High-beta plasmas in theta pinches and cusp experiments

High-β plasmasare those whose diamagnetic currents react strongly on the confining magnetic field. Examples of such plasmas, in which the field is largely excluded from the plasma, are those in linear θ pinches and cusp-compression devices. In the former, measurements show β (the ratio of particle and magnetic pressures) to lie typically in the range 0.6 to 0.9. The sheath which separates the plasma from the confining magnetic field has the dimension c/ωpi (the ion collisionless skin depth), indicating that sheath ions carry the current. θ-pinch and cusp experiments are described in which the basic limitation on plasma containment is end-or cusp loss. In both cases the dimension of the loss 'hole', out of which plasma flows, is found to be a few ion Larmor radii. A simple theoretical argument is given to show that gross flute instabilities depend sensitively on the collisionless or collision-dominated nature of the high-β plasma. In the case of θ pinches experimental evidence for this effect is given, and its relevance to stabilization of a toroidal θ pinch is discussed.