Finding ultrahigh vacuum systems for next-generation gravitational wave detectors

Plans are underway for the next generation of gravitational wave detectors (GWDs). The Einstein Telescope in the European Union, and the Cosmic Explorer in the United States — the proposed successor for the Laser Interferometer Gravitational-Wave Observatory (LIGO) — will require hundreds of kilometers of ultrahigh vacuum beam pipes. Scarcia et al. are looking for the best material for constructing these.

Author Carlo Scarcia says simply scaling up the vacuum layouts and practices of previous GWDs is not economically viable.

“The beam pipe vacuum system would cover more than one-third of the total cost of the experiment,” he said.

The researchers compared two ferritic stainless steels, which Scarcia describes as the “cousins” of the austenitic stainless steels typically used for vacuum applications. They measured the outgassing rates — the rate at which gasses escape the metals’ surfaces — for various molecules commonly found in ultrahigh vacuum regimes.

The ferritic stainless steels had remarkably low outgassing rates, often 2-4 orders of magnitude below their austenitic counterparts, sometimes falling below system sensitivity.

Maintaining an ultrahigh vacuum is an important component of GWDs. A photon needs to travel a long distance in the detector while minimizing the phase shift acquired by interacting with residual gas molecules, which is on the same order of magnitude as a gravitational wave and can lead to false detections.

Now that the group has shown the material itself can achieve the necessary vacuum conditions, they next plan to conduct vacuum studies on a 36-meter sample vacuum tube to conduct further tests in an operational scenario.

“This would mean a lot in terms of cost saving, and leaving that budget for making science, getting better optics, and planning with more freedom,” said Scarcia.

Source: “Study of the UHV compatibility of selected ferritic stainless steels for application in vacuum systems of future gravitational wave detectors,” by Carlo Scarcia, Giuseppe Bregliozzi, Paolo Chiggiato, and Ivo Wevers, Journal of Vacuum Science & Technology B (2025). The article can be accessed at https://doi.org/10.1116/6.0004698 .