Large orbit plasma stability theory

A new analysis of the low frequency flute modes for the large ion orbit migma plasma configuration is reported. Results are given for the triple distribution function of ions. The theory for the m=1 mode shows an important reduction of the instability gap, when compared with conventional small Larmor radius treatments. The Migma IV experiment, which did not experience flute instability, was well within the unstable density range of thermal plasma calculations, and partially overlaps or lies below the unstable gap of the new theory. The second part of the paper considers microinstabilities. An explanation of the stabilization, by a negative bias potential, of the Harris instability observed in Migma IV is suggested. A localized unstable radial mode driven by ion counterstreaming at the center is discussed. Single particle orbit behaviour for finite p2 has been studied to ascertain losses that may arise due to stochasticity. Results indicate accurate conservation of the radial adiabatic invariant for orbits with axial energy up to 25% of the total kinetic energy. Finally, some stability problems of the high density diamagnetic migma regime are outlined. (orig.)