Search

Nuclear instruments & methods in physics research / A 538(1-3), 45 - 64 (2005). doi:10.1016/j.nima.2004.09.003, A systematic study is presented on the superconductivity (sc) parameters of the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz cavities for the linear collider project TESLA. Cylindrical Nb samples have been subjected to the same surface treatments that are applied to the TESLA cavities: buffered chemical polishing (BCP), electrolytic polishing (EP), low-temperature bakeout (LTB). The magnetization curves and the complex magnetic susceptibility have been measured over a wide range of temperatures and dc magnetic fields, and also for di erent frequencies of the applied ac magnetic field. The bulk superconductivity parameters such as the critical temperature Tc = 9.26 K and the upper critical field Bc2(0) = 410 mT are found to be in good agreement with previous data. Evidence for surface superconductivity at fields above Bc2 is found in all samples. The critical surface field exceeds the Ginzburg-Landau field Bc3 = 1.695Bc2 by about 10% in BCP-treated samples and increases even further if EP or LTB are applied. From the field dependence of the susceptibility and a power-law analysis of the complex ac conductivity and resistivity the existence of two different phases of surface superconductivity can be established which resemble the Meissner and Abrikosov phases in the bulk: (1) coherent surface superconductivity, allowing sc shielding currents flowing around the entire cylindrical sample, for external fields B in the range between Bc2 and Bcohc3, and (2) incoherent surface superconductivity with disconnected sc domains between Bcohc3 and Bc3. The coherent critical surface field separating the two phases is found to be Bcoh c3 = 0.81Bc3 for all samples. The exponents in the power law analysis are different for BCP and EP samples, pointing to different surface topologies., Published by North-Holland Publ. Co., Amsterdam

22 pp. (2006)., The electromechanical model of the TESLA cavity has been implemented in FPGA technology for real-time testing of the control system. The model includes Lorentz force detuning and beam loading effects. Step operation and vector stimulus operation modes are applied for the evaluation of a FPGA cavity simulator operated by a digital controller. The performance of the cavity hardware model is verified by comparing with a software model of the cavity implemented in the MATLAB system. The numerical aspects are considered for an optimal DSP calculation. Some experimental results are presented for different cavity operational conditions.

Universität Hamburg, Diss., 2005; 159 pp., (2005). doi:10.3204/DESY-THESIS-2005-012, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

TESLA Reports 2005 33 pp. (2005)., The paper describes design and practical realization of low and high level software for laboratory purposes to control FPGA-based LLRF electronic equipment for TESLA. There is presented a universal solution for particular functional devices of the control system with FPGA chips. The paper describes architecture of software layers and programming solutions of hardware communication based on the proprietary Internal Interface (II) technology. Such a solution was used for the Superconducting Cavity Controller and Simulator (SIMCON) for TESLA experiment (Test Facility) in DESY. The examples of the build and tested software blocks were given in this paper. This documentation is a unity with TESLA Reports published in 2004 by the Elhep and describing the SIMCON hardware, ver.1.0. The paper was written in a form of a User’s Manual.

TESLA Reports 2005 4 pp. (2005)., We present results of multiobjective optimization of the International Linear Collider (ILC) which seeks to maximize luminosity at each given total cost of the linac (capital and operating costs of cryomodules, refrigeration and RF). Evolutionary algorithms allow quick exploration of optimal sets of parameters in a complicated system such as ILC in the presence of realistic constraints as well as investigation of various what-if scenarios in potential performance. Among the parameters we varied there were accelerating gradient and $Q$ of the cavities (in a coupled manner following a realistic $Q$ vs. $E$ curve), the number of particles per bunch, the bunch length, number of bunches in the train, etc. We find an optimum which decreases (relative to TESLA TDR baseline) the total linac cost by 22%, capital cost by 25% at the same luminosity of 3 × 10$^{38}$ m$^{–2}$ s$^{–1}$ . For this optimum the gradient is 35 MV/m, the final spot size is 3.6 nm, and the beam power is 15.9MW. Changing the luminosity by 10$^{38}$ m$^{–2}$ s$^{–1}$ results in 10% change in the total linac cost and 4% in the capital cost. We have also explored the optimal fronts of luminosity vs. cost for several other scenarios using the same approach

