Performance Studies of a Colloid Thruster System

Preliminary results are presented, of measurements of thrust (T) and specific impulse (Isp), produced by a single emitter prototype colloid propulsion system. The development of the colloid thruster propulsion package at Stanford University is in preparation for flighttesting on a 20-kg university nanosatellite, to be launched in 2003. The half-kilogram Colloid Micro Thruster (CMT) package measures 10 cm x 10 cm x 20 cm, consumes 6 watts of maximum power for all on-board functions (including power processing, and microprocessor control system), and stores and provides controlled delivery of 10 ml of 2.0 M sodium-iodide/glycerol propellant to the emission source. The prototype unit tested here is highly modular, to allow testing of multiple thruster “core” designs. Here we report on the performance of a single-emitter core, consisting of a 150 μm OD / 75 μm ID stainlesssteel capillary needle charged to as high as 6 kV relative to the extractor which is maintained at spacecraft ground potential. The measured emission current is found to be exponentially dependent on the applied acceleration voltage, placing it outside of regimes studied previously. Performance mapping, carried out by two indirect methods of determining T and Isp (from steady-state measurements of current and mass flow, and from time-of flight measurements) indicates operation at a level of Isp = 200 sec and T = 4 μN – somewhat below design targets, due largely to the relatively low applied voltage used (~4.5 kV). A discussion is presented about interesting dynamical behavior seen at emission onset, when current "bursts" are observed.