Abstract
Charges as small as 1 pC degrade the performance of high precision inertial reference instruments when accumulated on their test masses (TMs). Non-contact charge management systems are required for the most sensitive of these instruments, with the TMs free-floating, and their charges compensated by photoelectrons in a feedback loop with a TM charge measurement. Three space missions have successfully demonstrated this technique: the Relativity Mission, Gravity Probe-B launched in 2004, the LISA Pathfinder launched in 2015, and the UV-LED mission launched on SaudiSat 4 in 2014; with the first two using the 254 nm Hg discharge line and the last one a set of 255 nm UV-LEDs. UV-LEDs represent a significant improvement over the discharge sources, in terms of reliability, lifetime, switching speeds, power consumption, weight, and volume. Charge management techniques that eliminate the charge measurement and feedback systems, referred henceforth as passive, reduce the complexities and disturbance effects introduced by these systems, and are thus the subject of active research and development work. Passive charge management (PCM) depends critically on the stability and reproducibility of the photoemission properties of a given system. In support of this work, we present comprehensive flight characterization data for a suite of 16 UV-LEDs in various configurations and 255 +/- 1 nm center wavelength. Flight data was acquired between December 2014 and December 2015 with the UV-LED instrument flown on SaudiSat 4. We back up our results with ground-based measurements performed in configurations comparable to the flight one between 4 September 2020, and 8 October 2020. All results confirm the excellent reliability of the UV-LEDs in the space environment, are fully consistent with the findings of ground studies, and support the approach of using LEDs for PCM. We find that the equilibrium potential of the TM, under illumination by the 255 nm LEDs, is independent of the UV intensity and reproduceable to about congruent to +/- 6 mV, or +/- 6 fC pF(-1), over periods of up to six months. The value of the equilibrium potential is dependent on the geometry of the electric field between the TM and its enclosure, and thus on the exact configuration of the PCM instrument.