The Laser Interferometer Space Antenna (LISA) is an ESA mission designed to detect gravitational waves from space. To initiate the science phase, six test masses (TMs) are precisely handled and released into near-perfect free fall by dedicated mechanisms known as the Grabbing, Positioning, and Release Mechanisms (GPRMs). The stringent requirements on the noise level affecting the TMs’ release acceleration are extremely ambitious, motivating the need to experimentally verify the feasibility of achieving such performance. To this end, a dedicated precursor mission, LISA Pathfinder (LPF), flew from 2015 to 2017 to test key technologies. However, during the LPF mission, most release tests exhibited anomalous release velocities, often exceeding the requirements. In addition, the TM repositioning tests also revealed a bi-stable behavior in the TM rotations, which depend on the repositioning direction. This effect is produced by an unexpected non-rectilinear motion of the GPRM end effector, characterized by a micrometric side motion at the reversal of its axial motion. The bi-stable behavior also contributes to a TM-GPRM end effector misalignment, producing unwanted contacts and increasing the probability of a non-compliant TM release. Previous analyses identified asymmetric friction forces in the side-guiding system of the GPRM end effector as the primary cause of this behavior. Starting from the LPF flight experience, the GPRM delta development project in view of LISA led to a redesign of the mechanism architecture, supported by numerical analyses and multi-body models. Since the rectilinearity of the end-effector motion has been identified as critical for flight operation, alternative side-guiding concepts are developed, analyzed, and tested experimentally to evaluate their impact on the overall mechanism performance. The correlation of the models with ground and flight experimental data strengthens the understanding of the guiding system behavior, providing pivotal insights for selecting the GPRM design baseline for LISA.

The Laser Interferometer Space Antenna (LISA) is a space-based gravitational-wave observatory that uses free-falling test masses as inertial references to detect mHz-frequency signals. Interactions between these test masses and galactic or solar energetic particles cause charge buildup and Coulomb forces, a major source of acceleration noise that must be accurately modeled. LISA Pathfinder measurements showed that the Poissonian test-mass charging noise, an important entry in the noise budget, was considerably larger than that in prelaunch simulations, indicating missing physical processes in early models. Emission of low-energy secondary electrons (LEEs) from test-mass and housing surfaces has been proposed as a key mechanism affecting charging and the sensor’s electrostatic response. We report a particle accelerator-based experiment that directly tests the LEE hypothesis by measuring proton-induced test-mass charging in a LISA-like Gravitational Reference Sensor geometry as a function of the test-mass electrostatic potential. A flight-representative test mass and the capacitive sensor electrode housing surrounding it are exposed in vacuum to monoenergetic 70–230 MeV proton beams, while test-mass charging currents are measured with femtoampere sensitivity. By varying the test-mass potential by volts during irradiation, we gain sensitivity to eV-scale charge carriers whose transport and collection depend on these fields. The measurements yield stable, reproducible picoampere-level charging currents and a clear dependence of the net charging rate on test-mass potential. This behavior directly supports the role of low-energy secondary electrons in test-mass charging and is consistent with the LEE-based interpretation of LISA Pathfinder data. These are the first controlled laboratory measurements of low-energy electron effects in test-mass charging for a LISA-like sensor and provide a benchmark for validating charging models and charge-management strategies for LISA and future space-based precision interferometry missions.