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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.

Polarization properties of coherent X-radiation produced by 72 MeV electrons interacting with a silicon monocrystal have been investigated experimentally at the superconducting Darmstadt linear accelerator S-DALINAC. Spatially 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 this radiation.

Parametric X radiation (PXR) produced by electrons with an energy of ${E}_{0}=4 \mathrm{MeV}$ interacting with the atoms in the (220) plane of a $20\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ silicon and a $55\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ diamond crystal and observed at an angle of $44\ifmmode^\circ\else\textdegree\fi{}$ by a Si(Li) detector has been investigated with respect to the interference with coherent bremsstrahlung (CB) that originates in the same interaction process between the incoming relativistic electron and the crystal. Since the energy of PXR and CB is identical, contributions of both types of radiation are indistinguishable. The newly derived analytical expressions describe the radiation which consists of a coherent superposition of PXR and CB. For the comparison of the experimental results with the theoretical predictions a Monte Carlo simulation taking into account all effects accompanying the radiation process has been performed. The comparison shows very good agreement between experiment and theory.

Parametric x rays (PXR) produced by bombarding silicon and diamond crystals with electrons of 30 to 87 MeV were detected at $180\ifmmode^\circ\else\textdegree\fi{}$ relative to the direction of the electron beam. It was found that the dependence of the intensity on the orientation of the crystal agrees with the predictions of the kinematical theory of PXR. The absolute intensity is twice as large as predicted. These findings can be explained considering dynamical effects that govern the x-ray crystal interaction. Additionally, x rays caused by self-diffracted transition radiation have been observed.

Channeling radiation from relativistic electrons has been measured in germanium at low electron energies (6.00, 9.00 and 10.18 MeV) using a 20 μm thick germanium crystal at the Darmstadt superconducting linear accelerator S-DALINAC. The results of the theoretical calculations (planar case) were compared with experimental data and are in reasonable agreement with the observed results. The channeling radiation energies scale as γ5/3 (γ is the ratio of the electron total energy and its rest mass) for the deeply bound states (states localized in the vicinity of the atomic plane) and γ2 for states localized between the channeling planes. We have also measured the intensity of the 1-0 transition along the (1 1 0) plane as a function of the tilt angle (angle between electron beam and plane) and found that channeling radiation occurs even when the tilt angle is larger than the critical angle, due to multiple scattering.

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.

Polarization properties of parametric X radiation (PXR) have been investigated theoretically and experimentally at the S-DALINAC. For the first time analytical expressions describing the polarization properties of PXR have been derived. PXR is – under ideal conditions – 100% linearly polarized in every single point of its angular distribution. In reality, however, multiple scattering of the electrons passing through the crystal, the emittance of the electron beam, the crystal mosaicity and the detector acceptance decrease the polarization degree. In the present experiment PXR was produced by E 0 =80.5 MeV electrons interacting with a (2 2 0) plane of a 13 μm silicon crystal and observed at an angle of about 20° by a CCD polarimeter. For the comparison of the experimental results with the theoretical predictions a Monte Carlo simulation taking into account the experimental conditions has been performed. The comparison shows very good agreement.

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 spectral-angular and the linear polarization properties of channeling radiation emitted by 30.8 MeV electrons incident on a 13 μm thick silicon crystal at small angles with respect to various major crystal axes have been investigated. The transition from axial to planar channeling radiation properties is well described by a two-dimensional many-beam calculation based on a periodic string potential. At energies above the strongest planar channeling radiation lines coherent bremsstrahlung linearly polarized perpendicular to the polarization of the corresponding planar channeling radiation is observed and found to be well reproduced by the model calculations. Axial channeling radiation of the 〈110〉 axis shows a small amount of linear polarization due to the `double-well' potential in agreement with the calculations.

