Two approaches for the passive charge management of contactless test masses

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 〖eV〗_TM^max, where V_TM^max is the maximum allowable potential required for normal operation of the TM. In its optimized configurations and following an exposure of < 30 sec 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 minutes to converge to zero TM potential from 100 mV, with a similar drift of  2.0 mV/day. For reference, V_TM^max≅80mV for the LISA and LISA pathfinder sensors. 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.

Flight and ground demonstration of reproducibility and stability of photoelectric properties for passive charge management using LEDs

“…In sections 3 and 4 we describe and discuss respectively the flight and ground experiments and their results that demonstrate the reproducibility to ∼ =±6 mV ( ∼ =±0.2 pC for 28 pF) of the TM charge under 255 nm UV illumination, for our system configuration, and 0 V TM enclosure bias. These results add confidence to two proposed PCM approaches: (a) single source low energy photoelectrons [4] and (b) balanced dual source high energy photoelectrons [4,19]. Conclusions are presented in section 4.…”

“…For the 255 nm LEDs, an incident beam of power P photon = 50 µW generated a current of I electron = 3.49 pA for our film samples [28], resulting in a value of Ψ = (3.40 ± 0.35) × 10 −7 , somewhat low compared with other work [29][30][31][32][33][34][35]. Tests with similar gold coatings performed in a system with better vacuum and more comparable geometry gave [4]: Ψ = (5.9 ± 0.2) × 10 −7 , the value we will use henceforth. The reflectivity coefficient is the ratio of reflected photons to incident ones: R ≡ P reflected /P incident .…”

“…The instrument has been optimized for PCM by coating both TM and its housing with the same material-in this case gold. We find that the work function of the deposited Au film is centered at about 4.6 eV [4] and spans from 4.3 eV to 4.9 eV [28]. Note that the values for the Au work function differ significantly by author: 4.3-4.9 eV [29], 4.86 eV [30], 5.38 eV [31], 5.31-5.37-5.47 eV (for the 111, 110, and 100 crystal orientations respectively) [32], 5.1 eV [33], 4.83 ± 0.02 eV [34,35].…”

“…In this paper we present satellite flight data, backed up by ground experiments, that validate the use of UV-LEDs for the PCM technique [4], (known also as self-adaptive [19]), a significantly simplified approach, that will improve instrument performance while substantially reducing mass and complexity. The PCM techniques have been developed specifically for instruments with spherical TMs, TM to housing gaps larger than 1 cm, and no electrostatic sensing or activation systems.…”

“…A collaboration of Stanford University, NASA and KACST [3], the mission was launched in June 2014. For validation, ground-based measurements were performed in between September and October 2020, in the apparatus also used for the demonstration of two passive charge management (PCM) approaches [4].…”