Flight demonstration of charge management and of passive charge management of insulated test masses in inertial sensors using 254 nm UV-LED electron photoemission

The UV-LED small satellite mission, a collaboration of Stanford University, NASA and KACST (King Abdulaziz City of Science and Technology, Saudi Arabia) has demonstrated the precise control of the potential of electrically isolated test masses (TMs) that is essential for the operation of space accelerometers and drag-free sensors. Accelerometers and drag-free sensors were and remain at the core of geodesy, aeronomy and precision navigation missions as well as gravitational science experiments and gravitational wave observatories. TMs accumulate charge through cosmic radiation, the triboelectric effect and other mechanisms, thereby downgrading the measurement precision of these high sensitivity instruments. The UV-LED mission and prior ground testing demonstrates that AlGaN UVLEDs operating at 255 nm are superior to mercury vapor lamps because of their smaller size, lower power draw, higher dynamic range, and higher control authority. We show flight data that demonstrates AC charge control (UV-LEDs and bias are AC modulated with adjustable relative phase) between a spherical test mass and its housing. We demonstrate the ability to control the test mass potential on an 89 mm diameter spherical test mass over a 20 mm gap in a drag-free system configuration. The test mass potential was measured with an ultra-high impedance contact probe.

For TMs requiring very low residual charge, 0.1 pC to 10 pC, a control loop consisting of a charge measurement system and a source of charges is presently utilized. However, the charge measurement system adds to the instrument complexity, is limiting the range of critical design parameters and is in itself a source of disturbance for the measurement. The UV-LED mission also demonstrated a passive bipolar charge reduction method, using UV generated photoelectrons, that converges repeatably to less than 0.1 pC from both positive and negative charges and does not require continuous TM charge measurement. Finally, the key electrical and optical characteristics of the UV-LEDs showed less than 7.5% change in performance after 12 months in orbit.