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 temperature dependence of planar channeling radiation of 62.8 MeV electrons has been studied for silicon, germanium, and beryllium. The measurements have been performed using an uncollimated low-emittance cw beam from the superconducting electron linac S-DALINAC at the Technische Hochschule Darmstadt. Energies and linewidths of transitions between transverse bound states have been determined in the energy range between 40 and 230 keV for silicon and beryllium at temperatures between 12 and 330 K, and for germanium between 12 and 223 K. From the shift of the transition energies with temperature the mean thermal vibrational amplitudes of the atoms transverse to the channeling planes are determined by comparison with calculations using the many-beam formalism. Within experimental errors no directional dependence of the vibrations is observed. For silicon a Debye temperature of (535.2\ifmmode\pm\else\textpm\fi{}8.5) K at 12 K and (519.0\ifmmode\pm\else\textpm\fi{}10.8) K at 300 K has been derived. For germanium an increase from (232.7\ifmmode\pm\else\textpm\fi{}12.8) K at 12 K to (292.0\ifmmode\pm\else\textpm\fi{}16.4) K at 223 K is observed. Planar channeling radiation spectra from a beryllium crystal taken at 12, 220, and 300 K have been analyzed the same way yielding a Debye temperature of (1060\ifmmode\pm\else\textpm\fi{}50) K.

The intensity of channeling radiation (CR) was compared for diamond and ruby crystals and also with the intensity of parametric X-radiation (PXR) originating from the same diamond. The investigations were carried out with relativistic electrons provided by the injector of the S-DALINAC at energies between 3.0 and 9.0 MeV. Both types of radiation were observed by means of Si(Li) detectors placed at 0° and 44° with respect to the electron beam axis for CR and PXR, respectively. The highest photon flux Φ = 2 × 108 photons/(smm2) was found to result from CR of diamond crystals. Ruby crystals exhibit somewhat narrower X-ray lines but less intensity. A comparison with PXR reveals that — against predictions — its intensity is three orders of magnitude weaker than CR under the same conditions. The possibility to use the CR and PXR as intense tunable photon sources is discussed.

Utilizing 90{degree} Compton scattering the linear polarization of channeling radiation produced at the superconducting accelerator S-DALINAC with 62 MeV electrons in silicon and diamond has been measured in the energy range between 50 and 400 keV. Planar channeling radiation due to transitions involving transversal bound as well as unbound states is completely linearly polarized perpendicular to the channeling plane. Axial channeling radiation does not show linear polarization.

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.

Employing many-beam codes for transversely bound electrons moving through crystal lattices, we calculate channeling radiation (CR) and coherent bremsstrahlung (CB) intensities in the few and few-tens-of MeV region. CB is found to be of an intensity comparable to that of CR, and dominating the spectrum beyond the Lindhard angle in the form of well defined peaks.

Using a Compton polarimeter, the linear polarization of channeling radiation of 63 MeV electrons in silicon has been measured at the superconducting electron linear accelerator S-DALINAC. Planar channeling radiation due to transitions between transversal bound states as well as for free-bound transitions is completely linear polarized. Axial channeling radiation has no component of linear polarization.

The present paper reports on the recent progress that has been made with respect to the investigation of channeling radiation produced by MeV electrons. The advances are based on new accelerator techniques which provide high quality intense electron beams of low emittance. With these prerequisites it has become possible to study the more hidden effects occurring during the electron-crystal interaction and to investigate the possibility that channeling radiation could be a candidate for a monoenergetic intense photon source in the keV region. From the study of the intensity and linewidth of transitions within the channeling potential, the quantities 1/e-occupation length, equi-population length and X-ray coherence length can be deduced, and the various contributions which govern the scattering process can be extracted and evaluated. First observations of the polarization of channeling radiation between discrete states are also presented.

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.

At the new superconducting electron linear accelerator S-DALINAC polarization of channeling radiation from 65 MeV electrons in silicon has been measured for transitions between discrete transversal bound states for the first time. Using a Compton-polarimeter the linear polarization was determined for the (100) and the (110) plane. Within errors the channeling radiation is completely polarized in agreement with theoretical expectation.

