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Seminar, In the baseline design of the International Linear Collider (ILC) an undulator-based source is foreseen for the positron source in order to match the physics requirements. The baseline parameters are optimized for the ILC at $\sqrt{s} =$ 500 GeV, that means an electron drive beam of 250 GeV. Precision measurements in the Higgs sector, however, require measurements at $\sqrt{s} =$ 250 GeV, i.e. running with the electron drive beam only at 125 GeV, which imposes a challenge for achieving a high yield. Therefore the baseline undulator parameters have to be optimized as much as possible within their technical performances. In this bachelor thesis we therefore present a theoretical study on the radiation spectra of a helical undulator, based on the equation for the radiated synchrotron energy spectral density per solid angle per electron in the relativistic, far-field and point-like charge approximation. From this starting point the following undulator properties are examined: the deposited power in the undulator vessel, which can disrupt the functionality of the undulator magnets, the protective property of a mask on this disturbances and the number of positrons produced by the synchrotron radiation in a Ti-6Al-4-V target. Those quantities were evaluated for various values for parameters as undulator period, undulator length and magnetic flux in order to find optimal baseline parameter sets for $\sqrt{s} =$ 250 GeV.

22nd International Free Electron Laser Conference and 7th FEL Users Workshop, Durham, United States, 13 Aug 2000 - 18 Aug 2000; Nuclear instruments & methods in physics research / A 475(1-3), 368 - 372 (2001). doi:10.1016/S0168-9002(01)01541-8, This paper presents the conceptual design of a high power radiation source with laser-like characteristics in the ultraviolet spectral range at the TESLA Test Facility (TTF). The concept is based on the generation of radiation in a regenerative FEL amplifier (RAFEL). The RAFEL described in this paper covers a wavelength range of 200–400 nm and provides 200 fs pulses with 2 mJ of optical energy per pulse. The linac operates at 1% duty factor and the average output radiation power exceeds 100 W. The RAFEL will be driven by the spent electron beam leaving the soft X-ray FEL, thus providing minimal interference between these two devices. The RAFEL output radiation has the same time structure as the X-ray FEL and the UV pulses are naturally synchronized with the soft X-ray pulses from the TTF FEL. Therefore, it should be possible to achieve synchronization close to the duration of the radiation pulses (200 fs) for pump-probe techniques using either an UV pulse as a pump and soft X-ray pulse as a probe, or vice versa., Published by North-Holland Publ. Co., Amsterdam

49 pp. (2007)., Current permanent magnet material quality is insufficient to obtain field quali- ties in undulators, which satisfy FEL requirements. Therefore position and orienta- tion of magnets have to be carefully chosen in order to obtain mutual cancellation of field errors. In this paper we compare two different sorting schemes, simulated annealing and a straight forward paring method. They are applied to a 5 m pro- totype structure built for the European XFEL facility. The algorithms of these two methods are described in detail and the sorting results and the expected field qualities are carefully compared.

11 pp. (2005)., We have applied beam-based alignment techniques to correlate the position reference systems of quadrupoles and wirecanners in the undulator section of VUV FEL [1]. Procedure, error analysis and results obtained between October 8 and 11 are presented.

22 pp. (2005).

10th European Particle Accelerator Conference (EPAC 06), Edinburgh, Scotland, 26 Jun 2006 - 30 Jun 2006; TESLA-FEL Reports 13 pp. (2005).

6 pp. (2005).

TESLA-FEL Reports 13 pp. (2004)., The baseline specifications of European XFEL give a range of wavelengths be-tween 0.1 nm and 2 nm. This wavelength range at fixed electron beam energy of17.5 GeV can be covered by operating the SASE FEL with three undulators whichhave different period and tunable gap. A study of the potential for the extendingthe spectral range accessible to the XFEL in the direction of longer wavelengths ispresented. The extension of the wavelength range to 6 nm would cover the waterwindow in the VUV region, opening the facility to a new class of experiments. Thereare at least two possible sources of the VUV radiation associated with the X-rayFEL: the ”low (2.5 GeV) energy electron beam dedicated” and the ”17.5 GeV spentbeam parasitic” (or ”after-burner”) source modes. The second alternative, ”after-burner undulator” is the one we regard as the most favorable. It is possible to placean undulator as long as 80 meters after 2 nm undulator. Ultimately, VUV undulatorwould be able to deliver output power approaching 100 GW level. A beam from thisdevice could be run in pump-probe mode with the X-ray FEL.

