Propagating vortices in the unstable region of a quadrupole

Abstract

A feature of the plasma in the UMIST quadrupole has been the occurrence of isolated pulses of plasma outside the critical surface, where the plasma is expected to be unstable to interchange modes; isolated plasma structures should then 'fall' outwards and disappear rapidly. The observed pulses propagate along the quadrupole and show no outward motion. The authors propose that the pulses in fact represent vortices, and that their rotation can stabilise them against falling, much as a gyroscope is stabilised by spin. On this basis the authors predict their velocity from first principles and obtain agreement with experiment to an order of magnitude or better, depending on assumptions about the ion temperature; they also show that the vortices can maintain themselves against collisional dissipation by the energy released when plasma is transported outwards in their circulation, and on this basis predict an average potential drop of 0.3-1.5 V between centre and edge of the vortex: the observed value is in the range 0-5 V.