Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session NO4: DIII-D Tokamak |
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Chair: Randy Wilson, Princeton Plasma Physics Laboratory Room: 551AB |
Wednesday, October 31, 2012 9:30AM - 9:42AM |
NO4.00001: Overview of Recent DIII-D Experimental Results P. Gohil Recent DIII-D experiments have addressed critical ITER issues, increased our understanding of plasma transport physics, and improved the physics basis for steady state operation. These experiments have utilized several new diagnostic and hardware capabilities, such as real time steering of the ECH launcher mirrors, 75~Hz pellet ELM pacing IR/visible periscopes and high-resolution profile reflectometry. Progress in several key areas of research will be presented: understanding transport induced by \hbox{3-D} field perturbations at the plasma edge; determining reliable alternative techniques for ELM control that do not require internal coils, such as \hbox{QH-mode} and pellet pacing; improved control of runaway electrons and disruption control through pellet injection; increased understanding of core transport and model validation; improved operation of steady state, high $\beta_N$ plasmas; investigations into improving divertor performance through changes in the magnetic divertor geometry; and new results on high $Z$ material erosion. [Preview Abstract] |
Wednesday, October 31, 2012 9:42AM - 9:54AM |
NO4.00002: Latest Results on Resonant Magnetic Perturbation (RMP) Induced ELM Suppression on DIII-D R. Nazikian Recent experiments on DIII-D have revealed new insights into the suppression of edge localized modes (ELMs) by RMPs. A leading hypothesis for ELM suppression is that resonant fields open up islands at the top of the H-mode pedestal where the perpendicular electron velocity $V_e$ is small. These islands enhance transport, flatten the pressure profile and restrict the width of the pedestal sufficient to suppress the Peeling-Ballooning mode. Experiments with strong counter rotating plasmas maintained large $V_e$ across the plasma profile and RMP ELM suppression was not observed, consistent with the model. X-ray imaging measurements reveal structure inside the last closed flux surface that may indicate the presence of islands in ELM suppressed plasmas. A new lower limit for $n=3$ RMP ELM suppression of 1.3 kA in the I-coils provides a strong test of the requirement for ELM suppression based island and stochastic field line models. Rapid modulation of the RMP amplitude reveals a prompt response of the turbulence on the time scale of the $E_r$ change, indicative of turbulent transport modification as a precursor to the pressure profile evolution and ELM suppression. Experiments to open up additional $q_{95}$ windows of ELM suppression will be discussed. [Preview Abstract] |
Wednesday, October 31, 2012 9:54AM - 10:06AM |
NO4.00003: Edge Soft X-Ray Imaging Measurements of 3D Plasma Response to Resonant Magnetic Perturbations M.W. Shafer, E.A. Unterberg, A. Wingen, J.H. Harris, D.L. Hillis, T.E. Evans, D.M. Orlov Three-dimensional (3D) perturbations in the edge plasma due to the application of non-axisymmetric fields are measured in the \hbox{X-point} region in \hbox{DIII-D} by tangential imaging of Ultra-Soft (USXR) and Soft \hbox{X-Ray} (SXR) emission in an effort to understand the internal plasma response. To date, 3D structures are resolved by performing a differential measurement of the image for different toroidal phases of the applied perturbation. Forward modeling of the line-integrated image is used to directly compare to the measurements. Vacuum field calculations show perturbations of the boundary take the form of lobes extending outward from the un-perturbed separatrix and moving poloidally as the edge safety factor changes. These results match well with the USXR emission measurements. Using SXR emission measurements, structure is measured for the first time in the steep-gradient region inside the separatrix and is consistent with modeled internal 3D topological features. [Preview Abstract] |
Wednesday, October 31, 2012 10:06AM - 10:18AM |
