Optimization of a micro-retarding potential analyzer for high-density flowing plasmas

Summary form only given. This research addresses the limitations of existing retarding potential analyzers (RPAs) which can operate in plasmas with electron densities in excess of 1times10¹⁸ m ^-3, but are susceptible to space charge limitations. RPAs of a traditional gridded design have been implemented both experimentally and as in situ diagnostics to measure ion energy distributions, neutral particle flux, and species concentrations. The single-channel micro-retarding potential analyzer (SC-muRPA) developed has a channel diameter and electrode spacing on the sub-millimeter scale and can accurately operate in plasmas with densities of up to 1times10¹⁷ m^-3. To eliminate space charge effects and to increase the operating range of the SC-muRPA to densities above 1times10¹⁸ m^-3, a low transparency microchannel plate (MCP) has been incorporated as the floating electrode to produce the multi-channel micro-retarding potential analyzer (MC-muRPA) design. Improvements to the current collection theory for both the SC-muRPA and the MC-muRPA are also derived. Current-voltage curves are obtained for incoming flowing plasmas that range from near-stationary to hypersonic, with temperatures in the range of 0.1 to 10 eV, and with densities in the range of 1times10¹⁵ m^-3 to 1times10²¹ m^-3. The SC-muRPA current collection theory is validated by comparisons with the classical RPA theory as well as with 3D particle-in-cell simulations performed on an unstructured tetrahedral mesh. Determination of unknown plasma properties is based on a fuzzy-logic approach using generated look-up tables of current-voltage curves to converge towards the experimental curve. Thermal and sizing analyses were performed to confirm the elimination of melting and density gradients during experiments. RPA sizing recommendations are listed for a broad range of plasma properties, as well as the next it- ration of the muRPA design for high-density plume characterization