End-to-end Multiphysics simulation of photoelectric charge control: application to free-falling test masses in LISA

Gravitational reference sensors (GRS) consisting of a free-floating test mass (TM) are vital for Earth gravity recovery missions as well as the Laser Interferometer Space Antenna (LISA) gravitational wave observatory. The success of these missions relies heavily on the ability of the GRS to minimize all non-gravitational forces on the TM. Specifically, the Charge Management System (CMS) within the GRS strives to eliminate accumulation of charge due to energetic charged particles on the TM originating from cosmic rays and/or solar particles. These charges couple with stray electric fields within the GRS resulting in unwanted electrostatic forces on the TM. The use of contact-less charge control of the TM is desirable to mitigate further mechanical and thermal forces induced from contact with the TM. This is achieved using the photoelectric effect by illuminating the TM using UV LED light sources which causes emission of photoelectrons from the TM surface, hence reducing its overall charge. We seek to provide a simulation for this process entirely in the COMSOL Multiphysics modeling software. The presentation will aim to emphasize the necessity of the CMS for upcoming space missions, the methodologies of modeling the TM discharge process in COMSOL, and finally comparisons of simulation and experimental results on a LISA-like GRS installed at the University of Florida’s torsion pendulum facility.