Testing of the UV discharge system for LISA pathfinder

We present the results of tests that demonstrate the functionality and performance of the test-mass discharge system for LISA and LISA Pathfinder. In space, the LISA and LISA Pathfinder test-masses will accumulate charge from energetic charged particles. Using the photoelectric effect, a system of ultra-violet (UV) illumination will discharge the test masses. Testing of this UV discharge system, integrated with the LISA Pathfinder engineering model sensor in the torsion pendulum facility at the University of Trento has shown that: isolated test-masses can be charged positively or negatively using UV light; the rate of charging can be measured and is proportional to the UV power; the test-mass charge can be continuously held near zero with a combination of light illuminating the test mass and the surrounding sensor housing, without introducing excess force noise above the pendulum detection limit.