Density limit for electron plasmas confined by magnetic surfaces

Abstract

The density limit for electron plasmas confined by toroidal magnetic surfaces is investigated. In a cylinder, the well-known limit is the Brillouin density, nB≡ϵ0B2∕2me. In an axisymmetric torus, the confining region shifts outward in major radius, and this shift is shown to equal half the plasma radius when n∕nB≈ι2a∕R0, where ι=1∕q is the rotational transform of the magnetic field and a∕R0 is the inverse aspect ratio of the torus. In a nonaxisymmetric torus, electron confinement is found to be lost due to stochasticity effects when n∕nB≈(ι2∕8M2)(a∕R0)2∕δMN. The asymmetry amplitudes δMN are the fractional variations in n∕B2 on a magnetic surface in the poloidal mode number M and the toroidal mode number N≈ιM.