Increased Brownian force noise from molecular impacts in a constrained volume

TL;DR

This study demonstrates that residual gas damping and associated force noise increase significantly when a macroscopic test mass is in close proximity to surrounding walls, impacting precision experiments like LISA.

Contribution

The paper provides experimental measurements and numerical modeling showing increased Brownian force noise due to molecular impacts in constrained volumes.

Findings

Force noise is ~15 times higher than in infinite gas volume.

Damping coefficient increases as the gap between test mass and walls decreases.

Results are consistent with the model predictions.

Abstract

We report on residual gas damping of the motion of a macroscopic test mass enclosed in a nearby housing in the molecular flow regime. The damping coefficient, and thus the associated thermal force noise, is found to increase significantly when the distance between test mass and surrounding walls is smaller than the test mass itself. The effect has been investigated with two torsion pendulums of different geometry and has been modelled in a numerical simulation whose predictions are in good agreement with the measurements. Relevant to a wide variety of small-force experiments, the residual-gas force noise power for the test masses in the LISA gravitational wave observatory is roughly a factor 15 larger than in an infinite gas volume, though still compatible with the target acceleration noise of 3 fm s^-2 Hz^-1/2 at the foreseen pressure below 10^-6 Pa.