Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session RP1: Poster Session VIII |
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Room: Adam's Mark Hotel Grand Ballroom I & II 2:00pm |
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RP1.00001: NSTX, OTHER SPHERICAL TOKAMAKS, AND MAGNETIC CONFINEMENT |
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RP1.00002: National Spherical Torus Experiment (NSTX) Facility/Diagnostic Overview M. Ono The capabilities of the NSTX experimental facility and diagnostics continue to improve. The new TF joints are performing well at 4.5~kG. New in-board shaping coils were installed to produce plasmas with simultaneously high elongation $\sim 2.5$ and high triangularity $\sim 0.8$ needed for advanced operation. The EFC/RWM system with six external coils driven by three switching power amplifiers (1~kHz, 6~kA-turn) is now fully operational. With these new tools, we significantly expanded the NSTX operating parameters, achieving the highest controlled elongation of 2.75, a shape factor $q_{95}I_{p}/aB_{T}$ of 37~MA/m-T, plasma volume of 14~m$^{3}$, stored energy of 430~kJ, normalized beta of 7.4~\% MA/m-T, bootstrap current fraction of 60 \% at 700~kA, and longest plasma pulse length of 1.5~s or about 4 times the resistive skin time. In the area of the plasma diagnostics, ten additional Thomson scattering channels are providing detailed measurement of the H-mode pedestal and internal barrier regions. The 8 channel MSE diagnostic is providing crucial $j(r)$ measurements including high electron confinement reversed shear plasmas. A tangential microwave scattering system to measure electron-transport- relevant fluctuations is being commissioned. [Preview Abstract] |
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RP1.00003: Analysis of pellet induced perturbations in NSTX using soft X-ray techniques D. Stutman, K. Tritz, L. Delgado-Aparicio, M. Finkenthal, R. Bell, R. Kaita, S. Kaye, H. Kugel, B. LeBlanc, L. Roquemore, E. Synakowski, F. Levinton, S. Sabbagh A `multi-color' soft X-ray technique is being developed for perturbative transport studies on NSTX. The plasma is simultaneously viewed by soft X-ray arrays in different energy bands and modeling of the emission profiles used to propagate on fast time scale ($<$0.1 ms) the T$_{e}$ profile measured by laser scattering. Applied to Type-I ELM perturbations the technique shows good accuracy over tens of ms. The perturbed T$_{e}$ profile indicates fast `cold pulse' propagation in the outer plasma, with a marked slow down towards the axis. The incremental electron heat diffusivity has radial dependence quite opposite to that obtained from the power balance. The technique is further applied to perturbations produced by Li pellet injection. The pellet induced emission is recorded in three spectral ranges, with the range $>$0.1 keV providing an image of the pellet penetration, estimated to arise from C lines excited by charge exchange with Li neutrals. The higher energy profiles are used to estimate the T$_{e }$and the electron times impurity density perturbation. The capabilities of this technique and initial results from pellet perturbative experiments will be discussed. Work supported by US DoE grant DE-FG02-99ER5452 at JHU [Preview Abstract] |
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RP1.00004: L-H Transition and Turbulence Behavior in Ohmic H-modes on NSTX C.E. Bush, S. Kubota, R. Maqueda, T. Biewer, J. Boedo, K.C. Lee, S.J. Zweben, R. Raman, R. Bell, M. Bitter, E. Fredrickson, D. Gates, R. Kaita, S. Kaye, H. Kugel, B. LeBlanc, R. Maingi, S. Medley, J. Menard, L. Roquemore, V. Soukhanovskii, E. Synakowski, K. Tritz, K.M. Williams The L-H transition and H-mode behavior and turbulence have been studied in NSTX ohmically heated (OH) plasmas, which offer the advantage of an absence of the NBI sources of particles and momentum. The OH H-modes have density profiles that are peaked in the core, rather than the edge as in NBI-heated H-modes, making the core accessible for turbulence studies using correlation reflectometry. A decrease by more than a factor 2 in the core correlation length across the L-H transition was observed using correlation reflectometry. Concurrently gas-puff imaging (GPI) shows the edge becoming very quiescent as edge turbulent “blob” activity subsides. No consistent precursor MHD activity to the L-H transition has been observed for OH H-modes in NSTX. However, an edge rotation diagnostic showed the edge electric field becoming more negative up to 20 ms before the transition in several OH H-mode plasmas. Observations of Te, Ti, and Vf before and after the transition and from other diagnostics will be presented in discussions of the physics of NSTX OH H-modes. [Preview Abstract] |
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RP1.00005: Measurement and Analysis of Core Turbulence in NSTX S. Kubota, W.A. Peebles, N.A. Crocker, X.V. Nguyen, D.R. Mikkelsen, R.E. Bell, S.M. Kaye, B.P. LeBlanc, G.J. Kramer, E.J. Valeo Measurements of core turbulence using a homodyne radial correlation reflectometer (26-40 GHZ) and quadrature reflectometers (30, 42, 49 GHz) have been made in NSTX discharges (Ohmic, NB and RF heated L-modes, and Ohmic H-modes) which have peaked low density profiles for good core access. % Previous measurements in NB-heated L-mode discharges indicated radial correlation lengths ($L_{\mathrm{cr}}$) increasing from $\sim$2 to 10-15 cm over a radius from $\rho$$\sim$0.7 to 0.4. This range of values is typical for most L-mode discharges observed. However for Ohmic H-mode discharges, a sudden decrease in $L_{\mathrm{cr}}$ in the core plasma is seen at the L-H transition. % Changes in fluctuation levels and spectra will also be compared with confinement properties determined via TRANSP. % Analysis of the reflectometer data will be aided by the use of a fast 2-D full-wave code [E.J.\ Valeo, G.J.\ Kramer, R.\ Nazikian, Plasma Phys.Control.\ Fusion 44, L1 (2002)]. % to map the reflectometer response to %turbulence inputs. % The long-term goal is a direct comparison between experimental results and turbulence predictions using the nonlinear gyrokinetic simulation code GYRO. [Preview Abstract] |
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RP1.00006: Study of NSTX electron density and magnetic field fluctuations using the FIReTIP system K.C. Lee, C.W. Domier, M. Johnson, N.C. Luhmann, Jr., H. Park, E.D. Fredrickson, S.A. Sabbagh, S.J. Zweben The Far Infra Red Tangential Interferometry and Polarimetry (FIReTIP) system on NSTX is capable of simultaneously measuring fluctuations of electron density and magnetic field. Magnetic fields fluctuations ($\tilde {B})$ can be isolated during MHD activity such as internal kinks, since interferometry provides electron density information while polarimetry provides Faraday rotation data along the same beam path. In this paper, a comparison study of the electron density fluctuations with the fluctuations measured by the Gas Puffing Image (GPI) at the boundary plasmas will be discussed. A discussion of the $\tilde {B}$measurements from 3 channels at different tangencies including comparisons with EFIT equilibrium parameters and the structure of MHD modes will be presented. [Preview Abstract] |
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RP1.00007: Spatially Resolved Measurements of NB Energetic Ion Distributions in NSTX Sid Medley, Robert Andre, Lane Roquemore The mass and energy resolving superimposed E$\vert \vert $B Neutral Particle Analyzer (NPA) on NSTX can be scanned over NPA sightline tangency radii from R$_{tan}$ = +125 cm to R$_{tan}$ = -75 cm on a shot-to-shot basis. This capability was used to measure the spatially resolved energy distribution Neutral Beam (NB) ions in both L-mode and H-mode discharges. In L-mode discharges, the NPA spectra exhibit classical slowing down and pitch angle scattering behavior in agreement with TRANSP code simulations. Also, the measured and TRANSP-calculated neutron emission rates are in good agreement. The same is true for H-mode discharges in which low n = 1-3, low frequency f $<$ 50 kHz MHD tearing mode activity is absent. However, when MHD activity of this type is present in H-mode discharges, the NPA spectra exhibit a significant depletion of energetic ions that depends on time, energy and spatial location. Concurrently, the TRANSP-calculated neutron emission rate generally exceeds measurements by $\sim $ 10-20{\%}. TRANSP analysis of these observations using a model for anomalous energetic ion diffusion as a function of energy, space and time will be presented. $^{\ast }$\textit{Supported by U. S. DOE Contract DE-AC02-76CH03073 } [Preview Abstract] |
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RP1.00008: Initial measurements of beam ion profile in NSTX with Solid State Neutral Particle Analyzer Array D. Liu, W.W. Heidbrink, D.S. Darrow, A.L. Roquemore, S.S. Medley The Solid State Neutral Particle Analyzer (SSNPA) array on the National Spherical Torus Experiment (NSTX) utilizes Si diodes coupled to fast digitizers to measure the energy distribution of charge exchange fast neutral particles (35$\sim $100KeV) at four fixed tangency radii (60, 90, 100, and 120cm) to obtain the corresponding beam ion profile. Noise reduction techniques required to operate in the tokamak environment and post-shot pulse height analysis (PHA) methods are described. The results have been compared with those on the scanning E//B type Neutral Particle Analyzer (NPA) and good agreement was achieved. The redistribution and loss of beam ions during MHD activity including sawteeth events and IRE's has been observed. An experiment to study redistribution caused by fishbone instabilities is planned. \textit{Supported by U. S. DOE Contract DE-AC02-76CH03073} [Preview Abstract] |
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RP1.00009: Pitch Angle Resolved Measurements of Neutral Beam Ion Loss from NSTX Plasmas Douglass Darrow Pitch angle resolved measurements of neutral beam ion losses from NSTX plasmas have been made under a range of MHD-quiescent conditions. The loss is at the injection energy (80 keV), and a constants-of-the-motion model of the beam ion phase space predicts the range of pitch angles seen, indicating the measurements are attributable to prompt loss. A loss at high pitch angle and localized in pitch angle is often observed during MHD activity. [Preview Abstract] |
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RP1.00010: Comparison of Full Lorentz Model and Guiding Center Model of Ripple Loss Simulations in NSTX P.W. Ross, D.A. Gates, R.B. White Simulations of energetic particle ripple loss in National Spherical Torus Experiment (NSTX) are presented. Theoretical calculations of large gyroradius effects on magnetic ripple induced particle loss are presented. Of particular interest is the comparison between the full Lorentz code GYROXY and the guiding center code ORBIT in accurately estimating ripple loss. Both codes use the EFIT equilibrium reconstruction of NSTX discharges with a measured magnetic ripple added. Comparison is made in the particle loss rates for the two cases and the validity of the guiding center approximation in NSTX is discussed. The relative magnitude of ripple loss to other loss mechanisms is also presented. [Preview Abstract] |
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RP1.00011: Wave Coupling for HHFW in NSTX S. Bernabei, R.E. Bell, J.C. Hosea, B. Leblanc, C.K. Phillips, J.R. Wilson, P.M. Ryan, D.W. Swain, S.A. Sabbagh It has been established in previous experiments that effective coupling of the waves to the plasma can vary widely HHFW (High Harmonic Fast Waves) in heating and driving current in NSTX as plasma conditions and antenna phasing are varied. Experiments in 2004 established a strong dependence of power absorption on antenna phasing (toroidal wavelength of the excited fast wave). Experiments during the 2005 experimental campaign continue to explore this dependence as well as the role played by discharge conditions. By analyzing the data base of HHFW shots several parameters have been identified that have an effect, such as antenna-plasma distance, plasma shape and magnetic field lines. Results from these experiments will be presented. In addition, the effect of direction of the magnetic with respect to the antenna on coupling and damping has not been explored: An experimental proposal to change the direction of the plasma current and/or the toroidal field is scheduled. Results from these experiments, which can clarify possible geometric effects will also be presented. [Preview Abstract] |
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RP1.00012: Electron Stored Energy/Energy Confinement Indication of HHFW Surface Power Deposition on NSTX J.C. Hosea, S. Bernabei, B. LeBlanc, C.K. Phillips, J.R. wilson, P. Ryan, D.W. Swain, D. Stutman Additional Thomson scattering measurements of the electron energy confinement time during and following HHFW pulses for both heating and current drive antenna phasing are being carried out on NSTX in order to confirm the earlier conclusion that the power delivered to the core plasma is reduced considerably for the current drive phasing case as contrasted to the heating phasing case. This result occurs even though the radial deposition of energy into the electrons in the core plasma is noticeably more peaked for current drive phasing (longer wavelength excitation) relative to that for heating phasing (shorter wavelength excitation) as is expected theoretically. Thus it indicates that surface/peripheral damping processes play a more important role for current drive phasing. Many processes are possibly contributing to this ``surface'' power loss – surface wave excitation, RF sheath dissipation, and parametric decay wave excitation to name a few. Evidence of parametric decay wave heating has been obtained but does not appear to account fully for the difference between the two phasings. The possible contribution of collisional damping of surface waves to the surface power loss will be explored. [Preview Abstract] |
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RP1.00013: NSTX Reflectometer Measurements of RF Waves in the Scrape-off Layer in Front of the HHFW Antenna Array J.B. Wilgen, G.R. Hanson, P.M. Ryan, D.W. Swain, S. Bernabei, N. Greenough, J.C. Hosea, J.R. Wilson The microwave reflectometer on NSTX, in addition to its primary function of measuring edge-density profiles, has been modified to monitor RF waves in the scrape-off layer in front of the 30 MHz High Harmonic Fast Wave (HHFW) antenna array. Access to the plasma is located on the horizontal midplane, between two current straps of the HHFW array. A broadband reflectometer covers the frequency range of 6-27 GHz, probing the density range from below 1 x10$^{17}$ m$^{-3}$ up to 8 x10$^{18}$ m$^{-3}$. RF wave-related signals are extracted from the reflectometer using a high-pass filter and preamplifier circuit, and then digitized at 100 MHz sampling rate. The reflectometer microwave signal exhibits 30 MHz sidebands, due to the modulation of the cutoff layer by the electrostatic component of the RF waves. In addition, parametric decay waves are detected at frequencies below the heating frequency, near 28 and 26 MHz. Dependence of the RF spectra on the antenna phasing and on the reflection location within the scrape-off layer will be presented and compared with similar spectra obtained from a floating Langmuir probe located in the HHFW antenna. Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725 [Preview Abstract] |
