Thermal gradient-induced forces on geodetic reference masses for LISA

TL;DR

This study experimentally investigates thermal gradient-induced forces on test masses for LISA, identifying dominant effects and confirming models to ensure the mission's sensitivity is maintained.

Contribution

It provides a detailed experimental and numerical analysis of thermal-gradient forces specific to LISA's environment, improving understanding of noise sources.

Findings

Radiometric effect confirmed and modeled accurately.

Outgassing effects are low and manageable.

Thermal forces do not threaten LISA sensitivity goals.

Abstract

The low frequency sensitivity of space-borne gravitational wave observatories will depend critically on the geodetic purity of the trajectories of orbiting test masses. Fluctuations in the temperature difference across the enclosure surrounding the free-falling test mass can produce noisy forces through several processes, including the radiometric effect, radiation pressure, and outgassing. We present here a detailed experimental investigation of thermal gradient-induced forces for the LISA gravitational wave mission and the LISA Pathfinder, employing high resolution torsion pendulum measurements of the torque on a LISA-like test mass suspended inside a prototype of the LISA gravitational reference sensor that will surround the test mass in orbit. The measurement campaign, accompanied by numerical simulations of the radiometric and radiation pressure effects, allows a more accurate and representative characterization of thermal-gradient forces in the specific geometry and environment relevant to LISA free-fall. The pressure dependence of the measured torques allows clear identification of the radiometric effect, in quantitative agreement with the model developed. In the limit of zero gas pressure, the measurements are most likely dominated by outgassing, but at a low level that does not threaten the LISA sensitivity goals.