The charge-management system on LISA-Pathfinder - Status & outlook for LISA

The electrostatic charging of the test masses in the Lisa Pathfinder inertial sensor has severe implications on the design, operation and noise behaviour of the sensor. The flight model (FM) charge management device (CMD) designed to keep the test masses at a low residual charge is currently being built at Imperial College, London. Detailed simulations and tests of the FM uv-lamps, the vacuum feedthroughs (F/T) and the distribution of light in the inertial sensor housing have been performed. Simulations show excellent mechanical robustness of the F/Ts and the lamp assembly. Vibration response has been calculated using finite element modelling on the F/T and the lamp assembly. Two lamps have been measured for performance before and after a vibration and shock test to flight acceptance specifications. The lamp performance is found to be unaffected by the vibration load. The distribution of light inside the sensor has been modelled to high accuracy in order to predict discharge rates for various voltage settings on the sensors electrodes. The noise of the uv-light as well as the long term variations have been accurately measured and the noise is found to be well below the required specifications. Furthermore developments towards modifying the charge management device for LISA will be presented. These developments include uv-leds or frequency tripled red laser diodes to replace the mercury lamps used in the current systems to achieve equivalent or superior performance at lower weight and power consumption. These technologies are however as yet untried in space and need thorough investigation. Lifetime, noise and uv output vs power consumptions are critical parameters. Experiments to measure these parameters have been set up and started and first results have been achieved.