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RP1.00014: Simulation of ECE frequency spectra for NSTX and comparison with new radiometer results Jakub Urban, Josef Preinhaelter, Gary Taylor, Steffi Diem, Linda Vahala, George Vahala Time evolution of ECE spectra in the 20-40GHz range is simulated for NSTX plasmas. The code is based on the full wave solution of the cold plasma wave propagation used for the determination of EBW-X-O and EBW-X mode conversion efficiencies using adaptive finite elements and on the determination of the effective radiation temperature from simultaneous solution of EBW ray evolution coupled to the integration of the radiative transfer equation. This method has been successfully used to determine the central temperature in NSTX from the detected EBW signal at 16.5GHz [1]. We obtained detailed information on how the ECE intensity is connected to the plasma parameters. Hence these simulations can test the applicability of using EBW for plasma diagnostics and for the determination of parameters useful for ECCD. \newline [1] J. Preinhaelter et al, 16th Topical Conf. on RF Power in Plasmas, Park City, Utah, B-05, in print. [Preview Abstract] |
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RP1.00015: Design and Testing of Electron Bernstein Wave Emission Radiometers for the National Spherical Torus Experiment and the TJ-II Stellerator J.B.O. Caughman, J.B. Wilgen, M.D. Carter, D.A. Rasmussen, P.M. Ryan, P.C. Efthimion, G. Taylor, A. Cappa, F. Castejon, A. Fernandez Efficient Electron Bernstein wave (EBW) mode conversion is important for viable electron heating of high $\beta $ plasmas, such as those on NSTX and TJ-II. Measurement of the thermal EBW emission from the plasma allows the EBW mode conversion efficiency to be determined, and also has the potential to offer a diagnostic for measuring electron temperature profile evolution. For NSTX, a dual-polarized quad-ridged broadband horn with a focusing lens is being used to measure the EBW emission at 20-40 GHz. A focused beam is needed to achieve efficient coupling at the mode conversion layer. The horn is mounted on a spherical base that can be moved up to 10 degrees in any direction. Emission from the plasma propagates through a glass lens and is focused on the horn. For TJ-II, a design using a corrugated horn configuration is being considered. For both systems, the field pattern from the horn or horn-lens combination has been measured with a 3-D scanning system in an effort to minimize the beam waist at the plasma edge. Details of the experimental results and future plans will be presented. [Preview Abstract] |
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RP1.00016: Discrete compressional Alfven eigenmode spectrum in NSTX and DIII-D Nikolai Gorelenkov, Eric Fredrickson, William W. Heidbrink Sub-cyclotron frequency instabilities of Compressional Alfven Eigenmodes (CAE) observed in the similarity experiments on National Spherical Torus (NSTX) and DIII-D [W.W. Heidbrink, et.al. submitted to Nuclear Fusion] are analyzed numerically applying an ideal MHD code NOVA. The code uses the numerical equilibrium and recovered main properties of these modes predicted by the theory [N.N. Gorelenkov, et.al., Nucl. Fusion, v.42. 977 (2002)]. The discrete spectrum was obtained for CAEs, which are characterized by three quantum numbers (M,n,s), where M, n, and s are poloidal, toroidal, and radial mode numbers, respectively. In the analyzed cases, observed mode frequency splitting between s and s+1 branches is consistent with the one numerically obtained. Poloidal mode number splitting, i.e. between M and M+1 modes, is larger by a factor of two, which is possibly due to neglecting the Hall term. Obtained mode structures are used for the numerical stability analysis with the NOVA-K kinetic code. CAE properties and their implications are discussed. [Preview Abstract] |
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RP1.00017: Effect of Fast-ion Distribution Function on Beam Driven Instabilities in NSTX E. Ruskov, W.W. Heidbrink, E.D. Fredrickson, D. Darrow, S. Medley, N. Gorelenkov The deuterium beam distribution function is modified from shot to shot while keeping the total injected power to $\sim$2~MW. The experimental ``knobs'' are the beam energy (90 keV and 60 keV), the beam tangency radius, and the fraction of trapped beam ions, which is modified at a predetermined time by applying $\sim \ \ge 2$~MW of high harmonic fast wave (HHFW) heating. Neutral particle analysis confirms perpendicular acceleration of the beam ions. The neutral beams are injected into a helium L-mode plasma and produce a rich set of instabilities, including TAE modes, instabilities with rapid frequency sweeps or chirps, and strong, low frequency (10-20 kHz) fishbones. Fishbones are excited when $q_0 < 0$ and when the trapped beam-ion fraction increases; they are always present later in the discharge. However, TAE modes are excited only early in the discharge and, under some circumstances, they are suppressed by HHFW heating on a collisional time scale. In contrast with a Dipole experiment,\footnote{D. Maslovsky, B. Levitt and M. E. Mauel, Phys.\ Plasmas {\bf10} (2003) 1549.} the cyclotron heating has no effect on the chirping instabilities. [Preview Abstract] |
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RP1.00018: Three-wave interaction of fast ion driven modes in NSTX N.A. Crocker, W.A. Peebles, S. Kubota, E.D. Fredrickson Fast ions generated in fusion plasmas by heating techniques, and alpha particles in burning plasmas, can excite global modes that degrade fast ion confinement. While the mode dynamics have been well studied, nonlinear three-wave interactions between modes have received little attention. Such interactions can transfer energy, leading to damping and excitation. Using core localized reflectometry and edge magnetic measurements, we observe three-wave interactions between two distinct types of such modes in NBI-heated plasmas: energetic particle modes (EPMs), consisting of harmonics in frequency ($f)$ and toroidal mode number ($n)$, peaking at the $f\sim $ 17 kHz, $n$~= 1 fundamental, and higher frequency modes (HFMs) that peak at larger $f$ and $n$ and have a uniform $f$ and $n$ spacing of $\Delta f$~$\sim $ 17 kHz and $\Delta n$ = 1. This spacing suggests three-wave interactions are occurring [see EJ Strait, Plas. Phys. {\&} Cont. Fus. (1994)], which we verify by calculating the bicoherence. Analysis also indicates these interactions organize the HFMs into a toroidally localized wave-packet whose envelope is stationary in the EPM toroidal rotation frame. [Preview Abstract] |
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RP1.00019: Parallelization of MHD spectral analysis at NSTX Seth Veitzer, Jonathan Menard, Peter Stoltz The ability to perform between-shot analysis of NSTX plasmas can be limited by the amount of time it takes to complete the analysis. For instance, MHD fluctuation spectral analysis is presently too time consuming if the complete toroidal array is included in the analysis and the processing is done in serial. We show here a method of parallelizing the MHD mode calculation, written in the IDL language, which gives up to 5x speedup over the serial case for the entire computation. This method decomposes the problem in the temporal domain, which provides better improvement in performance than decomposition in the spatial domain. However, this decomposition requires all of the parallel processors to load all of the MHD data prior to starting the computation. For this reason, further improvement can be achieved with the addition of a parallel MDSPlus data server, which we also describe. [Preview Abstract] |
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RP1.00020: Overview of MAST results and future plans Brian Lloyd The MAST experimental programme is focused both on physics studies for ITER and on addressing key issues for the long term potential of the spherical tokamak such as non-solenoidal start-up, current drive and plasma exhaust. ITER physics studies cover confinement scaling, transport physics including the generation and sustainment of transport barriers, pedestal physics, scrape-off layer transport, error fields and performance limiting instabilities such as ELMs, neo-classical tearing modes etc. These studies are carried out in close collaboration with international partners including joint experiments with other devices. Substantial upgrades to MAST have been implemented during the last two years including a new divertor, centre column and error field correction coils as well as many diagnostic enhancements. An upgrade to the neutral beam heating system is well-advanced. An overview of the latest MAST results in the areas described above, exploiting the new MAST capabilities, will be presented together with future plans. [Preview Abstract] |
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RP1.00021: Neutral beam heating and current drive on MAST M.R. Tournianski, Rob Akers, G. Cunningham, P.G. Carolan, D.L. Keeling Heating and NBCD studies on MAST benefited from recent diagnostic upgrades such as T$_{i}$ and V$_{\phi }$ profiles on the ion Larmor radius scale by improved CXRS and bremsstrahlung imaging to measure $Z_{eff}$ profiles. The operational flexibility of MAST has also been improved by implementation of digital plasma control and real time optical edge detection and position control. These enhancements, combined with the access offered by the large MAST vessel, have been exploited for the study of off-axis heating and NBCD in vertically displaced SND plasmas. These had a neutron yield and stored energy comparable to up-down symmetric DND discharges with on-axis NBI. The extreme features of low aspect ratio devices can have a strong effect on the behaviour of fast ions, potentially influencing their heating and NBCD properties. The experimental results and modelling of off-axis NBI in SND (upper and lower) and on-axis NBI in DND will be presented and compared in terms of heating and NBCD. [Preview Abstract] |
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RP1.00022: Status of the TST-2 spherical tokamak and future plans Y. Takase, A. Ejiri, Y. Adachi, N. Ishii, H. Kasahara, H. Nuga, T. Oosako, M. Sasaki, Y. Shimada, N. Sumitomo, I. Taguchi, H. Tojo, J. Tsujimura, M. Ushigome, T. Yamada After completing EBW heating and current drive experiments (8.2 GHz at a 100 kW power level) at Kyushu University, TST-2 was moved back to the University of Tokyo. After performing upgrades to magnetic field coil power supplies and RF heating systems, TST-2 has resumed operation. Development of a start-up scenario without the use of the central solenoid will be continued. Since high power in the electron cyclotron frequency range is presently not available, the use of RF power at lower frequencies (around 20 MHz or around 200 MHz) will be explored to supply enough source of plasma during the start-up phase when the vertical field is not adequate to maintain plasma equilibrium. Electron heating by the high-harmonic fast wave will be pursued using an antenna capable of exciting waves with different wavenumbers. Recently, four 200 MHz transmitters were transferred from the JFT-2M tokamak. These will be used for plasma current start-up experiments using RF power in the lower-hybrid frequency range. [Preview Abstract] |
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RP1.00023: Heating Properties of Merging Startup in TS-3/4/5 High-Beta ST Experiments Yasushi Ono, Eiichiro Kawamori, Alexander Balandin Cause and mechanism for high-power reconnection heating studied using merging startup of two STs in the TS-3/4/5 experiments. Our 2-D ion temperature (Ti) measurement by three polychrometors with ICCD cameras revealed two hot-Ti spots in the reconnection outflow (downstream) regions. The bipolar reconnection outflow was observed to collide with the reconnected field lines, forming fast shock structures that satisfied the Rankin-Hugoniot's condition. The reconnection transformed a part of poloidal magnetic energy of merging STs into their thermal energy probably through the fast shock/ viscosity damping of reconnection outflow. The ion heating energy as well as the merging speed was observed to increase inversely with toroidal (guiding) field component Bt of the merging STs. As Bt was decreased, ion gyroradius increased and finally exceeded the sheet thickness during sheet compression of the ST merging. It caused the significant increase in sheet resistivity, reconnection speed and outflow speed that directly determined the ion heating power of merging. Another finding was rapid decrease in beta of low-q ST right after its reconnection heating. This high-beta collapse is related with the Taylor relaxation to force-free equilibria of spheromak and low-q tokamak On the other hand, the high-q ST was observed to confine most of thermal energy injected by the reconnection heating. Consequently, final beta of the merging ST increased with Bt after this high-beta relaxation. [Preview Abstract] |
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RP1.00024: Progress on the Lithium Tokamak Experiment R. Kaita, R. Majeski, T. Gray, T. Kozub, H. Kugel, R. Parsells, C. Priniski, J. Spaleta, J. Timberlake, L. Zakharov, V. Soukhanovskii, S. Krasheninnikov, L. Baylor, R. Maingi The goal of the Lithium Tokamak eXperiment (LTX) is to produce tokamak discharges with near-zero recycling, and investigate the consequences of operating under this extreme condition for plasma transport and stability. A major component of LTX is a conducting copper shell with a dynamically-bonded stainless steel liner as the plasma facing component (PFC). Fabrication of a prototype shell section has demonstrated that the tolerances required for conformity to the plasma shape can be maintained. A lithium coating will provide the low recycling PFC on LTX. The goal of depositing 100 nm lithium layers within the five minute interval between discharges has been achieved. Details of the progress in these areas and the status of other LTX tasks will be reported. [Preview Abstract] |
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RP1.00025: Effects of Lithium Plasma-Facing Surfaces on Particle Confinement in CDX-U T. Gray, R. Kaita, R. Majeski, H. Kugel, J. Spaleta, J. Timberlake, V. Soukhanovskii, R. Maingi Recent experiments on the CDX-U spherical torus have successfully achieved a significant reduction in recycling with large-area liquid lithium plasma-facing surfaces. The effects of a liquid lithium toroidal limiter and evaporative lithium coatings on overall density and $\tau_p^*$ will be presented. Such conditions have also demonstrated the need to improve plasma fueling. To address this challenge, a supersonic gas injector, based on a Mach 8 Laval nozzle design,[1] has been installed on CDX-U. The fueling efficiency of the nozzle compared to standard gas puffing will be compared. [1] M. Baumgartner, Ph. D. thesis, Princeton University (1997) [Preview Abstract] |
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RP1.00026: Plasma Equilibrium Re-constructions of CDX-U J. Spaleta, L. Zakharov, R. Kaita, R. Majeski, T. Gray Recent CDX-U experiments have focused on the effects of liquid lithium as a plasma first wall in tokamak environments. Evidence indicates that liquid lithium limiter operation in CDX-U provides access to a plasma with a much reduced level of wall re-cycling (see poster by T. Gray). Work has been on-going to create the first ever plasma equilibrium re-constructions of the CDX-U device using the ESC equilibrium code. The CDX-U re-constructions will be the first to use a novel calibration technique to account for time-dependent localized eddy currents near magnetic sensors. The goal is to contrast the equilibrium profiles obtained during liquid lithium and solid lithium limiter operation, looking for evidence of plasma current profile broadening as expected from transport simulations for very low re-cycling plasmas. A comparison of plasma equilibria during liquid lithium and solid lithium operation will be presented. An overview of the CDX-U diagnostics, including a new diamagnetic coil, and a description of the magnetic signal calibration technique used to account for eddy current contributions will also be shown. [Preview Abstract] |
