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When electrical arcs occur in space, a plasma expands away from the arc-site, neutralizing adjacent surfaces (a current), and causing a current to be produced at the arc-site (source of neutralization current). The speed of this plasma expansion depends on the plasma species, which in turn depend on the ionizable materials near the initial electrostatic discharge (ESD) site. Based on laboratory experiments undertaken as part of the U.S. round-robin experiments on plasma propagation speed, a scenario for arc plasma propagation and arc current profiles is presented. It is found that the complex arc current profiles invariably seen in laboratory arcs are due to a multicomponent plasma, where each plasma species expands away from the arc-site supersonically and with approximately constant velocity. Apparent slowing of the arc plasma seen in high-speed video cameras is caused by the density depletion of the lightest (most rapid) plasma component first and heavier (slower) plasma components later. Electron currents onto surfaces originate at the arc-site, and the conductive arc plasma is a conduit for these currents. Sudden, simultaneous onset of arc currents at all distances from the arc-site is the result of blowoff currents, making all surfaces more positive, which then attract ambient electrons. Sudden, simultaneous cutoff of arc currents at all distances from the arc-site is the result of collapse of the plasma due to conditions at the arc-site. The ionization at the arc-site during the arc is seen to be rapidly variable, with variations on the nanosecond timescale. This model not only makes the varied plasma velocities reported in the literature understandable, but it also makes predictions about the arc radiofrequency interference (RFI), contamination produced by the arcs, and the total charge in an arc possible. Arc-site materials are suggested which, being hard to ionize and with massive ions, minimize arc currents and maximize arc current rise times.

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The Mumbo space environment simulation chamber discussed here comprises a set of tools to calibrate a variety of low flux, low energy electron and ion detectors used in satellite-mounted particle sensors. The chamber features electron and ion beam sources, a Lyman-alpha ultraviolet lamp, a gimbal table sensor mounting system, cryogenic sample mount and chamber shroud, and beam characterization hardware and software. The design of the electron and ion sources presented here offers a number of unique capabilities for space weather sensor calibration. Both sources create particle beams with narrow, well-characterized energetic and angular distributions with beam diameters that are larger than most space sensor apertures. The electron and ion sources can produce consistently low fluxes that are representative of quiescent space conditions. The particle beams are characterized by 2D beam mapping with several co-located pinhole aperture electron multipliers to capture relative variation in beam intensity and a large aperture Faraday cup to measure absolute current density.