  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 LED light source to control the test mass potential,
taking advantage of their high bandwidth to phase-lock the light output to
100kHz 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
