Microstability of β ∼ 1 tokamak equilibria

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 similar to 1. However, a beta similar to 1 state may be unstable to various pressure- and current-driven instabilities or have unfavourable microstability properties. To explore these possibilities, we generate beta similar to 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.

Automated summary

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 similar to 1. However, a beta similar to 1 state may be unstable to various pressure- and current-driven instabilities or have unfavourable microstability properties. To explore these possibilities, we generate beta similar to 1 equilibria and investigate their stability.