Measurements of test mass charging in the LISA Pathfinder mission

LISA Pathfinder, a precursor to the space-based gravitational wave observatory, LISA, operated at the L1 Lagrange point between the Sun and the Earth between January 2016 and July 2017. The ESA-led mission successfully demonstrated the technology required to achieve the ambitious goals of LISA, most notably the pure free-fall of test masses undisturbed by relative accelerations above a few fm/s² in the milli-Hz band. Electrical charging of the test masses by cosmic rays and solar particles gives rise to electrostatic forces that, uncorrected, can compromise this performance.

We present measurements of the test mass charging rates made in orbit and compare with predictions from high-fidelity, high-energy particle simulations using the GEANT4 software toolkit. Combining charge measurements with the results of in situ measurements with a dedicated particle detector allows us to probe fluctuations in the charging rate driven by cosmic ray variations. We also identify a charging dependence on the electric field around the test mass caused by low-energy secondary electrons emitted from its gold-coated surfaces and surroundings.