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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

14 pp. (2008)., We discuss non-conventional diagnostics for the beam line between the interaction point and the beam dump of CLIC. The main focus is put on the beamstrahlung monitor, but also other systems such as a coherent pair monitor, tail monitors, and interferometric beam dump thermometer are considered.

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.

5 pp. (2006)., The possibility is presented of using electro-optics techniques to increase laser-wire scanning rates for intra-train scanning at the international linear collider (ILC). The requirements include the preservation of the laser beam mode quality and the ability to work with laser beam powers of order 10 MW. The first ideas of a possible device configuration are presented together with a survey of Electro-Optic (EO) materials and first experimental results using linear prisms.

9 pp. (2006)., TPMON is part of EUROTeV’s Work Package 5 that covers diagnostics. The aim of the task will be to investigate the feasibility of a precision bunch train timing measurement at 30 GHz, aiming for an accuracy of 10 fs. Tests of the electronics will be carried out at CERN’s CTF3. Initial work is concentrating on studying the optimum method of intermediate frequency phase detection.

12 pp. (2005)., To keep the emittance growth in the ILC main linac at a reasonably low level ($∆ε_y$ < 10 nm in 90% of all cases), Dispersion Free Steering (DFS) seems to be insufficient. In this report the use of emittance tuning bumps in addition to DFS is investigated. Simulations show that two dispersion bumps may be enough to reduce the emittance growth from 50 nm, after DFS, to approximately 10 nm, as desired. The effect of an additional wakefield bump is also studied giving very good results: an emittance growth of 1.5 nm or less in 90% of all cases. Furthermore, the robustness of the tuning bumps is investigated, showing that the emittance target is reached even in presence of noise during or after the bump tuning.

9 pp. (2005)., The study of ground motion and its impact on the International Linear Collider (ILC), where $e^+ e^-$ beams of the order of nanometers must collide with high luminosities, may not be negligible. All sources of vibration, including ground motion, can cause the beams to simply miss each other at the interaction region (IR).In this paper, we describe a program of site characterization and comparison, via measurement of ground motion spectra, using inertial broadband seismometers. The emphasis is on using the same equipment and data analysis techniques as applied to all the sites studied. Our database of ground motion measurements for each site is available to the scientific community.

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

73 pp. (2005)., This report summarises the status of the work in the seven scientific Work Packages of EUROTeV as presented during the ILC-European Regional Meeting at Royal Holloway in June 2005. The purpose of the meeting was to monitor the progress and to contrast the developments inside EUROTeV with the worldwide developments of the GDE. The presentations of the entire meeting are available from http://www.pp.rhul.ac.uk/workshop/.

25 pp. (2005)., A spin rotator which preserves the overall optic axis is presented. This rotator uti- lizes three bends, two solenoid pairs, and two reflectors, which are used to remove the cross plane coupling introduced by the solenoids. The design requirements for such a rotator considered as a part of the ILC are summarized. It is shown how the solenoids have to be set up to achieve longitudinal IP polarization taking into account non-zero crossing angles at the interaction region and a Linac following the curvature of the earth. Furthermore, the stability requirements for mechan- ical and electrical imperfections are analyzed. A spin rotator of the investigated type can reliably provide all the spin orientations required if tolerances of 1μm for lateral displacement of the quadrupoles and roll tolerances of 0.25mrad are met. These numbers should not be considered as the final installation tolerances since how these figures are affected by beam-based alignment techniques utilizing BPMs has not been investigated yet.

25 pp. (2005)., In this paper, we perform a detailed study of the power losses along the post- collision extraction line of a TeV $e^+e^-$ collider with a crossing angle of 20 mrad between the beams at the interaction point. Five cases are considered here: four luminosity configurations for ILC and one for CLIC. For all of them, the strong beam-beam effects at the interaction point lead to an emittance growth for the outgoing beams, as well as to the production of beamstrahlung photons and $e^+e^-$ pairs. The power losses along the 20 mrad extraction line, which are due to energy deposition by a fraction of the disrupted beam, of the beamstrahlung photons and of the $e^+e^-$ coherent pairs, were estimated in the case of ideal collisions, as well as with a vertical position or angular offset at the interaction point.

