Performance of the electrostatic control force system for LISA Test Masses: experimental results from LISA Pathfinder and new on-ground measurements

The LISA mission will detect low frequency gravitational waves by interferometrically measuring the distance between free-falling Test Masses (TM) in space. These are six, 2 kg gold-platinum cubes, contained in three spacecrafts which fly in a triangular formation, 2.5 million km from one another. The TMs are in nearly perfect free-fall and are followed by the spacecrafts, there to shield them and keep them aligned with the interferometers. Spurious forces introduced by the spacecraft on the TMs are a major source of noise for the measurement and must be limited to the absolute minimum. Several important sources of noise are linked to the system of forces used to control the TM dynamics. The whole system is usually referred to as electrostatic force actuation. The forces are generated by applying voltages to electrodes surrounding the TMs: noise in these voltages end up being one of the main disturbances for LISA. This thesis tackles actuation noise from multiple directions. First, it contains the analysis of experiments performed on LISA Pathfinder (LPF), LISA precursor mission. Dedicated measurement campaigns allowed an in depth characterization of actuation noise, in particular voltage gain fluctuations and additive voltage noises. Good agreement with the current actuation noise model was found: through this, we estimate that actuation explains between 20 and 40% of LPF noise, and project the noise in the LISA configuration. A detailed investigation of possible non-linearities in actuation voltages is reported. They were first measured in the laboratory on a LPF Front-End-Electronics (FEE) copy. Then, their impact on LPF and LISA was evaluated through simulations and development of analytic models, revealing that while they should not be a dominant noise source in LPF, they can become significant under certain conditions. The second part of the thesis focuses on measuring actuation-related forces with the 1TM torsion pendulum, which was re-activated after a shut-down of seven years and updated in many aspects. It was then used for two measurement campaigns. The first was a demonstration that the instrument has the capability of measuring torque noise from actuation gain fluctuations, a test that could be previously performed only in space. The campaign successfully recovered fluctuation from some home-made actuation electronics, and it is planned to use the pendulum to test the new LISA FEE. Finally, the 1TM facility was used for a second test campaign, to characterize the noisy charge fluctuations of the TM, induced by illumination with UV light. This is used in flight to control the TM charge, and can be a source of noise if too much power is used. The campaign completes a much wider characterization done with the 4TM torsion pendulum and demonstrates the accuracy of our models for this source of noise.