Microstability of $\beta \sim 1$ 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 unfavorable microstability properties. To explore these possibilities, we generate $\beta \sim 1$ equilibria and investigate their stability. Initially, we study an analytical technique that was used in the past to generate $\beta \sim 1$ equilibria and outline its limitations. Hence, we demonstrate the generation of high-$\beta$ equilibria with the computer code $\texttt{VMEC}$. We then analyze these equilibria to determine their stability against the infinite-$n$ ideal ballooning mode. We follow that by engaging in a detailed microstability study, beginning with assessments of electrostatic ITG and TEM instabilities. We observe interesting behavior for the high-$\beta$ equilibria -- stabilization of these modes through two distinct mechanisms. Finally, we perform electromagnetic gyrokinetic simulations and again observe stabilizing trends in the equilibria at high $\beta$. These trends are different from their lower $\beta$ counterparts and offer an alternative, potentially favorable regime of tokamak operation.