TESLA Reports 2004 21 pp. (2004)., In this report we estimate long- and short-range wake functions for new elements to be used in TESLA Test Facility (TTF) – II. The wake potentials of the LOLA-IV structure and the $3^{rd}$ harmonic section are calculated numerically for very short bunches and analytical approximations for wake functions in short and long ranges are obtained by fitting procedures based on analytical estimations.

9 pp. (2004).

TESLA Reports 2003 9 pp. (2003)., This paper presents a FPGA based DSP system for realtime simulation of superconducting accelerator’s cavity. The super- conducting linacs require sophisticated control systems for maintaining the constant amplitude and phase of accelerating field in the accelerator’s cavities. The debugging of these systems on real hardware can be both difficult and dangerous.To allow testing of the real LLRF systems in the real time and with different cavity parameters, the FPGA based system has been developed.

TESLA Reports 2003 13 pp. (2003)., Experiences concerning RRR degradation and gas absorption of high purity niobium welded at different electron beam (EB) facilities are summarized. The oxygen and nitrogen content is increased as a rule at the welding seam. The absorption of hydrogen takes place not directly at the welding seam but in the heat‐affected area. The RRR degradation is moderate, if the total pressure is better than 5×10$^{−5}$ mbar and the preparation is done under clean condition. A series of welding was done in the new DESY EB facility, which was specifically designed to meet the requirements for the welding of niobium cavities for TESLA. High vacuum (between 2×10$^{−8}$ and 2×10$^{−6}$ mbar) allowed improving the niobium purity at the welding seam, in which the RRR were increased from 350 to ca. 370‐380. The distribution of RRR, gases, grain size and thermal conductivity in the welding area are analyzed. The repair of a burned hole in the cavity is also discussed.

TESLA Reports 2003 11 pp. (2003)., The cavity control system for the TESLA - TeV–Energy Superconducting Linear Accelerator project is initially introduced in this paper. The FPGA – Field Programmable Gate Array technology has been implemented for digital controller stabilizing cavity field gradient. The cavity SIMULINK model has been applied to test the hardware controller. The step operation method has been developed for testing the FPGA device coupled to the SIMULINK model of the analog real plant. The FPGA signal processing has been verified according to the required algorithm of the reference MATLAB controller. Some experimental results have been presented for different cavity operational conditions.

TESLA Reports 2003 24 pp. (2003)., A mode-locked titanium-sapphire laser with 15 fs pulse width is used to determine the temporal profile of the ultrashort electron bunches in the linear TESLA Test accelerator. The experimental setup is described and first results are presented on the synchronization of the Ti:Sa laser with the radio frequency of the superconducting accelerator and on the coincidence measurements between the laser pulses and the coherent transition radiation pulses generated by the electron bunches. For the next step of the experiment, the electro-optic sensor will be mounted inside the vacuum pipe of the linac to permit a direct measurement of the electric field carried by the relativistic bunches.

TESLA Reports 2003 19 pp. (2003)., The cavity control system for the TESLA - TeV–Energy Superconducting Linear Accelerator project is initially introduced. The elementary analysis of the cavity resonator on RF (radio frequency) level and low level frequency with signal and power considerations is presented. For the field vector detection the digital signal processing is proposed. The electromechanical model concerning Lorentz force detuning is applied for analyzing the basic features of the system performance. For multiple cavities driven by one klystron the field vector sum control is considered. Simulink model implementation is developed to explore the feedback and feed-forward system operation and some experimental results for signals and power considerations are presented.

21 pp. (2003).