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RP1.00027: The Phase II Campaign for the Pegasus Toroidal Experiment B. Squires, E. Unterberg, D. Battaglia, M. Bongard, S. Burke, N. Eidietis, R. Fonck, G. Garstka, B. Kujak-Ford, B. Lewicki, G. Winz The Pegasus upgrade has been completed with the installation of a new 130 MV-A IGCT switched OH power system to provide a programmable V$_{loop}$(t) with sub-ms response time. New diagnostics include a multichannel bolometer array, an imaging visible bremsstrahlung array, an X-ray pulse height analyzer, and an improved 1mm interferometer. Initial operation has demonstrated active control of all coil currents and hence more control over plasma formation. The present campaign is using these new capabilities to develop scenarios to access the high I$_{p}$/I$_{TF}$, high I$_{N}$, high $\beta _{t}$ regime of interest. The Phase I performance of I$_{p} \quad \sim $ 0.15 MA has been recovered with $<$ 1/2 the V-s used previously. V$_{loop}$(t) programming has demonstrated increasing control of the plasma formation and mitigation of tearing modes which had constrained the operational space. An array of plasma gun current injectors is planned as a DC-helicity source to extend the effective use of V-s from the OH solenoid and test a new plasma formation scheme. [Preview Abstract] |
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RP1.00028: Non-Inductive Startup Via a Plasma Gun DC Helicity Source on the Pegasus Experiment N.W. Eidietis, G. Fiksel, R.J. Fonck, G.D. Garstka, E.A. Unterberg, G.R. Winz Developing a non-inductive startup technique is important for the ultralow-A Pegasus ST experiment, and the ST concept in general. Two low impurity, high I ($\sim $1 kA) plasma guns have been installed in the lower divertor region of Pegasus to test toroidal current drive via DC helicity injection during plasma startup. Aided by a high magnetic stacking factor, the dual gun array provides a toroidal current of 15-20 kA. A transition from discrete helical current streams to a uniform reconnected plasma is observed, with a doubling of the net toroidal current. Relaxation to a tokamak-like plasma state was not observed at this low current, but is expected as the net current is raised to provide a poloidal field greater than the vacuum vertical field. Experimental attempts to attain a relaxed tokamak-like configuration are concentrating on optimizing a 2-3 gun assembly at very low field strengths. Design requirements are presented for a 12 gun array to be installed in Pegasus next year, which is projected to provide I$_{p} \quad >$ 0.1 MA. [Preview Abstract] |
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RP1.00029: Development of an Active Plasma Control System for Pegasus M.W. Bongard, S.P. Burke, R.J. Fonck, B.T. Lewicki The Phase II Pegasus ST experiment includes fully programmable power supplies for all magnet coils. These will be integrated with a digital feedback plasma control system (PCS), based on the PCS in use on DIII-D, to provide active feedback control of the plasma evolution. The initial goal is to control I$_{p}$(t), R(t), and Z(t). The feedback cycle consists of: 1) sampling magnetic diagnostics, 2) applying a response matrix derived from equilibrium reconstructions, 3) accounting for induced vessel currents and power supply frequency responses, and 4) adjusting the current demand, all on a timescale that is fast compared to the shot duration of $\sim $25 ms. Data is sampled via a 500 kHz, 96-channel CPCI digitizer from DTACQ Solutions, Ltd. The power supply control signals are generated by 16 analog waveform generators, with the option to utilize 32 digital I/O lines in the future. The PCS digitizer is controlled via a system of Linux-based computers that perform requisite computation-intensive tasks and interface to the existing LabVIEW control codes via a TCP/IP network link. [Preview Abstract] |
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RP1.00030: Electron Temperature Diagnostics on the Pegasus Toroidal Experiment D.J. Battaglia, M.W. Bongard, R.J. Fonck, D.J. Den Hartog A soft X-ray (SXR) Pulse Height Analysis (PHA) system has been implemented to measure the electron temperature on the Pegasus Toroidal Experiment. The detector is a silicon drift diode (SDD) mounted on a bellows. The SDD detector is well suited for high resolution (139 eV at 5.9 keV), high count rate (10$^{6}$ cps) X-ray spectroscopy and therefore is able to obtain time-resolved temperature measurements on the order of a millisecond. The detector is radially scannable which permits profile measurements on a shot-to-shot basis with a spatial resolution as low as a few centimeters. Temperatures in the range of 300 eV -- 1 keV should be measurable with the PHA system. Temperatures below 300 eV can be measured using oxygen and carbon line ratios with SXR Ross filter spectroscopy. A Thomson-scattering system is also being designed for future implementation. The first generation of the diagnostic will include a 10 J, 40 ns Q-switched ruby laser ($\lambda $ = 694.3 nm) and a single-spatial-channel avalanche photodiode detector/spectrometer system. [Preview Abstract] |
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RP1.00031: Planned EBW Heating and Current Drive Experiments on Pegasus G.D. Garstka, R.J. Fonck, B.T. Lewicki, S.J. Diem, P.C. Efthimion, G. Taylor The Pegasus Toroidal Experiment provides an attractive opportunity for investigating the physics and implementation of electron Bernstein wave (EBW) heating and current drive in an overdense ST plasma. The toroidal field of 0.07-0.15 T on axis will provide fundamental resonant absorption of 2.45 GHz waves. The new plasma control system will provide a stable plasma edge to support resilient EBW coupling; initial tests will focus on the O-X-B mode conversion scenario. Experiments with up to 1 MW of RF power will address fundamental issues concerning EBWs in ST experiments. These include edge coupling, nonlinear effects (such as parametric instabilities) at the edge, ray propagation, deposition locations, and current drive efficiency, which may be as large as 60 kA/MW at high T$_{e}$. The proposed hardware is made up in large part of pieces from the PLT lower hybrid system. These include two 450 kW klystrons and associated systems, recirculators, and power transmission equipment. [Preview Abstract] |
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RP1.00032: Modeling of EBW Propagation and Damping on Pegasus S.J. Diem, P.C. Efthimion, G. Taylor, R.J. Fonck, G.D. Garstka Numerical modeling of electron Bernstein wave (EBW) propagation and damping on the very-low-aspect ratio Pegasus Toroidal experiment has been explored using the GENRAY ray tracing code and CQL3D Fokker-Planck code in support of planned heating and current drive experiments. Calculations were performed for 2.45 GHz waves launched with a 10 cm poloidal extent for a variety of equilibrium configurations. Poloidal launch scans show that driven current is a maximum when the poloidal launch angle is between 10 and 25 degrees, supporting a launcher placed near the midplane. Calculations predict that 400 kW of coupled EBW power will drive 10 kA of plasma current in plasmas with an I$_{tf}$ of 90-150 kA. RF-driven current densities reached 20-100 kA/cm$^{2}$ between a normalized minor radius of 0 to 0.2 where the central density and temperature are 4.5e19 m$^{-3}$ and 310 eV, respectively. Current drive was primarily via the Fisch-Boozer mechanism. Initial results of O-X-B mode coupling calculations will also be presented. [Preview Abstract] |
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RP1.00033: ST Formation via Driven Relaxation of a Screw Pinch: Overview and Design Considerations of a Proposed Concept Exploration Experiment S. Hsu, X. Tang, M. Kostora, T. Intrator, G.A. Wurden, D. Lazanja, A. Boozer An ST with a plasma center column (PCC) could potentially overcome all the disadvantages of a material column. Along with CHI, a PCC could: (1)~form an ST via driven relaxation of a screw pinch (SP), (2)~carry inboard TF coil current, (3)~assist ST sustainment in combination with rf/NBI, and (4)~eliminate neutron shielding and column replacement. Achieving these goals will require considerable research. We propose to study the physics of (1). Our concept is to form a symmetry-axis SP between two electrodes, each coaxial with annular CHI electrodes. By programming SP/CHI electrode currents/voltages, the SP will undergo driven relaxation resulting in an ST-PCC equilibrium. After discussing the project motivation/impact, goals, physics basis/assessment (see poster by X.~Tang), and plans for ST-PCC equilibria/stability numerical modeling, we will focus on design considerations/challenges of key hardware systems: (1)~SP source consisting of electrodes and axial field coils; (2)~CHI electrodes and bias field coils; (3)~power supplies; (4)~interface between outboard TF windings and SP electrodes; (5)~flux-conserving boundary with electrical breaks for SP/CHI electrodes and shape-optimized for equilibrium/stability; and (6)~diagnostics for characterizing ST-PCC formation. [Preview Abstract] |
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RP1.00034: Physics Design Bases of a Spherical Torus with a Plasma Center Column Xianzhu Tang, Allen Boozer, David Lazanja, Scott Hsu Low aspect ratio toroidal pinches such as the standard ($q>1$) and the ultra-low-q ($q < 1$) Spherical Tori or Tokamaks (ST), would have a far more robust reactor engineering design if a plasma center column (PCC) can be used in place of a material center post. Biased electrodes across the plasma center column would drive a plasma current to produce the toroidal magnetic field in lieu of the TF coils. Two ongoing efforts are the proto-sphera experiment in Europe and the proposed ST-PCC concept exploration experiment at LANL (see accompanying poster by S. Hsu, et al). The operation of such reactors is naturally divided into two distinct phases: formation by driven-relaxation (helicity injection) and sustainment by auxiliary current drive and heating such as rf and NBI. The design constraints of the ST-PCC are primarily motivated by the formation rather than the sustainment physics. With a Taylor-relaxed plasma as the baseline case, we illustrate both analytically and numerically the three essential factors in guiding the design. First, the flux amplification factor determines the aspect ratio of the ST-PCC. Second, the plasma elongation gives the most freedom in shaping the q profile. Two examples are the standard spherical Tokamak with q$>$1 throughout the plasma and the ultra-low-q Tokamak with q much less than unity for bulk of the plasma. Third, the vacuum bias magnetic flux plays the second most important role in modifying the q profile. [Preview Abstract] |
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RP1.00035: MCX |
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RP1.00036: The Maryland Centrifugal Experiment : status and plans Catalin Teodorescu, Andrew Case, Richard Ellis, Adil Hassam, Robert Lunsford, Raymond Elton, Joydeep Ghosh, Hans Griem The Maryland Centrifugal Experiment (MCX) studies supersonic rotation of plasma produced by the application of a steady state electric field perpendicular to a linear confining magnetic field. MCX has achieved high density ($n_{e }>$ 10$^{20}$ m$^{-3}$ ) fully ionized plasmas rotating supersonically with azimuthal velocities $v_{\phi }$ in the range of 100 - 250 km/sec with ion temperatures typically 30 eV and sonic Mach numbers ($v_{\phi }$/$v_{ti})$ in the range of 1 to 3 and Alfv\`{e}n Mach numbers ($v_{\phi }$/$v_{A}$ ) of somewhat less than unity. Plasmas remain stationary for milliseconds, much longer than MHD instability timescales. MCX has implemented extensive new diagnostics including a multi-chord ion Doppler spectrometer, arrays of magnetic probes, an H$_{\alpha }$ emission array of detectors, and a two color interferometer. Results will be reported on velocity profiles and related MHD activity. A higher voltage (20 kV) discharge capacitor bank is being tested and results on velocity limits will also be reported. The major upgrade plans include increasing the midplane magnetic field to 1 T and installing extensive surface conditioning. Work supported by USDOE. [Preview Abstract] |
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RP1.00037: H$_\alpha$ Emissions Diagnostic for the Maryland Centrifugal eXperiment Ryan Clary, Andrew Case, Richard Ellis, Adil Hassam, Robert Lunsford, Catalin Teodorescu H$_{\alpha}$ detectors have been designed and constructed for the Maryland Centrifugal eXperiment (MCX) using high-speed photodiodes. Currently, light is collected from various positions along the axis in nominally radial directions in order to obtain general diagnostic information about MCX. We present a basic description of the instrument design and typical data traces in the parameter space of the experiment. The axial core in MCX is charged to a high voltage with the high voltage feeds placed asymmetrically at one axial end. In addition, MCX exhibits two modes of operation: an ``ordinary'' mode (O-mode) and a ``high rotation'' mode (HR-mode). In view of this, we will present observations related to axial symmetry, general H$_ {\alpha}$ emission levels in HR-mode vs. O-mode, \& correlation between H$_{\alpha} $ emission characteristics and shot mode-types. We also propose a possible design for a multi-cord instrument array that will aid in determining a radial neutral density profile. [Preview Abstract] |
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RP1.00038: Spatially-resolved spectroscopic measurements of plasma rotation in MCX* J. Ghosh, A. Case, R.F. Ellis, R.C. Elton, H.R. Griem, A. Hassam, R. Lunsford, C. Teodorescu Earlier spectroscopic measurements [1] of averaged plasma rotation velocities on the Maryland Centrifugal Experiment (MCX) have been supplemented to include radial resolution with a five-channel fiber-optic collection system. Detailed information from each view is made possible by an 8X increase in spectral resolution, using a 2-m spectrograph and a 2400/mm grating. Inversion of the integrated chordal emissions into a radial dependence are performed by two methods: (a) an iterative comparison of the measured emissions with a summation of assumed emissions in five concentric zones, and (b) a combination of Abel-like matrices inversions [2]. Preliminary results show mean velocities of 45 +/-10 km/sec for both C+ and C++ ions. A major advance is the measurement of a radial shear in rotational velocity as large as 9 km/sec/cm, which is in the range theoretically predicted to achieve MHD stability. Also determined is a radial distribution of carbon ions, indicating a dominant location near the central core. Supporting spatially-resolved values for electron density are derived from the Stark widths of hydrogen Balmer-series spectral lines. *Sponsored by DoE. [1] J.Ghosh, et al., Phys. of Plasmas v.11, p. 3813 (2004). [2] R.Bell, Rev. Sci. Inst. v.86, p. 558 (1995); v. 68, p. 1273 (1997). [Preview Abstract] |
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RP1.00039: Supercritical Alfven Ionization Velocities observed on MCX R. Lunsford, R.F. Ellis, A. Case, R. Clary, A. Hassam, C. Teodorescu It has been theorized that Alfven's Critical Ionization Velocity (CIV) stands as a limit curtailing the speed at which a centrifugally confined plasma can rotate. While this limit in speed has been observed in the early experimental runs of MCX (Maryland Centrifugal eXperiment), by modification of the input parameters we have been able to access a high rotation discharge mode (HR mode) whose azimuthal velocity exceeds the CIV by greater than a factor of two. Previous analysis (C. Teodorescu, et al, unpublished) has shown that the observed average rotation velocities bifurcate about the CIV predicted for MCX geometry. In all cases the ordinary mode (O-mode) speeds are below the CIV boundary, and the HR-mode speeds are above. The O-mode CIV limitations are thought to be due to plasma and neutral boundary interactions at the insulating surfaces. This suggests that the ability to reach the supercritical state would be aided by a marked plasma detachment from the insulators. Thus, the presence of supercritical speeds supports the possibility of enhanced centrifugal confinement. [Preview Abstract] |
