RF Cable Heating Effects on a Micronewton-scale Torsional Thrust Stand

In order to test the performance of thrusters using radio-frequency (RF) electrical propulsion in the micronewton range, a thrust stand must be calibrated to accurately measure these thrusts. Confounding effects arise from the heating of power cables attached to the thruster and thrust stand which, due to linear expansion of the cabling, cause deviations in calibration that require accounting for. An overview of the calibration program for the LEAP torsional thrust stand is presented, showing that the stiffness of cabling attached to the thrust stand is of primary importance to thrust stand sensitivity. Factors controlling the amount of linear expansion heated RF cables undergo are tested in atmosphere in vacuum, finding that power and the amount of cable insulation are dominant. Finally, the cables are placed on the thrust stand and heated. It is shown that while the cable heating has no effect on the sensitivity of the thrust stand once at equilibrium, getting to that equilibrium requires hours of waiting and hundreds of apparent micronewtons of thrust stand deflection from the cable heating. By understanding that the thrust stand deflection increases approximately linearly during heating, this effect is shown to be correctable in data reduction.