TESLA Reports 2003 10 pp. (2003)., The fundamental knowledge contained in the previous paper “Cavity Control System Essential Modeling for TESLA Linear Accelerator” is applied for Matlabs’ Simulink implementation of the basic models for the cavity resonator. Elementary simulations of the cavity behavior are carried out and some experimental results for signals and power considerations are presented.

TESLA Reports 2003 19 pp. (2003)., The cavity resonator modeling for the TESLA - TeV–Energy Superconducting Linear Accelerator project is initially introduced. The electromechanical model including Lorentz force detuning and beam loading is applied for analyzing the basic features of the plant. The continuous SIMULINK and digital MATLAB model implementation is developed as a reference for the hardware simulator based on the FPGA – Field Programmable Gate Array technology. The step operation mode has been applied for testing of the FPGA device coupled to the software control block. The FPGA signal processing has been verified according to the desired model of the real cavity plant. The numerical aspects have been investigated for the efficient design. Some experimental results have been presented for different cavity operational conditions. The following considerations have lead to the synthesis of the efficient algorithm for the cavity model, then implemented in the FPGA system.

11 pp. (2003)., The paper is an introduction to the optimization methods of the linear accelerator cavity control system. Three distinct time periods of cavity operation are considered; filling with the EM field energy, field stabilization for the accelerated beam, and field decay. These periods are considered as different states of the cavity corresponding to adequate solutions of differential equation modeling the cavity behavior. The cavity could be operated by several different methods in each work phase:During the filling – feedback and feed-forward alone, feedback and feed-forward together, self-tuning;During the flattop – feed-forward and feedback alone or together,During the decay – detuning and quality factor may be measured.The optimization is understood as a choice of the most efficient way of the cavity control during each period. The control may be done in terms of minimum power consumption from the klystron during whole work cycle and efficient field stabilization in the cavity, during flattop period. The introductory analysis of the cavity operational modes in three mentioned periods is presented in this paper.Additionally, the alternative, more precise algorithm of the cavity field detection is proposed. The cavity field is estimated as a result of intermediate frequency signal envelope decoding.All considerations in this paper deal with a single cavity system in noiseless and deterministic conditions.

The 2001 Particle Accelerator Conference, PAC2001, Chicago, USA, 18 Jun 2001 - 22 Jun 2001; 3771-3773 (2001)., This paper reports on recent beam measurements of higher order modes in the TESLA Test Facility (TTF) accelerating modules. Using bunch trains of about 0.5 ms with 54MHz bunch repetition and up to 90% modulated intensity, transverse higher order modes are resonantly excited when the beam is offset and their frequency on resonance with the modulation frequency. With this method, the trapped modes can be excited and their counteraction on the beam observed on a wide-band BPM downstream of the module. Scanning the modulation frequency from 0 to 27MHz allows a systematic investigation of all possible dangerous modes in the modules.

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; 1673-1675 (2002)., The rf control analysis tool consists of a set of library blocks to be used with SIMULINK. The tool allows to study the performance of a given rf control design. The library blocks include models for the superconducting cavities, the rf power source, the beam, and the rf feedback system.The rf control relevant electrical and mechanical characteristics of the cavity are described in form of timevarying state space models. Included are perturbations from Lorentz force detuning and microphonics with the appropriate parameters for several mechanical resonances. The power source is calibrated in terms of actual power and includes saturation characteristics and noise. An arbitrary time structure can be imposed on the beam current to reflect a macro-pulse structure and bunch charge fluctuations. For rf feedback several schemes can be selected: Generator driven system or self-excited loop, traditional amplitude and phase control or I/Q control. The choices for the feedback controller include analog ordigital approaches and various choices of filters for the gain stages. Feed forward can be added to further suppress repetitive errors. The results of a performance analysis of the TTF Linac rf system using these tools are presented.

