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 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 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.

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.

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.

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

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

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.

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.

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.

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.

The 18th International Conference on High Energy Accelerators, HEACC2001, Tsukuba, Japan, 26 Mar 2001 - 30 Mar 2001; TESLA Reports 2001 35-37 (2001)., The construction of the 33 km long 500 GeV centre-of-mass energy (upgradeable to 800 GeV) superconductinglinear collider TESLA at DESY with an integrated X-ray FEL facility of 0.1 nm wavelength has been proposed by an international collaboration. The collider will consist of more than 21000 superconducting 9-cell 1.3 GHz RF cavities assembled in about 1800 cryomodules containing also a superconducting magnet package with a quadrupole and dipole steering coils. The paper describes the cryogenic distribution system necessary for cooling to 2 K, mainly consisting of twelve 2.5 km long cooling chains (cryogenic units) of cryomodules, and gives information on the heat load budget, on operating modes as well as on the energy upgrade philosophy.

The 2001 Particle Accelerator Conference, PAC2001, Chicago, USA, 18 Jun 2001 - 22 Jun 2001; 1074-1076 (2001)., Superconducting cavities are highly susceptible to small changes in resonance frequency due to their narrow bandwidth. At the proposed linac for the TESLA Linear Collider [1] the frequency changes resulting from mechanical deformations caused by Lorentz force detuning of the pulsed cavities will be of the order of the cavity bandwidth at the design operating gradient close to 25 MV/ m. The additional power required for field control is of the order of 10 % and will be intolerably high for the planned upgrade to 35 MV/m which appears to be feasible in the near future. While passive stiffening of the cavities is already applied to the present cavity design, the further reduction of the Lorentz force detuning constant is technically challenging. Therefore we propose an active scheme which reduces the timevarying Lorentz force detuning to much less than one cavity bandwidth. If successful, the scheme will improve the power efficiency of the TESLA linac significantly.

TESLA Reports 2001 8 pp. (2001).

Universität Hamburg, Diss., 2001; 168 pp. (2001). = Universität Hamburg, Diss., 2001

The 7th European Particle Accelerator Conference EPAC 2000, EPAC 2000, Vienna, Austria, 26 Jun 2000 - 30 Jun 2000; Hamburg : DESY 1024-1026 (2000)., The proposed TESLA linear collider will use long bunch trains. Damping of these bunch trains at once requires a damping ring with a large circumference of 17 km and a moderate energy of 3-5 GeV. This unusual large ratio of circumference to energy leads to a considerable incoherent space charge tune shift, which in turn leads to an increase of the vertical emittance. Possible solutions to reduce the space charge force will be discussed. Their benefits and problems are investigated through numerical simulations., Published by DESY, Hamburg

The 7th European Particle Accelerator Conference EPAC 2000, EPAC 2000, Vienna, Austria, 26 Jun 2000 - 30 Jun 2000; Hamburg : DESY 327-329 (2000)., Since several years the development of seamless niobium cavity fabrication by hydro forming is being pursued at DESY. This technique offers the possibility of lower cost of fabrication and perhaps better rfperformance of the cavities because of the elimination of electron-beam welds, which in the standard fabrication technique have sometimes lead to inferior cavity performance due to defects. Several single cell 1300 MHz cavities have been formed from high purity seamless niobium tubes, which are under computer control expanded with internal pressure while simultaneously being swaged axially. The seamless tubes have been made by either back extrusion and flow forming or by spinning or deep drawing. Standard surface treatment techniques such as high temperature post purification, buffered chemical polishing(BCP), electropolishing (EP) and high pressure ultra pure water rinsing (HPR) have been applied to these cavities. The cavities exhibited high Q - values of 2 x 10$^10$ at 2K and residual resistances as low as 3 n$\Omega$ after the removal of a surface layer of app. 100 $\mu$m by BCP. Surprisingly, even at high gradients up to the maximum measured values of E$_{acc}$ $\approx$ 33 MV/m the Q-value did not decrease in the absence of field emission as often observed. After electropolishing of additional 100 $\mu$m one of the cavities reached an accelerating gradient of E$_{acc}$ $\geq$ 42 MV/m., Published by DESY, Hamburg

21 pp. (2000)., Experimental data on transverse emittance and beam density measurements for a variety of injector settings (photocathode drive-laser phase w.r.t. to peak electric field in the RF-gun cavity, magnetic fields settings of solenoids) are compared along with numerical simulations performed using the particle tracking code ASTRA [1].

