Aperture plane potential control for thermal ion measurements

The effects produced by the addition of an aperture plane to control the bias around an instrument for low-energy ion measurements on satellites collecting data in the plasmasphere and over the polar cap are analyzed. The analysis is based on the design and functions of the retarding ion mass spectrometer (RIMS) on the DE 1 satellite. The NASA Charging Analyzer Program, which treats the spacecraft as a cylinder, was used to generate one set of predictions. A second model involves solution of the Laplace equation with the spacecraft treated as a sphere. Both models were used to predict the barrier height expected at the aperture in response to the bias induced to offset the potential. Comparisons with plasma data show that in the thin sheath regime of the plasmasphere the detectors act as if the potential is shifted, thereby lowering the energy resolution of the instrument. Outside the plasmasphere the barrier height dominates the spin curve variations more than the potential. Partial potential control is available from induced bias apertures if control is active and guided by electron detectors with sensitivities in the 0.5-50.0 eV energy range.