[en] Beam tests and operation of superconducting cavities conducted since the third workshop on RF superconductivity (Argonne, Sep. 1987) are reported in this paper. The paper is concerned particularly with electron machines. Storage and acceleration of the beam are discussed, focusing on the CERN test in SPS, the DESY test in PETRA, the superconducting injector at Darmstadt, and the KEK beam tests in T-AR. Then, long-term performance of the cavity in the ring is discussed focusing on Eacc (max) and O-value, environmental conditions, and operational experience in T-MR. RF controllability is addressed, centering on the Robinson stability, cavity tuning loop, quench detection and interlocks, recovery procedure, field calibration, and phase adjustment. Higher order modes are also discussed. Superconducting cavities have been operated successfully in accelerators. It has been confirmed that the superconducting cavities can be used stably for experimental use. For more than 5000 hours the cavities have indicated no essential degradation of the cavity performance. The study of long-term performance should be continued in longer range of period. (N.K.)

[en] A short review is given of historical events in accelerator and cryogenic developments at both Stanford and Berkeley. Methods of refrigeration between 1.85 K and 4.5 K together with modern techniques and improvements are discussed. Where the decade of the 70's was the era of the screw compressor, the 80's can be considered that of the cold vacuum pump for superfluid cooling. Distribution methods are of major importance, and arguments can be made for bath or tube cooling, two-phase, thermo-syphon, supercritical or superfluid. System design affects reliability, safety and operating stability. Distribution costs and heat loads can be a large part of system totals. Some specific system descriptions are included. (author)

[en] For TRISTAN, fabrication and tests of five-cell niobium cavities have been carried on. Sixteen five-cell cavities were installed in the TRISTAN tunnel in the summer of 1988. Before the installation of superconducting cavities, TRISTAN has been operating with 104 nine-cell normal-conducting cavities with the maximum beam energy of 28.5 GeV. By adding the superconducting cavities, the beam energy has been upgraded from the Autumn of 1988 to 30, 30.4 and then 30.7 GeV. The second sixteen five-cell cavities were also fabricated, tested and installed in the tunnel at the beginning of August 1989. Cool down and commissioning of the cavities will start at the middle of October, and from November beam energy will be pushed up to 32 or 33 GeV. The present paper reports the test results of thirty-two 5-cell cavities before installation in the tunnel, and some of experience of the beam operation of the first sixteen 5-cell cavities. Vertical test results given include the statistics of accelerating field gradient, field strength for each cavity, RF losses of cavities relative to the peak surface field, etc. Two 5-cells were then combined and assembled in horizontal cryostats with input coupler, HOM couplers and frequency tuner. Some horizontal test results are also presented. (N.K.)

[en] The present paper outlines the recent preparation methods for superconducting cavities used in various laboratories and universities, and reports the problems of the cavity fabrication at KEK as an example of mass production. Preparation and handling are first addressed, focusing on material, fabrication, surface treatment, rinsing, clean environment, and heat treatment. Cavity production at KEK is then described, centering on defects on the surface and clean environments. Field gradients of more than 20 MV/m have been obtained by 1.5-3 GHz single cavities, for multi-cell cavities Eacc of 10 MV/m are available at any frequency range. The successful construction of thirty-two cavities for TRISTAN at KEK is due to the careful checking of the surface and quality control of all processes against the surface defects and contaminations. Eacc of 5 MV/m has been achieved by 94 % of the TRISTAN cavities at the first cold test, but 6 % of them had to be reworked because of the surface defects. These defects could not be detected by an X-ray photograph or visual inspections during the fabrication processes. (N.K.)

[en] Manufacturing tolerances, thermal stresses, acoustic noise, and cooling fluid pressure fluctuations all conspire to make the field in the cavity not precisely what the accelerator physicist has in mind. Tuners and control systems are the tools used to fight back: they regulate the field in the cavity to the desired magnitude and phase. Amplitude and phase stabilities are usually of greater concern in superconducting cavities than in copper cavities. The key to achieving a stable gradient and phase is feedback. A probe must be placed in the cavity itself to sense the present cavity status. Electronic control is then given the responsibility to correct for any measured disturbance. The electronic modulation of forward power has been implemented in a number of ways. Perhaps the easiest implementation to understand has two separate control loops, one for amplitude and one for phase (phase-amplitude loops). Other major electronic control devices include complex phasor modulator (CPM-amplitude loops), vector loop, and variable reactance. 'Slow' tuners are used when the tuning range of the 'fast' tuner plus electronic tuning is not enough to compensate for unpredictability or drift in the static frequency setting. (N.K.)

