The TESLA free electron laser

The TESLA free Electron Laser (FEL) makes use of the high electron beam quality that can be provided by the superconducting TESLA linac to drive a single pass FEL at wavelengths far below the visible. To reach a wavelength of 6 nanometers, the TESLA Test Facility (TTF) currently under construction at DESY is extended to 1 GeV beam energy. Because there are no mirrors and seed-lasers in this wavelength regime, the principle of Self-Amplified-Spontaneous-Emission (SASE) is employed. A first test of both the principle and technical components is foreseen at a photon wavelength larger than 42 nanometers. With respect to linac technology, the key prerequisite for such single-pass, high-gain FELs is a high intensity, diffraction limited, electron beam to be generated and accelerated without degradation. Key components are RF guns with photocathodes, bunch compressors, and undulators. The status of design and construction as well as both electron and photon beam properties are discussed. Once proven in the micrometer to nanometer regime, the SASE FEL scheme is considered applicable down to Angstrom wavelengths. This latter option is particularly of interest in context with the construction of a linear collider, which requires very similar beam parameters. The status of conceptual design work on a coherent X-ray user facility integrated into the TESLA Linear Collider design is briefly sketched.