  The design of the proposed linear collider TESLA is based on 9-cell 1.3 GHz
superconducting niobium cavities, operated in pulsed mode. Within the framework
of an international collaboration the TESLA Test Facility (TTF) has been set up
at DESY, providing the infrastructure for cavity R&D towards higher gradients.
More than 60 nine-cell cavities were tested, accelerating gradients as high as
30 MV/m were measured. In the second production of TTF-cavities the average
gradient was measured to be 24.7 MV/m. Two modules, each containing eight
resonators, are presently used in the TTF-linac. These cavities are operated in
pulsed mode: 0.8 ms constant gradient with up to 10 Hz repetitions rate. We
will focus on two aspects: Firstly, the cavity fabrication and treatment is
discussed, allowing to reach high gradients. Latest results of single cell
cavities will be shown, going beyond 40 MV/m. Secondly, the pulsed mode
operation of superconducting cavities is reviewed. This includes Lorentz force
detuning, mechanical vibrations (microphonics) and rf field control. Both
topics meet the upcoming interest in superconducting proton linacs, like the sc
driver linac for SNS (Spallation Neutron Source).