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RP1.00040: Magnetic Diagnostics on the Maryland Centrifugal Experiment (MCX): Upgrades and Initial Results A. Case, R. Clary, R.F. Ellis, A. Hassam, R. Lunsford, C. Teodorescu, S. King, J. Young The magnetic fluctuation diagnostic suite on MCX has recently undergone significant upgrades. A sixteen probe Bz array has been installed near the machine midplane. This array will allow resolution of plasma perturbations up to azimuthal mode number m=8. In addition we have four x,y,z triplet probes located along the machine centerline which provide information about the longitudinal distribution of Bdot activity. Preliminary data from the Bdot array indicates that the magnetic activity in the plasma varies considerably between the various machine operating modes. We present data showing evidence of global changes in the plasma Bdot activity associated with changes in machine operating modes, along with data on the azimuthal mode number spectrum derived from Bdot array measurements. [Preview Abstract] |
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RP1.00041: Neutral profiles in centrifugal confined plasma Sheung-Wah Ng, A. B. Hassam A simple model is developed for the penetration of neutrals into plasma for a rotating mirror configuration, for example, the Maryland Centrifugal Experiment (MCX). In this model, an inward confining force originating from the centrifugal force due to the rotation competes with the outgoing tendency of the plasma in the parallel direction. An exponential relation between the centrifugal confining force and the neutral density at the axis wall is demonstrated, with ionization and charge-exchange interaction being considered. A 2D code has been developed with ``recombination" layers simulating the physics close to the walls. In the crossfield direction, classical diffusion losses are allowed; the code however shows the signature of a scrape-off layer (SOL) in 2-dimensional simulation. How neutral dynamics is affected by supersonic rotation is of fundamental interest for the mirror configuration of centrifugally confined plasmas. [Preview Abstract] |
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RP1.00042: Innovation in fusion energy sciences: looking towards the future Simon Woodruff, Y.C. Francis Thio, Thomas R. Jarboe During the last 50 years, about 10{\%} of the fusion budget in the United States has been spent on experiments that investigated alternates to the tokamak and stellarator. The alternates comprise not only open and closed magnetic configurations but also inertial concepts, and more recently hybrids of both magnetic and inertial (e.g. MIF). This concept innovation remains a necessary component of a healthy fusion program: even as ITER goes ahead, the mission of the world fusion program remains to produce an economic fusion reactor, the path to which still remains unclear. For the present-day vision of the tokamak reactor, an order of magnitude increase in performance is still required in order to make it attractive in the near-term. This paper summarizes recent advances in alternates and presents the arguments supporting concept innovation (or configuration optimization) as necessary to make headway towards the goal of economic fusion energy. [Preview Abstract] |
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RP1.00043: L-Mode Confinement in Toroidal Devices Joseph Gorman A loss-cone in velocity space exists in toroidal devices. Particles having low velocities parallel to the main confining field, B, can gradBXB drift, somewhat uncompensated, toward the scrape-off layer. An analysis of this effect leads to the formulation for the plasma confinement time, $\tau$, in seconds viz., $\tau$ = 10$^{15}$ BTr/n where B is the confining magnetic field in Tesla, T is the electron temperature in eV, r is the plasma minor radius in meters and n is the plasma density in m$^{-3}$. This equation predicts the plasma confinement times of stellarators and tokamaks,in the L-Mode, that agree with experiments to within about a factor of two. The agreement extends to data from experimental fusion devices, big and small, conducted world-wide during the past several decades. The origins of this semi-empirical formula and the good fit it provides to the experimental data will be presented and discussed. [Preview Abstract] |
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RP1.00044: The stability of zonal flows in finite beta tokamak edge plasmas Parvez Guzdar, Robert Kleva The generation of zonal flow in finite beta plasmas is an active area of research. We have developed a time-dependent code to investigate the stability of zonal flows in finite beta plasmas. The code can investigate different profiles of the zonal flow as well as the effects of magnetic shear. The focus of the investigation will be tokamak edge plasmas where finite beta effects are enhanced by steep pressure gradients and high safety factor q. We will present results of the stability of the zonal flows as a function of the two key dimensionless parameters, the classical ideal ballooning stability parameter and the diamagnetic parameter. The relevance of this study to edge transport will be discussed. [Preview Abstract] |
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RP1.00045: Geodesic Acoustic Propagation and Ballooning Mode Formalism M.B. Li, P.H. Diamond, G.G. Young, M. Arakawa Relevance of ballooning formalism (BMF) in nonlinear interaction of toroidal electromagnetic drift waves in the presence of zonal flows and Geodesic Acoustic Oscillation (GAO) is critically examined from a physical argument of radial propagation of wave packets. To achieve the quasi-translational invariance of poloidal harmonics which is necessary for the BMF, the geodesic curvature induced transfer [1] of fluctuation energy in radial direction should occur faster than the time scale of physical interest. Of course, this does not happen necessarily in drift-Alfven (DALF) turbulence simulations [2]. This observation casts considerable doubts on the applicability of various codes based on the BMF concept to nonlinear electromagnetic problems. \newline \newline [1] B. Scott, Phys. Letters A 320 (2003) 53. [2] B. Scott, New J. Phys 7 (2005) 92. [Preview Abstract] |
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RP1.00046: BASIC PLASMA: TURBULENCE, TRANSPORT, SHOCKS, NONLINEAR PHENOMENA |
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RP1.00047: Superresolution algorithm for turbulence in\-vest\-igat\-ions D. Block, I. Teliban, A. Piel, V. Naulin In general, spatio-temporally high resolved data is required to investigate processes in plasma turbulence. For most diagnostics, however, the maximum spatial resolution is limited by probe dimensions and constraints on probe array construction. To overcome limitations in spatial resolution this contribution describes a superresolution algorithm which uses the high temporal resolution of a multi-probe system to enhance the spatial resolution. The performance of the algorithm is benchmarked with synthetic data, data from plasma turbulence simulations and experimental data obtained from a Langmuir probe array with 64 probes. Significant improvements in amplitude, trajectory and shape of individual coherent structures in a turbulent system are observed. [Preview Abstract] |
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RP1.00048: Investigation of edge ExB sheared flow development in the TJ-II stellarator M. Angeles Pedrosa, Arturo Alonso, Eduardo Calder\'{o}n, Alexander A. Chmyga, Nicolai B. Dreval, Leonid Eliseev, Teresa Estrada, Carlos Hidalgo, Ludmila Krupnik, Alexander V. Melnikov, Roberto Octavio Orozco, Jose Luis de Pablos, Stewart J. Zweben Experiments carried out by means of Langmuir probes in the TJ-II stellarator have shown that above a plasma density threshold the level of turbulence decreases with a concomitant development of ExB perpendicular sheared flow (i.e. naturally occurring shear layer). The effect of density on sheared flows has been observed in different plasma magnetic configurations and regimes [1, 2]. The reversal in the ExB rotation has been 2-D visualized using Ultra Fast Speed cameras. Estimated velocity of observed blobs, rotating predominantly in the perpendicular direction, is in the range of 10$^{3}$--10$^{4}$ m/s. Heavy Ion Beam Probe and reflectometry measurements also show a strong dependence of edge radial electric fields and plasma rotation with plasma density. [1] C. Hidalgo et al., Phys. Rev. E 70 (2004) 067402. [2] M.A. Pedrosa et al., Plasma Phys. Control. Fusion 47 (2005) 777. [Preview Abstract] |
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RP1.00049: A Basic Physics Study of Zonal Flows in CLM Vladimir Sokolov, Xiao Wei, Amiya K. Sen, Khare Avinash A basic physics experimental study of zonal flows [1] associated with ITG (ion temperature gradient) drift modes has been performed in the Columbia Linear Machine. The difficult problem of detection of zonal flows (ZF) has been solved via a novel diagnostic using the paradigm of FM (frequency modulation) in radio transmission. Using this and Discrete Short Time Fourier Transform, we find a power spectrum peak at ITG (‘carrier’) frequency of $\sim 120kHz$ and FM sidebands at frequency of $\sim 2kHz$. We have definitively identified ZF with azimuthal (poloidal) and axial (toroidal) symmetry and very low frequency. However, the stabilizing effect of ZF on the parent ITG modes appears to be small and no significant isotopic effects are seen. A theoretical model for the above will be presented. The research was supported by U.S. Department of Energy Grant No. DE-FG02-98ER-54464.\\ \newline [1] P.H. Diamond, S-I Itoh, K.Itoh and T.S.Hahm, Plasma Phys.Controlled Fusion 47, R35 (2005). [Preview Abstract] |
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RP1.00050: Observation of Zonal Flow Formation in a Cylindrical Magnetized Plasma Jonathan H. Yu, Christopher Holland, George R. Tynan, Michael J. Burin An azimuthally symmetric sheared flow is observed to spontaneously develop in a turbulent cylindrical magnetized plasma, with no apparent source of external angular momentum. The shear layer is maintained against ion-ion viscosity and ion-neutral flow damping by electrostatic Reynolds stress, which is due to collisional drift turbulence. Measurements show that as the magnetic field is increased, the shear layer develops near the radial edge of the plasma. Simultaneously, collisional drift turbulence is produced from coherent drift eigenmodes due to harmonic generation and three wave interactions. An inverse energy transfer from turbulent, small spatial scales to large spatial scales drives the zonal flow, and this energy transfer rate will be measured in future work. Simulations of collisional drift turbulence demonstrate zonal flow formation via merging of tilted vortices, and the resulting azimuthal velocity profile is consistent with Mach probe measurements and time-delay velocity estimates. This set of measurements may provide insight into the structure formation process of zonal flows that are often observed in the edge of tokamaks. [Preview Abstract] |
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RP1.00051: Experimental and numerical studies of neutral gas depletion due to neural gas collisional heating in an inductively coupled plasma Masashi Shimada, George R. Tynan The significant neutral gas temperature rise has been observed in various noble gases in the center of an inductively coupled plasma (ICP) chamber and the actual neutral gas pressure has been obtained by considering the thermal transpiration effects with this neutral gas increase. A plate which consists of 8 vertical optical ports with collimating lens and a movable optical fiber probe have been developed and used to measure axial and radial profile of gas temperature in an ICP respectively. When thermal transpiration is accounted for, our neutral pressure measurement shows the gas pressure remains the same as the fill pressure when plasma is on. Since the neutral gas presumably follows the ideal gas law, p=nT, our results therefore imply that the neutral gas density is significantly reduced in the central region of the discharge chamber. 0D/1D neutral gas heating model has been developed and compared with experimental results. DSMC (Direct Simulation Monte Carlo) has been carried out to simulate the ion acceleration in collisional pre-sheath and charge exchange/elastic collisions with background neutral gas. [Preview Abstract] |
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RP1.00052: On particle temperature in CSDX Alexander James, George Tynan The Controlled Shear Decorrelation eXperiment (CSDX) was constructed to study collisional drift turbulence in a linear, uniform magnetic field. Ion and neutral temperatures play a role in the viscous damping and ion-neutral flow drag respectively, and thus are relevant to understanding drift turbulence in this simple system. In this poster we present high resolution (0.01 Angstrom) emission spectroscopy measurements of Ar-I and Ar-II emission from this device and then deconvolve the results with the instrument response function to find ion and neutral temperatures. Both temperatures are found to increase with magnetic field in the device. The peak ion and neutral temperatures are 1.0 eV and 0.5 eV at a magnetic field of 1kG. The observed temperatures are explained in terms of a simple volume-averaged collisional heating model. [Preview Abstract] |
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RP1.00053: Validation of Drift Turbulence Simulations with Laboratory Plasma Experiments George Tynan, Christopher Holland, Jonathan H. Yu, Troy Carter, Lincan Yan First-principles simulations of drift turbulence are being developed with the intention of predicting the rates of turbulent transport in magnetic confinement systems. The underlying physics is expected to exhibit complex dynamics, including the formation of a saturated turbulent state from given free energy sources and dissipation mechanisms, the development of a self-regulating balance between turbulence and organized structure formation via nonlinear turbulence/structure interactions, and bifurcations between states of turbulent transport. Thus experiments that examine aspects of these complex dynamics in isolation can be useful in validating turbulence simulations. Here we provide detailed comparisons of drift turbulence-zonal flow interactions in the CSDX laboratory plasma device with similar interactions that occur in a two-field fluid collisional drift turbulence simulation. Experiment and simulation show strikingly similar shear layer formation mechanisms, supporting the basic theoretical picture of zonal flow formation via a nonlinear transfer if kinetic energy to large spatial scales. We also provide initial comparisons of drift turbulence measurements and simulation from the larger LAPD plasma device, and we discuss future plans for simulation validation efforts using this approach. [Preview Abstract] |
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RP1.00054: Numerical Linear Device: Numerical simulation of turbulent plasmas in linear devices Naohiro Kasuya, Masatoshi Yagi, Kimitaka Itoh, Masafumi Azumi, Sanae Itoh Structural formation mechanisms in magnetically confined plasmas are crucial issues in plasma physics. We have been developing a 3-dimensional numerical simulation code called Numerical Linear Device (NLD), which models a simple cylindrical plasma configuration. The 3-field (density, potential and parallel velocity of electrons) reduced fluid model is adopted. Using this code, quantitative comparison of turbulence characteristics with experimental results, such as identification of the observed instabilities, nonlinear saturation level of turbulence, wavenumber spectra and balance of momentum transport are analyzed. For describing the competition between drift and interchange modes, gravitation terms due to the magnetic curvature are added. Parameter scan predicts the experimental condition for excitation of turbulence. It is found that ion-neutral collision is an important parameter to excite drift wave turbulence, and ion viscosity weakly stabilizes the turbulence. Nonlinear simulation gives saturated turbulence. Turbulence characteristics of linear devices LMD and ECH in Kyushu Univ. and CSDX in UCSD will be discussed. [Preview Abstract] |