558 pp. (2004).

16 pp. (2003)., Cavity-type beam position monitors were developed to measure the beam alignment at the undulator section of DESY’s TESLA Test Facility (TTF). This paper describes some theoretical aspects and the experimental setup. The results of some in-beam measurements at the TTF are presented and pros and cons of the monitor concept are discussed.

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.

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.

13 pp. (2002). doi:10.3204/PUBDB-2017-07167, A new design for a multistage High-Gain Harmonic Generation (HGHG) scheme is proposed. The main difference with previous HGHG schemes is that in our scheme the HGHG technique can be applied more than once in a HGHG chain (single bunch scheme). This is consequence of the fact that the growth of the energy spread due to the HGHG process in our case is much less than initial energy spread, and exponential growth rate in the main undulator is practically the same as without stage sequence. Problems relating to X-ray HGHG FEL are discussed. Our studies have shown that the frequency multiplication process produces a noise degradation proportional to at least the square of the multiplication ratio. This prevents operation of HGHG FEL at a very short wavelength range. The results presented in this paper have demonstrated that the HGHG FEL approach is quite adequate for the VUV coherent source, but not scalable to X-ray devices.

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.

8 pp. (2002)., A collimation scheme for the protection of the undulators at the TESLA Test Facility phase 2 (TTF2) is discussed. The transverse and energy collimation, and the protection against off-energy and mis-steered bunches are proposed in a beam line which, besides the collimation elements, integrates the fast feedback system and matches the beam to the undulator entrance.

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; 2682-2684 (2002)., The new generation of X-FEL synchrotron radiation sources requires long undulator sections. The temperatures have to be controlled over long distances to ensure an uniform temperature distribution. At the TESLA project the undulators are located in tunnels. The tunnels should be built with a cross section as small as possible to reduce the tunnel costs. The limited space prevents the installation of space consuming air ductsbetween the undulator sections and the air conditioning equipment. The requirements for the temperature stability of the other tunnel sections are much lower which motivates a separation of the normal tunnel section from the undulator section. A dividing wall is the regular solution but it has some big disadvantages. A wall will complicate the monorail operation or impede the smoke removal in case of fire. A new air conditioning system will be installed and tested in the FEL tunnel of the TESLA Test Facility phase 2 (TTF2). The calculations of the temperature distribution will be presented.

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; AIP 2706-2708 (2002). doi:10.1063/1.1638357, To perform a proof of principle experiment of a SASE based Free Electron Laser operating at wavelength between 70-160 nanometer a 15 m long permanent magnet undulator has been installed in the TESLA Test Facility (TTF) linac phase 1. The type of magnets used (NdFeB) is known to be sensitive to radiation damages if exposed to high energy electrons. Already beam losses in the order of 10−6 of the nominal TTF beam current (64μA) are critical for the undulator and can cause an irreversible damage of its magnets after a few months of operation. To protect the undulator against radiation a two stage spoiler-absorber collimation system has been implemented to the beamline in front of the undulator. In this paper the design and the performance limits of the collimator system are presented., Published by AIP

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; 1966-1968 (2002)., To perform a proof of principle experiment of a SASE based Free Electron Laser operating a permanent magnet undulator has been installed in the TESLA Test Facility (TTF) linac. The type of permanent magnets (NdFeB) used is known to be sensitive to irradiation. Already losses of the order of 10−6 at nominal TTF beam current can cause a degradation of the undulator magnets after a few month of operation. To protect the undulator against radiation a collimation system in front of the undulator removes the electrons with large betatron motion. To detect beam haloor dark current escaping the collimators a beam loss monitor (BLM) system based on photomultiplier has been developed. During the past two years the BLM system has been improved in its electronic components and detectors. It has become a standard tool for linac operation and is nowintegrated part of the linac protect system. In this paper the design, the operation experiences and the performance limits of the system are presented.

