Design and Simulation of Pulsed UV LED Charge Control for the LISA Gravitational Reference Sensor

Capacitive inertial reference sensors in space are a necessary technology for gravitational wave observations. They consist of a test mass (TM) in free fall surrounded by an electrode housing. In the space environment, the TM accrues electric charge that eventually pollutes the science measurement. To minimize electrostatic force noise contributions, it is necessary to maintain a near-neutral TM charge relative to the housing. The TM can be discharged in a contact-free manner, exploiting ultraviolet light via the photoelectric effect to preserve instrument sensitivity. Understanding the physics of UV light-based charge control is critical to the success of LISA, a gravitational wave detector in space to be launched in the early 2030s.

A technology demonstration mission for LISA, LISA Pathfinder (LPF) flew from 2015-2017 and was successful. To improve LPF methods of charge control for LISA, UV LEDs will be used instead of Hg lamps due to their high bandwidth capability. It is desired to pulse the UV LEDs at 100 kHz to exploit electric fields already present in the Gravitational Reference Sensor (GRS) to facilitate photoelectron current direction and to reach lower UV light power levels. To achieve this, a custom analog current source hardware had to be developed to run the UV LEDs in such a manner. The custom current source has been developed for flight with control software and an opto-mechanical structure to house and fiber couple the UV LEDs and house the electronics.

Numerical and analytical modeling of charge movement within the LISA GRS was performed and advanced charge control methods were validated, resulting in a charge-induced force noise below the allotted budget. The model describing TM charge is uses LPF GRS apparent yield data and environment radiation data. In addition, experiments on the University of Florida torsion pendulum were performed and validated the model. Results will include a comparison of the simulated charge control performance with the allocated noise budget, a comparison of the new advanced modes of charge control with LPF heritage modes, experimental results that match the simulation, and a discussion of the use of these charge control schemes on LISA.