Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session CO4: DIII-D Tokamak |
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Chair: Michael Mauel, Columbia University Room: Regency VI |
Monday, November 2, 2009 2:00PM - 2:12PM |
CO4.00001: Overview of Recent DIII-D Experimental Results M.E. Fenstermacher DIII-D experiments in 2009 addressed critical ITER issues and physics understanding needed for extrapolation to future devices. Multiple schemes for rapid plasma shutdown were demonstrated including massive gas injection, large shattered D$_2$ pellets and impurity filled shell pellets. Detailed particle balance experiments show dramatically reduced wall uptake in ITER relevant H-mode compared with L-mode. Joint DIII-D/JET experiments showed no dependence of pedestal pressure width on $\rho^*$, indicating a favorable scaling to ITER. Torque from non-resonant magnetic perturbations improved access to QH-mode at low rotation. DIII-D demonstrated low voltage startup with ECH assist and low $\ell_i$ plasma \mbox{rampdown}, as well as solenoidless startup. Progress on qualifying Hybrid scenario plasmas for $Q=10$ in ITER included $\beta_N = 2.5$ with ELM suppression by RMPs. Advances in physics understanding included: 1) systematic $q_{min}$, $q_{95}$ scans showing the dependence of $n_e$ and $T_e$ profiles on $q(r)$, 2)~plasma response to non-axisymmetric fields, 3)~validation of core turbulence and thermal transport models and 4)~intrinsic rotation studies. [Preview Abstract] |
Monday, November 2, 2009 2:12PM - 2:24PM |
CO4.00002: Particle Control and Carbon Transport Experiments on DIII-D S.L. Allen, E.A. Unterberg, N.H. Brooks, M.A. Mahdavi, D.G. White, J.W. Davis, A.A. Haasz, B. Fitzpatrick As part of the 2009 Joint Research Target, DIII-D, along with MIT and NSTX have completed a series of experiments on particle control and transport. On DIII-D, both dynamic (calculated particle balance) and static (pumps closed or regenerated) particle balance experiments were carried out in both L- and H-mode with cryopumping. We find that the exhaust obtained from both techniques is comparable. We find that the uptake can be large in ohmic and L-mode, but in H-mode, the wall retention flux is very small, which is promising for long pulse burning plasma experiments. Also, in support of tritium control research for ITER, we have started a process to qualify the DIII-D internal components for an anticipated air bake (350$^{\circ}$, 10~Torr) in 2010. We find issues with some copper components, but most components are compatible. Air baking removes the co-deposited carbon that can be rich in tritium in ITER with carbon walls. [Preview Abstract] |
Monday, November 2, 2009 2:24PM - 2:36PM |
CO4.00003: Observations of Thermal Transport Enhancement in Stochastic Boundary Experiments at DIII-D and TEXTOR O. Schmitz, H. Frerichs, M.E. Austin, B.D. Bray, T.E. Evans, T.H. Osborne, H. Reimerdes, M.E. Fenstermacher, C.J. Lasnier, M.W. Jakubowski, S. Mordijck, R.A. Moyer, E.A. Unterberg Comparison of stochastic boundary experiments in TEXTOR L-modes to DIII-D H-modes shows on both experiments a $q_{95}$ resonance in the pedestal pressure $p_e$ which is driven by a resonant decrease of the pedestal electron temperature $T_e$ as $q_{95}$ is varied. This decrease in $T_e$ is correlated to an increase in the modeled stochastic layer width while the electron density does not show a strong $q_{95}$ resonance. The $T_e$ decrease is only seen for DIII-D in an ITER similar shape at high triangularity as opposed to an increase in $T_e$ and a small effect only on the thermal transport for low triangularity plasmas. This indicates significant shape dependence for the $q_{95}$ resonant thermal transport features in stochastic boundary experiments for suppression of type-I edge localized modes by RMP at DIII-D. [Preview Abstract] |
Monday, November 2, 2009 2:36PM - 2:48PM |
CO4.00004: Correlation Between Density Pump-out and Free Streaming Particle Transport in Low Collisionality Resonant Magnetic Perturbation H-modes S. Mordijck, R.A. Moyer, E.A. Unterberg, T.E. Evans Experimental pedestal density data shows a decrease in the gradient within the transport barrier during RMP H-mode, as compared to an ELMing H-mode. Recent modeling, with SOLPS5 and TRIP3D, indicates that this change is the result of an increase in particle transport. This increase in transport is the consequence of the creation of open field lines inside the traditional separatrix. The magnitude ($\sim0.1\,$m$^2$/s) and radial extent of this free-streaming transport are well correlated with experimental changes. In this paper, we present a more systematic study, where we compare the increase in particle transport calculated with TRIP3D, directly with the changes in pedestal density. We notice that the experimental density pump-out during RMP H-mode is linearly correlated with the increase in free streaming transport for low collisionality plasmas. [Preview Abstract] |