TESLA Reports 2002 55 pp. (2002)., We present the calculation results for a lossless TTF cavity-coupler unit, for combination cavity-coupler units and for an 8-cavity accelerating module, at the frequency band corresponding to third dipole band of the 9-cell TTF cavity. Experimental investigations of the copper TESLA Test Facility cavity, equipped with two higher order mode couplers, are also presented, for the original and the modified (“mirrored”) downstream higher order mode couplers.Here we developed the procedure for the determination of resonant frequencies and Q-factors using the dependence of the complex transmission coefficient S$_{m,n}$(f) or the complex reflection coefficient S$_{m,m}$(f) on frequency in the frequency band under investigation. These dependencies were obtained from the numerical calculation of S- parameters of lossless TESLA Test Facility (TTF) modules in the third dipole mode frequency band and from the experimental investigation of the copper TESLA Test Facility cavity equipped with two higher order mode couplers in the first, second and third dipole mode frequency bands, second quadrupole and second monopole mode frequency band. The procedure permits us to determine resonant frequencies and Q- factors (Q$_o$, Q$_{Load}$ and Q$_{ext}$) using a large enough frequency step (∆f) with only few frequency points located on the resonance curve corresponding to the high Q-factor resonance.

TESLA Reports 2002 165 pp. (2002).

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; 1915-1917 (2002)., A measurement System for detecting dark currents, generated by the TESLA cavities is proposed. It is based on the cryogenic current comparator principle and senses dark currents in the nA range. Design issues and the application for the CHECHIA cavity test stand are discussed.

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; 876-878 (2002)., In order to test the performance of a superconducting linac, the TESLA Collaboration has built and operated a 120 m long linac which includes an RF electron gun, two 12mlong superconducting accelerator modules, two bunch compressors and a 13.5 m long undulator for a VUV FEL. This linac serves as a test bench for high gradient operation of 9-cell superconducting cavities with full beam loading as required for the TESLA design. Results of recent running periods will be summarized in this paper.

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; 2481-2483 (2002)., Electrical power transmission systems produce stray magnetic fields around the lines. In a three phase AC system the sum of line currents is always zero, but the spatial arrangement of the wires create a displacement of the magnetic field. This causes interferences to adjacent systems, especially to accelerator beams with weak focusing, which require a magnetic and electrical environment with low parasitic fields. A multi coaxial line system has not this disadvantage. Therefore a coaxial gas insulated line (KOGIL) for medium and high voltage has been developed in a cooperation between DESY Hamburg and Darmstadt, University of Technology, High Voltage Laboratory. A prototype was built up for 20 kV and 25 MVA. The measurements and simulations as well as the possible applications of this new power transmission system will be presented.

TESLA Reports 2002 15 pp. (2002)., The TESLA collider[1] is a large scale project with a large amount of complex and partially innovative systems. In a project of such characteristics, the stage of test of its various parts and the commissioning of the whole facility requires a considerable amount of effort, expertise and time. Moreover, a prompt delivery of luminosity to the experiments depends strongly on a successful commissioning. Therefore, preparation of a detailed commissioning plan and schedule together with some account of the personnel resources needed is mandatory. This study should be included in the plans of the construction and installation work of the TESLA collider. A working group of the TESLA collaboration meeting held in Saclay in April 2002 started to address these problems. Neither time, personpower nor cost have been considered in detail. In principle, the commissioning of completed parts of the collider has to be adapted to the installation schedule. Nevertheless, we suggest to consider the possibility to partially change the installation schedule in order to commission some sections earlier. Moreover, some additional infrastructure is necessary to close and shield parts of the tunnel, and to dump low energy beams. The commissioning of the XFEL part has not yet been considered at this stage of the study.