5 pp. (2002)., Radiation damage to undulator systems made of permanent magnet materials like NdFeB or SmCo is a critical issue in SASE projects. It is of even more interest in high-duty cycle machines using superconducting accelerators such as the TESLA Test Facility (TTF), which is a prototype for the future X-FEL at TESLA. This paper reports on experience on the undulator system of the VUV-FEL of TTF-1, which ended on May 6, 2002 after nearly 3 years of operation. The radiation exposure of the undulator system was recorded continuously and meticulously over the whole installation period in the TTF linac. Results of these dosage measurements as well as magnetic measurements after de-installation are reported. A comparison with magnetic field data taken prior to installation is given.

5 pp. (2002)., This paper presents the progress of the Self-Amplified Spontaneous Emission and Regenerative Amplifier Free Electron Laser experiments at the TESLA Test Facility (TTF). In the second part, the layout of the soft X-ray user facility TTF2 and the status of construction work is described.

11 pp. (2002)., A large X-ray FEL lab is under consideration within the TESLA project and is supposed to be operated in parallel with the TESLA linear collider. There will be five SASE FELs and five conventional spontaneous undulators. A conceptual design study has been made for the undulator systems for these X-FELs. It includes segmentation into 6.1 m long undulator ‘cells’. Each consists of a 5 m long undulator ‘segment’, a separate quadrupole, one horizontal and one vertical corrector, and a phase shifter. These items are presented and discussed.

9 pp. (2002)., An X-ray diagnostic station will be installed for each of the XFEL undulator beamlines at TESLA. Primary purpose of the X-ray diagnostics is to provide an additional tool for alignment and commissioning of the numerous undulator cells along an XFEL beamline independently from electron-beam-based alignment procedures. Both methods will complement one another. The X-ray diagnostic station will be a sensitive instrument generating essential input for the undulator control system. The diagnostic station will be located about 120 m downstream from the last undulator cell. Total flux measurements will verify the XFEL's gain. Analysis of the spectral and spatial distribution of the spontaneous radiation of individual or several consecutive undulator segments will be used to optimize angle and position of the electron beam trajectory, to verify the magnetic gap, and to adjust the phase match between two undulator segments. The two latter purposes cannot be served by electron-beam-based alignment.

2 pp. (2001).

9 pp. (2001)., A single-pass free-electron laser operating in the self-amplified spontaneous-emission (SASE) mode at around 100 nm is currently under test at the TESLA Test Facility at DESY. After first observation of SASE in February 2000, the photon beam has been characterized by different techniques. We present the methods of VUV photon diagnostics that were used to measure the spectral and angular distribution of the photon beam and how these properties are affected by the electron beam energy and orbit in the undulator.

12 pp. (2001)., The laser alignment system ALMY was tested at the 15 m long undulator section of the TESLA test facility. The positions of the undulator modules relative to each other have been determined with a precision of 0.1 mm , limited by the accuracy of the mechanical support of the sensors. Additionally, ALMY allows one to measure movements or drifts over several days and we found that the undulator components are stable within 10 μm . The resolution of the sensors is better than 2 μm over a distance of 15 m .

20 pp. (2001).

7 pp. (2001)., The TESLA Test Facility (TTF) at DESY is a facility producing sub-picosecond electron pulses for the generation of VUV or soft X-ray radiation in a free electron laser (FEL). The same electron pulses would also allow the direct production of high-power coherent radiation by passing the electron beam through an undulator. Intense, coherent far-infrared (FIR) undulator radiation can be produced from electron bunches at wavelengths longer than or equal to the bunch length. The source described in this paper provides, in the wavelength range 50– 300 μm , a train of about 1– 10 ps long radiation pulses, with about 1 mJ of optical energy per pulse radiated into the central cone. The average output power can exceed 50 W . In this conceptual design, we intend to use a conventional electromagnetic undulator with a 60 cm period length and a maximum field of 1.5 T . The FIR source will use the spent electron beam coming from the VUV FEL which allows one to significantly extend the scientific potential of the TTF without interfering with the main option of the TTF FEL operation. The pulses of the coherent FIR radiation are naturally synchronized with the VUV pulses from the main TTF FEL, enabling pump-probe techniques using either the FEL pulse as a pump or the FIR pulse as a probe, or vice versa.