The superconducting cw-electron-accelerator S-DALINAC at Darmstadt became operational in January of 1991. It has been used extensively since then to deliver beam to the various experimental installations for nuclear and radiation physics associated with the accelerator. In order to fulfill the requirements of the experiments beam from the nA range up to 40 μA and at energies between 2.5 and 84 MeV could be provided in true cw operation. Energies above 100 MeV could be achieved in accelerator test runs. A special time structure of the electron beam was used for Free Electron Laser (FEL) physics. Spontaneous emission has been observed routinely. The principle of operation of the accelerator is sketched briefly while main emphasis is on specific problems of a superconducting recirculating electron accelerator and their solutions. The operating performance of the superconducting accelerator cavities like the gradients achieved and the amplitude and phase stability of the accelerating field in the presence of microphonic perturbations are discussed in detail including the influence of radiation pressure. For the two recirculating beamlines simple devices were developed to provide variable path lengths for the electron beam in order to optimize the reinjection phase for the successive acceleration in the linac.

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.

Channeling radiation from 72 MeV electrons was measured using the new superconducting electron accelerator at Darmstadt (S-DALINAC). Radiation spectra were obtained for electrons channeled along the main axes and planes in a 13 μm thick silicon crystal. Good agreement between theory, i.e. many-beam calculations, and experiment can be reported for planar channeling radiation. The radiation at the transition from planar channeling in the (110) plane and the (111) plane to channeling along the 〈110〉 axis was also investigated. The spectra show a continuous change from pure planar radiation to the more intense axial channeling radiation, with new lines occurring in the (110) spectra for angles between the electron direction and the 〈110〉 axis smaller than about 10 mrad. The line energies depend on the angle to the axis and range from about 50 to several hundred keV.

Presently an FEL operating in the near infrared (λ = 2.5 5μm) is under construction at the superconducting accelerator which has originally been designed for nuclear physics experiments requiring electron beams of low peak currents. The redesign of the present injection with the purpose of achieving higher peak currents for the operation of the FEL and the resulting requirements for the individual injection components will be described. Furthermore the experimental results obtained with an injection test stand consisting of the electron gun, the chopper-resonator and some electron beam diagnostics are discussed.

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.

At the superconducting electron accelerator at Darmstadt, the intensive production of photons caused by channeling has been investigated since beams of high quality and low emittance have become available. First results were obtained by bombarding thin silicon and diamond crystals of 10 and 50 μ thickness, respectively, with electrons with 3.0 ⩽ E 0 ⩽ 7.7 MeV and beam intensities ranging from a few nA to 30 μA. Axial-channeling radiation was observed in the direction of the 〈111〉 and 〈110〉 axis of Si and diamond, respectively, by means of a Si(Li) detector placed under 0° with respect to the beam axis. The energies of the observed lines from 3 to 30 keV caused by 3p-1s, 2p-1s, 2p-2s, 3d-2p and 4s-3p transitions are understood in terms of the single-string approximation. In the case of diamond, the channeling radiation intensity on top of the bremsstrahlung background exhibits an enhancement of I ch / I br = 5:1. The maximum photon intensities detected so far with the help of thermoluminescence detectors amount to 2 × 10 10 photons/s and hence a nearly monochromatic photon source (with ΔE / E ≅ 0.1) has been produced.

We describe the main design characteristics of a proposed superconducting recyclotron for electrons with a variable energy from 1o to a maximum of 13o MeV, a duty cycle of 1oo% and an energy spread of 2▪10−4 at maximum energy. The 3 GHz superconducting accelerating units, which are the main building blocks for the 4o MeV linac and the 1o MeV injector, are of a design which is expected to be free of electron multipacting. The design energy gradient of these units is 5 MeV/m. First experimental results on single cell accelerating cavities of the proposed spherical shape are given.