NO4.00004: Modeling Edge Plasma Response to 3D Fields in \hbox{DIII-D} N.M. Ferraro, T.E. Evans, L.L. Lao, A.D. Turnbull, M.J. Lanctot, M.W. Shafer, E.A. Unterberg, R. Nazikian, R.A. Moyer, D.M. Orlov Modeling of plasma response to applied non-axisymmetric fields show significant displacement of edge temperature and density profiles. The calculated displacements, often of two centimeters or more in H-mode pedestals with parameters appropriate to DIII-D, are due to the helical distortions resulting from stable edge modes being driven to finite amplitude by the applied fields. At low toroidal mode numbers (particularly $n=1$ and $n=2$) these displacements are greater in magnitude, and typically different in phase, than the distortions of the separatrix manifolds predicted from vacuum modeling. Comparison of these results finds good agreement with experimental measurements from Thomson scattering, beam emission spectroscopy, and soft x-ray imaging. Implications for ITER and resonant magnetic perturbation edge localized mode suppression are discussed. The plasma response is calculated using the M3D-C1 code, which implements a resistive two-fluid model, and self-consistently includes the plasma, separatrix, and scrape-off layer. [Preview Abstract] |
Wednesday, October 31, 2012 10:18AM - 10:30AM |
NO4.00005: Time Evolution of the H-mode Pedestal Characteristics in Type I ELM Discharges on DIII-D T.H. Osborne, R.J. Groebner, S.P. Smith, P.B. Snyder, D.M. Thomas, J.A. Boedo, T.L. Rhodes, H. Stoschus, Z. Yan The time evolution of H-mode pedestal profiles leading up to a Type I edge localized mode (ELM) is studied in low ELM frequency discharges with electron cyclotron or neutral beam heating. A continuous increase of the pedestal pressure gradient, $\nabla p^{PED}$, and/or the edge transport barrier, ETB, width was observed in all cases. Simulations and Li-beam measurements indicated a continuous increase in pedestal current, resulting in conditions consistent with destabilization of a peeling-ballooning mode. $P^{PED}$ evolution was dominated by changes in the $n_e$ profile while $T_e$ stopped evolving after about 25\% of the inter-ELM period, near the time of onset of density fluctuations observed by BES. However the $T_e^{PED}$ and $\nabla T_e$ varied with heating power. $P^{PED}$ in higher $I_p$ discharges grew mainly through $\nabla p$ increases. In this case $\nabla p$ was below the KBM limit predicted by the EPED model early in the inter-ELM period, although all cases were near the EPED predictions at the ELM. The evolution of $n_e$ and $T_e$ in higher $I_p$ cases was consistent with the paleo-classical model predictions. [Preview Abstract] |
Wednesday, October 31, 2012 10:30AM - 10:42AM |
NO4.00006: Measurements and modeling of the divertor heat flux width in DIII-D M.A. Makowski, C.J. Lasnier, J. Nichols, A.W. Leonard, T.H. Osborne, P.B. Snyder The relationship between measured divertor heat flux and upstream SOL profiles is explored to test critical pressure gradient models of radial energy transport in \hbox{DIII-D}. We are developing a physics based model that extends the well validated kinetic ballooning mode pedestal paradigm up to the separatrix and into the SOL. Evidence in support of this model is that the measured separatrix pressure gradient scales similarly to the critical pressure gradient for the infinite-$n$ ballooning mode obtained from the BALOO code. Recent results have revealed how the heat flux width scales with plasma parameters [1]. The main result is that $\lambda_{sol}\sim B^{-1}_p$ and is independent of all other parameters including machine size. Here $\lambda_{sol}$ is the heat flux width in the scrape-off-layer (SOL) and $B_p$ is the poloidal field at the outer mid-plane. New measurements of the heat flux width versus the injected power demonstrate a weak dependence, $\lambda_{sol}\sim P_{inj}^{-0.1}$, consistent with other experiments. \vskip6pt\noindent [1] T. Eich et al., Phys. Rev. Lett. {\bf 107}, 215001 (2011); M.A. Makowski, Phys. Plasmas {\bf 19}, 056122 (2012). [Preview Abstract] |
Wednesday, October 31, 2012 10:42AM - 10:54AM |
