A Charge Management System for Gravitational Reference Sensors – Design and Instrument Testing

Abstract

Gravitational reference sensors (GRSs) are imperative to Earth geodesy missions and gravitational wave observations in space. A typical GRS consists of a test mass (TM) surrounded by a capacitive electrode housing to perform sensitive relative position measurements and to apply small forces to the TM. This paper specifically discusses advancements in the charge management system (CMS) for the GRS being used in the LISA mission. Space radiation accumulating charge on the TM will eventually generate unwanted forces on the TM due to stray electric fields in the spacecraft. Thus, the TM charge must be kept close to a zero potential. The TM charge will be controlled in a contact-free manner by shining UV light and exploiting the photoelectric effect. A major design improvement for future missions is using UV LEDs, which can be pulsed. This facilitates more advanced charge control schemes for a continuous science measurement. The UV LEDs are housed in an aluminum block and controlled with supporting electronics via the charge management device (CMD). The CMD needs to be integrated with the spacecraft computer and needs to contain redundancy to survive the 10+ year LISA mission. The CMD is a NASA deliverable for the ESA mission and has begun the process of technology advancement and testing. The unit has custom PCBs designed to supply both continuous and pulsed current to the UV LEDs, readback telemetry data, manage CMD power needs, and synchronize with the spacecraft computer to communicate with spacecraft operators. The system achieved TRL 4 at the end of 2018 and surpassed all requirements for performance, redundancy, and lifetime. The system is capable of generating stable and robust square UV light pulses, has the capacity to drive the UV LEDs at their full dynamic range, and meets requirements on power, pulse properties, stability, and commanding speed. This technology features novel discharge methods and improvements on past missions in terms of noise level and continuous science measurements. The CMD design and testing results in this study are critical to the success of LISA, future Earth geodesy missions, and future science missions relying on precision inertial sensor technology.