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RP1.00055: Long-Range and Local Dynamics of Fluctuations and Particle Transport in a Large Scale Laboratory Plasma M. Gilmore, L. Yan, N.A. Crocker, W.A. Peebles Experimental evidence for long-time correlations in fluctuations and turbulent flux, persisting hundreds of autocorrelation times, in the Large Plasma Device is presented. The plasma has three distinct radial regions, each of which exhibits different behavior on local as well as the long time scales. In the center, significant long-range correlation is found, but intermittency is moderate. At large radii, outside of the main plasma column, fluctuations and transport are dominated by convective avaloids, with significant long-range correlation, and high intermittency. Separating the two regions, is a layer where a sheared E$\times $B flow exists. It is shown that long-range correlation in the central region fluctuations and flux appears to be consistent with a Kolmogorov-type turbulent spectrum, as opposed to an avalanche-driven system. Additionally, these measurements suggest that the avaloids originate at the plasma edge, and do not begin as avalanches in the column core that spill out of the plasma. [Preview Abstract] |
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RP1.00056: Experimental Investigation of Active Feedback Control of Turbulent Transport in a Magnetized Laboratory Plasma S. Xie, M. Gilmore, S. Will, P. Ram, A.G. Lynn, C. Watts, C.T. Abdallah Many toroidal fusion devices now routinely generate edge and/or core transport barriers, where heat and particle transport are reduced far below Bohm diffusion levels. However, minimal particle transport is not necessarily desirable, since it can lead to core impurity accumulation, or alpha particle buildup. Ideally, active, stable control over the transport, rather than simple minimization, could be obtained. To this effect, research is now underway to investigate active control of particle transport. Turbulence and transport dynamics are, of course, strongly nonlinear, and apparently not deterministic. However, modern nonlinear control methods now exist, such as chaotic control and fuzzy control, which do not rely on a model of the system dynamics to affect stable control. Experiments are being conducted in the new HELCAT (HELicon-CAThode) linear device at UNM. HELCAT is a 4 m long device, with B $<$ 0.22 T, and cathode-produced densities, n $\sim $ 1-5$\times$10$^{12}$ cm$^{-3}$. Sheared flows, generated via biased concentric rings, will be utilized to modify the transport. Fluctuations and flux will be monitored with probe arrays. Initial experiments in a smaller, low B-field device to characterize the electrostatic fluctuations from a chaos perspective will be presented. [Preview Abstract] |
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RP1.00057: Magnetic field scaling of turbulence and transport in the Large Plasma Device L. Yan, M. Gilmore, N.A. Crocker, W.A. Peebles, T. Carter, G.Y. Antar Understanding the magnetic field dependence of turbulent-driven transport is a topic of significant interest to the plasma community. The LArge Plasma Device (LAPD: He$^{+}$ plasmas, a = 50cm, L = 17 m, n$_{e0} \quad \sim $ 1.5 10$^{12}$ cm$^{-3}$, T$_{e0} \quad \sim $ 15eV) at UCLA is ideally suited for such an investigation since it is highly reproducible and allows precise variation of magnetic field over a large dynamic range. A detailed study in the edge of LAPD has been performed where magnetic fields are varied from 500 to 1500 G, in 50 G increments. A peak in radial particle transport is observed, where the density gradient is largest and sheared azimuthal flow is maximum. This peak transport flux increases with reducing magnetic field. At the same location the turbulent correlation length also increases at lower magnetic field. Further analysis will include measurement of the fluctuation probability distribution functions and the application of conditional averaging to search for large scale structures such as avaloids. [Preview Abstract] |
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RP1.00058: Effects of Edge DC Biasing on Plasma Rotation and Transport in a Toroidal Geometry. Ashild Fredriksen, Claudia Riccardi We report results from experiments performed to study how a change in boundary conditions is affecting the plasma states in the toroidal geometry of the Blaamann device in Tromso. The boundary condition was changed by applying a DC bias on a limiter extended around the entire poloidal circumference of the plasma column. Two distinctly different plasma potential states were found. One state was associated with a bias at or negative with respect to the floating potential of the limiter, and a small ion saturation current. The other state was associated with a positive bias with respect to the floating potential, near or in the electron saturation regime of the limiter. In the latter case the potential minimum in the middle of the cross-section was significantly less negative than in the case of ion-saturation current to the limiter. On the other hand, the grounded limiter provided the best confinement properties, for which the density maximum was significantly higher than for both more positive and more negative biases. This state also had the lowest fluctuation levels, and near zero poloidal velocities close to the boundaries, as well as the smallest radial, anomalous particle transport. [Preview Abstract] |
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RP1.00059: Suppression of cross-field diffusion resulting from biased rotation of a cylindrical plasma J.E. Maggs, T.A. Carter, R.J. Taylor Results reported here are from experiments conducted in the LAPD at the BaPSF (Basic Plasma Science Facility). The LAPD produces a cylindrical, 17 meter long, plasma column using an electron beam emitted from an oxide coated cathode. The configuration used in these studies is a helium plasma with a uniform axial magnetic field. The plasma surrounding the main plasma core is rotated by externally applying a bias between an electrically floating section of the vacuum chamber wall and the cathode. The rotating plasma reaches a steady state within 1-2 t$_{c}$ after the application of the bias, where t$_{c}$ is the parallel particle confinement time, L/c$_{s}$. The edge gradient is observed to dramatically steepen due to the application of the bias. The unbiased plasma column exhibits an extended edge gradient with a scale length of about 100 ion gyro-radii, while the biased plasma can exhibit gradient scale lengths as small as 5-10 ion gyro-radii. End losses in the cylindrical plasma play a dominant role in the plasma dynamics, and the extended radial profile observed in the unbiased plasma requires cross-field particle diffusion at Bohm rates. The profile steepening observed in the rotating, biased plasma is consistent with complete suppression of anomalous cross-field particle diffusion. [Preview Abstract] |
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RP1.00060: Suppression of turbulent particle flux during biased rotation in LAPD T.A. Carter, J.E. Maggs, R.J. Taylor The edge plasma in LAPD is rotated through the application of a bias voltage (typically 100V-200V) between the plasma source cathode and the vacuum vessel wall. Without bias, cross-field turbulent particle transport causes the density profile to extend well past the cathode edge, with a fairly gentle gradient ($L_n \sim 10$~cm). As the bias voltage is applied and increased past a threshold value, the measured density profile steepens dramatically ($L_n \sim 2$~cm) at a radius near the peak of the flow shear. Turbulent transport flux measurements in this region show that the flux is reduced and then suppressed completely as the threshold is approached. As the bias voltage is increased further, the measured turbulent transport flux reverses direction. The amplitude of the density and azimuthal electric field fluctuations is observed to decrease during biased rotation, the product of the amplitudes decreasing by a factor of 5. However the dominant change appears in the cross-phase, which is altered dramatically, leading to the observed suppression and reversal of the turbulent flux. Detailed two-dimensional turbulent correlation measurements have been performed using the high repetition rate (1~Hz) and high reproducibility of LAPD plasmas. In unbiased plasmas, the correlation is localized to around 5~cm radially and a slightly smaller distance azimuthally ($\rho_s \sim 0.5-1$~cm). During biased rotation, a dramatic increase in the azimuthal correlation is observed, however there is little change in the radial correlation length. [Preview Abstract] |
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RP1.00061: Drift-Alfven wave transport studies in the Large Plasma Device Stephen Vincena, Walter Gekelman The Large Plasma Device (LAPD) at UCLA is a unique testbed for performing controlled and detailed transport experiments. Tailored density gradients have been created within an effectivly infinite (perpendicular to B$_0$) plasma using biased electrodes. Spontaneous density and potential fluctuations (drift waves) are observed in the steepest density region. These waves are correlated with cross-field particle flux and lead to a relaxation of the density gradient. The process of growth and disruption repeats in a semi-periodic fashion. Since the plasma beta is typically near the electron-to-ion mass ratio, the waves are of the drift-Alfven type and have both density and magnetic fluctuations. The background plasma is pulsed once per second for months at a time which allows for high spatial and temporal resolution measurements of the entire process, and for different background magnetic fields strengths, ion species, gradient scale lengths, and electron temperatures. [Preview Abstract] |
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RP1.00062: Internal Shear Flows in the Large Plasma Device Eric Lawrence, Walter Gekelman, Patrick Pribyl, Stephen Vincena Plasmas that have localized shear flow have long been of interest to both the fusion and space plasma physics communities. A small barium oxide coated nickel emissive cathode ($d \sim 10$ cm) was constructed to create a rotating plasma in the core of the background plasma column ($n_e \sim 3 \times 10^{12}$ cm${}^{-3}$, $0.4$ kG${} \le B \le 2.5$ kG, $d \sim 60$ cm, and $L \sim 18$ m) of the Large Plasma Device (LAPD) at UCLA. The background plasma is formed by a pulsed DC discharge from a large ($d \sim 0.7$ m) emissive cathode. The small cathode is located 12 m downstream from the background plasma source. It is surrounded by a ring anode to create $E \times B$ rotation in a 1 cm wide annular ring within the main plasma core. Previous laboratory shear flow experiments were done in devices that could only support electrostatic modes, while this experiment supports Alfv\'{e}n modes. Diagnostics include magnetic, Mach, and Langmuir probes, and laser induced fluorescence. Research supported by the Department of Energy and conducted at the Basic Plasma Science Facility at UCLA. [Preview Abstract] |
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RP1.00063: PLIF Studies of Flowing Plasmas Brett Jacobs, Walter Gekelman, Stephen Vincena A narrow sheet of pulsed plasma current $( \delta < R_{ci}, \delta \sim c/ \omega_{pe})$ is investigated in a strongly magnetized plasma column $(n \sim 3x10^{12} cm^{-3}, D > 500 R_ {ci}, B_{0} \sim 1kG, T_{rep} = 1sec)$ with a PLIF (Planar Laser Induced Fluorescence) diagnostic. The current is carried by electrons and is pulsed on after the background highly ionized, quiescent plasma is established. The formation of the current channel results in a deep density depression $(\delta n / n \sim .5)$ and triggers a complex flow pattern in the plasma. A 100ns (stretched) pulse from a tunable dye laser ( FWHM = .0018nm) is shaped into a thin slab by cylindrical optics and resonantly excites an electronic transition in a plane of argon ions. The fluorescence from the ions is recorded by a fast I-CCD camera as the laser's wavelength is tuned across the resonance line. In these experiments the ion flow pattern and distribution function is measured at thousands of spatial positions by sythesizing the I-CCD images of the LIF light. The short pulse length of the laser allows for measurement of the evolution of the flow, which occurs on the timescale of hundreds of microseconds. *Research supported by the Department of Energy and conducted at UCLA's Basic Plasma Science Facility on the LAPD. [Preview Abstract] |
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RP1.00064: Interaction of High Amplitude Alfv\'{e}n Waves with Density Striations David Leneman, Walter Gekelman, Patrick Pribyl, Bret Jacobs Examining the basic physics of Alfv\'{e}n waves propagating in non-uniform plasmas is important to understanding a wide range of phenomena in space. Using a multi-turn loop antenna we launch high amplitude (B$_{wave}$/B$_{0}$ = 10$^{-3})$ and measure the radiation pattern as the wave interacts with a density striation. The experiment is conducted in the Large Plasma Device (LaPD) at the University of California, Los Angeles. The cylindrical device produces a uniform magnetic field of up to 2.5~kG and noble gas plasmas 0.5 m in diameter and 18 m long by means of a pulsed, cathode-anode discharge, with densities of 3x10$^{12}$/cm$^{3}$. Striations are easily produced in the plasma by introducing a metal paddle into the plasma source. Plasma production is inhibited there and the deficit of plasma persists along the field lines, which connect back to the paddle. The radiation pattern is observed to rotate in the presence of the striation. We present LIF measurements of the ion distribution to explore ion heating and drifts. This experiment is carried out at the Basic Plasma Science Facility. [Preview Abstract] |
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RP1.00065: Laser-Produced Colliding Plasmas on LaPD Andrew Collette, Walter Gekelman, Stephen Vincena The expansion and interaction of dense plasmas in the presence of a magnetized background plasma is important in many astrophysical processes, among them shocks which transport energy. We study the collision of two dense, laser-produced plasmas expanding perpendicular to the background magnetic field, each with an Alfv\'{e}n Mach number of approximately 0.5. The plasmas are launched off of two carbon targets, 9cm apart, by a short pulse of laser energy (Nd:YAG, 1J 8ns). Experiments are currently in progress in a small test chamber at UCLA (background plasma n$\approx $3$\times $10$^{12}$, 3 meters long, B$_{0}<$700G) and will shortly be migrated to the LaPD (LArge Plasma Device; n$\approx $3$\times $10$^{12}$, 18 meters long, 70cm diameter, 400G$<$B$_{0}<$2.5kG). Additionally, previous investigations of laser-produced plasmas on LaPD have identified complex current systems associated with their expansion, which radiate Alfv\'{e}n and Lower Hybrid waves. We present an analysis of the shocks and waves produced by these interactions and their effect on the background plasma. Work supported by the NSF and done at the BAPSF (UCLA). [Preview Abstract] |
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RP1.00066: Supercritical Radiative Shocks Do Not Exist R. Paul Drake The concept of a supercritical regime for radiative shocks arose historically in the context of media that are optically thick to x-rays. Such a shock has an upstream state, a precursor region, a density jump, a downstream cooling zone, and a downstream steady state. From thermodynamic arguments, the temperature T$_p$ in the (presumed near LTE) precursor region cannot exceed that in the final downstream state, T$_f$. Fluid dynamics analysis shows that T$_p$ might have any value up to T$_f$. If one uses an ``equilibrium diffusion'' model for the radiation transport (a single group diffusion model), one concludes that there is a ``supercritical'' regime in which T$_p$ = T$_f$ and in which the temperature gradient across the density jump provides the required upstream energy flux. However, the equilibrium diffusion model cannot be valid near such a shock. By simple analysis of the energy balance, or by more reasonable treatment of the radiation transport, one can show that such shocks never actually reach the supercritical regime, which is better viewed as a limiting case. This research was sponsored by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through DOE Research Grant DE-FG52-03NA00064, and other grants and contracts. [Preview Abstract] |