TESLA Reports 2002 61 pp. (2002)., One of the sources of transverse emittance dilution in the superconducting e$^+$e$^−$ linear collider TESLA are transverse wakefields of higher frequency modes (HOM) excited in the accelerating cavities by long trains of intense bunches. Traditionally HOM couplers (HOMC) have been designed to damp modes from the 1$^{st}$ and 2$^{nd}$ dipole bands, however measurements have shown that modes with unexpectedly high Q-factors and R/Q values occur in the 3$^{rd}$ dipole band. The present work quantitatively analyses the fields and their damping in the frequency range of the 3$^{rd}$ dipole band (~2580 MHz). In our investigation we use an S-parameter concept. For the calculations the TESLA Test Facility (TTF) accelerating module is split into several components, such as the nine-cell cavity, different types of HOM and fundamental mode couplers, cylindrical and coaxial bellows, cold windows etc., for which the S-parameters are independently calculated using the MAFIA and Microwave Studio (MWS) codes. Using the scattering matrixes of all these components, we have the ability to calculate the scattering matrix of complex sub- systems or of complete modules. The frequency dependence of the S-parameters permits us to find resonant frequencies and Q-factors of complex devices and to study the field distribution in cavities and along waveguides for a given excitation. An understanding of the damping mechanisms in the 3$^{rd}$ dipole band was obtained and an improved design of the HOM couplers is proposed in order to suppress those 3$^{rd}$ dipole band modes with high values of R/Q.

TESLA Reports 2002 155 pp. (2002).

TESLA Reports 2002 6 pp. (2002).

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; 1473-1475 (2002)., Measurements in the TESLA Test Facility (TTF) have shown that modes with unexpectedly high Q-factors and R/Q values occur in the third dipole band. The present work quantitatively analyses the fields and their damping in the frequency range of the third dipole band (~2.5 GHz). In our investigation we use a scattering (S)-parameter concept. For the calculations the TTF accelerating module is split into several components, forwhich the S-parameters are independently calculated. Using the S-matrices of all these components, we have the ability to calculate the S-matrix of complex systems or of complete modules. The frequency dependence of the Sparameters permits us to find resonant frequencies and Qfactors of complex devices. An understanding of the damping mechanisms in the third dipole band was obtained and an improved design of the higher order mode (HOM) couplers was proposed in order to suppress those third dipole band modes with high values of R/Q.

TESLA Reports 2002 4 pp. (2002)., The shape of TESLA accelerating structure can be improved to decrease maximal surface magnetic field by sacrificing and increasing maximal surface electric field. This sacrifice may be necessary because the magnetic field is a hard limit but electric field emission can be decreased by processing. For the case of superconducting cavities RF magnetic strength should be of a more concern.

Universität Hamburg, Diss., 2001; 85 pp. (2001). = Universität Hamburg, Diss., 2001

; TESLA Reports 2001 15 pp. (2001)., We review the current status of the R&D results and design studies on the TESLA superconducting e$^+$e$^−$ linear collider project.

TESLA Reports 2001 16 pp. (2001)., A test bench for the test of single TTF/FEL-cryomodules is proposed. The general equipment and the layout of the cryogenic supply are described.

Universität Hamburg, Diss., 2001; 216 pp., (2001). doi:10.3204/DESY-THESIS-2001-055

TESLA Reports 2001 13 pp. (2001).

The 18th International Conference on High Energy Accelerators, HEACC2001, Tsukuba, Japan, 26 Mar 2001 - 30 Mar 2001; TESLA Reports 2001 19-21 (2001)., The Tesla linear collider requires 600 pieces of 10MW L-Band klystrons operating at a pulse duration of 1.5ms and a repetition rate of up to 10Hz with an efficiency of the order of 70%. Since these performances are not achievable with a single beam klystron, a multi beam klystron (MBK), which uses many low perveance electron beams in parallel in one vacuum envelope, was developed by THOMSON TUBES ELECTRONIQUES (TTE).The advantage of this solution is the low voltage required and the high efficiency compared with a single beam klystron. THOMSON TTE has developed and manufactured the TH1801. This MBK is in operation at the Tesla Test Facility at DESY, where it was tested at full pulse duration of 1.5ms. It reached 10MW at 117kV and 131A with an efficiency of 65%. This paper reports on the tubes, the test results and the operation experience of the TH1801 multi beam klystrons built until now.