The linewidth of parametric X-ray radiation has been determined experimentally applying an absorption technique. Using a copper foil of 27.7 μm thickness and tuning the energy of the PXR peak across the K-absorption edge of copper by tilting a 55 μm thick diamond crystal, the variance of the PXR line at 8.98 keV photon energy was found to be σ = 48 eV. The diamond crystal has been bombarded with electrons of 6.8 MeV kinetic energy delivered by the superconducting linear accelerator S-DALINAC at Darmstadt. The experimental yield of the intensity amounts to N = 0.91 × 10 −5 photons/(electron sr). From the measured variance the spectral density in the peak is deduced to be J = 0.95 × 10 −7 photons/(electron sr eV).

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.

Plasmas offer the possibility of high acceleration gradients. An intriguing suggestion is to use the higher plasma densities possible in solids to get extremely high gradients. Although solid-state plasmas might produce high gradients they would pose daunting problems. Crystal channeling has been suggested as one mechanism to address these challenges. There is no experimental or theoretical guidance on channeling for intense electron beams. A high-density plasma in a crystal lattice could quench the channeling process. An experiment has been carried out at the Fermilab NICADD Photoinjector Laboratory to observe electron channeling radiation at high bunch charges. An electron beam with up to 8 nC per electron bunch was used to investigate the electron-crystal interaction. No evidence was found of quenching of channeling at charge densities two orders of magnitude larger than that in earlier experiments.

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

For possible applications channeling of electrons has been investigated with respect to the maximum achievable channeling-radiaton intensity per crystal thickness and also for high electron-bunch charges. Using diamond crystals of various thickness up to 204 μm and electron energies of 9 and 10 MeV the photon intensity of the 1–0 transition of the (110) plane was found to be (1.12 ± 0.17) 10−1 photons/e/sr at 9 MeV and (1.56 ± 0.25) 10−1 photons/e/sr 10 MeV. The maximum intensity is expected at these electron energies for a thickness between 500 and 800 μm, respectively. In a further experiment that is currently under development at the A0 TESLA Test Facility at Fermilab the upper intensity limit for the production of channeling radiation by relativistic electrons of typically 16 MeV energy interacting with thin (∼ 18 μm) diamond and Si crystals is investigated. It is the aim to explore channeling under extreme conditions and thus to use channeling as a test bench to study the basics of channeling-plasma acceleration. For electron charge densities up to 8 nC/bunch, which is nearly six orders of magnitude larger than employed so far, resulting in a photon flux of 2 · 1010 photons/sr pulse at a photon energy between 15 and 25 keV no evidence has been found of significant quenching of channeling.

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.

Using the continuous wave electron beam of the new Darmstadt superconducting electron accelerator and high intensity axial channeling in a diamond crystal, the possibility for the production of an intense tunable photon source in the x‐ray region is investigated. Due to low emittance of the machine and its high current it has become possible, for the first time, to produce channeling radiation in the energy region of a few keV with an intensity of up to 2×1010 photons/s in an x‐ray energy window of 10%. This is six orders of magnitude more than what has been observed previously.

Applying an absorber technique, the experimental shape and width of a parametric x-radiation line has been determined. The 9 keV radiation was produced by bombarding a diamond crystal of 55 μm thickness with electrons of 6.8 MeV. The variance of the spectral line distribution was found to depend on the tilt angle of the crystal and to have a magnitude of σ=51 eV. Simulations based on a Monte Carlo method exhibit that the observed variance is mainly influenced by multiple scattering of electrons passing through the crystal (≈43 eV) and the finite detector opening (≈18 eV), leaving for the intrinsic linewidth a value of the order of 1 eV. The spectral density of the line was found to be J≈10−7 photons/(electron×sr×eV).

