The conceptional design of the proposed linear electron-positron collider TESLA is based on 9-cell 1.3 GHz superconducting niobium cavities with an accelerating gradient of Eacc ≥ 25 MV/m at a quality factor Q0 ≥ 5 · 10 . The design goal for the cavities of the TESLA Test Facility (TTF) linac was set to the more moderate value of Eacc ≥ 15 MV/m. In a first series of 27 industrially produced TTF cavities the average gradient at Q0 = 5 · 10 9 was measured to be 20.1 ± 6.2 MV/m, excluding a few cavities suffering from serious fabrication or material defects. In the second production of 24 TTF cavities additional quality control measures were introduced, in particular an eddy-current scan to eliminate niobium sheets with foreign material inclusions and stringent prescriptions for carrying out the electron-beam welds. The average gradient of these cavities at Q0 = 5·10 9 amounts to 25.0±3.2 MV/m with the exception of one cavity suffering from a weld defect. Hence only a moderate improvement in production and preparation techniques will be needed to meet the ambitious TESLA goal with an adequate safety margin. In this paper we present a detailed description of the design, fabrication and preparation of the TESLA Test Facility cavities and their associated components and report on cavity performance in test cryostats and with electron beam in the TTF linac. The ongoing R&D towards higher gradients is briefly addressed.

The proposed linear electron–positron collider TESLA is based on 1.3GHz superconducting niobium cavities for particle acceleration. For a centre-of-mass energy of 500GeV, an accelerating field of 23.4MV/m is required which is reliably achieved with a niobium surface preparation by chemical etching. An upgrade of the collider to 800GeV requires an improved cavity preparation technique. In this paper, results are presented on single-cell cavities which demonstrate that fields of up to 40MV/m are accessible by electrolytic polishing of the inner surface of the cavity.

Abstract Three electropolished one-cell cavities were,measured before and after in-situ bakeout under ultra-high vacuum conditions. Before bakeout the cavities showed,a strong reduction in quality factor at fields above 25 MV/m. After the bakeout,the Q drop,was,no,longer present and gradients of up to 39 MV/m were achieved. This indicates that electropolishing yields highest accelerating gradients in niobium,cavities only in combination,with in-situ bakeout. 1 ELECTROPOLISHING OF NIOBIUM

Copper cavities with a thin niobium film - as used in the large electron positron collider LEP - would be also attractive for future linear colliders, provided the decrease of the Q-value with the accelerating gradient could be reduced. We aim at extracting the important parameters that govern this decrease. The dependence on the RF frequency is studied by exciting 500 MHz and 1500 MHz cavities in different modes. In addition we combined RF measurements for two 1500 MHz cavities of different RF performance with microscopic tests (AFM,TEM) on samples cut out of the same cavities. Their microstructural characterization in plan-view allows the extraction of the grain size and the defect densities.

Power couplers for the 352 MHz LEP2 superconducting RF cavities have been plagued by vacuum and electron outbursts which are attributed to multipacting. Processing of these couplers has been a lengthy operation which was often needed again after high power running even if only for a relatively short time. We report here on recent progress made in improved production methods of coupler parts and special treatment of surfaces, as well as practical tests and simulations of geometrical coupler modifications

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.

For increasing the energy of the Large Electron Positron Ring (LEP) at CERN superconducting accelerating cavities are being installed. Most of the 216 cavities, developed at CERN and manufactured in industry, have undergone successful acceptance tests. By sputter-coating high-purity niobium onto a copper substrate accelerating fields of 6 MV m(-1) with a quality factor Q(0) of 3.4 10(9) at 4.5 K and at a frequency of 350 Mhz were obtained. To reach these specifications it was necessary to develop surface preparation and sputtering techniques able to cover the 5.5 m(2) of a cavity with a defect-free niobium layer of 1.5 mm thickness. Assembly and all preparations have to be done in a clean room of class 100. The cavities work in the ultra-high vacuum (similar to 10(-10) mb) of the LEP ring. The working temperature is 4.5 K. Higher performance for a low price has been obtained as compared to that for cavities made of niobium sheet material. Copyright (C) 1996 Elsevier Science Ltd.

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.<>

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.< >