Speaker
Description
At the core of the LISA gravitational wave observatory are isolated test masses that act as mirrors for a 2.5 million km baseline interferometer. The test masses must be kept in near pure free-fall: isolated from all unwanted force disturbances above the femto-Newton level. Electrical charging of the test masses produced by cosmic ray impacts gives rise to electrostatic forces that can reduce the sensitivity of the observatory to gravitational waves. In previous work, GEANT4 simulations have been used to predict the test-mass charging rate in orbit. We revisit these simulations, comparing the results with measured test-mass charging rates in the LISA Pathfinder technology demonstration mission during 2016 and 2017. Combining charge measurements with the results of in situ measurements with a dedicated particle detector allow us to probe fluctuations in the charging rate driven by cosmic ray variations. The observations made in orbit confirm the importance to this seemingly simple problem of physical processes from eV to TeV energy scales.
