Negative Mass Instability with Bθ Field

Earlier negative mass instability calculations with a mirror magnetic field have been extended to also include a Bθfield. The plasma is contained in a torus, of elliptical cross section, which is radially thin. From a new dispersion relation it is found that: (a) for a cold plasma the negative mass instability persists even for large Bθ, although the growth rate goes as Bθ−1; (b) for a finite velocity spread plasma, the addition of a Bθ field allows much higher densities, because the stability criterion becomes independent of density. These results appear to explain the beam disruption observed on the Stevens Institute plasma betatron surprisingly well.