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 DI3: Gyrokinetics and Core Turbulence |
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Chair: Martin Greenwald, Massachusetts Institute of Technology Room: Ballroom BC |
Monday, October 29, 2012 3:00PM - 3:30PM |
DI3.00001: Multichannel Transport in L-mode and I-mode Plasmas at Alcator C-Mod and Comparison with Gyrokinetic Simulations Invited Speaker: A.E. White New experiments at Alcator C-Mod are challenging and expanding our understanding of electron, impurity particle, and momentum transport. Understanding these coupled transport channels is important, since alpha particles will slow down mostly on electrons in ITER intrinsically rotating, alpha-heated plasmas. Core density fluctuations are reduced in high performance (H98$\sim$1) I-mode plasmas by up to 30$\%$ compared to L-mode. At the L-I transition, the core turbulence changes lead the edge turbulence changes. This is in contrast to observations at L-H transitions, where the edge turbulence changes lead the core turbulence changes. It is also found that L-mode and I-mode plasmas are both ITG dominant, with lower linear ITG growth rates in I-mode. TRANSP analysis indicates that core electron heat transport is reduced in I-mode compared to L-mode, with little change in ion heat transport. This indicates that significant electron heat transport is driven by ITG turbulence in L-mode. However, impurity particle transport is similar in L-mode and I-mode, suggesting that tracking changes in ITG drive alone does not describe as well the transport in this channel. Additionally, in L-mode plasmas, small $<$ 20$\%$ changes in line averaged density lead to dramatic differences in the on-axis rotation, due to a hollowing of the radial profile at marginally lower density. This change in the shape of the rotation profile in L-mode plasmas does not appear to occur at the ITG/TEM boundary, contrary to similar phenomena seen in Ohmic plasmas. For the first time, local, long-wavelength density and electron temperature fluctuations in the core plasma (0.4 $<$ r/a $<$ 0.9) at C-Mod have been measured with reflectometry and correlation ECE. These new fluctuation measurements at C-Mod will allow for better testing and validation of transport models. Gyrokinetic simulations are in progress to interpret the new multichannel and multifield turbulence/transport results from C-Mod. [Preview Abstract] |
Monday, October 29, 2012 3:30PM - 4:00PM |
DI3.00002: Search for the Missing L-mode Edge Transport and Possible Breakdown of Gyrokinetics Invited Speaker: R.E. Waltz While GYRO simulations of typical core ($0 < r/a < 0.7$) DIII-D L-modes seems to be in good agreement with experiment, simulated low-k ($k_\theta\rho_s < 1$) transport and turbulence intensity is more than 5-fold lower than experimentally inferred levels in the near edge L-mode ($r/a=0.7-0.95$) DIII-D shot 128913 [1]. Global edge slice GYRO simulations of this and the well-studied discharge 101391 [2] are presented here to document the shortfall. TGLF transport code simulations over a large L-mode database indicate this short fall is not atypical so that L-mode edges transit to H-like pedestal profiles contrary to experiment. High edge e-i collisionality stabilizes the TEM modes so that diffusivities ($\chi$) decrease like $T^{7/2}/n$ to the cold edge. The very high magnetic shear and density gradients stabilize the ITG despite the very high temperature gradient drive and high $q$. High-$k$ ETG can make-up for the shortfall in the electron but increases ion transport very little. Near L-edge transport is highly local. Focusing on local simulations at $r/a=0.9$, the ion channel short fall can exceed 10-fold. An artificial 10-fold increase in collisionality is needed to reach the expected resistive g-mode scaling with $\chi$ increasing like $nT^{-1/2}$. Identical GYRO drift kinetic ion simulations (suppressing the gyroaverage) are close to experiment levels suggesting a possible breakdown of low-frequency gyrokinetics. Formulation of a nonlinear theory of 6D drift-cyclotron kinetics following the fast time scale of the gyrophase to test the breakdown of 5D gyrokinetics with reduced model simulations is presented.\par \vskip6pt \noindent [1] C.\ Holland, A.E.\ White, {\em et al.}, Phys.\ Plasmas {\bf 16}, 052301 (2009).\par \noindent [2] R.E.\ Waltz, J.\ Candy, C.C.\ Petty, Phys.\ Plasmas {\bf 13}, 072304 (2006). [Preview Abstract] |
Monday, October 29, 2012 4:00PM - 4:30PM |
