Impact of plasma shaping on tokamak microstability

TL;DR

This study uses gyrokinetic simulations to explore how plasma shaping, especially elongation, affects microinstability stability in tokamaks, revealing both destabilization and stabilization mechanisms relevant for high-performance plasma operation.

Contribution

The paper provides the first detailed gyrokinetic analysis of flux-surface shaping effects on microstability in JT-60SA-like equilibria, including a benchmark with the GKV code.

Findings

Increased elongation destabilizes electrostatic fluctuations with peaked pressure profiles.

Shaping stabilizes kinetic ballooning modes through enhanced FLR effects.

At high beta, elongation can reduce magnetic shear stabilization, affecting ballooning stability.

Abstract

We have used the local-$\delta{f}$ gyrokinetic code GS2 to perform studies of the effect of flux-surface shaping on two highly-shaped, low- and high-$\beta$ JT-60SA-relevant equilibria, including a successful benchmark with the GKV code. We find a novel destabilization of electrostatic fluctuations with increased elongation for plasma with a strongly peaked pressure profile. We explain the results as a competition between the local magnetic shear and finite-Larmor-radius (FLR) stabilization. Electromagnetic studies indicate that kinetic ballooning modes are stabilized by increased shaping due to an increased sensitivity to FLR effects, relative to the ion-temperature-gradient instability. Nevertheless, at high enough $\beta$, increased elongation degrades the local magnetic shear stabilization that enables access to the region of ballooning second-stability.