In the early 1990s, DESY began to develop a new technology: radio frequency accelerator technology based on superconducting cavities made of niobium, which are cooled to approximately 2 K (−271 °C) with liquid helium. The first accelerator on this basis was a test facility for superconducting linear accelerators at DESY to test the principle of self-amplified spontaneous emission (SASE) of X-ray laser light. The SASE theory was developed and refined at DESY and at institutes in Russia, Italy and the USA from 1980 onwards. In 2000 to 2001, the test facility at DESY was the first free-electron laser in the world to produce light flashes in the vacuum ultraviolet and soft X-ray range. Today, the FLASH facility produces ultrashort light pulses in the soft X-ray range for seven experimental stations. Since 2020, it has been expanded to further optimise the properties of the radiation (FLASH2020+ project).