Electrostatic gyrokinetic simulation of ultra-high-beta(β~1) equilibria

We present a microstability analysis of ultra-high-beta(β~1) equilibria for tokamaks. These equilibria have previously been studied in the context of MHD stability (Hsu, Artun and Cowley PoP 3,266 (1996)), however no microstability calculations have been performed. We use linear gyrokinetic stability calculations performed with the GS2 (bitbucket.org/gyrokinetics/gs2) code to examine how susceptible these equilibria are to turbulence caused by microinstabilities. We examine the stability of these equilibria to two major sources of electrostatic turbulence: Ion-Temperature Gradient modes and (Collisionless) Trapped-Electron modes. To understand the trend with a changing beta, we compare these ultra-high-beta equilibria with an intermediate beta(β~0.1) and a low-beta(β~0.01) equilibrium at two different radial locations: the inner core(Normalized radius ρ = 0.5) and the outer core(ρ = 0.8) for two different triangularities: δ = 0.4 and δ = -0.4. We find that the ultra-high beta equilibria are stable to both the ITG and TEM mode over a wide range of gradient scale lengths(R/L_T and R/L_n). To justify our findings and build intuition, we compare various figures of merit such as the local shear, field-line curvature, precession drift and relate their values to the behaviour of the electrostatic modes.