The 7th European Particle Accelerator Conference EPAC 2000, EPAC 2000, Vienna, Austria, 26 Jun 2000 - 30 Jun 2000; Hamburg : DESY 2028-2030 (2000)., Operation with very short bunches for the TESLA X-ray FEL generates wakefields in a frequency regime above the threshold for Cooper pair break-up (750 GHz) in superconducting Niobium. The impact of these wakefields on the cavity quality factor is investigated and an estimate of the additional heat load at the 2 K level in the TESLA linac during FEL-opration is given., Published by DESY, Hamburg

The 7th European Particle Accelerator Conference EPAC 2000, EPAC 2000, Vienna, Austria, 26 Jun 2000 - 30 Jun 2000; Hamburg : DESY 2199-2201 (2000)., For the TESLA Test Facility at DESY modulators, which generate high voltage, pulses of 130 kV with an electrical power of up to 16.8 MW are used. The pulses have a repetition rate of up to 10 Hz and a width of 1.7 ms. Since it is not possible to take the pulsed energy directly from the mains the required energy is stored in capacitor banks to be released during the pulse. Power supplies are needed to recharge the capacitor banks of the modulators without disturbing the mains. In particular the low repetition rate of the pulses has to be suppressed. Tobe independent from the repetition rate the power supplies have to operate in a constant power mode. The DC charging current into the capacitors decreases during the loading with the same factor with which the voltage increases. In this way the current taken from the threephase mains is constant. Different types of power supplies have been investigated at DESY and are described here. These types are antiductors with secondary diode bridges, switched mode power supplies, diode bridges plus series switched mode supplies, diode bridges with SCR bridges in sequential phase control., Published by DESY, Hamburg

The 7th European Particle Accelerator Conference EPAC 2000, EPAC 2000, Vienna, Austria, 26 Jun 2000 - 30 Jun 2000; Hamburg : DESY 1107-1109 (2000)., Higher order modes (HOM) can have a destructive influence on the beam emittance in the future linear colliders. Therefore it is important that the quality factor of the high impedance modes be damped under a safe level. Although attentive care was taken in designing the superconducting 9-cell cavities and the HOM couplers for the TESLA collider, a high Q and high impedance mode was discovered in some cavities at the TESLA Test Facility (TTF). In order to discover the reason for which this particular mode is very little damped, a series of investigations have been carried out. The mode frequency is higher for the studied cavities comparatively to others, where this mode seems to be properly damped. Field distribution measurements at the separate cavities show a quasi-trapping of the mode. The simulations andmeasurements done at individual cavities show that for certain boundary conditions the field is minimum at the location of both HOM couplers in a tube., Published by DESY, Hamburg

The 7th European Particle Accelerator Conference EPAC 2000, EPAC 2000, Vienna, Austria, 26 Jun 2000 - 30 Jun 2000; Hamburg : DESY 2016-2018 (2000)., A chain of four non-resonantly coupled 7-cell cavities is thought to replace the 9-cell cavities considered in the standard design of the accelerating structures for the TESLA main linac. The main advantages of these so called superstructures are the reduced number of power couplers and the increased fill factor in the linac. This paper deals with the damping of the higher order modes (HOM) to a level where they cause no harm on the beam. Calculations have been performed in order to set a safety limit to the quality factors. The measurements done on a copper model of the superstructure show that the required damping level can be achieved., Published by DESY, Hamburg

The 7th European Particle Accelerator Conference EPAC 2000, EPAC 2000, Vienna, Austria, 26 Jun 2000 - 30 Jun 2000; Hamburg : DESY 462-464 (2000)., The TESLA linear collider with integrated X-ray FEL is based on superconducting accelerating cavities. Behind the positron production target normal conducting cavities have to be used in order to cope with high particle losses and with focusing solenoids surrounding the cavities. With the long rf pulse (800$\mu$s flat-top) the achievable gradients are limited by rf power and heat load restrictions. Beam dynamics calculations of a standing wave preaccelerator will be discussed. The proposed preaccelertor includes an electron-positron beam separator section and is followed by a long transfer line and a matching section into a superconducting booster linac in which the positrons are accelerated to the damping ring energy of 5 GeV. The overall layout of the injector linac and the optimization of relevant parameters, as well as beam dynamics calculations will be presented., Published by DESY, Hamburg

