Femto-ampere Photocurrent Measurements for the LISA GRS

The Laser Interferometer Space Antenna (LISA) will be the first gravitational-wave detector in space. LISA consists of three spacecraft, separated by 2.5 million km. Using interferometric measurements of the relative acceleration of test-mass (TM) free-falling inside spacecrafts, the mission will detect low frequency gravitational waves. The team at University of Trento is responsible for one major subsystem of the mission, the Gravitational Reference Sensor (GRS). The core of the GRS is a gold-platinum TM inside an electrode housing (EH) which contains pairs of electrodes, both gold coated, for sensing and actuation. The TM is expected to accumulate 10-100 e/s of positive charge from cosmic rays and solar radiation. The GRS will compensate for this charging via photoelectric emission of electrons from UV light injection. Characterizing the TM-EH photocurrent discharge system is the focus of several ground-testing campaigns. Torsion pendulums are used to measure the charge on a TM, providing indirect measurements of the photocurrent; however, this process is difficult. In this poster, we present measurements from a recently-developed facility designed to directly measure the photocurrent with resolution below 1 fA. By testing electrical configurations, this experiment can also explore effects of actuation/sensing potentials on the discharge process. Broadly, this facility serves as a test bed to develop measurement techniques for characterizing the 6 GRS copies before spacecraft integration.