Herbert Lengeler

First Results on a Superconducting RF-Test Cavity for LEP

In 1979 a feasibility study for the application of superconducting accelerator cavities to LEP was started at CERN. In a first series of measurements using a single cell test cavity (f= 500 MHz) quality factors of 2.1 × 109 and maximum acceleration fields of 4.6 MV/m were reached at 4.2 K. A short survey of the experimental layout and the results are given. For the measurements a chain of carbon resistors and solid state X-ray detectors was used for temperature and X-ray mapping of the cavity surface. Together with a visual inspection of the cavity interior during operation a better insight in field breakdowns mechanism and electron loading could be obtained.

Superconducting niobium cavities of improved thermal conductivity

The construction of large scale superconducting accelerators asks for reliable and economical manufacturing procedures for the superconducting cavities. The improvement of thermal conductivity of the niobium used can be of great benefit in that respect. We have built and tested single cell cavities at 500 MHz and 3 GHz from a newly available niobium sheet material of improved thermal conductivity. At 500 MHz a maximum accelerating field of 13.0 MV/m at 4.2 K was achieved. At 3 GHz and 1.5 K as a best result 18.7 MV/m were obtained. A total of 15 cavity tests were carried out and the results clearly exceed field values obtained so far with standard reactor grade niobium at CERN and Wuppertal. Measurements of the thermal conductivity of niobium at helium temperatures are discussed, the temperature dependence of the surface resistance of high purity niobium is given, the observed field and Q limitations and electron loading phenomena are outlined.

Status Report of the Superconducting 5-Cell Acceleration Structure at CERN

At CERN a 5 cell 500 MHz superconducting accelerating cavity has been constructed. At present the cavity is installed in the PETRA storage ring at DESY for a test run which will start middle of March. The cavity is equipped with a high power coupler and with 5 higher order mode couplers, two of the loop type and three of the antenna type. Frequency tuning and regulation are achieved by varying the length of the two end-cells. In a test of the complete cavity-cryostat system at CERN, acceleration fields of 2.8 MV/m and Q0-values above 109 have been reached. The cryostat losses remained below 10 W.