The 18th International Conference on High Energy Accelerators, HEACC2001, Tsukuba, Japan, 26 Mar 2001 - 30 Mar 2001; TESLA Reports 2001 25-27 (2001)., The Tera-eV Energy Superconducting Linear Accelerator (TESLA), a 33 km long superconducting linear electron/positron collider of 500 GeV (upgradeable to 800 GeV) centre of mass energy, presently in the planning phase at DESY, will consist of about 21000 superconducting RF 9-cell cavities of pure Niobium. Each cavity of about 1 m length has to be cooled in a 2.0 K helium bath and operated at 1.3 GHz with 5 Hz beam repetition rate. The cavities, equipped with their individual helium vessels made from titanium, RF input couplers , tuners and magnetic shielding, will be assembled in groups of 12 in about 16 m long cryostats (cryomodules) with thermal radiation shielding at 5-8 K and 40-80 K. Part of the cryomodules will also be equipped with superconducting quadrupoles and steering coils. About 10% of the cryomodules will contain cryogenic valves and instrumentation required for cooling purposes. The paper presents design, construction and assembly details of the cryomodules, which are based on the experience with somewhat shorter cryomodules, already in use at the TESLA Test Facility (TTF) at DESY.

; TESLA Reports 2001 6 pp. (2001)., We review the current design of the interaction region of the TESLA superconducting linear collider. We describe in particular the recent upgrades of the collimation section and the beam extraction lines.

The 18th International Conference on High Energy Accelerators, HEACC2001, Tsukuba, Japan, 26 Mar 2001 - 30 Mar 2001; 4-8 (2001)., The cavity research and development which was essential to achieve the performance goals of a Q$_0$ of 10$^{10}$ at an accelerating gradient of 23,5 MV/m for TESLA-500 will be reviewed. Results from the 1,3 GHz nine-cell superconducting niobium cavities will be shown. For TESLA-800 the specifications are a Q$_0$=5×$10^9$ for the accelerating gradient of 35 MV/m. It will be shown that this is achieved regularly in electropolished one-cell cavities. First promising results on nine-cell cavities are shown. For TESLA-800 it is necessary to increase the fill factor of the linac. This is achieved via the superstructure concept. The first succesful test with a piezoelectric tuner to compensate the frequency detuning during the rf pulse is shown.

TESLA Reports 2001 15 pp. (2001).

TESLA Reports 2001 19 pp. (2001)., We present a preliminary design of the superconducting final focusing quadrupole magnets for TESLA and all their associated cryogenics.

The 18th International Conference on High Energy Accelerators, HEACC2001, Tsukuba, Japan, 26 Mar 2001 - 30 Mar 2001; TESLA Reports 2001 32-34 (2001)., The Tera-eV Energy Superconducting Linear Accelerator (TESLA) is a 32 km long superconducting linear electron/positron collider of 500 GeV centre of mass energy (upgradeable to 800 GeV), presently in the planning phase at DESY [1,2]. About 21000 superconducting RF 9-cell cavities have to be cooled with superfluid helium at 2 K. The cavities are assembled in groups of 12 into about 16 m long cryo-modules. The roughly 1800 cryomodules are arranged in an underground accelerator tunnel in 12 about 2.5 km long cryo-units. Except for the 2 first cryo-units every 2 cryounits will be supplied from one refrigerator in a refrigerator hall above ground. The first two cryo-units have individual refrigerators in individual refrigerator halls. The design goals for the cryoplants are reliability, simplicity and low installation and operating cost.

