Microstability of β ~ 1 tokamak equilibria

Abstract

High-power-density tokamaks offer a potential solution to design cost-effective fusion devices. One way to achieve high power density is to operate at a high $\beta$ value (the ratio of thermal to magnetic pressure), i.e. $\beta \sim 1$ . However, a $\beta \sim 1$ state may be unstable to various pressure- and current-driven instabilities or have unfavourable microstability properties. To explore these possibilities, we generate $\beta \sim 1$ equilibria and investigate their stability. First, we demonstrate the generation of high- $\beta$ equilibria with the computer code VMEC. We then analyse these equilibria to determine their stability against the infinite- $n$ ideal-ballooning mode. We follow that by engaging in a detailed microstability study using the GS2 code, beginning with assessments of electrostatic ion-temperature-gradient and trapped election mode instabilities. We observe interesting behaviour for the high- $\beta$ equilibria – stabilization of these modes through two distinct mechanisms – large negative local shear and reversal of electron precession drift. Finally, we perform electromagnetic gyrokinetic simulations and observe enhanced stability in the outer core of high- $\beta$ equilibria and absence of kinetic ballooning modes in the negative-triangularity, high- $\beta$ equilibria. The enhanced outer-core stability of high- $\beta$ equilibria is different from their lower- $\beta$ counterparts and offers an alternative, potentially favourable regime of tokamak operation.