Gyrokinetic studies of the effect of beta on drift-wave stability in NCSX

TL;DR

This study uses gyrokinetic simulations to explore how plasma beta influences drift-wave stability in NCSX, revealing that higher beta can marginally stabilize some modes but also lowers critical gradients, with implications for stellarator performance.

Contribution

It provides the first detailed analysis of plasma beta effects on ITG and TEM stability in NCSX geometry using gyrokinetic simulations, including benchmarking with other codes.

Findings

High beta marginally stabilizes adiabatic-electron ITG modes.

Lower critical gradient for kinetic-electron ITG modes at high beta.

Electrostatic and electromagnetic mode dependencies are similar.

Abstract

The gyrokinetic turbulence code GS2 was used to investigate the effects of plasma beta on linear, collisionless ion temperature gradient (ITG) modes and trapped electron modes (TEM) in National Compact Stellarator Experiment (NCSX) geometry. Plasma beta affects stability in two ways: through the equilibrium and through magnetic fluctuations. The first was studied here by comparing ITG and TEM stability in two NCSX equilibria of differing beta values, revealing that the high beta equilibrium was marginally more stable than the low beta equilibrium in the adiabatic-electron ITG mode case. However, the high beta case had a lower kinetic-electron ITG mode critical gradient. Electrostatic and electromagnetic ITG and TEM mode growth rate dependencies on temperature gradient and density gradient were qualitatively similar. The second beta effect is demonstrated via electromagnetic ITG growth rates' dependency on GS2's beta input parameter. A linear benchmark with gyrokinetic codes GENE and GKV-X is also presented.