KAGRA

KAGRA (or Kamioka Gravitational Wave Detector, formerly called the Large Scale Cryogenic Gravitational Wave Telescope or LCGT) is a gravitational-wave detector in Japan that began full operation in February 2020. It is a Michelson interferometer with 3 km arms, similar to the LIGO detectors and Virgo. As a fourth such detector, more data will be collected on each GW detection, which (among other things) will assist in determining the direction of the event's source. It is entirely underground, installed in an enlarged mine, and it uses cryogenic mirrors to help reduce noise, specifically mirrors made of sapphire, which along with their suspension mounting will be kept at 20 K during operation. It aims for a sensitivity capable of detecting two to three events a year. Tunnel excavation was completed in 2014, the detector began test operation (without the cryogenic cooling) in 2016, and installation of the fully-sensitive (cryogenic) instrument was completed in October 2019. KAGRA began its second run May 23, 2023 in coordination with planned LIGO and Virgo runs, but an issue prevented KAGRA from achieving its target sensitivity and the run was concluded after four weeks, which were useful for testing. Plans were further delayed by an earthquake in early 2024. KAGRA resumed operation June 12, 2025 at the commencement of Observing Run O4c with LIGO and Virgo, running at least until mid August 2025. After upgrades, Observing Run O5 is planned to begin in 2027 or 2028. As of 12/2025, I have not seen news of any KAGRA detection, but the improvements planned for O5 may bring it into the sensitivity range that LIGO had when it made its first detections. Such equipment upgrades of all four detectors require considerable time between such runs, and the pandemic lengthened one such interval.