Charging of free-falling test masses in orbit due to cosmic rays: results from LISA Pathfinder

TL;DR

This paper reports on measurements of test mass charging in space during the LISA Pathfinder mission, revealing how cosmic rays and environmental factors influence charge accumulation over 17 months, with implications for future gravitational wave observatories.

Contribution

It provides the first detailed analysis of test mass charging due to cosmic rays in space, including correlations with environmental data and projections for long-term charging behavior.

Findings

Test masses had initial negative charge after release.

Charging rates varied with cosmic ray flux and environmental conditions.

Correlation between particle detector data and charging rates enables long-term projections.

Abstract

A comprehensive summary of the measurements made to characterize test mass charging due to the space environment during the LISA Pathfinder mission is presented. Measurements of the residual charge of the test mass after release by the grabbing and positioning mechanism, show that the initial charge of the test masses was negative after all releases, leaving the test mass with a potential in the range $-12$ mV to $-512$ mV. Variations in the neutral test mass charging rate between $21.7$ e s$^{-1}$ and $30.7$ e s$^{-1}$ were observed over the course of the 17-month science operations produced by cosmic ray flux changes including a Forbush decrease associated with a small solar energetic particle event. A dependence of the cosmic ray charging rate on the test mass potential between $-30.2$ e s$^{-1}$ V$^{-1}$ and $-40.3$ e s$^{-1}$ V$^{-1}$ was observed and this is attributed to a contribution to charging from low-energy electrons emitted from the gold surfaces of the gravitational reference sensor. Data from the on-board particle detector show a reliable correlation with the charging rate and with other environmental monitors of the cosmic ray flux. This correlation is exploited to extrapolate test mass charging rates to a 20-year period giving useful insight into the expected range of charging rate that may be observed in the LISA mission.