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 β + ℓi/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 β + ℓi/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.