A slow toroidal -Z pinch experiment

Abstract

A slow (1.6 ms to peak current) toroidal theta -Z pinch is described, interest being centred on 'transitions' between stable and unstable states. Provided the electron temperature is above a certain critical value ( approximately 3.5*105 degrees K), the behaviour is little different from that of a Tokomak operating in its so-called unstable mode. The discharge goes through repeated instability cycles ( approximately 400 mu s period) of oscillatory, contraction and expansion phases. The electrons are heated (to several hundred eV max.) during the contraction phase and cooled during the expansion phase. Ions are accelerated (to perhaps several keV) during the expansion phase and either lost or 'cooled' during the oscillatory phase. With emphasis on the instability cycle, the basic measurements and distributions are described. An approximation for the magnetic surfaces is obtained, electron and 'ion' temperatures as functions of time are determined, and particle and energy containment times tau p and tau e are deduced. The overall hydromagnetic stability is shown to be consistent with the Kruskal-Shafranov limit. Both tau p and tau e are approximately 1 ms prior to the onset of a voltage spike. The mean value of tau p is approximately 2 ms, but the mean value of tau e is only approximately 100 mu s.