Validation of nonlinear gyrokinetic simulations of L- and I-mode plasmas on Alcator C-Mod

A J Creely,A E Hubbard,S J Freethy,J Candy,C Sung,G D Conway,N Cao,A E White,P Rodriguez-Fernandez,N T Howard,G M Staebler,J W Hughes,J E Rice

doi:10.1063/1.4977466

Abstract

New validation of global, nonlinear, ion-scale gyrokinetic simulations (GYRO) is carried out for L- and I-mode plasmas on Alcator C-Mod, utilizing heat fluxes, profile stiffness, and temperature fluctuations. Previous work at C-Mod found that ITG/TEM-scale GYRO simulations can match both electron and ion heat fluxes within error bars in I-mode [White PoP 2015], suggesting that multi-scale (cross-scale coupling) effects [Howard PoP 2016] may be less important in I-mode than in L-mode. New results presented here, however, show that global, nonlinear, ion-scale GYRO simulations are able to match the experimental ion heat flux, but underpredict electron heat flux (at most radii), electron temperature fluctuations, and perturbative thermal diffusivity in both L- and I-mode. Linear addition of electron heat flux from electron scale runs does not resolve this discrepancy. These results indicate that single-scale simulations do not sufficiently describe the I-mode core transport, and that multi-scale (coupled electron- and ion-scale) transport models are needed. A preliminary investigation with multi-scale TGLF, however, was unable to resolve the discrepancy between ion-scale GYRO and experimental electron heat fluxes and perturbative diffusivity, motivating further work with multi-scale GYRO simulations and a more comprehensive study with multi-scale TGLF.

Highlights

With a new generation of fusion devices on the horizon, the need for steady state scenarios operating with high energy confinement, but low impurity confinement, is apparent
Previous work at C-Mod found that ion temperature gradient (ITG)/trapped electron mode (TEM)-scale gyrokinetic simulations (GYRO) simulations can match both electron and ion heat fluxes within error bars in I-mode [White PoP 2015], suggesting that multi-scale effects [Howard PoP 2016] may be less important in I-mode than in L-mode
In addition to ohmic heating, auxiliary ion cyclotron range of frequency (ICRF) heating is applied, initially at 1.6 MW, and stepping up to 3.5 MW, which initiates the transition to I-mode

Summary

Introduction

With a new generation of fusion devices on the horizon, the need for steady state scenarios operating with high energy confinement, but low impurity confinement (to avoid fuel dilution and radiative losses), is apparent. The I-mode is a high energy confinement regime of plasma operation that is regularly obtained on the Alcator C-Mod tokamak.. I-mode plasmas achieve energy confinement times comparable to or exceeding H-mode, without as strong of a confinement time degradation with increased input power. I-mode is generally run with the ion B Â rB drift away from the active X-point (unfavorable rB drift), which enables a more robust access to the I-mode confinement regime.. I-mode is accessible below 3 T, but even small power increases generally lead to a H-mode transition.

Results

Discussion

Conclusion