Numerical modeling and experimental demonstration of pulsed charge control for the space inertial sensor used in LISA

Electrostatic charge control of isolated free-falling test masses is a key enabling technology for space-based gravitational missions. Contact-free electrostatic charge control can be achieved using photoelectron emission from metal surfaces under illumination with deep UV light. A contact-free method minimizes force disturbances that can perturb measurements or interrupt science operations. In this paper, we present charge control experiments using a gravitational reference sensor geometry relevant to the Laser Interferometer Space Antenna (LISA) gravitational wave observatory in a torsion pendulum apparatus. We use a UV light-emitting diode light source to control the test-mass potential, taking advantage of their high bandwidth to phase lock the light output to 100 kHz electric fields used for capacitive position sensing of the test mass. We demonstrate charge-rate and test-mass potential control by adjustment of the phase of the light with respect to the electric field. We present a simple physics-based model of the discharging process which explains our experimental results in terms of the UV light distribution in the sensor, surface work functions, and quantum yields. A robust fitting method is used to determine the best-fit physical parameters of the model that describe the system.