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RP1.00067: Irreversible dissipation at collisionless shocks: Nonlinear ion-acoustic instability Li-Jen Chen, Abigail Rymer, William Kurth, Donald Gurnett A number of wave instabilities have been proposed to account for irreversible dissipation across collisionless shocks. However, the most dominant instability has not been identified. For example, observations at Earth have not been able to determine the exact wave mode due to overlapping of the corresponding characteristic frequencies. We use Cassini observations at the bow shocks of Saturn and Earth to rule out a majority of previously proposed instabilities and identify the most dominant instability as the nonlinear ion-acoustic instability. Electron distributions at the bow shocks of Earth and Saturn, and interplanetary shocks are shown to have a common flat-topped component. We carry out particle simulations to demonstrate that the nonlinear ion-acoustic instability can produce turbulent wave fields that efficiently heat electrons and produce flat-topped electron distributions. Our observation and simulation results indicate that the nonlinear ion-acoustic instability is the most important contributor to irreversible dissipation across collisionless shocks. [Preview Abstract] |
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RP1.00068: Anisotropic Radiation Spectra from Strong Weibel Turbulence Mikhail Medvedev We refer the end state of the nonlinear Weibel instability to as the ``Weibel turbulence''. This state is observed in a wide variety of plasma environments, ranging from laser-plasma interaction systems and Fast Ignitor to collisionless unmagnetized shocks in space driven by cosmic explosions --- supernovae and gamma-ray bursts. Strong Weibel turbulence is characterized by quasi-steady-state, high-amplitude magnetic field inhomogeneities residing on a plasma skin depth scale. The dynamics and evolution of these fields is governed by strong nonlinear interactions of the associated current filaments, and is accompanied by efficient anisotropic heating of the electrons. We study the radiation emitted by these electrons and present a fully analytical calculation of the radiation spectrum. The anisotropy of the electron PDF and the magnetic filament orientation are taken into account. We obtained that the power spectrum $P (\omega)$ varies from $\propto\omega^1$ to $\propto\omega^0$ as the viewing angle $\Theta$ (with respect to the mean direction of the current filaments) varies from $0$ to $\pi/2$. We stress that the spectral analysis will provide accurate diagnostics of the plasma conditions in Fast Ignitor, astrophysical shocks and other systems. [Preview Abstract] |
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RP1.00069: Plasma Physics of Photoionized Gases by Short X-ray Pulses D.V. Romanov, V. Yu. Bychenkov, W. Rozmus, C.E. Capjack, R. Fedosejevs A study of nonequilibrium plasma that is created by a femtosecond, linearly polarized x-ray pulse in a gas jet target is presented. The x-ray photoionized gas is described by the anisotropic electron distribution function (EDF) that exists on the picosecond time scale, i.e. until particle collisions establish isotropic velocity distribution. This EDF gives rise to electromagnetic Weibel instability and electrostatic two stream instability. Linear theory and nonlinear evolution of these instabilities are described analytically and by multidimensional particle-in-cell (PIC) simulations. Static magnetic field generation and terahertz nonmonohromatic video-pulse irradiation from such plasmas is predicted. The model accounts for thermo-EMF at the edge of a plasma and the anisotropic EDF due to photoionization. Results from 3-dimensional PIC simulations show new unexpected scenarios of Weibel and two-stream instability nonlinear evolutions in these anisotropic plasmas. [Preview Abstract] |
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RP1.00070: Nonadiabatic Ponderomotive Barriers Ilya Dodin, Nathaniel Fisch Ponderomotive, or wave barriers produced by stationary high-frequency radiation are most often viewed as potential barriers conserving the particle energy. This conservation property is only approximate though, and we show that its violation renders a remarkable and previously unexplored flexibility for manipulating particles by electromagnetic fields. We show that irreversible wave barriers can exhibit features of a Maxwell demon and can be employed for current drive, selective confinement, cooling, and separation of constituents in gases and plasmas. While being of immediate applied interest, these possibilities as well entail fundamental questions about striking quantum-like nonlinear dynamics, which we show classical particles to exhibit in high-frequency fields. [Preview Abstract] |
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RP1.00071: Autoresonant Excitation and Control of Multiphase Nonlinear Waves Lazar Friedland Plasmas and fluids are examples of extended systems described by nonlinear differential equations possessing many nontrivial solutions. The question of reaching and controlling a particular stable solution (pattern) in this set of solutions by starting from simple initial/boundary conditions is fundamental to many applications. A general recent approach to nonlinear pattern formation is based on capturing the system into resonance with slow external perturbations followed by a continuing self-synchronization (autoresonance) in space and/or time. Applications of this paradigm exist in vorticity dominated flows, plasmas, planetary dynamics, atomic and molecular physics. The synchronization means excursion in the system solutions space with possible emergence of the desired nonlinear state. I will describe new developments in the field and recent applications to excitation of multiphase nonlinear waves in the Korteweg-de-Vries and nonlinear Schrodinger systems. [Preview Abstract] |
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RP1.00072: Equilibrium statistical mechanics of self-consistent wave-particle system Yves Elskens The equilibrium distribution of $N$ particles and $M$ waves (e.g. Langmuir) is analysed in the weak-coupling limit for the self-consistent hamiltonian model $H = \sum_r p_r^2 /(2m) + \sum_j \omega_j I_j + \varepsilon \sum_{r,j} (\beta_j / k_j) \cos (k_j x_r - \theta_j)$ [1]. In the canonical ensemble, with temperature $T$ and reservoir velocity $v < \inf_j {\omega_j/k_j}$, the wave intensities are almost independent and exponentially distributed, with expectation $\langle I_j \rangle = k_{\rm B} T / (\omega_j - k_j v)$. These equilibrium predictions are in agreement with Monte Carlo samplings [2] and with direct simulations of the dynamics, indicating equivalence between canonical and microcanonical ensembles. \par\noindent [1] Y. Elskens and D.F. Escande, Microscopic dynamics of plasmas and chaos (IoP publishing, Bristol, 2003). \par\noindent [2] M-C. Firpo and F. Leyvraz, 30th EPS conf. contr. fusion and plasma phys., P-2.8 (2003). [Preview Abstract] |
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RP1.00073: Numerical Simulation of Driven Electron Acoustic Waves. F. Valentini, D.H.E. Dubin, T.M. O'Neil Eulerian and PIC code simulations of the electron acoustic wave (EAW) are presented. This novel, low amplitude, BGK wave has the approximate dispersion relation $\omega \simeq 1.3 \, \bar{v}_e k$, where $\bar{v}_e$ is the electron thermal velocity and the wave number $k$ is assumed to be small (i.e., $k \lambda_D \ll 1$).\footnote{J.P. Holloway and J.J. Dorning, Phys. Rev. A {\bf 44}, 3856 (1991).} Within linear theory, the wave is heavily Landau damped, but the damping does not occur for a BGK wave since the electron distribution is flat (i.e., $\partial f_e / \partial v = 0$) in the immediate vicinity of the phase velocity. Simulations of a collisionless plasma show that an EAW is excited by a low amplitude resonant driver if the driver is applied over a long enough time (several trapping periods). When collisions are included in the simulation, successful excitation requires a sufficiently large driver amplitude. The trapping period for the driver must be short compared to an effective collision time--the time for small-angle Coulomb scattering to produce velocity diffusion over the width of the trapped-particle plateau. [Preview Abstract] |
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RP1.00074: Thomson Scattering Detection of Ponderomotively Driven KEEN Waves in a Laser Produced Plasma J.L. Kline, B.B. Afeyan, W.A. Bertsche, R.P. Johnson, N.A. Kurnit, D.S. Montgomery, V. Savchenko, K. Won, C. Niemann Kinetic simulations using ponderomotively driven excitations have~discovered the existence of stable, nonlinear, multimode coherent~ structures in plasmas named Kinetic Electrostatic Electron Nonlinear~ (KEEN) waves.$^{1}$ For a given wave number drive, they form and persist~ for drive frequencies in a broad range of ($\omega $,k) space lower in~ frequency that electron plasma waves and much higher than ion~ acoustic waves. Experiments were conducted on TRIDENT by~Polymath Research Inc., in collaboration with LANL, to detect these~waves driven by the beating of two laser beams.~The two lasers used~had 527 and 600 nm wavelengths which is predicted to drive waves in~the proper KEEN wave excitation band.$^{1}$ A nitrogen gas Raman cell was~used to convert a 527 nm laser beam to~600 nm. We drove KEEN waves and detected them with~263 and 351 nm Thomson scattering in nitrogen/hydrogen and helium gas~jet plasmas. This presentation will cover experimental conditions and~diagnostic attributes associated with the detection of KEEN waves.~We will discuss the properties of KEEN waves as a function of probe~beam intensity and as a function of pump-probe overlap time. Supported by DOE Academic Alliance Grant DE-FG03-03NA00059 and LANL~under Contract No. W-7405-ENG-36. $^{1}$ B. Afeyan et al., Proc. IFSA (2003, Monterey, CA), 213, Amer Nucl Soc, 2004. [Preview Abstract] |
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RP1.00075: Nonlinear oscillations in a unijunction transistor (UJT) circuit John Zielinski, Mark Koepke Phenomena such as plasma waves\footnote{T Tsuru, Nonlinear resonance phenomena of elect. plasma oscillations by beam modulation, J. Phys. Soc. Japan, 40, 548, 1976.} and oscillations in electric circuits which employ a plasma component\footnote{M Wendt, I Axnas, S Torven, Amplitude collapse of nonlinear double-layer oscillations, Phys. Rev. E, 57, 4638, 1998. } can be described by a differential equation with nonlinear dissipative and restoring force terms. The UJT oscillator circuit developed by Koepke and Hartley\footnote{ME Koepke, DM Hartley, Experimental verification of periodic pulling in a nonlinear electronic oscillator, Phys. Rev. A, 44, 6877, 1991} is also described by a similar equation. During the past year efforts have been made to understand the following aspects of this circuit's operation: 1) Determining conditions which lead to oscillation onset and termination (amplitude collapse). 2) Analytic and numerical modeling. 3) Characterizing the capacitances associated with the emitter-base junctions. 4) Exploring the relationship between this circuit and astable multivibrators. [Preview Abstract] |
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RP1.00076: Observation of plasma hole transition induced by microwave power modulation Shinji Yoshimura, Masayoshi Tanaka Spontaneous formation of stationary vortex structure with density depletion, or \textit{plasma hole}, has been observed in a cylindrical magnetized plasma. We have so far reported the flow velocity field, the vorticity distribution and the strong electric field resulting from the breaking of quasi-neutrality condition. The detailed transitional behavior of plasma hole formation is not entirely revealed yet. Here we report the results on microwave power modulation experiments to investigate the detailed change in density, plasma potential and flow velocity field of the plasma hole. Temporal evolutions of those quantities were measured by using a Langmuir probe, an emissive probe and a directional Langmuir probe. By increasing the microwave power to a certain threshold value, the density depletion characterizing the hole structure was formed along the central axis of the cylindrical plasma. The plasma potential started building up a bell-shaped distribution simultaneously, resulting in ExB rotation of the plasma. It should be emphasized that a definite inward-directed flow, which implies an anomalous viscosity of the plasma, was observed. A general relation between radial flow and viscosity is derived through the analysis of ion fluid equation, suggesting a possible means of determination of viscosity coefficient. [Preview Abstract] |
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RP1.00077: Nonlinear Stability Behaviour of the Rotation Dynamics in the Collisional Tokamak Edge Layer Cuma Yarim, Umur Daybelge, Albert Nicolai Toroidal and poloidal rotations of a collisional tokamak edge plasma with steep temperature and density gradients are investigated via the fluid equations. In the stationary case, multiple valuedness of the poloidal velocity near the separatrix and its catastrophic properties are studied in scope of the theory. The effects of stochasticity in temperature distribution on both toroidal and poloidal speeds and trajectory bifurcations in phase space are discussed. [Preview Abstract] |
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RP1.00078: Shock wave acceleration in weakly ionized plasmas Nirmol K. Podder, Anastasia V. Tarasova, Ralph B. Wilson IV A significant increase in velocity and considerable decrease in amplitude result in a supersonic electromagnetic shock wave when it is allowed to traverse into the positive column of an argon glow discharge plasma. The simultaneous increase in velocity and decrease in amplitude of the shock in plasma is not fully understood. In this work, a comprehensive set of measurements is performed on the plasma, shock wave, and their correlation. Plasma parameters are measured through the use of Langmuir probes, while the shock parameters are monitored through a multipoint laser deflection technique. These measurements enable the isolation of the plasma and gas parameters responsible for the shock modification in plasma. [Preview Abstract] |
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RP1.00079: Quasicoherent Nonlinear Interactions in Ion Density Fluctuations Ilker Uzun, Frederick Skiff Using Laser Induced Fluorescence(LIF) diagnostics, we present the phase-space resolved measurement of ion density fluctuations in a linearly magnetized device. These experiments are conducted in singly ionized ArII plasma, which is created by an inductively coupled plasma source, immersed in 1KG magnetic field along the axis of a cylindrical chamber. Cross and auto spectral estimates are calculated after the digitization of LIF signals collected from two independent periscopes having axially aligned mobility. The cross-power spectrum has a distinctive, broad peak near the drift wave frequency($\omega^{*}$) that can be dissected into two components; the one having a long wavelength consistent with drift wave theory, and the other, which we call the `kinetic component,' with a shorter correlation length and ion particle velocity dependence. Bispectrum and bicoherence calculations, on the other hand, suggest a quasicoherent nonlinear interaction between the kinetic and the fluid components. As the neutral pressure and thus the ion-neutral collision frequency increase, we observe a certain threshold where the kinetic component vanishes from the cross-power spectrum. Meanwhile, in bicoherence calculations, the nonlinear coupling of $\omega^{*}$ to its sub-harmonics is taken over by another nonlinear interaction that is between $\omega^{*}$ and its second harmonic. We propose an explanation for these changes in the spectra and bispectra. [Preview Abstract] |