Measurements have been performed at the superconducting Darmstadt electron linear accelerator (S-DALINAC) to investigate systematically channeling radiation produced by bombarding natural diamond crystals with thicknesses of 13, 20, 30, and 55 \ensuremath{\mu}m with electrons at 5.2 and 9.0 MeV. Planar channeling from the (110) and (111) planes was studied for a variety of transitions with respect to their energy, intensity, and linewidth. Axial channeling from the $〈110〉$ axis could be detected as well. It was found that the intensity increases as a function of the crystal thickness, and values up to 7.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}2}$ photons/esr could be obtained, which is the highest intensity at low electron energies achieved so far. The intensity increases with electron energy as ${\ensuremath{\gamma}}^{\frac{5}{2}}$. The $\frac{1}{e}$ occupation length deduced from the photon yield as a function of the crystal thickness was found to be ${l}_{\mathrm{occ}}\ensuremath{\approx}29 \mathrm{and} 85$ \ensuremath{\mu}m for planar and for axial channeling, respectively. These values are by far the largest ever observed. Comparison with a quantum mechanical theory of channeling radiation exhibits fairly good agreement for the intensity and linewidth provided that contributions caused by electronic scattering and Bloch wave broadening, which actually are largest for diamond, are properly taken into account. It turns out that multiple scattering dominates in the planar case and single scattering for the axial channeling. The coherence length could be deduced to be of the order of 0.7 \ensuremath{\mu}m, which is about a factor of 2 larger than observed before in silicon.

Parametric x-ray radiation of type B has been produced with an electron beam of energies between 3.5 and 9.1 MeV from the superconducting accelerator S-DALINAC and diamond of thickness $55\ensuremath{\mu}\mathrm{m}$. The photon intensity and its energy dependence were determined as a function of the tilt angle of the crystal. The intensity maximum varies with ${\ensuremath{\gamma}}^{2}$ and is about 3 orders of magnitude smaller than channeling radiation. Comparison with theoretical predictions exhibits very good agreement after taking into account effects caused by multiple scattering of the electrons in the crystal.

The ratio of multiple to single ionization, KL n /KL 0 , has been measured for a variety of elements (9≤ Z ≤ 29) as function of electron impact energy. We found that the ratio is decreasing with increasing energy and that it can be described by the expression σ(KL n )/ σ (KL 0 ) = a n + b n (ln E ) -1 . The energy independent terms, a 1 and a 2 , are in g agreement with shake-off calculations.

We report on the first operation of a small superconducting linear accelerator which serves as a pilot project for the Darmstadt superconducting recyclotron. A five niobium accelerating structure operated at 3 GHz was used to accelerate a chopped 200 keV electron beam to an energy of 850 keV. The Q value of the structure and its accelerating field (at 1.8 K) remained at 4 × 109 and 5.7 MV/m respectively during one year. Design, fabrication and test of the accelerating structure are discussed. A description of the pilot accelerator including its gun, chopper, and cryostat is given.

Particle induced X-ray emission methods (PIXE) have been extended to relativistic electrons to induce X-ray emission (REIXE) for quantitative trace-element analysis. The electron beam (20 ≤ E0≤ 70 MeV) was supplied by the Darmstadt electron linear accelerator DALINAC. Systematic measurements of absolute K-, L- and M-shell ionization cross sections revealed a scaling behaviour of inner-shell ionization cross sections from which X-ray production cross sections can be deduced for any element of interest for a quantitative sample investigation. Using a multielemental mineral monazite sample from Malaysia the sensitivity of REIXE is compared to well established methods of trace-element analysis like proton- and X-ray-induced X-ray fluorescence analysis. The achievable detection limit for very heavy elements amounts to about 100 ppm for the REIXE method. As an example of an application the investigation of a sample prepared from manganese nodules — picked up from the Pacific deep sea — is discussed, which showed the expected high mineral content of Fe, Ni, Cu and Ti, although the search for aliquots of Pt did not show any measurable content within an upper limit of 250 ppm.