Electrostatic charging of cubic test masses in the LISA mission

A Monte Carlo simulation of electrostatic charging of the LISA test masses by cosmic-ray protons and alpha particles has been developed using the Geant4 toolkit. A positive charging rate of 58+/-17 +e/s (elementary charges per second) was obtained with a 250 eV threshold for the production of secondary particles. This charging rate is five times larger than that found in previous simulations. The difference is mainly explained by the larger test mass investigated in this study, the tracking of lower energy secondary electrons and the wider range of cosmic-ray energies considered. We examine the significance of this mean charging rate and its associated fluctuations in terms of spurious Lorentz and Coulomb forces, and relate these to constraints in the sensor design and the discharge mode for the LISA test masses.