LIGO Document G1901913-v1

Future Laser Interferometer Gravitational Wave Observatories & Vacuum Requirements

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The Laser Interferometer Gravitational Wave Observatory (LIGO) comprises a pair of large facility observatories in Washington and Louisiana dedicated to gravitational wave (GW) astronomy and astrophysics, funded by the U.S. National Science Foundation. A century after Einstein predicted the existence of gravitational waves, LIGO detected these ripples in the fabric of spacetime resulting from two massive binary black holes colliding almost 1.3 billion years away, birthing a new era of GW astronomy. For this achievement LIGO founders were awarded the 2017 Nobel Prize in Physics. Since the first detection in 2015, over a dozen black hole mergers have been recorded, in addition to neutron star collisions, marking a significant breakthrough for multi-messenger astronomy. Concepts of third generation GW instruments are undergoing research and development, with the promise to expand humanity's ability to listen to the cosmic symphony of gravitational waves out to the very edge of the universe.

An NSF workshop was held at the LIGO Livingston site in January to explore potential novel vacuum system solutions for 3G observatories. Cost effective solutions are required for the design, construction and operation of these large vacuum systems, proposed to be a factor of ten larger than the current systems in the U.S. (LIGO), Europe (Virgo) and Japan (KAGRA). Technologies developed and employed in the existing GW observatories have been shown to meet stringent requirements of vacuum integrity, low hydrogen and heavy molecule outgassing, minimal particulate generation, low vibration, and stray light optical absorbance for successful operation. However, extrapolating costs from current lengths to 40km/arm of vacuum beamtube indicates the need to investigate a wide range of technologies and materials that could significantly lower the final cost of 3G observatories such as the Cosmic Explorer in the U.S. and the Einstein Telescope in the E.U. Two classes of solutions for the vacuum enclosures were examined: 1) the first design concept is an extrapolation of the single-wall vacuum pipe in the present generation of detectors; 2) the second design concept involves double-walled or nested vacuum pipes that would separate the atmospheric load from the stringent UHV properties needed for the inner wall. Pumping solutions and surface treatments were examined for both concept designs with an emphasis on potential hardware and treatments that could lower total costs but still meet the stringent vacuum requirements.

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