Results from the NRL Floating Probe on SPEAR III : High time resolution measurements of payload potential

Abstract

We present results from the NRL Floating Probe (FP) which made high time resolution measurements of spacecraft charging and discharging on the Space Power Experiments Aboard Rockets (SPEAR III) payload. SPEAR III was specifically designed to study the physics of spacecraft charging phenomena. We know that spacecraft in the ionosphere can charge to high levels (up to a few kilovolts). Charging occurs naturally in the auroral region due to high-energy streaming electrons or during the operation of active experiments (ion/electron beams or electromagnetic tethers). Charging and discharging events are often impulsive in nature and a method fast enough to track these potential changes will be extremely important. The FP consists of a metallic sphere containing a high-impedance amplifier and a capacitive divider network for scaling large voltages to the range that solid state circuits can handle. The probe can be used for either positive or negative polarity measurements. The 'worst case' time-response is associated with negative charging since the probe must collect ions to stay in contact with the local plasma. Modeling of the FP indicates that it has a time response faster than 1 ms for the entire range of negative charging normally associated with spacecraft (0 to -2 kV). We will discuss the FP design, construction, and theory of operation. SPEAR III also carried an ElectroStatic particle Analyzer (ESA) for monitoring the incoming ion distribution and a comparison of these two measurements is presented.