NO4.00007: The Dynamics of Turbulence and Flow During the L-H Transition Z. Yan, G.R. McKee, J.A. Boedo, D.L. Rudakov, G.R. Tynan, P.H. Diamond, R.J. Groebner, T.H. Osborne, G. Wang, L. Schmitz Comprehensive 2D turbulence and flow measurements are obtained before, during and after the L-H transition. An ion gyro-radius and density scan was performed on DIII-D to investigate the physics of the L-H transition and the threshold dependence on $B_T$ and $n_e$. The amplitude of long wavelength density fluctuations scale approximately with $\rho^*$. Stronger poloidal turbulence flow shear is found at low density. The poloidal turbulence velocity spectrum changes from GAM at $\sim$300$\,$ms before the L-H transition to zonal flows near the transition. About 100$\, \mu$s before the transition, the Reynolds stress gradient, inferred from 2D BES velocimetry, increases rapidly. The poloidal velocity and shear likewise peak near the same time, consistent with the prediction that Reynolds stress drives a zonal flow that triggers the L-H. The turbulence and shear dynamics leading to a typical L-H transition will also be compared with limit-cycle-oscillations between L- and H-modes observed under certain discharge conditions. [Preview Abstract] |
Wednesday, October 31, 2012 10:54AM - 11:06AM |
NO4.00008: Plasma Fluctuation Measurements in Ion Stiffness Experiments using Phase Contrast Imaging A. Marinoni, J.C. Rost, M. Porkolab, K.H. Burrell, J. Candy, T.C. Luce The Phase Contrast Imaging (PCI) diagnostic on the DIII-D tokamak has recently been modified to image density fluctuations near the plasma mid-radius, thus enabling the investigation of core turbulence. Results are presented on core fluctuations in experiments exploring ion profile stiffness [1], i.e. the degree of sensitivity of ion temperature profiles to heat flux variations. In these experiments, plasmas were heated by neutral beams (NBI) configured to provide both high and low input torque; the injected NBI power was varied at constant torque to evaluate profile stiffness. A preliminary analysis indicates a decreased stiffness at high rotation in the outer half of the plasma. The toroidal rotation depends primarly on torque, with little or no dependence on input power. The amplitude of fluctuations increases with decreasing rotation, and the power spectra at similar torque have quantitatively similar shapes and values with little dependence on input power. Correlation lengths depend neither on torque nor input power. PCI power spectra and correlation lengths are evaluated and compared to non-linear gyro-kinetic simulations using the GYRO code.\par \vskip6pt \noindent [1] J.E.\ Kinsey, {\em et al.}, Bull.\ Am.\ Phys.\ Soc.\ {\bf 56}, 282 (2011). [Preview Abstract] |
Wednesday, October 31, 2012 11:06AM - 11:18AM |
NO4.00009: Electron Transport Stiffness and Heat Pulse Propagation on DIII-D C.C. Petty, J.C. DeBoo, S.P. Smith, K.H. Burrell, A.E. White, J.C. Hillesheim, C.H. Holland Experiments on \hbox{DIII-D} have measured the stiffness of electron heat transport using a new method that combines heat pulse (HP) propagation and power balance (PB) analysis. Using a single modulated gyrotron, in addition to 5~cw gyrotrons, the radial profiles of $T_e$ oscillations from the fundamental to the 9$^{\rm th}$ harmonic are fit to determine the diffusion (D$_{\rm HP}$), convection (V$_{\rm HP}$) and damping coefficients. The $T_e$ gradient is then systematically scanned by varying the electron cyclotron heating profile on a shot-by-shot basis using the cw gyrotrons. Numerically integrating D$_{\rm HP}$ over this scan gives D$_{\rm PB}$, and the difference between the diffusive heat flux from D$_{\rm PB}$ and the total power-balance heat flux determines V$_{\rm PB}$. The ratio of D$_{\rm HP}$ to D$_{\rm PB}$ measures the transport stiffness, defined as the fractional increase in diffusive heat flux divided by the fractional increase in the $T_e$ gradient. In \hbox{L-mode} plasmas, a sudden increase in electron transport stiffness is seen when the $T_e$ scale length exceeds the theoretically predicted threshold value. Similar electron transport stiffness is observed with and without additional NBI. [Preview Abstract] |
Wednesday, October 31, 2012 11:18AM - 11:30AM |
