Modeling and performance of contact-free discharge systems for space inertial sensors

This article presents a detailed overview and assessment of contact-free ultraviolet light discharge systems (UVDSs) needed to control the variable electric charge level of free-flying test masses, which are part of high-precision inertial sensors in space. A comprehensive numerical analysis approach on the basis of experimental data is detailed. This includes ultraviolet light ray tracing, the computation of time variant electric fields inside the complex inertial sensor geometry, and the simulation of individual photoelectron trajectories. Subsequent data analysis allows determination of key parameters to set up an analytical discharge model. Such a model is an essential system engineering tool needed for requirement breakdown and subsystem specification, performance budgeting, onboard charge control software development, and instrument modeling within spacecraft end-to-end performance simulators. Different types of UVDS design concepts are presented and assessed regarding their robustness and performance. Critical hardware aspects like electron emission from air-contaminated surfaces, interfaces with other subsystems, and spacecraft operations are considered. The focus is on the modeling and performance evaluation of the existing UVDS onboard the LISA Pathfinder, a European Space Agency technology demonstrator spacecraft, which is in implementation phase. The results have motivated the design of a more robust discharge system concept for cubical test mass inertial sensors for future space missions. The analysis tools developed have been used for design optimization and performance assessment of the proposed design. A significant improvement of relevant robustness and performance figures has been achieved.