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We report upon the performance of an analog hadron calorimeter prototype, where plastic scintillator tiles are read out with wavelength-shifting fibers coupled to avalanche photodiodes. This prototype configuration has been tested using a positron beam at DESY with energies between 1 and 6 GeV. We present different detector calibration methods, show measurements for noise, linearity, and energy resolution and discuss gain monitoring with an LED system. The results are in good agreement with our simulation studies and previous measurements using silicon photomultiplier readout.

Narrow X-ray lines from helium-like argon emitted from a dedicated ECR source have been used to determine the response function of a Bragg crystal spectrometer equipped with large area spherically bent silicon (111) or quartz (10$\bar{1}$) crystals. The measured spectra are compared with simulated ones created by a ray-tracing code based on the expected theoretical crystal's rocking curve and the geometry of the experimental set-up., Version acceptee (NIM)

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

TESLA Reports 11 pp. (2004)., The proposed linear electron-positron collider TESLA is based on 1.3 GHz superconducting niobium cavities for particle acceleration. For a center-of-mass energy of 500 GeV an accelerating field of 23.4 MV/m is required which is reliably achieved with a niobium surface preparation by chemical etching. An upgrade of the collider to 800 GeV requires an improved cavity preparation technique. In this paper results are presented on single-cell cavities which demonstrate that fields of up to 40 MV/m are accessible by electrolytic polishing of the inner surface of the cavity.

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.

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.

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.

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.

22 pp. (2005).

The paper includes a description of predicted functionalities to be implemented in a universal motherboard (MB) for the next generation of LLRF control system for TESLA. The motherboard bases on a number of quasi-autonomous embedded executive modules. The modules are implemented in a few FPGA chips featured by the MB. The paper presents a practical design of the MB. The initial (basic) solution of the MB has the Cyclone as the chip where the board management is embedded. The board features communication modules – VME and micro, single chip PC with Ethernet. The board provides power supply for the FPGA chips. The board has fast internal communication between particular modules.

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).

Universität Hamburg, Diss., 2005; 134 pp., (2005). doi:10.3204/DESY-THESIS-2005-001, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

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.

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

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

We report on the design, construction and performance of a prototype for a high-granularity tile hadronic calorimeter for a future international linear collider detector. Scintillating tiles are read out via wavelength-shifting fibers that guide the scintillation light to a novel photodetector, the silicon photomultiplier. A prototype has been tested using a positron test beam at DESY. The results are compared with a reference prototype calorimeter equipped with multichannel vacuum photomultipliers. Detector calibration, noise, linearity and stability are discussed, and the energy response in a 1–6 GeV positron beam is compared with simulations. The present results demonstrate that the silicon photomultiplier is well-suited as photodetectors in calorimeters and thus has been selected for the construction of a 1 m 3 calorimeter prototype to operate in hadron beams.

6 pp. (2005).

TESLA Reports 40 pp. (2004)., A development project was carried out at DESY, aimed at reducing secondary electron emission and multipactor effects in TESLA couplers by TiN layers generation on surfaces which were not protected in this way previously. Thin TiN films on ceramic or metallic surfaces were produced by deposition from Ti vapor in low pressure ammonia. Selection of processing parameters and their effect on multipactor suppression in RF field have been studied using a multipactor test resonator at DESY. Appropriate values of deposition rate, substrate temperature, final layer tickness and chemical conversion procedure were selected. A significant reduction of multipactor time during RF tests was reached due to surface coating. Chemical analysis of TiN layers on both ceramic and metallic substrates has been performed using SIMS method. TiN coating of more than 80 waveguide and cylindrical RF windows were done for TTF2, TTF3 and TTF4 versions of TESLA couplers. Surface processing of flat waveguide windows for TTF2 resulted in significant reduction of multipactor effects and improvement of power transmission. Their good performance remained unchanged after 24-hour exposition to air. TiN coating of all vacuum-facing surfaces of a TTF2 coupler led to reduction of the indispensable RF conditioning time at a pulse length of 20 μs from typically 3 days down to 4.5 hours. The above activities are considered a preparatory step for the future large-scale anti-multipactor protective coating of TESLA couplers.

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.

The Tera Electronvolt Superconducting Linear Accelerator TESLA is the only linear electron-positron collider project based on superconductor technology for particle acceleration. In the first stage with 500 GeV center-of-mass energy an accelerating field of 23.4 MV/m is needed in the superconducting niobium cavities which are operated at a temperature of 2 K and a quality factor $Q_{0}$ of $10^{10}$. This performance has been reliably achieved in the cavities of the TESLA Test Facility (TTF) accelerator. The upgrade of TESLA to 800 GeV requires accelerating gradients of 35 MV/m. Using an improved cavity treatment by electrolytic polishing it has been possible to raise the gradient to 35 - 43 MV/m in single cell resonators. Here we report on the successful transfer of the electropolishing technique to multi-cell cavities. Presently four nine-cell cavities have achieved 35 MV/m at $Q_{0}$ ≥ $5$ × $10^{9}$, and a fifth cavity could be excited to 39 MV/m. In two high-power tests it could be verified that EP-cavities preserve their excellent performance after welding into the helium cryostat and assembly of the high-power coupler.One cavity has been operated for 1100 hours at the TESLA-800 gradient of 35 MV/m and 57 hours at 36 MV/m without loss in performance.

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.

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.

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.

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.

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.

Wakefields excited in the small aperture collimators may damage the beam. As pointed out by K. Bane and P. Morton the wakefields can be reduced considerably by tapering the steps and using the “step+taper” collimator geometry. The optimisation of this kind of geometry for TTF 2 collimators with different apertures is carried out in the paper. For the optimal geometries the non-linear near-wall wakefields are calculated. The numerical results are confirmed by comparison with analytical estimations for the fully tapered collimator in inductive regime.

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.

In the paper we illustrate some possible injection/extraction schemes in the Damping Ring of TESLA using RF deflectors. We illustrate different possible solutions using 2 or 3 RF frequencies in 2, 4 or 6 RF deflector groups both considering the bunches modeled as macro-particles without a finite length and considering a finite bunch length. We discuss, finally, the effects of the errors in the Damping Ring parameters such as the phase advance between the deflectors and the RF amplitude and phase.

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

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.

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.

Universität Hamburg, Diss., 2003; 205 pp., (2003). doi:10.3204/DESY-THESIS-2003-043, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg