Wolfgang Weingarten

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>>

Comparative measurements of niobium sheet and sputter coated cavities

Comparative measurements of sheet metal and sputter coated Nb cavities were performed at 500 MHz. The sputter coated ones had a higher low field Q, a stronger decrease of Q with the accelerating field, and showed no static magnetic field dependence of the Q value. Throughout the layer, larger amounts of impurities were found, which may explain the experimental results.

Superconducting 500 MHz accelerating copper cavities sputter-coated with niobium films

Thermal breakdown induced either by electron loading or by local defects of enhanced RF losses limits the accelerating field of superconducting niobium cavities. Replacing niobium with a material of higher thermal conductivity would be highly desirable to increase the maximum field. Therefore, cavities made of OFHC copper were coated by D.C. bias sputtering with a thin niobium film (1.5 to 5 μm). Accelerating fields up to 8.6 MVm-1were obtained without observing any field breakdown, the limitation being due to the available rf power. The Q values achieved at 4.2 K and low field were similar to those of niobium sheet cavities (i.e. \sim 2 \times 10^{9} ), but a fast initial decrease of Q to about 109was reproducibly experienced. Subsequent inspection of regions of enhanced rf losses revealed defects the origin of which is under study. The apparatus used for coating the cavities and the results obtained are presented and discussed.

Extension of the measurement capabilities of the quadrupole resonator

The quadrupole resonator, designed to measure the surface resistance of superconducting samples at 400 MHz has been refurbished. The accuracy of its RF-dc compensation measurement technique is tested by an independent method. It is shown that the device enables also measurements at 800 and 1200 MHz and is capable to probe the critical RF magnetic field. The electric and magnetic field configuration of the quadrupole resonator are dependent on the excited mode. It is shown how this can be used to distinguish between electric and magnetic losses.

Non quadratic RF losses in niobium sputter coated accelerating structures

Low field Q-values of more than 10/sup 10/ and maximum accelerating gradients E/sub a/ between 10 and 15 MV/m have been obtained in superconducting mono-cell and multi-cell accelerating cavities between 350 and 1500 MHz. The superconductor is niobium, which is magnetron-sputtered as a thin film (/spl sim/1-2 /spl mu/m) on a cavity made from copper sheet. The dependence of the slope of Q vs. E/sub a/ on temperature and frequency can be explained by RF magnetic flux trapped within intrinsic defects.<<ETX>>

Superconducting cavities for particle accelerators: achievements and problems

Abstract Superconducting RF accelerating cavities of different frequencies are presently operating on various accelerators world wide. The large majority of these cavities are made of niobium sheet, by lathe spinning and welding. In one case only, i.e. LEP2 at CERN, the majority of cavities is made of copper, internally coated with a thin layer of sputtered niobium. The main motivation for developing Nb-coated copper cavities mustbe found in the higher stability against quenching, consequent to the higher thermal conductivity of copper at liquid helium temperatures. Additional advantages are higher Q 0 values at low fields, insensitivity to trapped earth magnetic field and lower cost. Beside these advantages, the Nb-coated cavities unfortunately suffer from a faster Q 0 degradation with increasing the accelerating field. The performances of Nb-sheet and Nb-coated cavities are critically compared on the grounds of the large industrial production required by the LEP2 project.

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.

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>>

Various Methods of Manufacturing Superconducting Accelerating Cavities

We report on experience in superconducting cavity production methods gained in shaping, joining and thin film coating with various materials and techniques (Pb, Nb, Nb3Sn, NbN, NbTiN) with emphasis on their potential to reduce mass production costs.