NO4.00010: Control of Neoclassical Tearing Modes in DIII-D A.S. Welander, R.J. LaHaye, B.G. Penaflor, J. Lohr, V. Noraky, R. Prater, N.W. Eidietis, D.A. Humphreys, E. Kolemen, F. Turco New techniques have been developed on DIII-D for control of neoclassical tearing modes (NTMs). The NTM is a helical magnetic island formation that can occur on flux surfaces where the safety factor, $q$ is a rational number. An NTM can be suppressed by depositing electron cyclotron current drive (ECCD) on the $q$-surface by injecting microwave beams into the plasma. On DIII-D, steerable mirrors that reflect these beams into the plasma can be adjusted when the $q$-surface is moving to keep the ECCD aligned. Accurate tracking is made possible by equilibrium reconstructions that include measurements of the motional Stark effect and by estimating beam refraction. Three different algorithms can be employed to fine-tune alignment when NTMs occur. The first method adjusts ECCD alignment in steps until the island shrinks. The second method sweeps the alignment to find where ECCD has the biggest effect on the NTM. The third method uses temperature measurements by electron cyclotron emission. The gyrotrons are pulsed and the position of the resulting temperature pulses is compared to the position where the rotating NTM causes temperature fluctuations. Recent experimental results and directions toward robust disruption-free control will be presented. [Preview Abstract] |
Wednesday, October 31, 2012 11:30AM - 11:42AM |
NO4.00011: Neutral Beam-ion Prompt Loss Induced by Alfv\'en Eigenmodes in DIII-D X. Chen, W.W. Heidbrink, R.K. Fisher, D.C. Pace, C.C. Petty, M.A. Van Zeeland, G.J. Kramer, R. Nazikian Prompt beam-ion loss driven by toroidal and reversed shear Alfv\'en eigenmodes (TAE/RSAEs) have been observed on fast ion loss detectors (FILDs) in DIII-D. The losses, modulated at the TAE/RSAE frequencies, emerge quickly following beam turn on ($<$0.1 ms, consistent with a single poloidal transit period) and are correlated with different beams as the $q$-profile evolves during the discharge, indicating that those losses occur on the first drift orbit. Full orbit simulations with the SPIRAL code indicate that the observed losses are dominated by barely-confined trapped ions that are scattered by TAEs and RSAEs onto lost orbits that can intersect the FILD. Simulations show that the relative changes in pitch due to the modes being are larger than the changes in energy. In addition, losses at the beat frequencies between the TAEs and RSAEs are observed. Possible nonlinear interactions between the modes and fast ions are investigated in order to explain the losses at the beat frequencies. [Preview Abstract] |
Wednesday, October 31, 2012 11:42AM - 11:54AM |
NO4.00012: Particle assimilation study during shattered pellet injection on DIII-D N. Commaux, L.R. Baylor, T.C. Jernigan, D.A. Humphreys, N.W. Eidietis, P.B. Parks, J.C. Wesley, C.L. Lasnier, E.M. Hollmann, V.A. Izzo, R.A. Moyer Disruptions are an important issue for ITER. One of the most promising mitigation techniques is shattered pellet injection (SPI). SPI involves the injection of a massive frozen pellet ($\sim$3000 torr.L D$_2$) that is shattered just before entering the plasma into sub-millimeter solid fragments by bouncing on hard surfaces. The fragments are expected to penetrate deeper in the plasma than gas--thus rapidly increasing the density. Previous experiments have shown that SPI enables better assimilation of the particles by the plasma than gas injection but still lower than expected if the assimilation was adiabatic. The goal of this experiment is to study the influence of the MHD activity on particle assimilation during the injection by scanning $q_{95}$. MHD activity, particle assimilation and toroidal transport are studied using fast magnetics and spectroscopic measurements. [Preview Abstract] |
Wednesday, October 31, 2012 11:54AM - 12:06PM |
