For particle free vacuum systems, as e.g. systems containing superconducting cavities to be operated at high gradients, special care needs to be taken during pump down and venting. Neither should be particles introduced into the vacuum system, nor should particles already present within the system be moved. A series of measurements to study the movement of particles in long tubes during pump down and venting using an in-situ vacuum particle counter has been performed. Based on these measurements a set-up using flow controllers, diffuser and pressure gauges has been developed to avoid introducing particles into the vacuum system as well as moving existing particles during pump down and venting. This set-up can be operated manually as well as via a control unit. The electronics unit is an in- house development at DESY. It is usually connected to an oil free pump station. This set-up allows automated pump-down and venting of critical vacuum systems in a reliable and reproducible way, being faster than the procedures used so far.

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 European X-ray Free Electron Laser XFEL, a new international research facility, will be built at DESY/Hamburg. The XFEL will generate extremely brilliant and ultra short pulses of spatially coherent X-rays with tuneable wavelengths down to 0.1 nm, and exploit them for revolutionary scientific experiments at various disciplines. The basic process adopted to produce the X-ray pulses is SASE (Self-Amplified Spontaneous Emission). Therefore electron bunches are produced in a high-brightness gun, brought to high energy of about 20 GeV through a superconducting linear accelerator, and transported to up to 250 m long undulators, where the X-rays are generated. The beam vacuum system of the accelerator contains sections operated at room temperature as well as at 2 K in the areas of the superconducting accelerating structures, thus requiring an insulating vacuum system. In addition to standard UHV requirements, the vacuum system for this facility needs to preserve the cleanliness of the superconducting cavity surfaces. Therefore the preparation of all vacuum components for the 1.6 km long main linac includes cleaning of the components in a clean room to remove particles, installation into the accelerator in local clean rooms, and special procedures for pump down and venting. Further challenges are the undulator vacuum chambers filling more than 700 m, where a high surface quality with respect to surface roughness and thickness of oxide layers is mandatory to reduce wake field effects, and the vacuum systems for the various beam dumps, where exit windows acting as vacuum barriers of sufficient reliability need to be developed. In addition, a large amount of about 1.7 km of transport beam lines is required. The layout of the various vacuum sections as well as experience with prototype components will be described.

Massimo Altarelli, Reinhard Brinkmann, Majed Chergui, Winfried Decking, Barry Dobson, Stefan Düsterer, Gerhard Grübel, Walter Graeff, Heinz Graafsma, Janos Hajdu, Jonathan Marangos, Joachim Pflüger, Harald Redlin, David Riley, Ian Robinson, Jörg Rossbach, Andreas Schwarz, Kai Tiedtke, Thomas Tschentscher, Ivan Vartaniants, Hubertus Wabnitz, Hans Weise, Riko Wichmann, Karl Witte, Andreas Wolf, Michael Wulff, Mikhail Yurkov