Understanding and Mitigating the Neutral Gas Barrier in High-Density LAPD Plasmas

The Large Plasma Device (LAPD) parameter regime has greatly expanded following its 2020 upgrades, which included a Lanthanum Hexaboride cathode and a gas-puffing system. Recently, a mirror configuration has explored high-density (n ~ 6x10¹³ cm^-3, T_e ~ 2 eV) and warm plasma (n ~ 2x10¹² cm^-3, T_e ~ 17 eV) operational regimes. A fixed mirror ratio of three was employed at the plasma source, while the far-end ratio was systematically varied from one to seven. In the high-density regime, traditional gas feed using mass flow controllers introduced significant axial gradients. Diagnostics (Langmuir probes, microwave interferometer, fast-ion gauges, spectroscopy, Thomson scattering) revealed a dense neutral layer build-up at the leading edge of the expanding plasma, limiting spatial uniformity. A three-step neutral pumping process (edge injection, bulk ionization, leading-edge recombination/charge exchange) was identified, explaining the plasma's spatial and temporal evolution. Optimizing the gas-puffing system, cathode emissivity, and discharge power pushed this neutral barrier to LAPD's far-end, producing plasma with flat radial profiles and minimal axial variations over the 18 m column. These results demonstrate LAPD's unique capability to provide a robust platform for exploring novel plasma regimes.