[en] An approach towards industrial fabrication of superconducting Nb cavities for LEP was undertaken and the experience is described. Out of the first four Nb superconducting cavities to be installed in LEP two cryostats and cavities were produced by CERN and two more by industry. The results are essentially the same. The accelerating field and Q-value at the design value are between 5.3 and 9 MV/m and between 2.0 and 3.3 x 10⁹, respectively. The static cryostat losses were 16 W. The ancillary equipment (couplers, tuners) was produced by CERN. All the fabrication sequence applied at CERN has been used in an identical way by industry. (author)

[en] CEBAF, an electron accelerator under construction in Virginia, has 338 superconducting RF cavities contained in 42 1/4 cryomodules. Four cryounits which house eight RF cavities and a set of end caps make up a cryomodule. The thermal performance of a prototypical cryounit and end cap set is presented. (author)

[en] Los Alamos National Laboratory is engaged in two distinct areas of rf superconductivity research and development. One program is devoted to studying the rf properties of high-temperature superconductors (HTS) and assessing their potential for accelerator-cavity applications. Our best results to date have come from YBa₂Cu₃O₇ films (0.8 μm) electron-beam deposited onto LaGaO₃ substrates. A typical surface resistance value is 0.2 ± 0.1 mΩ measured at 22 GHz and 4 K, which is only a factor of 2 - 4 higher than Nb. A second area of rf superconductivity R and D at Los Alamos involves fabricating and testing conventional Nb superconducting rf cavities. Facilities are being constructed for polishing, cleaning, and testing both small (3-GHz) and large (805-MHz) Nb cavities. Much of our effort is being devoted to understanding the proper techniques for welding, cleaning, and handling Nb cavities, to ensure good high-field performance. Nb cavity applications are foreseen in upgrades to LAMPF, compact free-electron lasers, and high-current cw proton linacs. A single 402.5-MHz Nb cavity (in a beam cryostat) is presently being acquired from industry to provide a momentum compactor for the low-energy pion beam at LAMPF. (author)

[en] The 10 MeV injector of the superconducting 130 MeV electron accelerator at Darmstadt (S-DALINAC) produced its first beam in August 87, when a phase locked operation of its 5-cell capture section and two 20-cell structures was achieved for the first time. Status reports have been presented after the installation of the first and the second cryomodule of the main linac each one containing two 20-cell accelerating structures. The present report gives a brief description of the accelerator focusing on its main design parameters and its present status of installation. Results from some 550 hours of machine time for accelerator tests and from 950 hours of injector beamtime for atomic and nuclear physics experiments are summarized. Main emphasis is laid on the experience which was gained during this time with the performance of the superconducting cavities and associated equipment like couplers, tuners and windows. The development of a modest infrastructure for cavity treatment is in progress. The report describes the present status of installations and how to improve them. An outlook on how to proceed in the completion of the accelerator is also given. (N.K.)

[en] Coherent instabilities are mainly caused by the long range wakefields excited by the beam bunches as they pass through the cavities. The best way to increase the threshold is to hasten the decay of the excited fields, in other words to damp the higher order modes of the cavities once the shunt impedance are fixed. Various devices have been developed during the last years. The main differences lie in the type of coupling - hole, probe or loop - and in the way of rejecting the accelerating mode. But all the last versions follow the same rule: they couple through the beam tube and not directly in the cells. The present report focuses particularly on the estimation of the extracted power by HOM couplers (waveguide coupling, loop or probe coupling), coupling via beam tube and its limits, estimation of the extracted power by propagating waves (results for a reduced iris aperture 3-cell cavity, those for a large iris multicell cavity), requirements for a HOM coupler (rf requirements, 'engineering' requirements), waveguide couplers developments, coaxial couplers developments, cost savings by reduction of the number of coupling ports and HOM couplers, and last developments at Saclay. (N.K.)