9 pp. (2000)., We report the development of different algorithms for the calculation of quality factors of eigenmodes in accelerating cavities, which have resonance frequencies above the cutoff frequency of the beam tubes. The analysis is based on a discretization of such cavity structures by the Finite Integration Technique (FIT), and the radiation at the open boundaries is systematically taken into account by different approaches in time and frequency domain. Results indicate that even single cell cavities of the TESLA type show Q-values of 103 and multicell cavities values in excess of 104. Thus these modes may cause considerable beam instabilities. Comparison with the conventional method of analyzing closed cavities and identifying modes with little change in frequency as function of bound- ary condition show qualitative differences. Some modes from the closed cavity model do not exist in the open structure and thus would be misinterpreted as trapped modes when only a closed cavity analysis is employed.

3 pp. (2000)., To ensure overlap between the photon beam and electron beam at the SASE-FEL at the TESLA Test Facility [1], several position-sensitive diagnostics components are installed along the beamline of the FEL. For the undulator part, a new type of waveguide beam position monitors (BPMs) is designed, tested, and installed inside the beam pipe of one undulator module. This paper proposes a method to calibrate these monitors with beam-based measurements

TESLA-FEL Reports 43 pp. (2000)., The various technologies available to build a SASE-type undulator for a 1Å wavelength are compared. This includes permanent magnets, superconducting and room-temperature electromagnets for both the planar and helical-field configurations. Following a 3D computation of the magnetic field, general expressions are given for the peak field as a function of the undulator period. The growth length and saturated length of the SASE radiation is computed by means of the simple analytical expressions of the 1D theory and of a 3D numerical simulation for each type of undulator technology and electron beam quality. It is found that the saturated length is almost independent of the electron energy in the 15–40 GeV range of the TESLA linear accelerator project. It is proportional to the undulator period. A reduction of the saturated length is predicted with additional FODO type focusing with an optimum beta function around 20 m. This large beta function allows the placement of the FODO quadrupole outside the undulator in a drift space located between two undulator segments. The tunability of the radiation through the change of undulator field is possible but it requires a longer undulator. The tunability imposes the use of magnetic phasing sections also located between the undulator segments. A method is presented for an absolute monitoring and alignment of the angle of the electron beam in each undulator section. It is based on the observation of the spontaneous emission separately from each undulator section. It has a precision of 1.4 μrad limited by the electron beam divergence.

14 pp. (2000)., XFEL undulators for the TESLA project require high quality in order to drive the SASE process into saturation. On the other hand, as large quantities are needed for the five SASE undulators, it is rather important not to oversize the undulator design. The magnet structure is the most delicate and most expensive part of an insertion device. This report proposes a magnet design for a SASE undulator with tunable wavelength in the 1 Å regime. The layout of the magnet structure is optimized for a 60 mm undulator period which requires a magnet volume of 400 $cm^3$ per period to gain a peak field of 1.33 T. An end pole configuration of the gap variable device is suggested so that both field integrals are trimmed close to zero for all gaps.

TESLA-FEL Reports 14 pp. (2000).

TESLA-FEL Reports 11 pp. (2000)., For SASE-FELs the total undulator length increases with decreasing wavelength. In the X-ray wavelength range,the optimum-function is of the order of 40 m, which makes a separate focusing structure the best choice. In prin-ciple, three periodic focusing structures can be considered, namely a singlet (FODO), doublet or a triplet structure.In this report, the three different types of focusing for the TESLA X-ray FEL parameters will be discussed.

6 pp. (2000)., The longitudinal radiative force of an electron bunch moving in an undulator has been investigated in [1] assuming an 1D density distribution. To obtain the contribution of pure curvature effects and to avoid singular fields, the 1D linear motion field was subtracted. To relate these results to the 3D case we present analytical and numerical field calculations using the field solver of TRAFIC4. The 1D and 3D cases have been calculated in the transient regime, the steady-state regime and the steady-state regime averaged over one undulator period for the 1 GeV parameter set of the TESLA FEL.

TESLA-FEL Reports 16 pp. (2000)., A study is presented to estimate the production effort of the magnetic structures for the undulatorsystems of the TESLA X-ray SASE FELs. The total magnetic length of the magnet structures is1405m. It is proposed to produce these huge quantity ‘turn key ready’ on an industrial scale. Thisimplies a resolute R&D effort. It should include the improvement and optimization of the materialquality of NdFeB material, the development of robustmagnetic measurement and fine tuningtechniques, which can be used in an industrial environment and an extended prototyping phase. Thesedevelopments are prerequisite for successful industrial production and are described in more detail. Anestimate is made for production capacities which are required to produce 1405m of magnetic structurewithin five years.