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RP1.00080: NSTX Research, Recent Results, and Advancing Plasma Science Through Complementarity with Other U.S. Program Elements E.J. Synakowski Presented here are highlights from 2005's research effort. Emphasis is placed on complementarity between NSTX and moderate aspect ratio devices that makes for a scientifically powerful whole and advances fusion science generally. This year, advantage was taken of increased shaping, control, and wall preparation capability that enables connection with and extension beyond the moderate aspect ratio database in key parameters such as plasma beta and shaping factors. The 2005 NSTX research approach was broad, with advances made in understanding of MHD and mode control, transport and turbulence, wave and wave-particle physics, fast- ion MHD, solenoid-free startup and sustainment, and boundary physics. Integration of key elements of each topic enabled advances towards a long-term goal of high beta, solenoid-free sustained operations. A description of progress and issues on each of these topics, especially as they take advantage of key similarities and differences with other elements within the U.S. and world programs, will be outlined. [Preview Abstract] |
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RP1.00081: Progress of NSTX Program in Physics Basis for 10-MA Devices Y.-K. M. Peng Recent progress in Spherical Torus (ST) plasma science has indicated relatively robust and attractive physics conditions in a number of topical areas including shaping, stability limits, energy confinement, self-driven current, sustainment, and divertor heat flux. This progress has enabled an updated projection of the plasma conditions of a 10-MA ST such as the Component Test Facility (CTF), which is a necessary step in the development of practical fusion energy. The results indicated designs with R$_{0}$ = 1.2 m, A = 1.5, elongation $\sim $ 3, B$_{T} \quad \sim $ 2 T, producing a fusion burn power of 140 MW, and a fusion neutron flux of 2 MW/m2, driven by 50 MW of combined neutral beam and RF heating and current drive power. The design uses a single-turn toroidal field coil center leg without a central solenoid, and will require physics data on solenoid-free plasma current initiation, ramp-up to, and sustainment at multiple MAs. An assessment of the ST physics basis to establish the design of such a 10-MA device and comparison with the present and planned achievements of the NSTX Program will be presented. [Preview Abstract] |
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RP1.00082: Development of Particle and Impurity Control Techniques for NSTX M.G. Bell, H.W. Kugel, D.A. Gates, R. Kaita, D. Mueller, C.H. Skinner, B.C. Stratton, R. Maingi, V.A. Soukhanovskii, R. Raman The existing techniques for impurity control in NSTX of bakeout, boronization at about 2-week intervals, and helium glow- discharge between shots, have been expanded. Boronization during bakeout, using glow-discharge decomposition of $\sim$10g of deuterated trimethyl boron (DTMB), yields an initial improvement over boronization on cold surfaces. However, the effects of hot and cold boronization become comparable as plasma exposure accumulates. A brief ($\sim$15min) boronization applied in the morning before an experiment using only about 1g of DTMB can then restore good conditions. Density control is also needed to achieve efficient current-drive for non-inductive sustainment. Repeated ohmic helium discharges reduce recycling from the plasma-facing surfaces. Injection of lithium pellets into these discharges has now been used to deposit lithium on either the center column or the lower divertor target. In subsequent deuterium plasmas with 4MW of NBI, the volume-average densities were reduced by factors 4 - 5 and the density profiles became peaked for both divertor and limiter configurations. [Preview Abstract] |
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RP1.00083: Charge Exchange Recombination Spectroscopy on NSTX R.E. Bell, D.W. Johnson, R. Feder, T.M. Biewer A high throughput, high spatial resolution charge exchange recombination spectroscopy diagnostic is operating on the National Spherical Torus Experiment. Two f/1.8 fixed-wavelength transmission grating spectrometers are coupled to thinned back-illuminated CCD detectors. Emission from C VI $^{ }$5291 {\AA} is measured along 51 sightlines viewing three neutral beam sources with 0.5-3 cm resolution from edge to core every 10 ms. 39 sightlines not viewing the neutral beams measure background emission. Spatial and absolute photometric calibrations are conducted in vessel. Wavelength and instrumental function calibrations are performed using a neon glow. The data analysis consists of fitting and modeling the background emission, fitting the charge exchange emission in the active view, a beam attenuation calculation, Zeeman correction, computation of the effective charge exchange cross section, and correcting for the effects of the energy dependent charge exchange cross section on ion temperature and velocity. Fully automated data acquisition and analysis codes provide between-shot availability of fully corrected profiles of T$_{i}$, V$_{\phi } $, N$_{c}$, and Z$_{eff}$. [Preview Abstract] |
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RP1.00084: Measurements of ion and electron temperature profiles on NSTX with an X-ray imaging crystal spectrometer M. Bitter, C. Bush, K.W. Hill, L. Roquemore, B. Stratton, D. Mastrovito, P. Beiersdorfer, M.F. Gu The prototype of a new X-ray imaging crystal spectrometer has been installed on NSTX to measure profiles of the ion and electron temperatures from spatially resolved dielectronic satellite spectra of ArXVII in the wavelength range from 3.9 to 4.0 A [1]. The spectrometer consists of a spherically bent 110-quartz crystal, with a radius of curvature of 389 cm and a diameter of 10 cm, and a 10 cm x 30 cm, two-dimensional, position-sensitive, multi-wire proportional counter. It projects an image of a large area of the plasma with an extension of +/- 40 cm below and above the horizontal mid-plane of NSTX onto the detector with a demagnification of 2.5. The resolution in the plasma is solely determined by the Bragg angle, the height of the crystal and its distance from the plasma; and it is about 3 cm, if the crystal is fully opened. The concept of this new spectrometer is also of interest for ion temperature measurements on ITER [2]. The paper will present results from profile measurements of the ion and electron temperature from NSTX discharges with pure ohmic heating as well as RF and neutral-beam heating. [1] M. Bitter et al., Rev. Sci. Instrum.75, 3660 (2004); [2] R. Barnsley et al., Rev. Sci. Instrum.75, 3743 (2004). [Preview Abstract] |
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RP1.00085: Upgrade to the NSTX Thomson Scattering Diagnostic Benoit LeBlanc, David Johnson The Multi-Pulse Thomson Scattering (MPTS) diagnostic has undergone a hardware upgrade providing 10 new spatial channels, which brings the total number of radial locations to 30. Mirror collection optics focuses the gathered light unto 36 existing fiber bundles and, until recently, only 20 of these were instrumented. In the selection of which unused bundles to add, priority was given to increasing the spatial resolution at the outer edge. Seven bundles viewing the outer edge and two viewing the high field side were selected. We took advantage of a feature of the optical/mechanical design permitting to rearrange the bundle output end into sub-bundles. One outer-edge viewing bundle was split in two to further increase the spatial resolution in the ``pedestal'' region, resulting in an adjacent channel spacing of 0.9 cm. Spatial resolution ranges from 0.9 cm to 3 cm in the region of interest. In contrast with the original 6-filter design, which includes one channel dedicated to Rayleigh scattering calibration, the new polychromators have four spectral channels and do not include a Rayleigh filter. Raman scattering will be used to calibrate the new spatial channels. [Preview Abstract] |
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RP1.00086: Tangential multi-color ``optical'' soft X-ray array for electron temperature and transport measurements L.F. Delgado-Aparicio, D. Stutman, K. Tritz, M. Finkenthal, R. Bell, D. Johnson, R. Kaita, H. Kugel, B. LeBlanc, L. Roquemore A multi-color ``optical'' soft X-ray array is being prototyped on NSTX for fast ($\leq$0.1 ms) measurements of the electron temperature profile, heat and particle transport, and MHD activity. Tests of a single-color array indicate that the optical device can have a signal-to-noise ratio superior to that of diode arrays. In addition, due to the compact nature of the diagnostic the plasma access is improved. We designed, built, and tested on NSTX, a tangential 48-channel ``optical'' soft X-ray array which views the same plasma volume at the mid-plane, in three different energy ranges determined by different SXR metallic foils. Depending on the electronics to be used, the diagnostic can have a time resolution ranging from a few $\mu$s to $\sim$0.1 ms, with spatial coverage from 0$\leq$r$\leq$1. We plan to obtain space and time resolved electron temperature measurements for perturbative transport studies. Initial results from a range of operational regimes in NSTX will be presented. [Preview Abstract] |
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RP1.00087: Plasma Flow Measurements on NSTX using the SWIFT (Shifted Wavelength/Interference Filter Technology) diagnostic Stephen Paul, Robert Kaita, Lane Roquemore, Brett McGeehan, Nobuhiro Nishino The crossed interference filter technique for measuring ion flows is being extended from individual analog channels (as implemented on Columbia University's HBT-EP device) to an entire 2-D view on NSTX. A 40, 500 frame per second Ultima SE CMOS digital camera, manufactured by Photron, Ltd. is being used to view He II emission at 486.6 nm from the inside edge of the plasma on both sides of the center stack. In this way, both blue and red shifts are being monitored so an unambiguous measurement of parallel flow is available. The image is split and filtered with separate interference filters with precisely calibrated opposite linear slopes. Careful white-plate calibration of the 64 x 64 pixel detector is necessary, but once completed, the ion velocity can be calculated very simply from the ratio of the intensities from the two images. By comparing these measurements with the Edge Rotation Diagnostic on NSTX, asymmetries in parallel flows between the inboard and outboard edges of the spherical tokamak plasma will be able to be measured. A second experiment involves measuring the outward radial flow of clusters of helium ions (`blobs') at the outer edge of the plasma. [Preview Abstract] |
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RP1.00088: The Motional Stark Effect (MSE) Diagnostic on NSTX Howard Yuh, Fred Levinton The adoption of the motional Stark effect (MSE) polarimetry diagnostic is due to its very good temporal and spatial resolution of the q- profile, combined with its exceedingly good accuracy. This has resulted in many important scientific contributions towards our understanding of stability and transport. This work describes the implementation of the MSE-CIF diagnostic on NSTX. Due to the low magnetic field on NSTX the implementation of the MSE diagnostic requires a different approach for the viewing optics and spectral filter. The diagnostic views a heating beam with 8 inch collection optics, imaged onto a fiber array. The optical system is configured to maximize the polarization fraction by reducing the Doppler broadening from the heating beam. This is done with a vertical aperture in front of the collection optics to reduce geometric Doppler broadening. In addition, a wide field Lyot spectral filter with high throughput and high resolution has been developed to achieve the necessary signal-to-noise. Results with the MSE-CIF diagnostic have been obtained at magnetic fields $\geq$0.3 Tesla with eight channels providing coverage from the magnetic axis to near the outboard edge. The number of spatial channels can be increased to 19 in the future. Results of various plasmas regimes including L-mode, H-mode, and reversed shear will be presented. [Preview Abstract] |
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RP1.00089: Progress towards measurement of ETG turbulence on NSTX D.R. Smith, R.E. Feder, E. Mazzucato, H.K. Park, L. Lin, C.W. Domier, M. Johnson, N.C. Luhmann, Jr. Installation of the high-k scattering system on NSTX is nearly complete. The system will measure density fluctuations on scales relevant to electron temperature gradient (ETG) turbulence. System components include a backward wave oscillator source providing $\sim\!150$ mW at 280 GHz and a superheterodyne receiver with five simultaneous detection channels and noise temperatures of $\sim\!5000$ K. The system is configured for tangential scattering with the probe beam and five scattered beams lying nearly on the toroidal midplane. The scattered beams will sample radial density fluctuations with wavenumbers $k_r\!\le\!20\;\mbox{cm}^{-1}$. The 6 cm diameter probe beam provides good $k$-space resolution at $\Delta k_r\! \le\!0.7\;\mbox{cm}^{-1}$. Excellent spatial localization can be achieved at small scattering angles due to the large toroidal curvature of the spherical torus geometry. Steerable launch and detection optics can position the scattering volume either near the magnetic axis at $\rho\!\approx\!0.1$ or near the edge at $\rho\!\approx\!0.8$. The system will measure fluctuations with $k_r\rho_e\!\le\!0.6$ and $\tilde{n}/n\!\ge\!10^{-4}$ to investigate the existence of ETG turbulence. The connection between ETG turbulence and electron thermal transport remains a controversial issue. $^*$This work was supported by the U.S. Department of Energy under contract numbers DE-AC02-76CH03073 and DE-FG02-99ER54518. [Preview Abstract] |
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RP1.00090: Fast visible 2-D camera imaging on NSTX A.L. Roquemore, R. Kiata, D. Darrow, S. Zweben, T. Gray, N. Nishino, R. Maqueda, C. Bush, L. Dorf, G. Wurden, K. Shinohara Initial applications of 2-D cameras on tokamak devices were mainly used to gain qualitative insight into global dynamics such as plasma formation and motion. Quantitative data were limited, due in large part to the time scales of most plasma events that were far faster than the frame rate of the camera and the inability to resolve structures at the characteristic plasma scale lengths. Recent advances in the development of fast cameras increased the frame rate up to megahertz rates while exposure times as short as 3nsec have been achieved with intensifiers. NSTX supports 8 fast visible cameras with framing rates ranging from 10$^3$ to 10$^6$ frames/sec with resolution from 64 X 64 to 1024 X 1080 pixels at the shortest exposure times. These cameras have been used to study plasma phenomena including edge and divertor turbulence, ELMs, and impurity transport. The most recent applications include studies of supersonic gas jets, pellet ablation, incandescent dust behavior, and measurement of fast ion loss of neutral beam injected particles. The capabilities of each camera will be presented and movies of plasma phenomena recorded with them will be presented. [Preview Abstract] |
