2017]. Simulations of detailed experimental geometries are enabled by use of the PSI-Tet code, which employs a high order finite element method on unstructured tetrahedral grids that are generated directly from CAD models. Further development of PSI-Tet and PSI-Tri will also be presented. [Preview Abstract] |
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JP11.00143: Computational optimization of global gyrokinetic particle code GTS. Aditya Krishna Swamy, Stephane Ethier, Weixing Wang, Edward Startsev, Wei-li Lee, Rajaraman Ganesh Electromagnetic microturbulence is an important source of anomalous ion and electron transport in tokamak plasmas. Gyrokinetic Tokamak Simulation (GTS), a global PIC code presents a first-principles based method to understand and predict such transport. Recently, the double-split-weight scheme that avoids the high-beta ``cancelation problem'' has been developed and implemented in GTS to study electromagnetic turbulence. Use of magnetic coordinates and a field-line following grid in GTS provides a highly efficient means to resolve a relatively larger set of modes in the same run. The misalignment of the field-line following grid with cylindrical grid, however, makes Fourier-filtering of single mode highly inefficient, and therefore makes benchmarking of linear modes with other codes time consuming. Recent algorithmic optimizations to align this subroutine with the 2-d domain have resulted in a significant performance improvement of \textasciitilde 20x, with an overall code speedup of \textasciitilde 3x. These and further improvements to the filtering capability, along with linear benchmarks of electromagnetic instabilities such as MTM and KBM will be discussed. [Preview Abstract] |
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JP11.00144: Progress on Schwarz-type coupling of core- and edge-region tokamak simulations for whole device modeling Lee Ricketson, Ammar Hakim, Jeffrey Hittinger The edge and core regions of a tokamak differ drastically on many fronts -- geometry and collisionality, to name only two among many. It is thus natural that different numerical methods are optimally suited to the simulation of each region. However, this creates a challenge for the pursuit of whole device modeling (WDM): How can one self-consistently couple two distinct codes to achieve a uniformly accurate description of the entire tokamak? In support of the ECP goal of coupling the codes GENE (core) and XGC (edge) for whole device modeling, we present such a coupling scheme inspired by classical additive Schwarz methods. While traditional Schwarz schemes require iteration to convergence inside a single time step, this is computationally intractable in the context of 5-D gyrokinetic simulations. We give evidence -- both analytic and empirical -- that if one is interested only in long-time (e.g. transport-scale) averages, then this expensive iteration can be avoided while retaining the scheme's convergence properties. We present numerical results from tests on both a 1-D model problem and 2-D simulations of the Hasegawa-Wakatani equations. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
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JP11.00145: Bringing global gyrokinetic turbulence simulations to the transport timescale using a multiscale approach Jeffrey Parker, Lynda LoDestro, Daniel Told, Gabriele Merlo, Lee Ricketson, Alejandro Campos, Frank Jenko, Jeffrey Hittinger Predictive whole-device simulation models will play an increasingly important role in ensuring the success of fusion experiments and accelerating the development of fusion energy. In the core of tokamak plasmas, a separation of timescales between turbulence and transport makes a single direct simulation of both processes computationally expensive. We present the first demonstration of a multiple-timescale method coupling global gyrokinetic simulations with a transport solver to calculate the self-consistent, steady-state temperature profile. Initial results are highly encouraging, with the coupling method appearing robust to the difficult problem of turbulent fluctuations. The method holds potential for integrating first-principles turbulence simulations into whole-device models and advancing the understanding of global plasma behavior. [Preview Abstract] |
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JP11.00146: Development of Multi-Scale Temporal Coupling Methods for XGC Benjamin Sturdevant, Scott Parker, Robert Hager, Choong-Seock Chang, Julien Dominski, Seung-Hoe Ku The development of multi-scale time integration methods for XGC is reported, including a recent implementation of an equation-free algorithm for XGCa. In this work, XGCa is used to evolve a 4D distribution function over short time intervals. The distribution function is then restricted to a set of low-order fluid moments, which are projected over a large time step. Finally, the fluid moments are transformed back to a fine-scale 4D distribution function to restart the simulation at the next time step. By enabling the use of a time step size much larger than in standard PIC methods, this algorithm has promise for large computational savings in transport time scale simulations. For the tokamak edge, however, the use of low-order fluid moments is inadequate, since the distribution function can be far from Maxwellian. In this case, coupling between a fine-scale kinetic code and a coarse-scale kinetic code is of interest. A key tool for accurate kinetic-kinetic coupling is particle resampling. Recent methods for particle resampling have been developed which accurately preserve low-order velocity moments and local features of a kinetic distribution function [1]. We include plans to further develop this work for kinetic-kinetic coupling. [1] Faghihi, et al. (2017). arXiv:1702.05198 [Preview Abstract] |
