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The aim of the present work is to investigate the influence of a tapered undulator on the main parameters of an free electron laser (FEL). The experiments were carried out at the Darmstadt FEL at an electron beam energy of 31 MeV and a wavelength of 7 μm . The effect of tapering depth on various important free electron laser characteristics such as the wavelength corresponding to the peak of the spectral distribution, the spectral width, the small signal gain and the electron energy extraction efficiency has been experimentally observed and quantitatively understood. It is shown for example that the wavelength shift is well described taking into account short pulse effects. Furthermore, for the first time, it has been found that for some values of the tapering depth the FEL efficiency exceeds that of the uniform undulator by more than 35% and 18% for negative and positive tapering, respectively. The experimental data of the spectral distributions for different tapering values are compared with predictions based on simulations for the interaction processes in the tapered undulator and with the experimental results recently obtained at the IR DEMO FEL at the Thomas Jefferson National Accelerator Facility.

A compact autocorrelator suitable for the measurement of the optical pulse length of the infrared free electron laser (IR-FEL) at the S-DALINAC is described. It allows to investigate the energy transfer process from the electrons into the laser light within the undulator of the FEL. The autocorrelator utilizes the second harmonic generation (SHG) and it provides the possibility of a background-free measurement at wavelengths between 4 and 10 μm . Simulations for three different crystals used for SHG show that the largest efficiency for SHG and the best temporal resolution is achieved with a ZnGeP2 crystal at a length of 2 mm . A test experiment at the free electron laser for infrared experiments confirmed the expected high efficiency of about 7.5% of SHG for the ZnGeP2 crystal as well as the easy handling and fast adjustment of the system.

A simple method for an absolute measurement of the cavity length of a Free Electron Laser (FEL) with a non-contact optical technique is presented. The injection current of a semiconductor laser diode is modulated with a proper frequency and amplitude resulting in a high-frequency (several GHz) intensity modulation of the emitted laser light. This effect is used to determine the absolute distance between the two positions of the FEL cavity mirrors. The method is based on the subsequent measurements of the phase difference (corresponding to the modulation frequency) between two laser beams reflected from different positions of the resonator mirrors. In this way, the length of the optical cavity of the FEL at the superconducting Darmstadt electron accelerator (S-DALINAC) has been determined with an absolute accuracy of less than ±100 μm at a total length of 15 m corresponding to a relative accuracy of better than ±10 −5 .

Polarization properties of coherent polarization X-radiation produced by a low-emittance electron beam of 72 MeV interacting with a silicon monocrystal have been investigated experimentally at the superconducting Darmstadt linear accelerator S-DALINAC. Spatially and energetically resolved measurements of the direction and the degree of linear polarization show good agreement with expectations based on the kinematical theory of coherent polarization radiation and rule out a quasi-Cerenkov nature of the radiation.

The Darmstadt cw FEL designed for wavelengths between 3 and 10 μm driven by the superconducting electron accelerator S-DALINAC first lased on December 1st, 1996 and has operated thereafter successfully in the wavelength region between 6.6 and 7.8 μm. The pulsed electron beam employed had a micro pulse length of about 2 ps, with a repetition rate of 10 MHz and a peak current of 2.7 A while its energy was varied between 29.6 and 31.5 MeV. A wedged pole hybrid undulator, with 80 periods each of 0.032 m length and a magnetic field strength of 0.15–0.4 T, was located in between a 15.01 m long optical cavity equipped with two high reflectivity (99.8%) mirrors of 0.05 m diameter. Due to the low beam current special care with respect to the electron and optical beam properties was necessary to meet the stringent conditions in order to reach a minute small signal gain of at least a few percent resulting in amplification. Saturation was obtained after about 2000 repetitions of the photon pulse inside the cavity. The Darmstadt FEL experiment is unique with respect to its variable electron beam macro pulse structure and it is thus the first linac-based FEL providing a cw photon beam. First experiments using the FEL were performed for characterizing laser light parameters as well as for tissue ablation. Macropulse lengths of 4–8 ms at an average outcoupled power of 100 mW corresponding to a peak power density of 100 MW/cm2 at a beam spot of 150 μm were employed to investigate the thermal ablation of bovine cornea.