NO4.00013: Transport and Stability Characteristics of High $q_{min}$ Steady-State Scenarios with Off-Axis NBI J.M. Park, J.R. Ferron, C.T. Holcomb, T.C. Luce, R.J. Buttery, L.L. Lao, R. Prater, F. Turco, M. Murakami DIII-D experiments show that off-axis neutral beam injection (NBI) improves access to high $\beta$ and fully noninductive current sustainment at elevated minimum $q$ ($q_{min}$) by broadening the plasma current and pressure profiles. Off-axis NBI modifies the underlying characteristics of transport and stability, as expected from theory. For on-axis NBI, the electron (ion) confinement time normalized to ITER H-mode scaling, $\tau_{th,H_{98}}$ increases (decreases) with $q_{min}$. For off-axis NBI, both power balance analysis and TGLF modeling indicate better ion but worse electron confinement, when compared with on-axis NBI at the same $q_{min}$. There appears to be a strong confinement scaling with Shafranov shift, the ratio of electron to ion heating, and the location of heat deposition. The broader pressure profile obtained from off-axis NBI helps to avoid low-order tearing modes and increases the calculated ideal-wall $\beta_N$ limit. Nonlinear optimization of confinement and stability to achieve higher $\beta_N$ ($>$4) with fully noninductive current drive will be discussed using theory-based scenario modeling validated against high $q_{min}$ discharges with on- and off-axis NBI. [Preview Abstract] |
Wednesday, October 31, 2012 12:06PM - 12:18PM |
NO4.00014: Impact of Torque and Rotation in High Fusion Performance Plasmas W.M. Solomon, K.H. Burrell, R.J. Buttery, J.S. deGrassie, A.M. Garofalo, G.L. Jackson, T.C. Luce, C.C. Petty, P.A. Politzer, E.J. Doyle, F. Turco A range of experiments on DIII-D have investigated the role of externally applied torque and the associated toroidal rotation on confinement and stability in plasmas with high levels of normalized fusion performance. In standard H-mode plasmas, the confinement is initially enhanced by increasing rotation, but saturates at intermediate levels of rotation. However, the same effect is observed over a wider range of rotation in advanced inductive plasmas. Both ion and electron confinement improves with high rotation. Surprisingly, experiments in quiescent H-mode (QH-mode) plasmas have found the opposite trend, with improved confinement, performance and reduced turbulence levels at low rotation. The different behavior suggests that $E\times B$ shear, rather than rotation, is needed for improving confinement. In particular, for these QH-mode plasmas, it is found that the $E\times B$ shear near the edge is maintained or enhanced with torque from non-resonant magnetic fields, even at low rotation. In all scenarios, no major difference is observed in confinement whether the plasma is initiated with high rotation and slowed down, or formed with low rotation from the beginning. [Preview Abstract] |
Wednesday, October 31, 2012 12:18PM - 12:30PM |
NO4.00015: ELM-free, Quiescent H-mode Operation in DIII-D Under Reactor-relevant Conditions Using Non-Axisymmetric Magnetic Fields from Coils Outside the Toroidal Field Coil K.H. Burrell, A.M. Garofalo, W.M. Solomon, M.E. Fenstermacher Application of static, non-axisymmetric magnetic fields (NAMFs) to DIII-D plasmas allows sustained quiescent H-mode (QH-mode) operation under reactor-relevant conditions of beta, collisionality and torque from neutral beam injection (NBI). QH-mode is an ideal plasma for next step devices, exhibiting H-mode confinement levels while operating without edge localized modes at constant density and radiated power. Peeling-ballooning mode stability theory suggests, and previous studies confirm, that QH-mode operation requires sufficient radial shear in the toroidal rotation near the plasma edge. In past experiments, this rotation shear was predominantly produced by torque from counter-directed NBI. In recent experiments, co-NBI torque was overcome by the counter torque due to neoclassical toroidal viscosity (NTV) produced by the NAMFs. The latest experiments have demonstrated that sufficient NTV torque can be created using NAMFs produced by coils outside the toroidal field coil. These new results open a path for QH-mode utilization in self-heated, burning plasmas, where toroidal momentum input from NBI will be small or absent. [Preview Abstract] |
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