TESLA Reports 2000 10 pp. (2000).

6 pp. (2000)., A self-amplified spontaneous emission (SASE) free-electron laser (FEL) is under construction at the Advanced Photon Source (APS). Five FEL simulation codes were used in the design phase: GENESIS, GINGER, MEDUSA, RON, and TDA3D. Initial comparisons between each of these independent formulations show good agreement for the parameters of the APS SASE FEL.

16 pp. (2000).

TESLA-FEL Reports 11 pp. (2000)., In SASE FELs, the radiation power has to saturate in a single pass through the undulator. In the VUV and X-rayregime, with undulators ranging in length from several tens to hunderds of meters, even a few percent increase ingain length could mean a lengthening of the undulator by several meters. Therefore, determining what increasesthe gain length is of great importance.The influence of the electron beam quality has been studied in detail in many papers. For the TESLA X-rayFEL, with undulator lengths of up to 300 m, the gain degradation due to undulator imperfections has to be studied.Previous studies, for example for the TTF-FEL, have focused on the influence of dipole and quadrupole errors. Inthis report, similar results of simulations for the TESLA X-ray FEL undulators will be presented. In addition, othersources of errors, mainly related to the multi-segmented character of the undulators, will be discussed and upperlimits will be given.

6 pp. (2000)., Three vacuum chambers for the VUV SASE FEL undulator sections at the TESLA Test Facility (TTF) were designed, built, tested and installed. Each chamber is 4.5 m long and of 11.5 mm thick. The inner diameter of the beam pipe is 9.5 mm. The rectangular chamber profile with a width of 128 mm is used to integrate beam position monitors and steerers. This is needed to provide a good overlap between the electron and the photon beam over the entire undulator length. The chambers are built in an aluminum extrusion technology developed for the insertion device vacuum chambers of the Advanced Photon Source. After manufacturing, special processing was performed to reach low outgassing rates (

5 pp. (1999)., As the constraint of a small transverse emittance becomes more severe, the higher the electron beam energy in an FEL. To compensate for the transverse and thus the longitudinal velocity spread, a compensation scheme has been proposed previously by Derbenev and Sessler et al., for Free Electron Lasers by introducing a correlation between the energy and the average betatron amplitude of each electron. This compensation scheme is based on a constant absolute value of the transverse velocity, a feature of the natural focusing of undulators, and does not include strong focusing of a superimposed quadrupole lattice. This paper focuses on the electron motion in a strong focusing lattice with a variation in the axial velocity. The resulting reduction of the compensation efficiency is analyzed using simulations. It is seen that the compensation scheme is not much affected if the lattice cell length is shorter than the gain length. For the results presented in this paper, the parameters of the proposed TESLA X-ray FEL have been used.

5 pp. (1999)., In order to get fully coherent radiation from the Free Electron Laser (FEL) amplifier starting from the shot noise, it is foreseen to implement a seeding option into the VUV FEL being under construction at DESY (DESY print TESLA-FEL 95-03, Hamburg, DESY, 1995, Seeding option for the VUV free electron laser at DESY: joint DESY and GKSS proposal; Available at DESY upon request only). It consists of an additional undulator, a bypass for electrons and an X-ray monochromator. This paper presents the results of optimization of the seeding option for the VUV FEL providing maximal spectral brightness at minimal shot-to-shot intensity fluctuations. Calculations are performed with three-dimensional, time-dependent simulation code FAST (Nucl. Instr. and Meth. A 429 (1999) 233).

4 pp. (1999)., For single-pass free-electron lasers (FEL), such as amplifiers and SASE devices, saturation of the radiation power has to be reached within the length of the undulator. Therefore, detailed knowledge of electron beam parameters is crucial. So far, simulations have been performed with a given rms emittance and energy spread. At short radiation wavelengths, bunch compressors are used to compress the electron beam to achieve the desired high peak currents. In addition, external focusing along the entire undulator is used to maintain a constant small radius. The rotation of phase space due to compression might lead to a significant part of the bunch in tails that could increase the gain length. Furthermore, it is in general not possible to match both the beam core and the tail to the focusing structure. In this contribution, the influence of these tails, both transverse and in energy, on the FEL performance will be investigated. Simulations will be performed for beam parameters that have been assumed for the TESLA Test Facility FEL at DESY.