19 pp. (2005).

15 pp. (2005)., Aim of the control system development for TESLA cavity is a more efficient stabilization of the pulsed, accelerating EM field inside resonator. Cavity parameters identification is an essential task for the comprehensive control algorithm. TESLA cavity simulator has been successfully implemented using high-speed FPGA technology. Electromechanical model of the cavity resonator includes Lorentz force detuning and beam loading. The parameters identification is based on the electrical model of the cavity. The model is represented by state space equation for envelope of the cavity voltage driven by current generator and beam loading. For a given model structure, the over-determined matrix equation is created covering long enough measurement range with the solution according to the least-squares method. A low-degree polynomial approximation is applied to estimate the time-varying cavity detuning during the pulse. The measurement channel distortion is considered, leading to the external cavity model seen by the controller. The comprehensive algorithm of the cavity parameters identification was implemented in the Matlab system with different modes of operation. Some experimental results were presented for different cavity operational conditions. The following considerations have lead to the synthesis of the efficient algorithm for the cavity control system predicted for the potential FPGA technology implementation.

13 pp. (2005)., I present here some preliminary studies of the application of Dispersion Free Steering (DFS) to a possible 250-500 GeV ILC main linac which follows the Earth’s curvature (assuming a constant radius). For a direct comparison, results for the same linac with a laser-straight geometry are also presented.

Universität Hamburg, Diplomarbeit, 2005; 56 pp., (2005). doi:10.3204/DESY-THESIS-2005-002, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

22 pp. (2005).

6 pp. (2005).

Universität Dortmund, Diplomarbeit, 2004; 110 pp., (2004). doi:10.3204/DESY-THESIS-2004-049, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

TESLA Reports 2004 9 pp. (2004)., Operational principles of stripline and pill-box cavity beam position monitors (BPMs) are discussed. Their theoretically best achievable position and time resolutions are computed. Numbers are estimated using design values of the proposed e$^{+}$ e$^{−}$ linear collider TESLA.

6 pp. (2004)., Requirements are formulated for a sampling hadron calorimeter with gaseous active medium and digital read-out as detector component at a future linear e+e−-collider. Monogap glass resistive plate chamber (RPC) prototypes equipped with 1 cm2 readout pads and operated in saturated avalanche and streamer modes are studied as a possible detector for digital hadron calorimetry. Operating characteristics of the prototypes such as induced charges, efficiencies and fired pad multiplicities are measured. Preferable RPC specifications are outlined.

13 pp. (2004)., In this paper, we study a possible microbunching instability in the TESLA Test Facility linac. A longitudinal space charge is found to be the main effect driving the instability. Analytical estimates show that initial perturbations of beam current in the range 0.5– 1 mm are amplified by a factor of a few hundred when the beam passes two bunch compressors. A method to suppress the instability is discussed.

Universität Hamburg, Diplomarbeit, 2004; 80 pp., (2004). doi:10.3204/DESY-THESIS-2004-014, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

22 pp. (2004)., The operation of a Free Electron Laser at the TESLA Test Facility requires the electron trajectory to be aligned with precision of few micrometer in overlap with the photon beam. To achieve this goal precise position monitoring of the electron beam in the narrow undulator gap is mandatory. A beam position monitor based on a new microwave concept was designed, built and tested. It consists of four ridged waveguides which couple by small slots to the magnetic field of the electron beam. The paper reports the concept, the design and tests of prototypes in a test bench in the laboratory and with beam. TTF FEL measurement results are also presented and possible improvements are discussed.

