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 Princeton field-reversed configuration (PFRC-2) device with fill gases at pressures near 1 mTorr proceeds through two phases: a slow one, characterized by a rise time $τ slow ∼ 100$ μs, followed by a fast one, characterized by $τ fast ∼ 10$ μs. The transition from slow to fast occurs at a line-integral-averaged electron density, _tn_e, near $2 × 10 11$ cm⁻³, independent of magnetic field. Over most of the range of experimental parameters investigated, as the PFRC-2 axial magnetic field strength was increased, RMF_o power decreased, gas fill pressure lowered, or lower atomic mass unit (AMU) fill gas used, the duration of the slow phase lengthened from 50 μs to longer than 10 ms after the RMF_o power began. The post-fast-phase maximum n_e increases with the fill-gas AMU, exceeding 5 × 10¹³ 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.