Mechanically amplified milli-Newton thrust balance for helicon plasma thrusters

Direct thrust measurements by means of a thrust balance are the golden standard for measuring thrust and concurrently the specific impulse in electric thrusters. To measure the thrust of a medium power (300-1000W) Helicon Plasma Thruster (HPT) we propose a thrust balance design based on the VAHPER concept. The thrust balance is designed to measure thrust between 1-100 mN for thrusters weighing up to 50 kg. The thrust balance has a angular magnification of 31x and is shown to behave like a damped harmonic oscillator with a torsional stiffness of 48.2 N-rad/m, moment of inertia of 12.2 kg-m and a natural frequency of 316 mHz. A variable damping system is able to provide up to critical damping with an optimal damping ratio of 0.78 corresponding to a settling time of only 1.8 s. Preliminary calibration shows a compliance of 160 μm/mN corresponding to about 100 μm/mN when loaded with a 5 kg thruster. Noise levels are of the order of 2-5 μm. To accommodate the particularities of a medium-power RF thruster the thrust balance design includes the following features: an optical displacement sensor, water cooling, liquid electrical connectors, on-board, in-vacuum electronics that account for auto-leveling, in-situ calibration and temperature measurements. Furthermore, a contact-less power transfer mechanism is proposed, based on resonant inductive coupling operating at 13.56 MHz to mechanically decouple the RF power supply line., promising high power efficiency (> 95%) with a large bandwidth (1 MHz). ∗PhD Student, dept. ing. Aeroespacial, mwijnen@pa.uc3m.es. †Assistant Professor dept. ing. Aeroespacial, jnavarr@ing.uc3m.es. ‡Associate Professor, dept. ing. Aeroespacial, pfajardo@ing.uc3m.es. §Professor, dept. ing. Aeroespacial, eahedo@ing.uc3m.es.