HIGH VOLTAGE SPACE PLASMA INTERACTIONS J.E.McCoy, Space Environmt.Of. NASA-JSC, Houston,Tx 77034 All large space structures in Earth orbit are immersed in a very tenuous ionized "gas". This ionized "gas" (called plasma) exists everywhere in space. Although so tenuous as to be completely insignificant for most purposes to date, this plasma provides a source of electric current carriers which can become very significant for large structures and/or high voltages. Adequate consideration of such effects should be included ...during design of the SPS. There have been two primary problems identified to result from plasma interactions; one of concern to operations in geosynchronous orbit (GEO), the other in low orbits (LEO). The two problems are not the same. Spacecraft charging has become widely recognized as a problem, particularly for communications satellites operating in GEO. The very thin (0.1-10/cc) thermal plasmas at GEO are insufficient to bleed off voltage buildups (>10 kv) due to higher energy charged particle radiation collected on outer surfaces. Resulting differential cHargfng/discharging causes electrical transients, spurious command signals and possible direct overload damage. An extensive NASA/Air Force program has been underway for several years to address this problem (1,2). At lower altitudes, the denser plasmas of the plasmasphere/ionosphere provide sufficient thermal current to limit such charging to a few volts or less. Unfortunately, these thermal plasma currents which solve the (GEO) spacecraft charging problem can become large enough to cause just the opposite problem in LEO. Ionospheric plasma densities exceeding one million/cc exist around spacecraft in LEO. Operation of large solar arrays at high voltage, for SPS developmental testing or LEO assembly/self-propulsion, could drive substantial leakage currents through this surrounding plasma (Fig. 1). The resulting power loss to these parasitic currents has been observed to exceed solar cell output capability for small (10 cm) test objects in the laboratory. Recent estimates of this effect for large arrays, based on limitation of the leakage currents by formation of space charge limited sheaths around the high voltage surfaces, indicate that such losses should remain within acceptable limits for very large (>100m) arrays (Fig. 2). Large (10m) scale lab tests in simulated LEO plasmas at JSC tend to support these estimates (Fig. 3), but much more detailed work remains to be done (3). Several other plasma effects have been observed which may become more important as design considerations for SPS than the basic parasitic plasma currents. Focusing of the currents collected within a specific electrostatic "lens" configuration produced by the sheath fields surrounding a high voltage panel has been observed to produce local concentrations of current which could potentially overload or damage a small area of cells within a larger string, even though the average current density "leaking" from the plasma to the entire array is less than the design limits. Fig. 4 is a tracing of relative current density contours observed on the face of a simulated solar array operating at -2,000V in an argon plasma of density about 105/cc. The panel area included in the figure is about 1 meter by 2 meters, at one end of the IXIOm panel. The total current flow measured to the entire panel indicated an average current density of 1.0 ma/m2 (0.1 pamp/cm2). Most of this current was concentrated within the roughly triangular region within the contours shown; with contour level #1 containing local current densities roughly 0.1 pamp/cm2, increasing linearly to more than 0.8 pamp/cm2 within contour #8. Currents outside contour level 1 dropped sharply, to probably less than 0.01 pamp/cm2throughout region 0.
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