Finite Larmor radius stability theory of ELMO Bumpy Torus plasmas

Abstract

An eikonal ballooning mode formalism is developed to describe curvature-driven modes of hot electron plasmas in bumpy tori. The formalism treats frequencies comparable to the ion cyclotron frequency, as well as arbitrary finite Larmor radius and field polarization, although the detailed analysis is restricted to E∥ =0. Moderate hot electron finite Larmor radius effects are found to lower the background beta core limit, whereas strong finite Larmor radius effects produce stabilization. The critical finite Larmor radius parameter with weak curvature is FR=k2⊥ ρ2hR/ Δb (1+P′∥/ P⊥) where k⊥ is the perpendicular wavenumber, ρh the hot electron Larmor radius, R the magnetic field radius of curvature at the hot-electron layer, Δb the magnetic field scale length in the diamagnetic well, and P′∥,⊥ are the parallel and perpendicular pressure gradients. The interchange instability arises if 1>FR>1−βcR/ [2Δ(1+P∥/P′⊥)], whereas all modes are stable if FR>1, where βc is the core plasma beta and Δ is the core plasma pressure gradient length.