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JP11.00147: Implementation of non-axisymmetric mesh system in the gyrokinetic PIC code (XGC) for Stellarators Toseo Moritaka, Robert Hager, Micheal Cole, Choong-Seock Chang, Samuel Lazerson, Seung-Hoe Ku, Seiji Ishiguro Gyrokinetic simulation is a powerful tool to investigate turbulent and neoclassical transports based on the first-principles of plasma kinetics. The gyrokinetic PIC code XGC has been developed for integrated simulations that cover the entire region of Tokamaks. Complicated field line and boundary structures should be taken into account to demonstrate edge plasma dynamics under the influence of X-point and vessel components. XGC employs gyrokinetic Poisson solver on unstructured triangle mesh to deal with this difficulty. We introduce numerical schemes newly developed for XGC simulation in non-axisymmetric Stellarator geometry. Triangle meshes in each poloidal plane are defined by PEST poloidal angle in the VMEC equilibrium so that they have the same regular structure in the straight field line coordinate. Electric charge of marker particle is distributed to the triangles specified by the field-following projection to the neighbor poloidal planes. 3D spline interpolation in a cylindrical mesh is also used to obtain equilibrium magnetic field at the particle position. These schemes capture the anisotropic plasma dynamics and resulting potential structure with high accuracy. The triangle meshes can smoothly connect to unstructured meshes in the edge region. We will present the validation test in the core region of Large Helical Device and discuss about future challenges toward edge simulations. [Preview Abstract] |
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JP11.00148: A symbiotic approach to fluid equations and non-linear flux-driven simulations of plasma dynamics Federico Halpern The fluid framework is ubiquitous in studies of plasma transport and stability. Typical forms of the fluid equations are motivated by analytical work dating several decades ago, before computer simulations were indispensable, and can be, therefore, not optimal for numerical computation. We demonstrate a new first-principles approach to obtaining manifestly consistent, skew-symmetric fluid models, ensuring internal consistency and conservation properties even in discrete form. Mass, kinetic, and internal energy become quadratic (and always positive) invariants of the system. The model lends itself to a robust, straightforward discretization scheme with inherent non-linear stability. A simpler, drift-ordered form of the equations is obtained, and first results of their numerical implementation as a binary framework for bulk-fluid global plasma simulations are demonstrated. [Preview Abstract] |
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JP11.00149: Introducing a distributed unstructured mesh into gyrokinetic particle-in-cell code, XGC Eisung Yoon, Mark Shephard, E.Seegyoung Seol, Kaushik Kalyanaraman XGC has shown good scalability for large leadership supercomputers. The current production version uses a copy of the entire unstructured finite element mesh on every MPI rank. Although an obvious scalability issue if the mesh sizes are to be dramatically increased, the current approach is also not optimal with respect to data locality of particles and mesh information. To address these issues we have initiated the development of a distributed mesh PIC method. This approach directly addresses the base scalability issue with respect to mesh size and, through the use of a mesh entity centric view of the particle mesh relationship, provides opportunities to address data locality needs of many core and GPU supported heterogeneous systems. The parallel mesh PIC capabilities are being built on the Parallel Unstructured Mesh Infrastructure (PUMI) [1]. The presentation will first overview the form of mesh distribution used and indicate the structures and functions used to support the mesh, the particles and their interaction. Attention will then focus on the node-level optimizations being carried out to ensure performant operation of all PIC operations on the distributed mesh. [1] D.A. Ibanez et al., ACM Transactions on Mathematical Software, 42(3), Article No. 17 (2016) [Preview Abstract] |