DI3.00003: Observation of a Critical Gradient Threshold for Electron Temperature Fluctuations in the DIII-D Tokamak Invited Speaker: J.C. Hillesheim A critical gradient threshold for electron temperature fluctuations, directly correlated with an abrupt increase in the electron heat flux and reproduced in gyrofluid and gyrokinetic predictions, has been directly measured for the first time in the core of a confined high-temperature plasma. In an experiment in the DIII-D tokamak where $1/L_{Te}=-\nabla T_e/T_e$ and toroidal rotation were systematically varied in an L-mode target discharge, long wavelength ($k_\theta\rho_s<0.4$) electron temperature fluctuations exhibit a threshold in $1/L_{Te}$: below they change little and above they abruptly increase, with the threshold at $1/L_{Te}=2.8\pm 0.4\,$1/m. In contrast, long wavelength density fluctuations changed little. Concurrent with the increase in $\tilde{T}_e/T_e$, the electron heat flux increases rapidly with $1/L_{Te}$. A critical threshold for the electron thermal diffusivity was found at $1/L_{Te}=3.0\pm 0.2 1/m$, within uncertainties of the value for $\tilde{T}_e/T_e$. This implies the increase in $\tilde{T}_e/T_e$ plays a causal role for the increased electron heat flux. In linear gyrofluid calculations the critical gradient behavior is governed by the dimensionless parameter $\eta_e=L_{ne}/L_{Te}$, with an instability threshold of $\eta_e\approx 2$, which agrees well with the experimentally observed sharp increase in $\tilde{T}_e/T_e$ at $\eta_e\approx 1.9$. Both $\tilde{T}_e/T_e$ and the electron heat flux show little sensitivity to rotation. Above the critical $1/L_{Te}$, measurements are consistent with $\nabla T_e$ driven trapped electron mode turbulence. Below the threshold, measurements of the crossphase angle between electron density and temperature fluctuations indicate multiple instabilities may be active in the different rotation cases. The array of fluctuation measurements provides strong multifield constraints for turbulence model validation studies. [Preview Abstract] |
Monday, October 29, 2012 4:30PM - 5:00PM |
DI3.00004: Nonlinear Upshift of Trapped Electron Mode Critical Density Gradient: Simulation and Experiment Invited Speaker: D.R. Ernst A new nonlinear critical density gradient for pure trapped electron mode (TEM) turbulence increases strongly with collisionality, saturating at several times the linear threshold. The nonlinear TEM threshold appears to limit the density gradient in new experiments subjecting Alcator C-Mod internal transport barriers to modulated radio-frequency heating. Gyrokinetic simulations show the nonlinear upshift of the TEM critical density gradient is associated with long-lived zonal flow dominated states [1]. This introduces a strong temperature dependence that allows external RF heating to control TEM turbulent transport. During pulsed on-axis heating of ITB discharges, core electron temperature modulations of 50\% were produced. Bursts of line-integrated density fluctuations, observed on phase contrast imaging, closely follow modulations of core electron temperature inside the ITB foot. Multiple edge fluctuation measurements show the edge response to modulated heating is out of phase with the core response. A new limit cycle stability diagram shows the density gradient appears to be clamped during on-axis heating by the nonlinear TEM critical density gradient, rather than by the much lower linear threshold. Fluctuation wavelength spectra will be quantitatively compared with nonlinear TRINITY/GS2 gyrokinetic transport simulations, using an improved synthetic diagnostic. In related work, we are implementing the first gyrokinetic exact linearized Fokker Planck collision operator [2]. Initial results show short wavelength TEMs are fully stabilized by finite-gyroradius collisional effects for realistic collisionalities. The nonlinear TEM threshold and its collisionality dependence may impact predictions of density peaking based on quasilinear theory, which excludes zonal flows.\\[4pt] In collaboration with M. Churchill, A. Dominguez, C. L. Fiore, Y. Podpaly, M. L. Reinke, J. Rice, J. L. Terry, N. Tsujii, M. A. Barnes, I. Bespamyatnov, R. Granetz, M. Greenwald, A. Hubbard, J. W. Hughes, M. Landreman, B. Li, Y. Ma, P. Phillips, M. Porkolab, W. Rowan, S. Wolfe, and S. Wukitch.\\[4pt] [1] D. R. Ernst et al., Proc. 21st IAEA Fusion Energy Conference, Chengdu, China, paper IAEA-CN-149/TH/1-3 (2006). http://www-pub.iaea.org/MTCD/Meetings/FEC200/th\_1-3.pdf\\[0pt] [2] B. Li and D.R. Ernst, Phys. Rev. Lett. 106, 195002 (2011). [Preview Abstract] |
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