DIFFERENTIAL AND ACTIVE CHARGING RESULTS FROM THE ATS SPACECRAFT

This study of spacecraft charging concentrates on the differential charging and artificial particle emission experiments on ATS-5 and ATS-6. It was found that differential charging of spacecraft surfaces generated large electrostatic barriers to spacecraft generated electrons, from photoemission, secondary emission, and thermal emitters. The electrostatic barrier is a potential minimum outside the charged spacecraft which causes low energy electrons to be trapped near the spacecraft. The large dish antenna on ATS-6 was identified as the source of the electrostatic barrier around the Environmental Measurements Experiment package. Daylight charging on ATS-6 was shown to have behavior suggesting the dominance of differential charging on the absolute potential of the mainframe. Electron emission experiments on ATS-5 in eclipse charging environments showed that the electron emitter could partially or totally discharge the satellite, but the mainframe recharged negatively in a few 10's of seconds. The equilibrium emitter current was found to be .3 microamps, substantially below the milliamp capability of the emitter. The limiting of the current and the time dependence seen in the ATS-5 potential during these operations were explained as the result of differential charging of the insulating surfaces on the spacecraft, and the creation of an electrostatic barrier by the differential potential. This barrier limited the artificially generated electron current to the point that the net flux to the spacecraft was again negative. Both the daylight charging events of ATS-6 and the eclipse electron emission experiments of ATS-5 were further analyzed with a simple time dependent model which showed that the barrier height quickly reached an equilibrium value which limited but did not completely stop electron emission. Average and differential potentials developed in time subject to the constraint that the barrier height remain constant. Ion engine operations and plasma emission experiments on ATS-6 were shown to be an effective means of controlling the spacecraft potential in eclipse and sunlight. Ion fluxes from the neutralizer and engine served to discharge differentially charged surfaces, eliminating barrier effects around the detectors, and improving the quality of the particle data. In the appendices, a study of intense fluxes of electrons, narrowly confined in energy and angle, are shown to be electrons generated at the surface of spacecraft, and in particular the insulating surface of the University of Minnesota detector. Also, a brief study of the midnight warm plasma population showed that field aligned particle streams are most commonly seen, with one observation of a cool isotropic population in eclipse. This population was hidden in sunlight by positive spacecraft potentials, except during ion engine operations.