Characterization of a six-emitter colloid thruster using a torsional balance

Colloid thruster micropropulsion will be tested in space by the New Millennium Program's Space Technology 7-Disturbance Reduction System mission (NASA). The objective of this program is to develop the technologies required for precision spacecraft position control, with accuracy orders of magnitude better than previously demonstrated. This capability will enhance or enable aggressive scientific programs like the Laser Interferometer Space Antenna and the Micro-Arcsecond X-ray Imaging Mission, which are centerpieces of the NASA Office of Space Science roadmap. The colloid thrusters for the Disturbance Reduction System project must produce thrust in the 2-20 muN range, with a maximum step size of 0.1 muN and a noise figure better than 0.1 muN/root(Hz) in the 1-30 mHz bandwidth. We have operated a six-emitter electrospray source and directly measured its thrusting properties with a torsional balance. We have demonstrated that this colloid thruster can cover the desired thrust range continuously and has a noise figure below 0.1 muN/root(Hz) in the 7 mHz-1 Hz bandwidth. Its noise spectrum is likely better than 0.1 muN/root(Hz) down to 1 mHz, but the intrinsic noise of the balance made it impossible to verify this. In another experiment, we compared thrust measurements obtained with both the torsional balance and the time-of-flight technique and found them to differ within 15%. This result supports the use of the inexpensive time-of-flight technique for measuring the thrust of colloid thruster, when modest accuracy is required.