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 KI3: Turbulent Transport Measurement and Modeling |
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Chair: Chris Holland, University of California, San Diego Room: Centennial II |
Tuesday, November 3, 2009 3:00PM - 3:30PM |
KI3.00001: Marshall N. Rosenbluth Outstanding Doctoral Thesis Award Talk: Simultaneous Measurement of Electron Temperature and Density Fluctuations in the Core of DIII-D Plasmas Invited Speaker: Multi-field fluctuation measurements provide opportunities for rigorous comparison between experiment and nonlinear gyrokinetic turbulence simulations. A unique set of diagnostics on DIII-D allows for simultaneous study of local, long-wavelength ($0 < k_\theta\rho_s < 0.5$) electron temperature and density fluctuations in the core plasma ($0.4 < \rho < 0.8$). Previous experiments in L-mode indicate that normalized electron temperature fluctuation levels ($40 < f < 400\,$kHz) increase with radius from $\sim$0.4\% at $\rho = 0.5$ to $\sim$2\% at $\rho =0.8$, similar to simultaneously measured density fluctuations. Electron cyclotron heating (ECH) is used to increase $T_e$, which increases electron temperature fluctuation levels and electron heat transport in the experiments. In contrast, long wavelength density fluctuation levels change very little. The different responses are consistent with increased TEM drive relative to ITG-mode drive. A new capability at DIII-D is the measurement of phase angle between electron temperature and density fluctuations using coupled correlation electron cyclotron emission radiometer and reflectometer diagnostics. Linear and nonlinear GYRO runs have been used to design validation experiments that focus on measurements of the phase angle. GYRO shows that if $T_e$ and $\nabla T_e$ increase 50\% in a beam-heated L-mode plasma ($\rho=0.5$), then the phase angle between electron temperature and density fluctuations decreases 30\%-50\% and electron temperature fluctuation levels increase a factor of two more than density fluctuations. Comparisons between these predictions and experimental results will be presented. [Preview Abstract] |
Tuesday, November 3, 2009 3:30PM - 4:00PM |
KI3.00002: Comparison of experimental measurements and gyrokinetic turbulent electron transport models in Alcator C-Mod plasmas Invited Speaker: We present results of core turbulence measurements and associated transport in Alcator C-Mod using phase contrast imaging (PCI) diagnostic. In order to interpret and better understand the measurements we quantitatively compared the results with extensive simulations with the gyrokinetic code GYRO through a synthetic PCI diagnostic. Both L-Mode and H-Mode plasmas were examined. The L-Mode experiments were carried out over the range of densities covering the ``neo-Alcator'' (linear confinement time scaling with density) to the ``saturated ohmic'' regime. The key role played by the ion temperature gradient (ITG) turbulence has been verified by measurements of turbulent wave propagation which was dominantly in the ion diamagnetic direction in the laboratory frame. The absolute fluctuation intensity also agreed with simulation within experimental error. In the saturated ohmic and H-Mode regime where ion transport dominates, the simulated ion and electron thermal diffusivities also agree with experiments after varying the ion temperature gradient or adding E$\times $B shear suppression within experimental uncertainty. However, in the linear ohmic regime where electron transport dominates, GYRO does not agree with experiments, showing significantly larger ion thermal transport and smaller electron thermal transport. Our study shows that although the electron temperature gradient (ETG) mode is unstable, the nonlinear simulation with k$_{\theta}\rho_{s}$ up to 4 does not raise the electron thermal diffusivity to the experimental level. Further work to explore even higher-k ETG regimes with GYRO is underway and the results will be presented. Work supported by U. S. DOE under DE-FG02-94-ER54235 and DE-FC02-99-ER54512. [Preview Abstract] |
Tuesday, November 3, 2009 4:00PM - 4:30PM |
KI3.00003: Probing Plasma Turbulence by Modulating the Electron Temperature Gradient Invited Speaker: Validating transport models is an essential step toward accurate predictive capability of plasma transport in tokamak plasmas. Experiments with a single key parameter varied and the turbulence response measured provide excellent data sets for model validation studies. One such experiment on DIII-D will be discussed where the electron temperature gradient was systematically varied and the turbulence response documented and compared with predictions from the gyrokinetic turbulence code GYRO. The temperature gradient scale length $a/L_{Te}$ was varied at the plasma mid-radius by repetitively switching from ECH deposition just inside to just outside the mid-radius which modulated $a/L_{Te}$ at essentially fixed $T_e$. Electron density fluctuations at wavenumbers $k_\theta =\,$5--6~cm$^{-1}$, typically associated with TEM turbulence, were measured with a Doppler backscattering system (DBS) and were well correlated spatially and temporally with the variation produced in $a/L_{Te}$. The turbulence amplitude modulation was spatially localized, peaked between the two ECH deposition regions, and was consistent with expectations based on $a/L_{Te}$ being a drive term for TEM turbulence. The DBS lab frame frequency was observed to increase when $a/L_{Te}$ decreased. However, this is likely a result of variations in poloidal $E\times B$ motion rather than changes in the turbulence frequency. A direct comparison of GYRO predictions using a synthetic diagnostic module developed for DBS shows agreement within experimental uncertainties with measured turbulence amplitude variation but disagreement in spectral shape and considerable disagreement between predicted heat fluxes and power balance analysis. This is an area of ongoing investigation. Additional experiments at larger plasma radii were carried out where modulation of temperature fluctuations were also seen and will be discussed. [Preview Abstract] |
Tuesday, November 3, 2009 4:30PM - 5:00PM |
KI3.00004: Fluctuation-Induced Particle Transport and Density Relaxation in a Stochastic Magnetic Field Invited Speaker: Particle transport and density relaxation associated with electromagnetic fluctuations is an unresolved problem of long standing in plasma physics and magnetic fusion research. In toroidal fusion plasmas, magnetic field fluctuations can arise spontaneously from global MHD instabilities, e.g., tearing fluctuations associated with sawtooth oscillations. Resonant magnetic perturbations (RMP) have also been externally imposed to mitigate the effect of edge localized modes (ELMs) by locally enhancing edge transport in Tokamaks. Understanding stochastic-field-driven transport processes is thus not only of basic science interest but possibly critical to ELM control in ITER. We report on the first direct measurement of magnetic fluctuation-induced particle transport in the core of a high-temperature plasma, the MST reversed field pinch. Measurements focus on the sawtooth crash, when the stochastic field resulting from tearing reconnection is strongest, and are accomplished using newly developed, laser-based, differential interferometry and Faraday rotation techniques. The measured electron particle flux, resulting from the correlated product of electron density ($\delta $n) and radial magnetic fluctuations ($\delta $b$_{r})$, accounts for density profile relaxation during these magnetic reconnection events. Surprisingly, the electron diffusion is 30 times larger than estimates of ambipolarity-constrained transport in a stochastic magnetic field. A significant ion flux associated with parallel ion flow velocity fluctuations ($\delta $v$_{i,//})$ correlated with $\delta $b$_{r}$ appears responsible for transport larger than predictions from the quasi-linear test particle model. These results indicate the need for improved understanding of particle transport in a stochastic magnetic field. Work performed in collaboration with W.X. Ding, W.F. Bergerson, T.F. Yates, UCLA; D.J. Den Hartog, G. Fiksel, S.C. Prager, J.S. Sarff and the MST Group, University of Wisconsin-Madison. [Preview Abstract] |
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