Giorgio Cavallari

Superconducting niobium sputter-coated copper cavity modules for the LEP energy upgrade

Experience from the construction, assembly, and tests of two superconducting cavity modules for the Large Electron Positron colliding beam accelerator (LEP) are given. Each module consists of four individual four-cell 352 MHz Nb sputter-coated Cu cavities equipped with an RF power coupler, higher-order-mode (HOM,) dampers, and a frequency tuner, all housed in a single cryostat. The demountable HOM dampers of a new type designed for sputter-coated cavities allow Q/sub ext/ of 9000 for the HOMs with the largest (R/Q). Q values are higher (4.5 to 11*10/sup 9/) than those for similar Nb sheet cavities up to the maximum accelerating fields obtained (6 to 9.5 MV/m). The field limitation is electron loading and never thermal breakdown. Results on vertical tests of individual cavities are reported (Q value, maximum accelerating fields, residual resistance). They are complemented by results on horizontal tests of individual cavities, and on the fully equipped klystron-driven four-cavity module.<<ETX>>

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 cavities for the LEP energy upgrade

The technology of sputter coating of Cu cavities with Nb has been developed at CERN. The advantages of this technique have led CERN to order 168 of such cavities in industry. After an initial phase of technology transfer and of prototype development, the series production has been started in fall 92 by the three contractors. The results of the bare cavity tests are reported. Fixed and movable 120 kW power couplers (MC) have been designed, manufactured and put into operation. Various models of higher order mode (HOM) couplers have been developed to cope with foreseen increase of the beam intensity. Special care is given to the conditioning of power couplers and of HOM couplers before installation in the machine.<<ETX>>

Long-term test of a LEP prototype superconducting cavity in the CERN SPS

Using a superconducting LEP (Large Electron Positron Collider) prototype cavity, an e/sup +/ beam was accelerated at E/sub a/=5.5 MV/m. The SPS (Super Proton Synchrotron) proton accelerator/proton-antiproton collider/LEP injector was chosen for the implementation of the cavity. After 8000 h at 4.5 K E/sub a//sup max/ remained unchanged (7.1 MV/m); the Q-value (5 MV/m) was 2/3*10/sup 9/. No major system failure occurred. RF feedback guaranteed very low fluctuations of the cavity field amplitude and phase.<<ETX>>

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.