The effects of dilution on turbulence and transport in C-Mod ohmic plasmas and comparisons with gyrokinetic simulations

Main ion dilution has been predicted by gyrokinetic simulations to have a significant effect on ion thermal transport in C-Mod ohmic plasmas. This effect was verified experimentally with a specific set of experiments on C-Mod in which ohmic deuterium plasmas across the linear ohmic confinement (LOC)...

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Bibliographic Details
Published in:Physics of plasmas 2015-07, Vol.22 (7)
Main Authors: Ennever, P., Porkolab, M., Candy, J., Staebler, G., Reinke, M. L., Rice, J. E., Rost, J. C., Ernst, D., Fiore, C., Hughes, J., Terry, J.
Format: Article
Language:English
Subjects:
Confinement
Deuterium
Dilution
Electron energy
Fluxes
Ion temperature
Plasma physics
Rotating plasmas
Rotation
Simulation
Stability
Stiffness
Temperature gradients
Transport
Turbulence
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Summary:Main ion dilution has been predicted by gyrokinetic simulations to have a significant effect on ion thermal transport in C-Mod ohmic plasmas. This effect was verified experimentally with a specific set of experiments on C-Mod in which ohmic deuterium plasmas across the linear ohmic confinement (LOC) through the saturated ohmic confinement (SOC) regimes were diluted by seeding with nitrogen gas (Z = 7) injection. The seeding was observed to increase the normalized ion temperature gradients (ITGs) by up to 30% without a corresponding increase in the gyrobohm normalized ion energy flux, indicating a change in either the stiffness or the critical ion temperature gradient associated with ITG turbulence. The seeding also reversed the direction of the intrinsic toroidal rotation in plasmas slightly above the normal intrinsic rotation reversal critical density. GYRO simulations of the seeded and unseeded plasmas show that the seeding affected both the critical gradient and the stiffness. For plasmas in the LOC regime, the dilution primarily increased the critical gradient, while for plasmas in the SOC regime the dilution primarily decreased the stiffness. At r/a = 0.8, where the experimental fluxes were above marginal stability, local GYRO predicted and experimental energy fluxes agreed, except for Qi in the SOC regime where GYRO under-predicted the experimental energy flux. At r/a = 0.6, where the experimental fluxes were close to marginally stable, local GYRO predicted ITG modes to be strongly unstable and are responsible for both Qi and Qe (with Qi > Qe), as opposed to the experiment where Qi 
ISSN:1070-664X
1089-7674
DOI:10.1063/1.4926518