Sub-millinewton thrust stand and wireless power coupler for microwave-powered small satellite thrusters

The design and performance of a thrust stand for characterizing low-power electric propulsion thrusters are presented. The thrust stand is capable of sub-millinewton resolution for devices on the order of 1 kg. The architecture is based on a counter-weighted hanging pendulum design, a variant of the standard hanging pendulum that employs a counterweight to increase force resolution. Thrust is measured in a displacement mode using the change in position of the pendulum arm as measured by an optical displacement sensor. Passive eddy-current damping is used to offset oscillations and decrease setting time. An in situ calibration rig using known masses is used to calculate thrust. The thrust stand features an adjustable counterweight for in-vacuum sensitivity adjustment. In addition, the design of a broadband (600-2490 MHz) wireless microwave power coupler is presented. The device eliminates stiffness and thermal drift introduced by coaxial cables-typically the leading source of error in testing low-power microwave and radio frequency-powered thrusters. The thrust stand and coupler were tested using an electron cyclotron resonance magnetic nozzle thruster operating with xenon at flow rates from 1 to 10 sccm and powers ranging from zero (cold gas thrust) to 40 W. The resulting measurements showed a force resolution of ∼10μN over a range of thrusts from ∼14 to 600 µN.