Gyrokine.c Analysis of the Linear Ohmic Confinement Regime in Alcator C Mod *


 Gillian Ashlyn Stewart
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1 Gyrokine.c Analysis of the Linear Ohmic Confinement Regime in Alcator C Mod * Miklos Porkolab in collabora.on with J. Dorris, P. Ennever, D. Ernst, C. Fiore, M. Greenwald, A. Hubbard, E. Marmar, Y. Ma, Y. Podpaly, M.L. Reinke, J. Rice, J. C. Rost, and N. Tsujii, MIT, PSFC and J. Candy, G.M. Staebler and R. Waltz, General Atomics References: L. Lin et al, Phys. Plasmas 16, (2009) M. Porkolab et al, EPS, Strasbourg, July 2011 P. Ennever, et al, UP , at this meehng *Work supported by the US Department of Energy Presented at APSDPP meeting, Salt Lake City, Nov 15, 2011, JO
2 Energy confinement.me in ohmically heated plasmas in tokamaks follows a linear scaling with density, followed by a saturated regime; Can gyrokine.c theory (codes) explain this result? C Mod data
3 Work up to 2009 November APS DPP meeting TRANSP analysis in the linear ohmic regime indicated that! i "! e However, nonlinear GYRO code simulations for the measured profiles predicted the opposite, namely (L. Lin et al, PP 16, (2009))! e <! i Varying the T i profile and its gradient up to 30 % did not help, ITG modes (and hence ion transport) remained dominant over TEM/ETG and electron transport Varying Te/T i, collisionality (Z eff ) or β e did not help
4 Measured transport coefficients at low densities deviate form the GYRO code prediction in spite of varying R/L Ti, R/L ne DiffusiviHes averaged at r/a = , Z imp = 12, Z eff varies with density (Lin, 2009)
5 New experiments with density and temperature profiles measured by Thomson scattering (T e ) and HIREX (T i ); For best agreement with theory (GYRO) for the impurity species we use Z imp = 8 Z eff = 2.7 GYRO range GYRO range
6 TGLF predicts χ i to be strongly reduced with increasing Z eff as long as Z imp is not too high (< 10) so the main ion species is diluted; χ e is only weakly affected; TGLF is the Trapped Gyro Landau Fluid model developed by G.M. Staebler, J. E. Kinsey and R. Waltz, Phys. Plasmas 14, (2007) Experimental Z eff corresponds to red
7 Extensive transport studies with TGLF find: Using an effective impurity Z imp 12 results in negligible deuteron dilution and confirms the results of Lin et al, ie, χ i > χ e Collisionality and/or T e /T i are not the decisive factors in determining the relative magnitudes of χ i and χ e E r (r) ( 14 kv/m) deduced from toroidal rotation measurements has negligible effect on transport in CMod Need to confirm results with nonlinear GYRO Given that in the past TGLF predicted transport was calibrated against GYRO in the ITG dominated regimes, we needed to study how GYRO compares with TGLF in the electron transport dominated regime The two codes are in reasonably good agreement, with some quantitative deviations
8 GYRO and TGLF are in good agreement, showing the same trend of reducing ion transport as the main ion species (deuterium) is depleted at higher Z eff with moderate Z i (=8) but both predict negligible transport for r/a < 0.5
9 Measured heat fluxes of ions and electrons in reasonable agreement with nonlinear GYRO and TGLF predictions
10 Predicted heat fluxes in good agreement with experiment if the gradients of T e and n e are increased in GYRO by 1020% above nominal values, well within experimental error
11 Experimentally measured power spectrum obtained with Phase Contrast Imaging (PCI) in good agreement with global nonlinear GYRO as interpreted with a synthetic PCI module n e =0.64x10 20 m  3 Experimental PCI spectrum Global GYRO spectrum Doppler shifted with E r = 14 kv/m, as measured Low frequencies at f < 50 khz and k< 2 cm  1 may not be dishnguishable from edge turbulence components measured by GPI (P. Ennever, UP ) Power spectrum versus k " averaged over khz
12 Summary The linear ohmic confinement regime was revisited in CMod After extensive analysis with TGLF/GYRO we find that using an effective impurity ion species Z i 8, electron modes (TEM), rather than ITG modes dominate as a result of significant (up to 40%) dilution of the main ion species (deuterons) In agreement with experiment, electron thermal diffusivities and heat fluxes dominate over those of ions in the radial range r/a= The fluctuation spectrum measured with Phase Contrast Imaging is in agreement with synthetic PCI predictions from global GYRO In the inner plasma core (r/a < 0.5) transport predicted by TGLF/ GYRO is too small relative to experimental values; potential candidates for enhanced transport may be current driven drift waves (U /C S = 25) or mild sawtooth activity, or?