Colloid thrusters for the new millennium, ST7 DRS mission

The objective of the NASA JPL Space Technology 7-Disturbance Reduction System (ST7-DRS) mission is to demonstrate: (1) test mass trajectory control that deviates from purely gravitational trajectory by less than 3×10^-14m/sec²/Hz^0.5 [1+(f/3 mHz)²] over a frequency range from 1 mHz to 30 mHz; and (2) spacecraft position control within 10nm/Hz^0.5 over the same frequency range. To achieve these objectives, two technologies must be advanced and tightly integrated. These technologies are the gravitational reference sensors (GRS) being developed by Stanford University and the colloid micro-thruster (CMT) propulsion system in development at Busek Co. Inc. The DRS system will be integrated by JPL and fly on an ESA spacecraft called the LISA pathfinder. This work reports on the progress in the propulsion technology area. The ST7-DRS microthrusters develop thrust by emitting and electrostatically accelerating positively charged nanosized particles, generally referred to as colloids; hence these devices are known as colloid thrusters. Each colloid thruster on the DRS must produce thrust in the 2μN to 20μN range, be smoothly adjustable throughout this range with a maximum thrust increment of less than 0.1 μN and produce thrust noise at a level lower than 0.1 μN/√Hz in the 1 mHz to 30 mHz frequency interval. The DRS uses 8 colloid thrusters arranged in 2 clusters. Each thruster has an independent propellant storage and management subsystems and a power processing unit (PPU). Each cluster has a digital control and interface unit (DCIU) that receives thrust commands from the GRS computer and sends them in analog form to the appropriate thrusters within the cluster. Each cluster also has a field emission cathode based on carbon nanotubes that provides space charge neutralization for the positively charged stream of colloids. The 10:1 dynamic thrust range, the continuous thrust adjustability and noise requirements represent unique challenges that require significant advances in several technology areas, including extremely precise propellant flow control (<0.01μl/min), noise free, high voltage DC/DC converters (<10kV) and stable colloid emission (<0.1μN). We have demonst- rated that we meet the required thrust, thrust noise, and adjustability which we measured using a newly developed torsional thrust stand. We have further demonstrated the necessary functionality of the integrated system at a breadboard level. The performance, the system and the subsystem designs are described.