Linear micro-stability properties of a high β 1GW spherical tokamak

Spherical tokamaks (STs) offer an attractive route towards a compact high performance fusion reactor as historically they have achieved high plasma beta and strong shaping that facilitate plasmas with high bootstrap current. While scaling laws can be used to estimate confinement in a reactor relevant ST, their value is limited if the plasma regime is a substantial extrapolation from the parameter space used to generate these laws. In reality the confinement will be set by the turbulence that would arise in such a device, and any reduced transport model needs to describe the relevant characteristics of the dominant turbulence.

The local initial value gyrokinetic solvers, GS2 and CGYRO are used to analyse the nature of the modes that arise in a conceptual 1 GW ST. The dominant linear instabilities are found to be a collisional micro-tearing mode (MTM) and a kinetic ballooning mode (KBM) at the ion scale below k_yρ_s = 1. Towards the electron scale around k_yρ_s = 5, a collisionless MTM is found. Furthermore, when k_yρ_s > 10 the equilibrium examined is found to be completely stable, with no electron temperature gradient (ETG) mode. The KBMs and high k_yρ_s collisionless MTMs are suppressed by the anticipated level of flow shear, while the low k_yρ_s collisional MTMs are robust and are therefore expected to be the dominant source of transport nonlinearly. These low k_yρ_s MTMs have extended eigenfunctions in ballooning space indicating that nonlinear simulations may require very high radial resolution approaching the electron scale.

The dependence of these linear modes on different equilibrium parameters are examined to determine possible routes to stabilise each mode. With this information, potential options for generating a more stable equilibrium are discussed and a marginally stable plasma equilibrium is found linearly.