8 pp. (2004)., In the beginning of 2003, the ILC-TRC published its second report on the status of R&D towards the current proposed designs for an $e^+e^−$ linear collider [1]. The nearly 500 page report was the end result of over one year of intense work by over 30 scientists from the various linear collider related laboratories around the world. Charged with making a critical review of the machine designs, while identifying outstanding R&D items, the end result in itself represents a unique resource of information for anyone interested in the subject (both expert and novice alike). In the following report, I will summarise the findings of the committee, with perhaps a slightly personal perspective. Those readers who are interested in the subject are encouraged to read at least the executive summary (on which this report is loosely based), if not the entire report.

TESLA Reports 2003 6 pp. (2003).

2003 Particle Accelerator Conference, PAC2003, Portland, USA, 12 May 2003 - 16 May 2003; Hamburg : DESY 34-36 (2003)., The Photo Injector Test facility at DESY Zeuthen (PITZ) was built in order to study the production of high brightness electron beams, which are substantial for the successful operation of Free Electron Lasers (FEL) and linear colliders. The photoinjector at Zeuthen is based on a 1.5-cell L-band rf cavity with coaxial rf coupler equipped with emittance compensating solenoids, a laser capable to generate long pulse trains, an UHV photo cathode exchange system, and various diagnostics tools. The current goal of PITZ is a full characterization of the electron source, which will be installed at the TESLA Test Facility Free Electron Laser (TTF2-FEL) in autumn 2003. In the running periods before the gun is delivered toTTF2-FEL, the rf performance and the beam parameters will be measured in detail. The results presented in this contribution contain the measurements of dark current, driving laser parameters, beam charge, beam size along the facility, transverse emittance, momentum and momentum spread. The electron beam measurements will be presented in comparison with beam dynamics simulations., Published by 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.

6 pp. (2003)., Double slits with different separations, crossed slits and circular apertures have been used to study the transverse coherence of the VUV light of the SASE Free Electron Laser at the TESLA Test facility at DESY. The resulting diffraction patterns are converted to visible light by a Ce:YAG crystal and imaged by a high-resolution CCD camera. The visibility of the diffraction patterns indicates a high degree of transverse coherence. Measurements have been taken at various operating modes and wavelengths of the FEL. A numeric FEL simulation code has been used to calculate the wavefronts of the light at the exit of the undulator. By propagating the wavefronts through the optical setup, the diffraction at the double slits is computed with the code GLAD. Good agreement with the measurements is found.

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.

TESLA Reports 2003 14 pp. (2003).

Universität Hamburg, Diss., 2003; 169 pp., (2003). doi:10.3204/DESY-THESIS-2003-046, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

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.

TESLA Reports 2003 8 pp. (2003)., The paper illustrates some considerations concerning the design of RF Deflectors for TESLA Damping Ring. Scaling laws are applied to investigate the RF Deflector properties in term of length, filling time and dissipated power as a function of the iris diameter. Two optimized cells operating on the $π$ / 3 and $π$ / 2 modes are finally shown.

17th IEEE Particle Accelerator Conference, PAC 97, Vancouver, USA, 12 May 1997 - 16 May 1997; 65 pp. (1997)., For the SB-Linear-Collider project (SBLC) at DESY a redesign of the damping ring has been performed, mainly in order to optimise the dynamic aperture. The lattice of the proposed damping ring has a DBA structure, similar to the 3rd generation light sources ESRF, APS and SPRING-8. The damping ring is build up with 6 superperiods. The straight sections between these periods are used for the injection,ejection and rf-cavities. In turn, each superperiod consists of 6 DBA cells. The straight sections of these cells are used for the installation of the wigglers, which are needed for the damping. With the installation of overall 150 m of wigglers (B$_0$ = 2 T and $\lambda_0$ = 0.2 m) the equilibrium emittances are $\epsilon_{x,0}$ = 6.5 $\centerdot$ 10$^{−10}$ m $\centerdot$ rad and $\epsilon_{y,0}$ = 1.3 $\centerdot$ 10$^{−11}$ m $\centerdot$ rad (the corresponding normalised emittances are : $\epsilon_{x,n}$ = 3.8 $\centerdot$ 10$^{−6}$ m $\centerdot$ rad and $\epsilon_{y,0}$ = 7:6 $\centerdot$ 10$^{−8}$ m$\centerdot$rad . The damping time results in $\tau_y$ $\leq$ 3.4 ms. The dynamic acceptance including misalignment and magnet errors is in the range of A$_x$ $\leq$ 72 mm $\centerdot$ mrad and A$_y$ $\leq$ 80 mm $\centerdot$ mrad.