The 2001 Particle Accelerator Conference, PAC2001, Chicago, USA, 18 Jun 2001 - 22 Jun 2001; 1101-1103 (2001)., The superstructure is a viable option for the TESLA linear collider because of a high effective gradient and a reduced number of rf components. However, the high number and close proximity of passband modes impose challenging demands on the rf control system. The control problem is complicated by the fact that the cavity probe signal does not exactly reflect the actual accelerating voltage experienced by the beam due to the different coupling of the generator, pickup probe and beam to the FM passband modes. The digital control system developed for the standard 9-cell cavity is not adequate for operation of the superstructure. As discussed in this paper, an additional filter is needed to guarantee robustness and stability. Based on rf control simulations the filter characteristic is optimized.

TESLA Reports 2001 7 pp. (2001).

TESLA Reports 2001 5 pp. (2001).

The 2001 Particle Accelerator Conference, PAC2001, Chicago, USA, 18 Jun 2001 - 22 Jun 2001; TESLA Reports 2001 2020-2022 (2001)., The vertical beam profile distortions induced by wakefield effects in linear colliders (the so-called “banana effect”) generate a beam-beam instability at the collision point when the vertical disruption parameter is large. We illustrate this effect in the case of the TESLA linear collider project. We specify the tolerance on the associated emittance growth, which translates into tolerances on injection jitter and, for a given tuning procedure, on structure misalignments. We look for possible cures based on fast orbit correction at the interaction point and using a fast luminosity monitor.

Universität Hamburg, Diss., 2001; 158 pp., (2001). doi:10.3204/DESY-THESIS-2001-052

The 18th International Conference on High Energy Accelerators, HEACC2001, Tsukuba, Japan, 26 Mar 2001 - 30 Mar 2001; 11-15 (2001)., TESLA, a 33 km long 500 GeV centre-of-mass energy superconducting linear collider (upgradeable to 800 GeV) with an integrated X-ray FEL facility of 0.1 nm wave length, has been proposed by an international collaboration to be built at DESY. The backbone of this collider are more than 21000 superconducting 9-cell RF cavities, operated at 1.3 GHz with 5 Hz beam repetition rate, and about 800 superconducting magnet packages with quadrupoles and steering dipoles. The paper gives an overview of the cryogenic system necessary to cool these elements to 2 K. The system mainly consists of about 1800 cryomodules assembled in twelve 2.5 km long cryogenic units, supplied from 7 refrigerators.

TESLA Reports 2001 28 pp. (2001).

The 2001 Particle Accelerator Conference, PAC2001, Chicago, USA, 18 Jun 2001 - 22 Jun 2001; 11-15 (2001)., After a brief introduction to the basic concept of Free Electron Lasers operating in the Self-Amplified Spontaneous Emission (SASE) mode, the paper presents the rapid progress achieved during the past 3 years in successfully proving this FEL process at shorter and shorter wavelengths, meanwhile reaching 80 nm. The technical challenges are discussed to be overcome in building FEL facilities for wavelengths much below the visible, and these challenges are compared with the present state-of-the-art. Finally, approved and proposed SASE FEL projects are briefly summarized.

TESLA Reports 2001 17 pp. (2001)., The Tera-eV Energy Superconducting Linear Accelerator (TESLA), a 32 km long superconducting linear electron/positron collider of 500 GeV (upgradeable to 800 GeV) centre of mass energy, presently in the planning phase at DESY, will consist of about 21000 superconducting RF-9-cell cavities of pure Niobium, cooled in a 2.0 K helium bath. The cavities will be assembled in groups of 12 in about 16-17 m long cryostats (cryomodules). Among other things, the cryomodules will be equipped with thermal shields at 40/80 K and 5-8 K temperature levels respectively.Before their installation in the TESLA tunnel, the accelerator cryomodules have to be qualified after the assembly in random tests. The qualification includes the check of the general mechanical dimensions and the measurement of the cryogenic performance of all systems, in particular, the performance of the cavities.In addition, all about 21000 single cavities have to be tested before the assembly in the cryomodules at a rate of about 24 per day.The paper presents the layout of the test facility and, in particular, the layout of the helium distribution system and the test benches.