Stability Properties of Anisotropic Pressure Stellarator Plasmas with Fluid and Noninteractive Energetic Particles

Abstract

The three-dimensional (3-D) VMEC code has been modified to model an energetic species with a variant of a Bi-Maxwellian distribution function that satisfies the constraint B·∇ℱh = 0, and the 3-D TERPSICHORE stability code has been extended to investigate the effects of pressure anisotropy in two limits. The lower limit is based on a purely fluid Kruskal-Oberman (KO) energy principle (ignoring the stabilizing kinetic integral), and the upper limit is obtained from an energy principle in which the hot particle pressure and current density refrain from interacting with the dynamics of the instability because their diamagnetic drift frequency is considered much larger than the dominant growth rate. We have specifically investigated the instability properties of a Heliotron device with a major radius of 3.9 m and total 〈β〉 ≃ 3.9%, where the energetic particle contribution 〈βh〉 varies from 0 to 1.3% for T‖/T┴ = 4. Both models demonstrate a significant stabilization of a global m/n = 4/2 mode as 〈βh〉/〈β〉 approaches 1/3, with the noninteracting (NI) hot particle model virtually reaching the point of marginal stability when 〈βh〉/〈β〉 ≃ 1/4. A variation of T‖/T┴ at fixed 〈βh〉/〈β〉 = 1/3 shows a noticeable decrease in the absolute magnitude of the unstable eigen-value in the range 1 ≤ T‖/T┴ ≤ 2 in the fluid KO model. The NI energetic particle model remains near marginality in the range 1 ≤ T‖/T┴ ≤ 4. The Mercier stability is consistent with the global mode calculations. Performance benchmarks of the TERPSICHORE code on several different computers are presented