The 2001 Particle Accelerator Conference, PAC2001, Chicago, USA, 18 Jun 2001 - 22 Jun 2001; 3096-3098 (2001)., The exact alignment of accelerator components is of crucial importance for the production of low emittance beams. Once a beam-line section is set up, a supplementary correction of misalignments implies the knowledge of its magnitude which is difficult to determine using conventional adjusting instruments. An excellent alternative to measure existing misalignments of accelerator components is to vary machine parameters and compare the behaviour of the beam with results obtained from a simulation. It is obvious that time consuming particle tracking programmes are notappropriate to reach this aim. Regarding computing time, the on-line simulation code V is advantageous compared to other beam dynamics programmes. The theoretical basis of V-Code, the “Ensemble Model”, consists of selfconsistent equations for the ensemble parameters that arederived from the Vlasov equation. The requirement to simulate misalignments such as offsets and tilts led to the development of the ALIGNMENT UTILITY which utilizes the solver of V-Code. The new utility enabled us to investigate the beam-line alignment of the TESLA Test Facility injector.This contribution presents the theoretical background and an illustrating example of the optimization process.

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 2001 Particle Accelerator Conference, PAC2001, Chicago, USA, 18 Jun 2001 - 22 Jun 2001; 2722-2724 (2001)., The TESLA linear collider incorporates an X-ray SASE FEL which demands challenging electron beam parameters (typically transverse emittance of 1.6 mm-mrd and peak current of 5 kA). For a realistic electron beam distribution at the entrance of the undulator, tracking has to be done from the cathode through the whole accelerator. Non-Gaussian beam profiles have to be taken into account as well as nonlinear effects such as space-charge, coherent synchrotron radiation field and wake fields. We have done this with several codes: Astra, for the low energy part (

2 pp. (2002)., Commissioning of the TESLA X-ray FEL undulator cells will start at low electron energy and long photon wavelength (~6nm) corresponding to the bottom end of TTF phase II operation. Electron and photon diagnostic schemes will be used to monitor the beam trajectory through the various undulator segments. Furthermore, photon diagnostics has to control all undulator gaps and prove the phase tuning of adjacent segments. Using higher harmonics of the spontaneous radiation of individual undulator segments, the photon diagnostic station can cover a wide spectral range and will be able to cope with the progression towards lower SASE wavelength (~1Å). The use of variable gap undulators allows to realize the photon diagnostic section in a single device located in the photon beamline downstream of the last undulator cell.

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; 1190-1192 (2002)., The transported dark current in a high-duty cycle accelerator as the TESLA Test Facility linac (TTF) could significately contribute to radiation damages of components along the beamline. In the past high dark currents emitted from the laser driven rf gun have been observed duringsubstantial time of linac operation. For a better understanding of the dark current transported through and lost in the entire linac, particularly in undulator magnets, numerical simulations are compared with experimental observations. In order to identify possible locations for collimators to remove the dark current, the beam and its halo have been investigated. The operation of an absorber recently installed in the dispersive section of TTF magnetic bunch compressor and its impact on the downstream collimator section is discussed.