The linear polarization of Parametric X-radiation (PXR) produced by 80.5 MeV electrons in a 13 μm thick silicon single crystal has been analyzed by means of a novel method of polarimetry exploiting directional information of the photoeffect in a charge coupled device consisting of 1.3×10 6 square pixels of 6.8 μm. The experiment was carried out at the Darmstadt superconducting linear accelerator S-DALINAC providing a low-emittance electron beam. The linear polarization of the (2 2 0) reflection observed in eight narrow angular bins between 20 ∘ and 21 ∘ with respect to the electron beam direction is consistent with complete local linear polarization. The orientation of the polarization plane, within measurement errors of typically 10 ∘ , varies over the diffraction pattern in such a way as to be expected from kinematical theory. The result of this experiment is in contradiction to the only other PXR polarization measurement performed so far.

The performance of a high transmission crystal monochromator to be used as part of a source of linearly polarized X-rays of moderately narrow bandwidth in the energy regime of 10–50 keV has been investigated using channeling radiation produced by 86 MeV electrons from the superconducting linear accelerator S-DALINAC at the Technische Hochschule Darmstadt. Best results on monochromatic photon yields have been obtained with cylindrically bent silicon monocrystals in Laue and with planar mosaic crystals in Bragg geometry, respectively. At energies between 20 and 30 keV monochromatized X-ray intensities of up to 1.5 × 10 10 photons ( s sr μ A ) have been obtained for axial channeling radiation from a 13 μm thick silicon radiator crystal. The measured yields of narrow band X-rays are such that an intense source of highly polarized planar channeling radiation could well serve to calibrate the linear polarization sensitivity of a CCD polarimeter.

The influence of the conversion of pump light of wavelengths below 400 nm into pump light with a central wavelength of 500 nm in a Europium doped quartzglass pump chamber has been investigated. The results are compared to the ones from an undoped quartzglass. The measurements resulted in a reduction of the slope efficiency by a factor of two for the doped quartzglass compared to the undoped one. Furthermore, the pump-pulse lengths around 80 μs lead to a decrease in the slope efficiency compared to pump-pulse lengths above 100 μs .

A new method of X-ray polarimetry based on the photoeffect in a finely segmented MOS charge coupled device (CCD) has been applied to monochromatic synchrotron radiation at HASYLAB at DESY and to planar channeling radiation at the superconducting electron linac S-DALINAC at Darmstadt. For the smallest pixel dimensions available today (6.8×6.8 μm 2 ) an analyzing power of 10% is observed at an energy of 33 keV. In addition to events from photoeffect in the thin depleted front layer of the CCD also diffusion spread events from much more abundant conversions deeper inside the chip were utilized for polarization analysis.

Utilizing electron beams delivered by the new S-DALINAC with energies of 32.8 and 38.4 MeV, the pulse structure, charge per pulse and energy resolution were studied and found to meet or almost reach the requirements needed. Despite the low intensity of this FEL experiment, spectra of spontaneous emission could be recorded for the first and third harmonic. An accumulation of intensity up to twelve times compared to a single photon pulse could clearly be identified as at least twelve round trips within the cavity. A length region with distinct amplification of the spontaneous emission intensity could be established unequivocally.

The Darmstadt superconducting electron accelerator S-DALINAC became fully operative at the end of 1990. Behind the main linac a free electron laser facility has been set up which will make use of an electron beam of energy between 35 and 50 MeV. In the beginning of 1991 the hybrid undulator was installed and measurements of the B- field show that all design specifications are fulfilled. The optical cavity having a length of 15 m and a Rayleigh length of 1.02 m is set up completely and also a photon beam line was built to transport the beam to the experimental site. Furthermore, the high current injection is implemented and the electron gun has already been used for many hours of beamtime. The electron beam has reached the energy required for FEL operation so that infrared radiation of 1.9 ps duration between 2.5 and 6 μm at a repitition rate of 10 MHz and a peak output power of hundreds of kilowatts will be produced. First measurements of the spontaneous spectrum are scheduled for fall 1991.

