Power-crowbar impacts on plasma characteristics in ZT-40

Detailed analysis of the impact of I/sub phi/ and I/sub theta/ power crowbars on the ZT-40 plasma is presented for the operational period between shots 1620-3600. It is demonstrated that in both aided- and self-reversed modes the main effect of the I/sub phi/ power crowbar is to lengthen the time to l/e of peak current, whereas the main effect of the I/sub theta/ power crowbar is to extend the reversal of B/sub phi/ at the wall, primarily in self-reversed operation. It is shown that this extension has the effect of also decreasing the current decay rate to the point where no distinction is seen between the two modes. A saturation is seen in the decay time (tau/sub l/e/ less than or equal to 0.4 ms) once the field reversal in either mode exceeds approximately 0.3 ms. Possible physical explanations of these effects are discussed