Linear gyrokinetic stability of a high $\beta$ non-inductive spherical tokamak

TL;DR

This paper analyzes the linear gyrokinetic stability of a high-beta spherical tokamak, identifying dominant micro-instabilities and their parametric dependence to inform predictive transport models for fusion reactors.

Contribution

It provides the first detailed linear gyrokinetic stability analysis of a high-beta spherical tokamak equilibrium, highlighting key micro-instabilities and their parametric behavior.

Findings

Kinetic ballooning modes and micro-tearing modes are dominant instabilities.

Equilibrium tuned to be marginally stable to all micro-instabilities.

Insights into micro-instability dependence aid in developing predictive transport models.

Abstract

Spherical tokamaks (STs) have been shown to possess properties desirable for a fusion power plant such as achieving high plasma ? and having increased vertical stability. To understand the confinement properties that might be expected in the conceptual design for a high $\beta$ ST fusion reactor, a 1GW ST plasma equilibrium was analysed using local linear gyrokinetics to determine the type of micro-instabilities that arise. Kinetic ballooning modes (KBMs) and micro-tearing modes (MTMs) are found to be the dominant instabilities. The parametric dependence of these linear modes was determined and from the insights gained, the equilibrium was tuned to find a regime marginally stable to all micro-instabilities at $\theta_0$ = 0:0. This work identifies the most important micro-instabilities expected to generate turbulent transport in high $\beta$ STs. The impact of such modes must be faithfully captured in first principles based reduced models of anomalous transport that are needed for predictive simulations.