Monday, November 2, 2009 2:48PM - 3:00PM |
CO4.00005: Dependence of Bootstrap Current, Stability, and Transport on the Safety Factor Profile in DIII-D Steady-state Scenario Discharges C.T. Holcomb, J.R. Ferron, T.C. Luce, P.A. Politzer, J.C. DeBoo, T.W. Petrie, C.C. Petty, R.J. La Haye, A.E. White, F. Turco, E.J. Doyle, T.L. Rhodes, L. Zeng A high beta, high gain steady state tokamak scenario with large bootstrap current fraction will have strong coupling between the current density and the pressure gradient through turbulent transport and the bootstrap current. To address this coupling experimentally, a scan of the safety factor minimum ($q_{min}$, from 1.1 to over 2) and edge value ($q_{95}$, from 4.5 to 6.5) was performed. The bootstrap current fraction increases with $q_{min}$ and $q_{95}$ by virtue of increasing density gradients. Compared to lower $q_{min}$, $q_{min}>2$ has lower $n=1$ stability limits, enhanced drift wave growth rates, higher low-$k$ density fluctuations, and lower confinement. At $q_{min}>2$ and $q_{95}=4.5$ the unsustainable condition $J_{BS} > J_{Total}$ occurs near the axis. These considerations suggest intermediate $q$ is the optimal operating point. [Preview Abstract] |
Monday, November 2, 2009 3:00PM - 3:12PM |
CO4.00006: Global Structure of a Stable, Driven Kink Mode: DIII-D Measurements and Model Validation M.J. Lanctot, H. Reimerdes, J.M. Hanson, G.A. Navratil, A.M. Garofalo, M.S. Chu, G.L. Jackson, R.J. La Haye, M.J. Schaffer, E.J. Strait, Y.Q. Liu, M. Okabayashi, W.M. Solomon Recent DIII-D measurements of the global structure of the non-axisymmetric plasma perturbation driven by applied $n=1$ magnetic fields enable the quantitative test of ideal MHD theory. Extensive magnetic measurements show that the ideal MHD code MARS-F predicts the plasma response within 20\% for values of beta up to 75\% of the beta limit calculated without a conducting wall, but overestimates the perturbed field at higher pressures. Experiments varying the pitch angle of the applied field at different values of plasma current demonstrate the plasma response depends primarily on the match of the applied field to the kink mode structure. Toroidally distributed soft x-ray measurements indicate the kink-like internal perturbation structure depends on the plasma pressure. The measurements are used to test kinetic stabilization models in the MARS-K code. [Preview Abstract] |
Monday, November 2, 2009 3:12PM - 3:24PM |
CO4.00007: Synergy in Two-Frequency Fast Wave Cyclotron Harmonic Absorption in DIII-D R.I. Pinsker, M. Choi, M. Porkolab, W.W. Heidbrink, Y. Zhu, F.W. Baity, J.C. Hosea Fast waves (FWs) at 60 MHz and at 90 MHz are coupled to DIII-D discharges for central heating and current drive at net FW power levels up to 3.5 MW. In 2~T discharges with fast deuteron populations from neutral beam injection, 4th and 6th deuterium cyclotron harmonic absorption on the fast ions competes with direct electron damping and with edge losses. If the fast deuterons are accelerated by absorption of 60 MHz (4th harmonic) FWs, adding 90 MHz power (6th harmonic) increases the fusion neutron rate by a increment larger than the sum of the increments observed with separate 90 MHz and 60 MHz pulses (synergy). Synergy in the global confinement is also observed. The regions of velocity space that are affected with the two-frequency FW heating are studied with fast-ion D$_{\alpha}$ spectroscopy and by detailed characterization of the dynamics of the neutron rate with modulated neutral beams. [Preview Abstract] |
Monday, November 2, 2009 3:24PM - 3:36PM |
