Design of a thrust balance for RF plasma thruster characterization

In this work we illustrate the design of a thrust balance, to be employed for the experimental characterization of low power helicon plasma thrusters developed at the Center for Aerospace Studies and Activities (CISAS) “G. Colombo” of the University of Padua. The nominal thrust level of the propulsive systems to be tested is estimated between 1 and 20 mN. The measurement of millinewton-range thrusts in presence of RF emissions magnetic fields from the thruster poses several engineering challenges as: (i) an highly sensible system is required, capable of detecting the low thrust levels produced by the thrusters with sufficient accuracy and stability; (ii) the necessity of feeding RF power to the thruster mounted on the balance imposes to deal with RF disturbances on the measurement systems and RF heating of the power line and of the mechanical structure of the balance; (iii) the magnetic field can interact with elements of the balance, hampering the measurement. We considered all these aspects with careful theoretical analyses, supported by experimental validation of the envisioned solutions. The work was thus structured as follow: (i) definition of the physical principles related to the measurement of millinewton-range thrusts in an RF environment; (ii) definition and preliminary characterization of the mechanical configuration of the system; (iii) definition of the measurement chain, in terms of mechanical interfaces, sensors, RF shielding and data handling, (iv) analysis of system criticalities; (v) design and experimental validation of the critical technologies involved in the system; (vi) detailed design of the thrust balance.