Laboratory study of the PFRC-2's initial plasma densification stages
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abstract
Initial plasma densification by odd-parity rotating magnetic fields (RMFo) 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 slow100s, followed by a fast one, characterized by fast10s. The transition from slow to fast occurs at a line-integral-averaged electron density, tne, near 21011 cm3, independent of magnetic field. Over most of the range of experimental parameters investigated, as the PFRC-2 axial magnetic field strength was increased, RMFo power decreased, gas fill pressure lowered, or lower atomic mass unit (AMU) fill gas used, the duration of the slow phase lengthened from 50 s to longer than 10ms after the RMFo power began. The post-fast-phase maximum ne increases with the fill-gas AMU, exceeding 51013cm3 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 ne behavior is discussed in relation to the E to H transition.
