Inertial sensor surface properties for LISA Pathfinder and their effect on test mass discharging

Maintaining the low acceleration noise target for LISA Pathfinder requires the removal of electrostatic charge from the test masses. The charge management device (CMD) has been designed to remove charge either continuously with a low noise impact, or intermittently at high discharge rates. Recent measurements and simulations have highlighted the sensitivity of the discharge performance to the inertial sensor surface properties. In response to this Imperial College London (ICL) and the University of Trento (UTN) have initiated a laboratory program to characterize the properties of representative surfaces in detail. The aim of this program is to ensure that the behaviour of the surfaces used in flight is well understood and that the discharge simulations and pre-flight measurements are representative of the in-flight performance. The discharge process has been simulated, taking into account surface properties and we use simulation results to understand the experimental results from the test mass discharge experiments performed using the Trento torsion pendulum. Finally, we describe a new concept to implement redundancy and ruggedness in a UV-LED based design for LISA, incorporating recent advances made for LISA Pathfinder charge management.