CO4.00008: Measurements of the Spatial Structure of Geodesic Acoustic Modes in DIII-D J.C. Hillesheim, W.A. Peebles, L. Schmitz, T.L. Rhodes, T.A. Carter Geodesic acoustic modes (GAMs) are linearly stable, turbulence driven modes exhibiting oscillating axisymmetric ($m=0$, $n=0$) $E\times B$ flows. They potentially play an important role in establishing the saturated level of turbulence in fusion plasmas. Two Doppler backscattering (DBS) systems at locations separated toroidally by 180$^{\circ}$ are aligned to make simultaneous measurements at the same radial location ($\rho\approx 0.8$) and wavenumber ($k_\perp\sim 4\,$cm$^{-1}$, $k_\perp\rho_s\sim 1$) in a beam-heated L-mode DIII-D plasma. Flow oscillations, which agree with the predicted GAM frequency scaling, correlate toroidally between the two DBS systems with an ensemble averaged cross-coherency of $\gamma\approx 0.6$ over 600 ms. The cross-phase between pairs of the DBS signals is consistent with the expected GAM structure. The radial variation in cross-phase agrees with descriptions of the GAM eigenmode as having an Airy function character with outward radial propagation; the measured radial wavelength is $\lambda_r\approx 2.8\,$cm and the calculated GAM characteristic length scale is $L_{GAM}=\rho_i^{2/3}L_T^{1/3}\approx 1.2\,$cm. [Preview Abstract] |
Monday, November 2, 2009 3:36PM - 3:48PM |
CO4.00009: Heat Transport in Off-axis EC-Heated Discharges in DIII-D M.E. Austin, K.W. Gentle, C.C. Petty, T.L. Rhodes, L. Schmitz, G. Wang In low-density H-mode discharges in DIII-D, ECH applied off-axis produces electron temperature profiles with strong peaking at the heating location and very slow penetration of heat into the core. This type of discharge is a counter example to the heat-pinch effect normally seen in tokamaks where off-axis heating propagates rapidly to the center. In a recent experiment on DIII-D, the conditions for producing these ``bat-eared" $T_e$ profiles were studied. It was observed that H-mode is a necessary condition; L-mode discharges exhibit the classic heat pinch. A region of low transport corresponds to the $q=1$ surface as verified by the sawtooth inversion radius. Results of transport analysis are presented as well as measurements of $n_e$ and $T_e$ fluctuations. [Preview Abstract] |
Monday, November 2, 2009 3:48PM - 4:00PM |
CO4.00010: Studies of Runaway Electron Confinement in MHD Disruption Simulations V.A. Izzo, E.M. Hollmann, A.N. James, D.G. Whyte, G. Olynyk, L.L. Lao Formation of a runaway electron beam during an ITER disruption is a major concern for machine survival, particularly because the avalanche growth of a seed population will be many orders of magnitude larger than in present devices. Enhanced fast-electron losses due to stochastic fields produced during the disruption, or from applied non-symmetric fields, can combat avalanche growth. The confinement of fast electrons is studied in the context of MHD simulations using a newly developed capability in the NIMROD code to track single particle orbits as the magnetic fields evolve. Macroscopic drift-orbit displacements associated with highly relativistic electrons play an important role in confinement by averaging over perturbing fields during a poloidal transit, thus allowing good confinement in the presence of stochasticity. Verification of the model for a small number of electrons compares the orbits with and without drift terms directly. Confinement of fast electrons during a gas-jet-induced disruption on Alcator C-Mod, and during a controlled error field ramp on DIII-D are presented. [Preview Abstract] |
Monday, November 2, 2009 4:00PM - 4:12PM |
CO4.00011: Solenoid-free Startup of DIII-D J.A. Leuer, N. Eidietis, D.A. Humphreys, A.W. Hyatt, G.L. Jackson, J. Lohr, P.A. Politzer, R. Prater, P.T. Taylor, M.L. Walker, G. Cunningham, D. Gates, D. Mueller, Y.-K. Oh, S.-W. Yoon, S.-H. Hahn, J.H. Yu Inductive plasma current startup to 170 kA was achieved in the DIII-D tokamak without the use of inboard poloidal field coils (solenoidless startup). This was achieved with strong preionization/heating using electron cyclotron (EC) power. For outside breakdown and moderated field null quality, flux conversion efficiency to plasma current using only diverter and vertical field coils is similar to DIII-D's ohmic startup. At higher flux states null quality degrades and flux/current efficiency is reduced, possibly from a reduced in EC/breakdown region coupling. EC current drive was minimal for the plasma regime studied. Preliminary solenoidless handoff experiments to neutral beam current drive were also explored. This research is expected to reduce central solenoid requirements for next generation devices and provide a basis for solenoidless startup of toroidal devices with integration of appropriate current drive techniques. [Preview Abstract] |
Monday, November 2, 2009 4:12PM - 4:24PM |
