Long-pulse operation of the PFRC-2 device

Studies of the time dependence of plasma density in long-duration plasma pulses were performed in the PFRC-2, a field-reversed-configuration device heated by odd-parity rotating magnetic fields. Long-pulse operation is made possible by a set of 8 superconducting internal passive flux-conserving rings, each with an inductive decay time of 1 sec and a critical current of 3 kA. With prefill hydrogen gas only, the line-average density rose to 2e12 cm$^{-3}$ in 1 ms and decayed to near 0 in about 10 ms. Using a PV-10 gas valve modified to provide supersonic gas injection, we have found operational regimes where in-discharge fueling with a single 10-ms-duration hydrogen puff produced stable high density (2e12 cm$^{-3}$) plasma discharges that persisted for 200 ms.