Physical review accelerators and beams 3(9), 092001 (2000). doi:10.1103/PhysRevSTAB.3.092001, 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 > 5E+9. 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 = 5E+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 = 5E+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., Published by American Physical Society, College Park, MD

The 7th European Particle Accelerator Conference EPAC 2000, EPAC 2000, Vienna, Austria, 26 Jun 2000 - 30 Jun 2000; Hamburg : DESY 2096-2098 (2000)., A superstructure of cavities has been proposed for the superconducting TESLA Linear Accelerator which will reduce the number of components (Input and HOM couplers) and also improve the average gradient. Thus the input power per input coupler has been drasticallyincreased: 700 kW ($T_{hf}$ =1.4 ms, $f_{rep}$=5 Hz) for the design operation and 1.4 MW for the possible upgrade version. In this paper two types of waveguide couplers were examined, including windows and bellows, taking the geometrical limitations of the cryomodule into account with respect to: maximum fields, cryogenic losses and the kick experienced by the beam., Published by DESY, Hamburg

The 7th European Particle Accelerator Conference EPAC 2000, EPAC 2000, Vienna, Austria, 26 Jun 2000 - 30 Jun 2000; Hamburg : DESY 58-62 (2000)., The effort of the international TESLA Collaboration at the TTF is focussed on the development of superconducting cavities to gradients of 25 MV/m and the operation of a linac with superconducting cavities as an integrated system test. Results on the performance of the nine cell superconducting cavities and on the R&D on single cells to gradients of 40 MV/m will be presented., Published by DESY, Hamburg

TESLA Reports 12 pp. (1999).

2 pp. (1999).

19th International Linear Accelerator Conference, LINAC 98, Chicago, USA, 23 Aug 1998 - 28 Aug 1998; 59 pp. (1998)., The aim of the TESLA Free Electron Laser (FEL) is to develop and realize an Angstrom wavelength, high gain FEL in parallel with the TESLA superconducting e$^+$e$^-$ linear collider. As a first step, an FEL for the VUV wavelength regime is now under construction at DESY, making use of the TESLA Test Facility (TTF). The VUV FEL at the TTF comes in two phases, which are both approved. The paper describes the over-all layout of each phase and the status of components.

19th International Linear Accelerator Conference, LINAC 98, Chicago, USA, 23 Aug 1998 - 28 Aug 1998; 59 pp. (1998)., Superconducting (s.c.) RF structures for the acceleration of electrons have been known for over 30 years (HEPL, Stanford). More than 10 years after their successful start a new generation (i.e. standing wave, 7r-mode cavities), made from niobium and operated at frequencies between 352 MHz and 3 GHz, was established at accelerators being operated for high energy physics (CERN, DESY, KEK) as well as nuclear physics (CEBAF, Darmstadt) experiments. Although the HEPL s.c. cavities were limited to a 10% duty cycle and the 2nd generation now allows continuous wave operation, the increase of the accelerating gradient was remarkably small with respect to the > 50 MV/m limit given by the physics of RF superconductivity. Thirty years ago HEPL cavities reached about 2 MV/m; 15 years later DESY cavities achieved 4 MV/m. And the large 338-cavity CEBAF installation is based on a 5 MV/m gradient, the commissioning of the accelerator being less than 10 years ago. Since 1992 the TESLA (TeV Energy Superconducting Linear Accelerator) collaboration has studied the fundamental problems in cavity fabrication as well as operation. Last year an 8 mA electron beam was successfully accelerated in a 15 MV/m module containing 8 s.c. 9-cell cavities. Two more modules (20 to 25 MV/m) will be installed. From recent cavity tests a gradient of 25 MV/m can be taken as state of the art. Cavity production, preparation, and installation was remarkably improved, a prototype linac (TESLA Test Facility Linac) for a large linear collider shows good performance.

19th International Linear Accelerator Conference, LINAC 98, Chicago, USA, 23 Aug 1998 - 28 Aug 1998; 59 pp. (1998)., The design goal of the 1.3 GHz 9-cell superconducting RF cavities for the Tesla Test Facility (TTF) is a gradient Eacc $\geq$ 15 MV/m at a quality factor of Qo $\geq$ 3$\centerdot$ 10$^9$. The cavities are operated in pulsed mode: 0.8 ms constant gradient with 10 Hz repetition rate.After the vertical acceptance test and prior to the assembly in the linac the cavities are tested in a horizontal cryostat fully equipped with helium vessel, high power input coupler, higher order modes coupler and tuning system. After installing module 1 the first 120 MeV beam has been delivered successfully.We report about the measured results in vertical and horizontal tests, the processing of the superconducting cavity system in the first module and the performance of the cavity system before, during and after operating the linac.

