Gas cooling of test masses for future gravitational-wave observatories

31 August 2021

How much exchange gas for cooling can gravitational wave detectors tolerate?

This question is answered by our publication in the journal "Classical and Quantum Gravity" and the answer could become relevant for the planned Einstein telescope. Many are familiar with the Brownian motion effect. The surfaces of the 40kg mirrors in gravitational wave observatories also show it: the mirror surfaces wobble and vibrate due to thermal energy. This type of Brownian motion limits the number of measurable gravitational waves. If one cools the mirrors, e.g. to -255°C, i.e. 18 Kelvin, the mirrors calm down considerably and the measurement sensitivity of the gravitational wave observatory increases. But how do you keep a mirror that is suspended by thin fibres in vacuum, which should not touch anything if possible, at such a low temperature? The problem arises because the mirror reflects very intense laser light and, unfortunately, absorbs some of it. This would inevitably heat it up if this thermal energy were not permanently dissipated. According to our calculations, which we performed in teamwork with colleagues at DESY, a 5 Kelvin helium gas pressure of less than 20µPa (2·10^(-8) mbar) does not significantly disturb the mirror. The cooling power is 10mW and still provides 10% of the necessary. The remaining 90% must be provided by radiative cooling and heat conduction through the thin fibres.

Publication:

Classical and Quantum Gravity 38 185003 (2021)