BART-LISA: Beam-like Application of Radiation on Test-masses of the Laser Interferometer Space Antenna (LISA)

The precise modeling of test mass (TM) charging is essential for the performance of the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave observatory operating in the millihertz frequency band. Building upon insights from the LISA Pathfinder (LPF) mission, which revealed significant discrepancies between in-flight measurements and Monte Carlo (MC) simulations, this work presents a dedicated beam test specifically designed to replicate and investigate the LISA TM charging process. The observed underestimation in previous models-up to a factor of four-has been largely attributed to the unaccounted contribution of low-energy electrons (LEE) emitted during cosmic ray (CR) interactions. Our proposed experiment uses a qualification model of the LPF Gravitational Reference Sensor (GRS), exposed to proton beams in the $70-230 \mathrm{MeV}$ energy range at the TIFPA Proton Therapy Center, to directly measure the charging current induced on the TM by the incident proton flux. The apparatus, operated under vacuum and equipped with a femto-amperometric readout system, is capable of resolving picoampere-level currents. Complementary GEANT4 simulations predict that LEE emission, modulated by the potential difference between the TM and its housing, can introduce measurable variations in the current at the $\mathbf{1 0 \%}$ level. This test thus offers, for the first time, the opportunity to experimentally validate the LEE contribution to TM charging, representing a critical step toward improving simulation models for future LISA operations.