CO4.00012: Identity Experiments in the Hybrid Regime on \mbox{DIII-D} and JET C.D. Challis, E. Joffrin, T.C. Luce, P.A. Politzer Hybrid plasmas have the potential for high fusion yield and long pulse tokamak operation. However, performance extrapolation to future devices depends on an understanding of the transport scaling with machine size, which may not be adequately described by existing ELMy H-mode scalings. An identity match and $\rho*$ scan has been performed in the hybrid regime on DIII-D and JET to investigate the core confinement. A similar plasma shape and NBI heating were used and $\nu^*$, $\rho^*$, $\beta$ and Mach number profiles were all matched within about 20\% at the plasma mid-radius. A $\rho^*$ range of roughly 2.5 was covered across the two devices for plasmas with $q_0\sim 1$ and normalized beta in the range $\sim$2.5-3.0. These experiments allow a comparison of transport in this domain on the two devices and an assessment of the $\rho^*$ dependence of core confinement. [Preview Abstract] |
Monday, November 2, 2009 4:24PM - 4:36PM |
CO4.00013: Scaling of H-mode Pedestal and ELM Characteristics With Gyroradius T.H. Osborne, R.J. Groebner, A.W. Leonard, P.B. Snyder, M.N.A. Beurskens, L.D. Horton, P. Lomas, S. Saarelma, L. Frassinetti, I. Nunes The dependence of the H-mode pedestal structure and ELMs on gyroradius ($\rho^* = \rho/a$) was examined in experiments combining data from the JET and DIII-D to produce a factor of 4 variation in $\rho^*$, while keeping the plasma shape, $q$, normalized pressure ($\beta$), collisionality, Mach number, and $T_i/T_e$ at the pedestal top constant. In this scan, the width of the steep gradient region of $T_e$ and $n_e$, $\Delta$, was independent of $\rho^*$ within uncertainties, $\Delta/a\sim (\rho^*)^{0.0\pm 0.15}$. The pedestal pressures and widths were in quantitative agreement with the EPED1 model in which the pedestal structure is set by combining the peeling-ballooning and kinetic ballooning stability thresholds. The ELM energy loss normalized to the pedestal energy decreased from 40\% to 10\% as $\rho^*$ decreased by a factor of 2 in DIII-D, but the trend did not continue in smaller $\rho^*$ on JET. [Preview Abstract] |
Monday, November 2, 2009 4:36PM - 4:48PM |
CO4.00014: Pedestal Density Fluctuations During Quiescent and ELMing H-mode Plasmas Z. Yan, G.R. McKee, M.W. Shafer, R.J. Groebner, P.B. Snyder, K.H. Burrell, T.H. Osborne, A.W. Leonard, T.L. Rhodes, L. Zeng Spatially resolved density fluctuation characteristics have been measured in the pedestal region of quasi-steady-state Type~I ELMing plasmas and ELM-free quiescent H-mode (QH) plasmas using 2D beam emission spectroscopy measurements. During Type~I ELMing plasmas, these fluctuations are modulated with the ELM cycles. Two distinct frequency bands (20-200~kHz and 250-450~kHz) are observed propagating in opposite directions. In QH-mode plasmas, discrete and coherent modes are observed in the pedestal region of particular discharges at relatively high-pedestal pressure. These modes appear from 50-250~kHz, peaking in amplitude around 150~kHz, with a uniform frequency separation of about 10~kHz. Observed characteristics of these modes will be compared with those from ELITE calculations of theoretically predicted pressure-gradient limiting instabilities, such as kinetic ballooning modes. [Preview Abstract] |
Monday, November 2, 2009 4:48PM - 5:00PM |
CO4.00015: Quiescent H-Mode Plasmas with Rotation Driven by Static Non-axisymmetric Fields A.M. Garofalo, K.H. Burrell, G.L. Jackson, A. Cole, W.M. Solomon, M.J. Lanctot, H. Reimerdes A quiescent H-mode (QH-mode) edge allows ELM-free operation of a plasma with good confinement and good particle exhaust. Until recently, QH-mode operation required rather strong plasma toroidal rotation in order for the edge velocity shear to exceed a minimum value [1]. However, rapid rotation may not be feasible in a self-heated burning plasma with little or no momentum injection from neutral beams. New DIII-D experiments in ITER-similar plasmas show that the neoclassical torque from static, nonresonant magnetic fields (NRMFs) provides a useful knob to change the edge rotation profile shear. NRMF application resulted in QH-mode operation with less than half the rotation (evaluated on top of the pedestal) of previous QH-mode without the NRMFs. At this low rotation, the NRMF torque may be amplified by entering the theoretically predicted $1/\nu$ collisionality regime.\par \vskip8pt \noindent [1] K.H.\ Burrell, {\em et al.}, Phys.\ Rev.\ Lett.\ {\bf 102}, 155003 (2009). [Preview Abstract] |
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