Measurement of current penetration during PDX discharge startup

The current penetration phase of PDX discharges is examined. The Fast-Ion Diagnostic Experiment has been used to measure the temporal evolution of the central q(r/a<0.4) and to show the effect of magnetic perturbations on fast ions. During plasma current penetration, a series of magnetic perturbations was observed in the plasma. When the plasma current was rising rapidly, the perturbations were accompanied by increases in βsub(theta)+lsub(i)/2 and decreases in the loop voltage, suggesting a rapid penetration of the plasma current. When the plasma current was rising slowly, a series of 'minor' disruptions occurred. These were accompanied by decreases in βsub(theta)+lsub(i)/2 and in the loop voltage, and increases in the plasma current. During this phase, current penetration may be enhanced by the change in the resistivity profile which accompanies the disruption. (author)