[en] The upgrading of LEP by s.c. cavities will require installation and operation of a few hundred 350 MHz, 4-cell cavities in the accelerator tunnel. It is at present anticipated to install eight cavities per rf-cell which have a length of ∼ 24 m. A tunnel slope of up to 1.5% and a tunnel diameter of 4.4 m have to be accommodated. For the design of adequate cryostats the following guiding lines were considered: up to eight cavities with their He tank could be housed in a common insulation vacuum. Cryostats should be modular and allow installation of individual cavities or groups of two cavities (with a total length not exceeding 6 m thus enabling normal transport inside the access pits and machine tunnel). A high accessibility to all critical parts like couplers, tuners and beam tube connections should be guaranteed. This requirement dictates a lateral access through the vacuum tank and thermal radiation shield which should also permit the removal and replacement of any one 4-cell cavity without disturbing the neighboring units. Cavity connections to the beam vacuum system as well as repairs should be possible under reasonably clean and dust-free conditions, particularly when keeping cavities under a slight overpressure of dry, dust-free protective gas. A test program was launched and a 1/5 scale model vacuum tank was constructed and tested. The main feature of this model was a frame and sealing skin design which offers complete accessibility to the inside of the vessel. The results obtained prompted the design and construction of a full size model which was completed in 1985 and proved the feasibility of the new concepts. A thin copper radiation shield mechanically clamped to the piping carrying the refrigerant and thus easily removable to meet the requirement of accessibility also proved adequate to intercept and evacuate the heat radiated by the vacuum tank. 4 references, 6 figures

[en] Three types of moderate vacuum failure were studied experimentally on a prototype of the sc cavity cryostats developed for LEP. The observations were interpreted using a simplified description of the liquid helium (LHe) bath and extrapolated to the worst possible pressure rise. This was verified in a last test by breaking the cavity vacuum in a fraction of a second with an 80 mm diameter valve. It was concluded that this simulated indeed the worst case and that, as a minimum, a rupture disk of 20 cm² is required on a low-impedance safety pipe to exclude excessive peak pressures and damage to the cavity. (author)

[en] The s.c. 4-cell cavities for LEP are designed to operate at 5 MV/m with a beam current of at least twice 3 mA. The matched W/sub ext/ is then 4 10⁶ resulting in a bandwidth of 90 Hz. The cavity has to be kept on its nominal frequency within a fraction of this bandwidth. The cavities are made from sheet metal and frequencies obtained after manufacturing have deviations of up to 200 kHz. An inelastic deformation to correct for the coarse manufacturing tolerances (200 kHz are corrected by a change of 5 mm in total length was foreseen. A specially designed tool allows even to change the length of each individual cell to correct deviations from the desired flat field distribution. The cavity is then mounted into the tuning support and the resonant frequency is adjusted with spacers to obtain the correct frequency at operational temperature with all vacua applied. 13 references, 8 figures

[en] After the commissioning of the LEP e⁺e^- collider at CERN, its energy will be upgraded by adding superconducting (s.c.) r.f. cavities. Obviously, there is a great incentive to test a prototype s.c. LEP cavity in one of CERN's existing accelerators, before a production in series of a large number of cavities (256) be launched. Such a test should prove the validity and the long-term reliability of the design in a real accelerator environment. The SPS proton accelerator/proton-antiproton collider/LEP injector was chosen for the implementation of the cavity, being at present CERN's largest accelerator. As a result the whole experiment (comprising Cavity, cryostat, refrigerator and r.f. system, installed in the tunnel about 60 m below ground) had to be remotely controlled from the accelerator surface buildings, in order not to interfere with the physics program. As long as a refrigerator was not yet delivered (during the first phase of the experiment) we used a 100 m long flexible He transfer line for cooldown with dewars located at the surface. Later on a cold box was installed 6 m off the cryostat, the compressor and control unit being placed in a surface building. For the operation of the SPS as a proton accelerator at high intensity (I_d.c. = 0.2 A), on magnetic cycles interleaved with cycles for lepton acceleration, the impedance of the s.c. cavity had to be reduced by several orders of magnitude. This was achieved by damping the cavity's fundamental passband mode impedances by an r.f. feedback including the tetrode power amplifier driving the cavity [1,2]. 10 refs., 5 figs

[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] Copper accelerating cavities will allow operation of the CERN Large Electron-Positron collider LEP at an energy of 55 GeV (giga electron volt) per beam. For ultimate upgrading to about 100 GeV the installation of superconducting accelerating cavities is planned. A preliminary design of the large cryogenic system for cooling at about 4.4 K is under way. The adopted principles are reviewed. Basic considerations concerning the refrigerator requirements, the layout of cold boxes and compressors, the liquid helium distribution over considerable distances, the cryostats and safety aspects in underground areas are described

[en] This paper reports on prototype LEP superconducting cavity installed in the SPS machine in order to gain experience with such a cavity in a real accelerator and also to provide more voltage for the SPS as LEP injector. In the initial experiments the cryostat was cooled down from the surface with dewars via a 100 m long flexible helium transfer line; for the final version a remotely controlled refrigerator has been installed in the tunnel, close to the cavity. The r.f. part is somewhat unusual because the cold cavity must not perturb the high intensity proton beam of the SPS; this is achieved using an r.f. feedback technique. Lepton beams were accelerated with the cavity, which showed no degradation of its performance as compared to the previous laboratory tests

[en] For the upgrading of LEP to energies beyond 55 GeV, s.c cavities will be used and it is planned to install 4 s.c. cavities in LEP after a first running in period. This paper presents results obtained with 350 MHz prototype 4-cell cavities. Besides the s.c. cavities a great effort has gone into auxiliary items like cryostats, main couplers, higher-order mode couplers and frequency tuners. Their layout and test results obtained in s.c. cavity measurements will be shortly presented. Some implications of the use of s.c. cavities in LEP will be discussed

[en] The article describes the development of superconducting accelerating cavities at CERN, working at 500 MHz and 350 MHz. (BHO)

[en] The results of the commissioning and first operation in LEP of a full-size superconducting cavity module are reported. It was operated in the laboratory at a total accelerating voltage of 32 MV. RF power generation, distribution and controls are similar to the LEP copper RF system already in operation. Liquid helium is supplied by a 1 kW refrigerator with the cold box in the klystron tunnel 30 m distant from the cryostat. Initial operating experience on the acceleration of LEP beams, higher order mode excitation, the RF and cryogenic system and controls are described. (author) 11 refs.; 2 figs