Superconducting (s.c.) RF structures for the acceleration of electrons have been known for over 30 years (HEPL, Stanford). More than 10 years after their successful start a new generation (i.e. standing wave, 7r-mode cavities), made from niobium and operated at frequencies between 352 MHz and 3 GHz, was established at accelerators being operated for high energy physics (CERN, DESY, KEK) as well as nuclear physics (CEBAF, Darmstadt) experiments. Although the HEPL s.c. cavities were limited to a 10% duty cycle and the 2nd generation now allows continuous wave operation, the increase of the accelerating gradient was remarkably small with respect to the > 50 MV/m limit given by the physics of RF superconductivity. Thirty years ago HEPL cavities reached about 2 MV/m; 15 years later DESY cavities achieved 4 MV/m. And the large 338-cavity CEBAF installation is based on a 5 MV/m gradient, the commissioning of the accelerator being less than 10 years ago. Since 1992 the TESLA (TeV Energy Superconducting Linear Accelerator) collaboration has studied the fundamental problems in cavity fabrication as well as operation. Last year an 8 mA electron beam was successfully accelerated in a 15 MV/m module containing 8 s.c. 9-cell cavities. Two more modules (20 to 25 MV/m) will be installed. From recent cavity tests a gradient of 25 MV/m can be taken as state of the art. Cavity production, preparation, and installation was remarkably improved, a prototype linac (TESLA Test Facility Linac) for a large linear collider shows good performance.

19th International Linear Accelerator Conference, LINAC 98, Chicago, USA, 23 Aug 1998 - 28 Aug 1998; 59 pp. (1998).