Two approaches for the passive charge management of contactless test masses

Abstract

Abstract Free floating test masses (TM) of inertial reference instruments accumulate charge mainly through the triboelectric effect during separation from their housings and, if in the space environment, from cosmic radiation. These charges will degrade the accuracy of high sensitivity accelerometers and drag-free sensors. We demonstrate in ground testing two passive bipolar charge management systems using photoelectrons emitted from gold coated surfaces under illumination by ultraviolet light emitting diodes (UV-LEDs) with 255 nm, 275 nm, and 295 nm central wavelengths. The first method uses fast photoelectrons, generated by two 255 nm UV-LEDs with adjustable-intensity (through fine-tuning of their excitation currents) and illuminating the TM and its housing respectively. A second technique uses slow photoelectrons generated by one UV-LED, of either 275 nm or 295 nm, directed at the TM. Fast and slow electrons are defined as having kinetic energies after photoemission above and below e V T M max , where V T M max is the maximum allowable potential required for normal operation of the TM. In its optimized configurations and following an exposure of <30 s to UV, the fast-photoelectron control system converges to zero TM potential from ±1 V with a drift of ≈1.5 mV/day. The slow-photoelectron system requires about 5 min to converge to zero TM potential from 100 mV, with a similar drift of ≈2.0 mV/day. For reference, V T M max ≅ 80 m V for the LISA and LISA pathfinder sensors. Insight into the slow photoelectrons charge control method is provided by our simple Gaussian model of the work function for gold films (that approximates their quasi-triangular measured shape): 4.6 eV center, 0.2 eV standard deviation, and 4.30 eV and 4.90 eV cutoffs. The longest wavelength (lowest energy) photoemission is obtained with the 295 nm LED (11 nm FWHM) at about 285 nm (4.35 eV), where only about 3.5% of the photons have an energy above the photoelectric threshold. While the two-source UV fast photoelectron method is a modification of the LISA Pathfinder approach, the slow photoelectron method is a new passive charge management technique. These two passive techniques were developed for instruments without sensing and activation systems. For instruments with electric fields surrounding the TM and more complex geometries additional adaptations are required.