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RP1.00091: Fast imaging of visible phenomena in NSTX R.J. Maqueda, C.E. Bush, L. Roquemore, K.M. Williams, S.J. Zweben Edge phenomena are important for global plasma confinement as well as power and particle handling and distribution to plasma facing components. High frame rate, 2-D imaging is a powerful tool to access the physics behind these phenomena which include: edge turbulence and ``blobs,'' ELMs, and MARFEs. This diagnostic is also useful in general plasma equilibrium and dynamics measurements, like those during Coaxial Helicity Injection discharges, and in pellet injection experiments. A new Phantom 7 fast-framing digital camera has been installed in NSTX which has been used at frame typically ranging between 68000 frames/s and 120000 frames/s and full discharge coverage (frames recorded for over 2 s). Examples will be presented showing the usefulness of this diagnostic for physics studies in the areas mentioned above. [Preview Abstract] |
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RP1.00092: Recent Analysis of Fast Time Series GPI Data on NSTX Kyron M. Williams, Ricardo Maqueda, Stewart J. Zweben, Charles E. Bush The gas puff imaging diagnostic (GPI) is used to study edge turbulence near the outer midplane of NSTX. The detectors include an ultra-fast camera and an independent array of 13 discrete phototubes, each of which views a 2 cm diameter region of the edge. These discrete detectors produce time series data with a 500 kHz digitization speed over 128 ms during a shot. Edge turbulence measurements were made with these discrete detectors under conditions of varying q-profiles, divertor configurations, NBI or RF heating conditions, plasma densities, and plasma currents. This poster will describe this data set and attempt to identify characteristic turbulence features in each of these conditions. [Preview Abstract] |
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RP1.00093: Bispectral analysis of the L-H transition as seen in the NSTX GPI data A.E. White, M.J. Burin, T.A. Carter, T.S. Hahm, J.A. Krommes, R.J. Maqueda, S.J. Zweben It was recently reported that the bicoherence of turbulent potential and density fields measured with Langmuir probes increases just prior to and during the L-H mode transition in DIII-D. By calculating the evolving 3 wave-coupling properties between turbulent and large scales with bispectral statistics, we can attempt to identify the formation of possible shear and/or zonal flows at the L- H transition. This poster will present bispectral analysis of the L-H mode transition in NSTX using the data obtained by the Gas Puff Imaging (GPI) diagnostic, with the goal of searching for coupling between modes which may lead to shear and/or zonal flows generated by turbulence. A radial and poloidal array of 13 detectors measures the HeI or D$_\alpha$ light emitted from the plasma and this array can be used to measure profiles of the bicoherence before, during, and after an L-H transition. The bicoherence measured using this collection of 1-D time series data will be compared to the 2-D images of the turbulence obtained from a high-speed camera. [Preview Abstract] |
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RP1.00094: Velocity Field Analysis of NSTX Gas Puff Imaging Data T. Munsat, R. Maqueda, J. Myra, T. Stoltzfus-Dueck, S. Zweben Time-resolved 2-D maps of velocity have been derived for fluctuation measurements in the edge plasma of NSTX using an optical flow technique. The velocity maps are derived from image sequences produced by the Gas Puff Imaging diagnostic, which covers a poloidal section of the outer plasma edge. The velocity maps are resolved to the same level as the data ($\sim$1 cm spatial resolution, 4 $\mu$s time resolution). Details and limitations of the optical flow technique are presented, including global velocity limits, aperture effects, and comparisons to other velocity derivation methods. Analysis of NSTX data, both in L-mode and H-mode plasmas, includes the comparison of radial and poloidal flow, implications for intermittent plasma transport, and statistical properties of the velocity distribution both in space and time. [Preview Abstract] |
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RP1.00095: Analysis of Flow Patterns in NSTX Edge Turbulence T. Stoltzfus-Dueck, N. Pomphrey, J. Krommes, S. Zweben, T. Munsat, R. Maqueda The dynamics of turbulent fluctuations near the edge of plasma confinement devices are thought to have a significant impact on plasma performance. The Gas Puff Imaging (GPI) diagnostic provides movies of density and temperature fluctuations in the tokamak edge region with high temporal ($\sim$4 microsecond) and spatial ($\sim$1-2 cm) resolution. The Principal Component Analysis (PCA) method provides an optimized representation of each movie in terms of empirically derived spatial `eigenfunctions' multiplied by time-dependent amplitudes. In this poster, new methods for the inference of a velocity field from the PCA representation of turbulent fluctuations, as well as implications related to the theory of edge turbulence, are discussed. [Preview Abstract] |
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RP1.00096: Blob birth and transport in NSTX: GPI data analysis and theory J.R. Myra, D.A. D'Ippolito, D.A. Russell, D.P. Stotler, S.J. Zweben, R. Maqueda, J. Boedo, T. Munsat Movies of blobs (i.e. convecting filamentary structures in the scrape-off-layer) taken with the gas-puff-imaging (GPI) diagnostic are used to extract blob parameters: birth zone, scale size, radial velocity $ v_x $ and (with DEGAS-2 modeling to infer plasma density and temperature from the He 5876 emission), density and temperature. These measured properties are compared with theory. It is shown that the birth zone and blob parameters are related to the local maximum of the edge $ \nabla ln \langle p \rangle $ suggesting blob generation by an underlying edge instability. The observed blobs are plotted on a theoretical regime diagram,\footnote{J.R. Myra, D.A. D'Ippolito, Lodestar Report \#LRC-05-105, May, 2005.} and mostly lie in the sheath-connected regime. The observed $ v_x $ are equal to, or exceed, a minimum velocity scaling predicted by theory. The excess depends on position and is qualitatively consistent with separatrix effects. However, some additional physics not in the present model also influences $ v_x $. [Preview Abstract] |
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RP1.00097: Simulation of convective cross-field transport, toroidal plasma flows, and dust dynamics in NSTX with UEDGE and DUSTT codes. A.Yu. Pigarov, S. Krasheninnikov, J. Boedo, R. Bell, S. Paul, A. Roquemore, V. Soukhanovskii, R. Maingi, C. Bush, T. Soboleva Fast intermittent convective cross-field transport has been observed in the outer SOL of NSTX and other tokamaks. It is expected that such kind of transport has ballooning like asymmetry and can be a cause of large parallel plasma flows in SOL. With UEDGE code, we perform multi-species fluid simulations in the LSN magnetic configuration of NSTX L-mode plasma using poloidally asymmetric profiles for anomalous transport coefficients and convective velocities and for some boundary conditions on the chamber wall. We present modeling results on SOL plasma flows originating from outer mid-plane, moving into inner divertor, and reaching M$\sim $1 at inner mid-plane. The UEDGE analysis of experimental NSTX data with newly developed 3D diagnostic tools (e.g. for bolometry) will be given. Also, as measured, dust particulates of micron size are unavoidably present in NSTX. We present results on simulation of dust dynamics, transport, and ablation with DUSTT code. The possible effect of dust on NSTX divertor plasma profiles is discussed. The research was supported by DoE Grants NRG5025 and DE-FG02-04ER54739 at UCSD. [Preview Abstract] |
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RP1.00098: RWM Critical Rotation Frequency and Beta Dependence in NSTX Aaron Sontag, S.A. Sabbagh, J.M. Bialek, W. Zhu, H. Reimerdes, A.M. Garofalo, J.E. Menard, D.A. Gates, R.E. Bell, M.G. Bell, B.P. LeBlanc, D.J. Battaglia The resistive wall mode (RWM) can be stabilized by maintaining the plasma toroidal rotation frequency ($\omega _{\phi })$ above a critical rotation frequency ($\Omega _{crit})$. Recent experiments on NSTX seek to determine $\Omega _{crit}$ and rotation profile effects through actively braking plasma rotation by the application of external magnetic fields. Results from these experiments indicate that maintaining $\omega _{\phi }$ at the q = 2 surface above $\omega _{A}$/4q$^{2}$ is a necessary condition for RWM stability where $\omega _{A}$ is the local Alfven frequency. This result is in agreement with a theoretical model derived from a drift-kinetic energy principle. Similarity experiments with DIII-D are being performed to examine the aspect ratio dependence of the $\Omega _{crit}$ scaling. When $\omega _{\phi }$ at the q = 2 surface drops below $\Omega _{crit}$, the growth of internal kink/ballooning modes can prevent the RWM from terminating the discharge. A small beta collapse which drops $\Omega _{crit}$, accompanies this mode growth allowing a recovery of RWM rotational stabilization while maintaining $\beta _{N} \quad > \quad \beta _{N}^{no-wall}$. [Preview Abstract] |
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RP1.00099: Error Fields and Locked Modes in NSTX Jong-kyu Park, Jonathan Menard, Russell Feder, George Labik, Christiane Ludescher, Aaron Sontag, Christina Tcheyan The full set of six mid-plane external error field and RWM control coils has now been installed and utilized on NSTX. Locked-mode threshold experiments performed with these coils imply that a 1-2 Gauss resonant 2/1 intrinsic error field is present in NSTX. The in-vessel B$_{R}$ sensor array and direct measurements of PF coil shapes indicate the presence of an effective shift of the lower primary vertical field coil (PF5) relative to the nominal machine centerline. The error field magnitude predicted by this shifted PF5 model is consistent with the results from locked-mode experiments, but the toroidal phase angle differs by as much as 60 degrees. Thus far, the 2/1 resonant error field threshold for mode locking has been measured as a function of plasma density in NSTX, and locking as a function of q, B$_{T}$, and elongation in LSN will be investigated in the near term. These studies will allow comparison to the threshold scaling derived from higher aspect ratio experiments. Plans for error field studies extended to higher-$\beta $ plasmas will also be discussed. Simultaneously, magnetic field characteristics of external error field coils are investigated in the context of how a desired and pure (n=1, 3) compensating field can be produced. This preliminary study will continue on more efficient pre-programmed and dynamic control of error fields in NSTX. [Preview Abstract] |
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RP1.00100: Improved Mode Number Identification of Low-frequency MHD Activity in NSTX S.P. Smith, J.E. Menard, S.A. Sabbagh, R.E. Bell Magnetohydrodynamic (MHD) oscillations have been measured with Mirnov coils in the National Spherical Torus Experiment (NSTX). An improved analysis algorithm has been developed to determine the poloidal and toroidal mode numbers (m and n) of these oscillations. First, the algorithm allows for a time- changing oscillation frequency of a given mode in order to accurately track the mode phase evolution. This allows the toroidal and poloidal mode numbers to be accurately tracked over much shorter time periods than can be achieved using simple windowed FFT techniques. Second, the low aspect ratio of NSTX causes the poloidal measurements to be highly asymmetric between inboard and outboard sensors. This toroidal effect is usually accounted for in high aspect ratio devices by a transformation of the poloidal angle ($\theta$), which includes a $\sin(\theta)$ term. Improved poloidal mode number identification is achieved here by including higher harmonics of $\sin(\theta)$ in the transformation. The weights of the harmonics and the dominant poloidal mode number are determined by performing an iterative search for the minimum error of the mode number fits. Together, m and n determine the dominant helicity of the modes. The oscillation frequency is then compared to the plasma rotation frequency (determined by charge exchange recombination) at the q surface corresponding to the measured helicity. [Preview Abstract] |
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RP1.00101: Study of Type I ELM Systematics Using Soft X-ray Analysis on NSTX K. Tritz, D. Stutman, L. Delgado-Aparicio, M. Finkenthal, R. Bell, R. Kaita, S. Kaye, B. LeBlanc, L. Roquemore, R. Maingi, S. Sabbagh NSTX plasmas exhibit a range of ELM behavior during H-mode discharges, including relatively large discrete phenomena classified as `Type I' ELMs. These ELMs can cause a reduction in the plasma stored energy of up to 15{\%} and can perturb the electron temperature profile by triggering a cold pulse that propagates radially inward on timescales of hundreds of microseconds. However, different operating regimes can exhibit smaller `Type I' ELMs which have a much smaller effect on the stored energy and electron temperature profile. The soft X-ray system on NSTX has the capability to examine the fast temperature perturbations and measure the propagation of these events via a `multi-color' technique which uses various X-ray filters to measure the incident X-ray spectrum with different energy cut-off thresholds. This technique is used to study the variety of `Type I' ELM behavior and relate the differences to NSTX plasma conditions. [Preview Abstract] |
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RP1.00102: Characteristics of ELM precursors on NSTX Philipp Ridha, Jonathan Menard, Dan Stutman, Kevin Tritz The precursor characteristics of Edge Localized Modes (ELMs) on NSTX were analyzed with Mirnov and USXR diagnostics in terms of toroidal mode number, growth rate, oscillation frequency, growth times before crash, and edge localization. Mode identification is especially difficult for most ELMs studied, as the precursor growth rates are often comparable to the oscillation period. Details of the mode identification process will be described. The Mirnov diagnostic does not tell whether the ELM is edge localized or not, thus the USXR array was used to discriminate between the edge and core plasma using an analysis of the X-ray emission with different metallic filters (Ti 0.4$\mu m$ - E $>$ 100eV, Be 10$\mu m$ - E $>$ 500eV, Be 100 $\mu m$ - E $>$ 1.2keV). Using the titanium filter, a strong correlation between Mirnov and USXR data during an ELM crash was observed. Analysis of the USXR data using a constrained tomographic inversion shows relative USXR fluctuation amplitudes from ELM precursors in the range of 1\% to 5\%. This analysis combined with an edge displacement model provides an estimate of the transient boundary displacements for typical ELMs of $<$ 5 mm. [Preview Abstract] |
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RP1.00103: Fast soft x-ray camera observation of fast and slow reconnection events on NSTX Brentley Stratton, Schwick von Goeler, Joshua Breslau, Eric Fredrickson, Wonchull Park, Leonid Zakharov Reconnection events on the National Spherical Torus Experiment (NSTX) are studied using data from a new soft x-ray camera diagnostic. The camera has a wide-angle tangential view of the plasma and can capture 300 images per discharge at rates up to 500000 frames per second. Two classes of m=n=1 reconnection events are seen: events such as sawteeth and internal reconnection events (IREs) characterized by rapid ($\sim $200 $\mu $s) reconnection, and events in which reconnection occurs on a much slower time scale (tens of ms) with a similar spatial structure. The mode evolution is reconstructed from the fast soft x-ray camera data. Nonlinear resistive MHD modeling with the M3D code and PEST code stability analysis is used to predict the growth rates and island structures of the fast and slow events, with the goal of understanding the conditions which lead to the two types of events. [Preview Abstract] |
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