Model calculation and simulation of nonobserved negative mass instability in migma

Abstract

The negative mass instability is investigated both analytically and numerically in two distinct regimes. The principal result of both studies is that the instability will cause significant disruption of orbits concentric with the axis of magnetic symmetry but will not disrupt orbits that pass near the axis. This is in agreement with experiments on both types of orbits. The numerical simulation uses a two-dimensional, electrostatic finite-size particle code. The particle pusher algorithm is time-centered and accurate to third order in the time step, which is 1 100 of a cyclotron period. The self-consistent electric field is obtained by means of a spatial Eulerian grid superposed on the particle positions. Area weighting and smoothing techniques are used to reduce the artificially high collision frequency caused by the large particle charges required for practical computation. The analytical study models a neutralized beam in a plasma background. The Larmor radius of the beam is significant in comparison with the scale of the magnetic field variation. The dephasing effect of passing through the magnetic axis rather than encircling it is shown explicitly.