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JP11.00150: Resistive MHD Stability Analysis in Near Real-time Alexander Glasser, Egemen Kolemen We discuss the feasibility of a near real-time calculation of the tokamak $\Delta'$ matrix, which summarizes MHD stability to resistive modes, such as tearing and interchange modes. As the operational phase of ITER approaches, solutions for active feedback tokamak stability control are needed. It has been previously demonstrated that an ideal MHD stability analysis is achievable on a sub-$\mathcal{O}($1s$)$ timescale, as is required to control phenomena comparable with the MHD-evolution timescale of ITER. In the present work, we broaden this result to incorporate the effects of resistive MHD modes. Such modes satisfy ideal MHD equations in regions outside narrow resistive layers that form at singular surfaces. We demonstrate that the use of asymptotic expansions at the singular surfaces, as well as the application of state transition matrices, enable a fast, parallelized solution to the singular outer layer boundary value problem, and thereby rapidly compute $\Delta'$. [Preview Abstract] |
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JP11.00151: Diagnosis of Acceleration, Reconnection, Turbulence, and Heating Mikal T. Dufor, Andrew J. Jemiolo, Amy Keesee, Paul Cassak, Weichao Tu, Earl E. Scime The DARTH (Diagnosis of Acceleration, Reconnection, Turbulence, and Heating) experiment is an intermediate-scale, experimental facility designed to study magnetic reconnection at and below the kinetic scale of ions and electrons. The experiment will have non-perturbative diagnostics with high temporal and three-dimensional spatial resolution, giving it the capability to investigate kinetic-scale physics. Of specific scientific interest are particle acceleration, plasma heating, turbulence and energy dissipation during reconnection. Here we will describe the magnetic field system and the two plasma guns used to create flux ropes that then merge through magnetic reconnection. We will also describe the key diagnostic systems: laser induced fluorescence (LIF) for ion vdf measurements, a 300 GHz microwave scattering system for sub-mm wavelength fluctuation measurements and a Thomson scattering laser for electron vdf measurements. The vacuum chamber is designed to provide unparalleled access for these particle diagnostics. The scientific goals of DARTH are to examine particle acceleration and heating during, the role of three-dimensional instabilities during reconnection, how reconnection ceases, and the role of impurities and asymmetries in reconnection. [Preview Abstract] |
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JP11.00152: Error field locking of real frequency tearing modes A.J. Cole, J.M. Finn, D.P. Brennan It has been shown [1] that the Maxwell torque on the plasma in the presence of an applied error field is modified significantly for tearing modes having real frequencies near marginal stability. In this poster we derive the tearing mode dispersion relation with pressure gradient, field line curvature and parallel dynamics both with and without perpendicular resistivity in the resistive-inertial (RI) and visco-resistive regimes, neglecting the divergence of the E × B drift. We find the usual Glasser effect, which involves real frequencies, is retained in this simplified model in both regimes, and that the existence of tearing modes with complex frequencies is related to nearby electrostatic resistive interchange modes. The interchange modes themselves are found to move into the sound wave continuum (for the case with no perpendicular resistivity) as the sound speed is increased. Results are also presented for the case of parallel dynamics with perpendicular resistivity, to investigate the tearing mode behavior when the sound wave continuum is discretized into a finite set of modes on the stable side of the frequency space. References: [1] J. M. Finn, A. J. Cole, and D. P. Brennan, PoP (Letters) 22, 120701 (2015). [Preview Abstract] |
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JP11.00153: Experimental Demonstration of Magnetic Reconnection in a Laboratory Scale with Guide Field Hyunsue Kim, Jan Egedal, Joe Olsen, Douglass Endrizzi, John Wallace Through a process called magnetic reconnection, the opposing solar wind and Earth magnetic fields annihilate and allow energetic solar particles to enter the magnetosphere. This energetic plasma can cause major disturbances to satellite communication networks and navigation systems, as well as electrical power grids. To better understand this process and prevent significant economic losses, NASA has launched the MMS Mission in 2015, a cluster of spacecraft which directly probes the reconnection sites in the magnetosphere. Though \textit{in situ} measurements of reconnection in space are essential to our understanding of the process, the mission comes at a cost of over {\$}1 billion. Thus, smaller laboratory experiments become essential to compliment the data acquired by MMS at relatively low cost. The Terrestrial Reconnection Experiment (TREX) currently aims to probe a similar configuration to dayside reconnection by adding a toroidal guide magnetic field, where under the right conditions, high frequency turbulent fluctuations are expected. Using a set of fast Langmuir probes to diagnose the fluctuations, the global structure of the plasma turbulence can be reconstructed. In this poster, an overview of the upgraded experiment and design progress of the fast Lprobe will be provided. [Preview Abstract] |
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