19th International Linear Accelerator Conference, LINAC 98, Chicago, USA, 23 Aug 1998 - 28 Aug 1998; 59 pp. (1998)., For the TESLA FEL operation very short intense bunches of electrons have to be accelerated. These bunches excite a broad spectrum of HOM (Higher Order Modes) up to frequencies of some THz. Two HOM couplers per cavity are foreseen in the present design proposal in order to extract some of the low frequency HOM from the superconducting accelerating structure. In this contribution an additional HOM broadband absorber is suggested which is to be installed between two cryogenic modules at a temperature of 70 K. Its task is to prevent that the really high frequency HOM are absorbed in the accelerator structure at the 2 K level. The absorption characteristics and the short range wake of four structures which make use of SiC as absorbing material are investigated using the MAFIA computer code. The proposed structures are easier to manufacture and better suited for the operation under vacuum conditions than the recently suggested waveguide array absorber.

19th International Linear Accelerator Conference, LINAC 98, Chicago, USA, 23 Aug 1998 - 28 Aug 1998; 59 pp. (1998)., The rf control system for the TESLA Test Facility employs a digital feedback system to provide flexibility in the choice of feedback algorithms and extensive diagnostics for rf system operation and exception handling. The control algorithm makes use of the state space formalism where the state describes the real and imaginary part of the cavity voltage (vector of the accelerating field) and the cavity detuning. The cavity detuning - which is time dependent due to the dynamics of lorentz-force-detuning in a pulsed cavity - can not be measured directly. Knowledge about the time-varying cavity detuning and other rf system parameters such as beam phase are derived by application of system identification to the uncalibrated measured cavity field and incident and reflected wave. In this process the calibrations for incident and reflected wave are determined which includes compensation for the finite directivity of the directional couplers.

19th International Linear Accelerator Conference, LINAC 98, Chicago, USA, 23 Aug 1998 - 28 Aug 1998; 59 pp. (1998)., The status of the layout of the linear collider project, TESLA, which employs superconducting accelerating structures, will be presented. Latest results from the R&D program on 1.3 GHz superconducting cavities, the accelerating gradients and quality factors which were achieved will be shown as well as the performance of the TESLA Test Facility linear accelerator.

19th International Linear Accelerator Conference, LINAC 98, Chicago, USA, 23 Aug 1998 - 28 Aug 1998; 59 pp. (1998)., During the winter shutdown 1997/98, the injection system of the H$^-$-Linac III was upgraded. At present two different kinds of H$^-$-sources are operated at DESY. The new cesium-free rf ion source is planned to operate parallel to the magnetron ion source.In addition a new MEBT (Medium Energy Beam Transport line) between two distinct RFQ accelerators and the Alvarez accelerator of Linac III was installed to match the beam of each ion source to the Alvarez acceptance. This scheme makes it possible to operate either H$^-$-source with the Alvarez linac with a miniumum of effort to switch from one to the other. The design of the transport line also facilitates the development and tests of newly proposed ion sources, for example deuterium and polarized proton sources.After assembly of the RFQs and ion sources as well as the new components for the MEBT, a relatively easy commissioning could be demonstrated, although the ions have to be transported on a long distance from the RFQ to the Alvarez linac. First results of measurements will be presented.

19th International Linear Accelerator Conference, LINAC 98, Chicago, USA, 23 Aug 1998 - 28 Aug 1998; 59 pp. (1998)., In the rf system for the TESLA Test Facility each klystron will supply rf power to up to 32 cavities. The superconducting cavities are operated in pulsed mode and at high accelerating gradients. The control of significant Lorentz force detuning and precise calibration and measurement of the vector sum are the main issues to be solved. Presently installed are 8 cavities in the first cryomodule of the linac which have been in operation since June 97. Initial commissioning has been conducted in a very short time due to the extensive diagnostics available in the digital the rf control system. The paper describes commissioning of the rf system with emphasis on the calibration of the vector-sum, adaptive feedforward, and the various diagnostic tools available. RF control performance and plans for the control of 24 cavities are also presented.