Laboratory study of the PFRC-2's initial plasma densification stages

Initial plasma densification by odd-parity rotating magnetic fields (RMF$_o$) applied to the linear magnetized PFRC-2 device with fill gases at pressures near 1 mTorr proceeds through two phases, a slow one, characterized by a rise time $ au_s sim 100$ $ mu$s, followed by fast one, characterized by $ au_f sim 10$ $ mu$s. The transition from slow to fast occurs at a line-integral-averaged electron density, n$_e$, near $2 imes 10^{11}$ cm$^{-3}$, independent of magnetic field. Over most of the range of experimental parameters investigated, as the PFRC-2 axial magnetic-field strength is increased, RMF$_o$ power decreased, gas fill pressure lowered, or lower atomic-mass (AMU) fill-gas used, the duration of the slow phase lengthened from $50$ $ mu$s to longer than 5 ms after the RMF$_o$ power began. The post-fast-phase maximum n$_e$ increases with the fill-gas AMU, exceeding 5 $ imes$ 10$^{13}$ cm$^{-3}$ for Ar. The slow phase is consistent with atomic physics processes and field-parallel sound-speed losses. The fast phase may be explained by improved axial confinement, possibly augmented by radial or axial contraction of the plasma. Another possible explanation, a large increase in electron temperature, is inconsistent with X-ray emission. The n$_e$ behavior is discussed in relation to the E to H transition.