Test results on the 9-cell 1.3 GHz superconducting RF cavities for the TESLA test facility linac

The design goal for the TESLA Test Facility Linac is an accelerating gradient of 15 MV/m at a quality factor of Q=3·10 ⁹. In the linac the cavities are operated in pulsed mode (constant gradient for 800 μs with 10 Hz repetition rate). The majority of the first 17 industrially produced cavities exceeded the specification. Several cavities reached 25 MV/m at Q>1·10¹⁰. Due to cleaning by high pressure water rinsing most of the cavities showed no field emission loading; the major limitation was thermal instability. In cavities with poorer performance, two types of field limitations were identified by temperature mapping and further surface analysis: tantalum impurity in the bulk niobium and defects in the equator welds. Eight cavities, equipped with main power coupler, HOM couplers and tuning system have been successfully tested in a horizontal cryostat in pulsed mode. Also here the majority of the cavities exceeded the design goal and gradients up to 25 MV/m could be achieved