TESLA-FEL Reports 8 pp. (1999)., In order to get the field quality required for the undulator of the TESLA Test Facility Free Electron Laser (TTF-FEL), two steps have been foreseen. The first step is to sort the magnets in such a way that the resulting field quality is as good as possible with the magnets at hand. The second step is to use techniques such as pole height adjustment and/or shimming to tune both x and y field components to the required specifications. The main purpose of the magnet sorting is to minimize corrections in the second step. Therefore, two transverse components of the field, ideally zero, are minimized by this procedure. In this paper we outline the magnet sorting procedure using simulated annealing.For the three undulator segments for phase I of the TTF-FEL geometrical constrains were properly taken into account. The results obtained for the magnets being used are presented.

2 pp. (1999).

8 pp. (1999)., The field of the undulator for the VUV–FEL at the TESLA Test Facility has to meet very tough tolerances in order to guarantee a close overlap between the electron beam and the laser field. Consequently the undulator was designed to have height-adjustable poles in order to allow for fine tuning of the vertical undulator field in such a way that the trajectory is straightened. The signature of local pole height and gap changes on the field distribution was investigated. It was seen that changes are not restricted to the pole itself. Its effect can be seen up to the next eight neighboring poles. In this contribution we describe an algorithm in detail, which allows the prediction of required pole height changes in order to correct for field errors. As input data field errors deduced from precise magnetic field measurements are used together with the signatures of pole movements. A band diagonal system of linear equations has to be solved to obtain the pole height corrections. For demonstration of the method the field of the 0.9 m long prototype structure was optimized to have a straight trajectory. Since only a sparse band diagonal system of equations has to be solved, the method has the potential to be used in very long undulators having 600 –1000 poles.

5 pp. (1999)., The electron beam trajectory inside an undulator with integrated strong focusing quadrupoles is disturbed by any kind of magnetic or alignment errors of the guiding field. The electron and photon beam must overlap over the entire undulator length to achieve an optimum output of the TTF FEL (A VUV Free Electron Laser at the TESLA Test Facility: Conceptual Design Rep., DESY Print TESLA-FEL 95-03, Hamburg, 1995). Therefore, it is necessary to measure and correct the electron beam trajectory. The orbit correction in the undulator is based on two principles of orbit measurement. The absolute position of the electron beam inside the undulator can be measured at 4 points of support with calibrated monitors. The second method of measuring the beam trajectory is a beam based alignment algorithm which uses relative orbit changes at 30 distributed beam position monitors along the undulator. A mismatching of the optic at the entrance of the undulator can be seen by measuring the beam size at different locations along the undulator.

7 pp. (1999)., In this contribution, the requirements on undulators for linac driven SASE FELs will be discussed. Differences between long and short wavelength SASE FELs will be worked out. The problematic influencing the choice of minimum gap and undulator peak field which are special for SASE FELs driven by multi GeV electron beams with sub picosecond pulses, peak currents of several kiloamperes will be pointed out. Special attention is given to the magnetic design of combined strong focusing undulators as are needed for VUV FELs. As an example results will be presented of the undulator for the VUV-FEL at the TESLA Test Facilty which has just been completed.

4 pp. (1999)., A highly desirable measurement in Free Electron Laser (FEL) experiments is the dependency of the radiation power along the undulator. Most designs of undulators prohibit detection of the radiation power within the undulator or extracting the electron beam at arbitrary positions. Transport of both, the radiation field and the electron beam, through the entire undulator, and thus an ongoing FEL interaction, is unavoidable. If there are many correction magnets distributed along the beam pipe, one can think of exciting a large orbit distortion downstream from any of these correctors. For a gain length comparable to or larger than the beta function this excitation of a coherent betatron oscillation might degrade the FEL amplification to a level, for which the radiation power does not further grow over the remaining length of the undulator. This paper presents the efficiency of this method for the parameters of the VUV FEL at the TESLA Test Facility.

TESLA-FEL Reports 24 pp. (1998).