Universität Hamburg, Diss., 2002; 58 pp., (2002). doi:10.3204/DESY-THESIS-2002-029, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

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; 2466-2468 (2002)., For TESLA a lot of equipment will be installed inside the accelerator tunnel which is not accessible during the four week run periods. During the following maintenance day time consuming trouble shooting is not possible and equipment will be exchanged instead of being repaired in place. Therefore the equipment has to be installed in electronic racks inside a container frame. This container will have a docking system to allow a quick exchange of the whole assembly via a monorail that will be installed inside the tunnel. In the TESLA Test Facility this principle will be investigated. The magnet power supplies will be installed in the tunnel. For this purpose the container frames with the electronic racks and a special docking system for quick exchange have been designed, and prototypes are being developed. The power supplies are modified to fit in the electronic racks.

2 pp. (2002)., During the past years, the experience gained with the TESLA Test Facility FEL resulted in several changes of its design. In addition, as the FEL is developed further to become a user facility during 2003, more details of future development of the FEL and the complete experimental hall for the users is taking shape. In this contribution, some of the aspects re briefly mentioned.

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; 1912-1914 (2002)., Laser wires will play an important role as the standard monitor for beam size measurements with micrometre resolution for the beam delivery system at any future linear collider. Some R&D work is still necessary to elevate preliminary laser wire designs to a compact, non-invasive and fast-scanning device. In this paper the latest R&D together with recent measurements and simulations are presented. Schemes to measure the beam size in a bunch train and from train-to-train are presented together with an evaluation of scanning techniques meeting these requirements. Results from simulations and measurements with a laser focus system and of a proposed Compton calorimeter are reported. Furthermore plans are outlined for the installation of a fast laser wire experiment at the PETRA accelerator at DESY.

4 pp. (2002)., The Phase-1 of the VUV Free Electron Laser at the TESLA Test Facility finishes in fall 2002. Phase-2, an extension of Phase-1 towards shorter wavelengths is under construction and will be ready for operation in 2003. A radiation wavelength as low as 6 nm will be obtained by raising the electron energy to 1 GeV. There will be only minor changes to the undulator system. Compared to Phase-1, six instead of three undulator segments will be installed. The integrated focusing system will be replaced by an electromagnetic doublet structure. We report about the changes of the undulator, the undulator vacuum system, the separated quadrupoles including a stretched wire alignment systems and the modifications to the beam diagnostic system consisting of pick up monitors and wire scanners.

6 pp. (2002)., We report on experimental evidence that the free-electron laser at the TESLA Test Facility has reached the maximum power gain of 10 7 while operating in the Self-Amplified Spontaneous Emission mode. Following the request of the first scientific users, the TTF FEL has been tuned in the wavelength range between 80 and 120 nm (vacuum ultraviolet, VUV), and saturation has been achieved in this entire wavelength range. At saturation the FEL emits short pulses with GW peak power and a high degree of transverse coherence. Also, the paper reports on the demonstration of controlled pulse length variation between 30 and 100 fs and on detailed intensity fluctuation studies in this operation mode.

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.

TESLA Reports 2002 11 pp. (2002)., Vertical emittance is a critical issue for future linear collider damping rings. Both NLC and TESLA specify vertical emittance of the order of a few picometers, below values currently achieved in any storage ring. Simulations show that algorithms based on correcting the closed orbit and the vertical dispersion can be effective in reducing the vertical emittance to the required levels, in the presence of a limited subset of alignment errors.

4 pp. (2002)., Two-color pump–probe experiments combining optical femtosecond lasers with short wavelength radiation from a free electron laser (FEL) are very attractive for sub-picosecond time-resolved studies. Since the synchronization between the two light sources to an accuracy of 100 fs is not yet solved, it is proposed to derive both radiation pulses from the same electron bunch. In the present work we focus on the special case where pump and probe beams are generated by the same electron bunch in the same insertion device. Specifically we propose to combine GW-level VUV FEL pulses between 150 and 90 nm wavelength and 10 MW -level third-harmonic radiation between 50 and 30 nm . This scheme does not require any special synchronization nor additional FEL hardware components since the nonlinear third-harmonic generation occurs naturally in the planar FEL undulator. Reflection optics are used for beam splitting and tunable delay, and the two harmonics are separated by using notch filters. The effect on the pulse duration is negligible.

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.