At the new superconducting 130 MeV electron accelerator, S-DALINAC, a free electron laser in the near-infrared region is presently under construction. It is intended to make use of an electron beam variable in energy between 35 and 50 MeV, a hybrid undulator (K ≥ 1.2; λu = 3.2 cm) constructed in the wedged pole configuration with 80 periods achieving a peak magnetic field on axis in excess of 0.4 T, and a nearly concentric optical cavity consisting of two spherical dielectric mirrors of 75 mm diameter separated by 15 m and mounted inside vacuum chambers. With this setup it will become possible to cover a wavelength region in the near infrared from 2.8–5.7 μm. The FEL will be operating continuously generating laser pulses of 1.9 ps duration at a repetition rate of 10 MHz. According to simulations the output peak power amounts to several hundred kilowatts. The properties of the FEL radiation will be unique regarding the region of wavelength, tunability, pulse time structure and cw mode. First experiments are expected to be carried out in 1991.

We report the experimental and numerical investigations of the influence of short pulse effects occurring in FELs in different operational regimes for electron bunch lengths which are of the order of the slippage distance. Several observables such as the small signal gain, the macropulse power and the spectral distribution of the FEL radiation were determined experimentally within the constraints of the stable focus regime at the infrared FEL at the S-DALINAC and for the limit cycle regime at the Dutch near infrared FEL FELIX. The experimental findings were compared to predictions of numerical simulations based on the 1D time dependent code FEL1D-OSC. The agreement between experiment and simulation is good. Furthermore, the simulations reveal a chaotic behavior of the macropulses for specific values of the slippage as well as period-doubling, two effects that are predicted to show up in the spectral distribution.

The Darmstadt IR FEL is designed to generate wavelengths between 3 and 10 μm and driven by the superconducting electron linear accelerator. The pulsed electron beam has a peak current of 2.7 A leading to a small signal gain of 5%. Currently, investigations of the energy transfer process inside the undulator are performed using the 1D time-dependent simulation code FAST1D-OSC. We present simulation results for the power vs. different desynchronization and tapering parameters as well as a comparison with experimental data from the S-DALINAC IR-FEL. Furthermore, a compact autocorrelation system assuring a background-free measurement of the optical pulse length is described. In a first test experiment at FELIX, the autocorrelator has been tested at wavelengths 5.7 ≤λ≤9.0 μm. The frequency doubling in a 2 mm-long ZnGeP 2 -crystal resulted in a time resolution of 300 fs and a conversion efficiency of 5%.© 2003 Elsevier Science B.V. All rights reserved. PACS: 41.60. Cr; 42.60. Fe; 42.65. Ky; 42.65. Re

We present experimental evidence that the free-electron laser at the TESLA Test Facility has reached the maximum power gain of 107 in the vacuum ultraviolet (VUV) region at wavelengths between 80 and 120 nm. At saturation the FEL emits short pulses with GW peak power and a high degree of transverse coherence. The radiation pulse length can be adjusted between 30 fs and 100 fs. Radiation spectra and fluctuation properties agree with the theory of high gain, single-pass free-electron lasers starting from shot noise.

A near-infrared Free-Electron-Laser is under construction at the newly erected superconducting 130 MeV electron accelerator. The first part of the accelerator went into operation last summer at the Institute of Nuclear Physics of the Technische Hochschule Darmstadt. Utilizing the electron beam tunable between 35 and 50 MeV and a hybrid undulator with K=1 and λ = 3.2 cm it becomes possible to build a cw-laser which will emit in a wavelength region from 5.24 to 2.57 μm. Presently the room temperature injection of the linac is converted into a high current injection, since peak currents of 2.7 A are required for the FEL experiment. A micropulse repetition rate of 10 MHz (300th subharmonic of the accelerator frequency) that matches the length of the optical cavity (15 m), together with the low emittance (en = 2 π mm mrad) and the low energy spread (ΔE ≤ 13 keV) required in the injector allows to transport a charge of about 10 pC per microbunch which is sufficient for the planned FEL experiment. As an undulator a hybrid system consisting of 80 periods of permanent magnet material and tunable poles of high permeable material is chosen. Simulations predicting the performance of the FEL exhibit a peak output power of 300 to 500 kW in the wavelength region considered. The scientific program deals with the development of the FEL, its components and a possible future use of the FEL radiation, which will be unique with respect to the features region of wavelength, tunability, pulse time structure and cw-mode.

Physical review letters 88, 104802 (2002). doi:10.1103/PhysRevLett.88.104802, Published by APS, College Park, Md.