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At the electron linac for beams with high brilliance and low emittance (ELBE) center for high-power radiation sources, the second version of a superconducting radio-frequency (SRF) photoinjector has been put into operation and has been routinely applied for user operation at the ELBE electron accelerator. SRF guns are suitable for generating a continuous wave electron beam with high average currents and high beam brightness. The SRF gun at ELBE has the goal to generate short electron pulses with bunch charges of 200–300 pC at typical repetition rates of 100 kHz for the production of superradiant, coherent terahertz radiation. The SRF gun includes a 3.5-cell, 1.3-GHz niobium cavity and a superconducting solenoid. A support system with liquid nitrogen (LN_{2}) cooling allows the operation of normal-conducting, high quantum efficiency photocathodes. We present the design and performance of the SRF gun as well as beam measurement results of the operation with Mg photocathodes at an acceleration gradient of 8 MV/m (4 MeV kinetic energy). In the last section, we discuss the SRF gun application for production of coherent terahertz radiation at the ELBE facility.

An XUV beamline based on high-harmonic generation (HHG) is presented. Recent observation of Ar5+ in XUV ionization experiments indicates high potential for XUV nonlinear optics and coherent diffractive imaging (CDI) with sub-femtosecond resolution.

The ongoing development of intense high-harmonic generation (HHG) sources has recently enabled highly non-linear ionization of atoms by the absorption of at least 10 extreme-ultraviolet (XUV) photons within a single atom (Senfftleben et al, arXiv:1911.01375). Here we investigate how the generation of these very intense HHG pulses in our 18-m-long beamline is aided by the reshaping of the fundamental, few-cycle, near-infrared (NIR) driving laser within a 30-cm-long HHG Xe medium. Using an incident NIR intensity that is higher than what is required for phase-matched HHG, signatures of reshaping are found by measuring the NIR blueshift and the fluorescence from the HHG medium along the propagation axis. These results are well reproduced by numerical calculations that show temporal compression of the NIR pulses in the HHG medium. The simulations predict that after refocusing an XUV beam waist radius of 320 nm and a clean attosecond pulse train can be obtained in the focal plane, with an estimated XUV peak intensity of 9 × 1015 W cm−2. Our results show that XUV intensities that were previously only available at large-scale facilities can now be obtained using moderately powerful table-top light sources., JPhys Photonics, 2 (3), ISSN:2515-7647

We present an optical parametric chirped pulse amplification (OPCPA) system delivering 4.4 TW pulses centered at 810 nm with a sub-9 fs duration and a carrier-envelope phase stability of 350 mrad. The OPCPA setup pumped by sub-10 ps pulses from two Yb:YAG thin-disk lasers at 100 Hz repetition rate is optimized for a high conversion-efficiency. The terawatt pulses of the OPCPA are utilized for generating intense extreme ultraviolet (XUV) pulses by high-order harmonic generation, achieving XUV pulse energies approaching the microjoule level.

10th International Particle Accelerator Conference, IPAC’19, Melbourne, Australia, 2 May 2019 - 24 May 2019; Journal of physics / Conference series Conference series [...] 1350, 012046 - (2019). doi:10.1088/1742-6596/1350/1/012046, Laser pulse shaping is one of the key elements to generate low emittance electron beams with RF photoinjectors. Ultimately high performance can be achieved with ellipsoidal laser pulses, but 3-dimensional shaping is challenging. High beam quality can also be reached by simple transverse pulse shaping, which has demonstrated improved beam emittance compared to a transversely uniform laser in the 'pancake' photoemission regime. In this contribution we present the truncation of a Gaussian laser at a radius of approximately one sigma in the intermediate (electron bunch length directly after emission about the same as radius) photoemission regime with high acceleration gradients (up to 60 MV/m). This type of electron bunch is used e.g. at the European XFEL and FLASH free electron lasers at DESY, Hamburg site and is being investigated in detail at the Photoinjector Test facility at DESY in Zeuthen (PITZ). Here we present ray-tracing simulations and experimental data of a laser beamline upgrade enabling variable transverse truncation. Initial projected emittance measurements taken with help of this setup are shown, as well as supporting beam dynamics simulations. Additional simulations show the potential for substantial reduction of slice emittance at PITZ., Published by IOP Publ.20382, Bristol

3rd European Advanced Accelerator Concepts workshop, EAAC 2017, Elba, Italy, 24 Sep 2017 - 30 Sep 2017; Nuclear instruments & methods in physics research / A 909, 107 - 110 (2018). doi:10.1016/j.nima.2018.02.043, Beam driven plasma acceleration is one of the most promising candidates for future compact particle accelerator technologies. In this scheme a particle bunch drives a wake in a plasma medium. The fields inside of the wake can be used to accelerate a trailing witness bunch. To maximise the ratio between acceleration of the witness to deceleration of the drive bunch, the so called transformer ratio, several methods have been proposed. The ones yielding the most favorable results are based on shaped drive bunches that are long in terms of the plasma wavelength. We present here methods to create such drive bunches employing temporally shaped UV-laser pulses for the extraction of electron bunches from a photo-electron gun. Theoretical considerations, experimental results and possibilities for further improvements are discussed., Published by North-Holland Publ. Co., Amsterdam

At the radiation source ELBE a superconducting radio-frequency photoinjector (SRF gun) was developed and put into operation. Since 2010 the gun has delivered beam into the ELBE linac. A new driver laser with 13 MHz pulse repetition rate allows now to operate the free-electron lasers (FELs) with the SRF gun. This paper reports on the first lasing experiment with the far-infrared FEL at ELBE, describes the hardware, the electron beam parameters and the measurement of the FEL infrared radiation output.

The latest generation of high-energy-class pulsed laser facilities, under construction or planned, such as EuPRAXIA, require reliable pump sources with high power (many kW), brightness (&gt, 1 MW/cm2/sr) and electro-optical conversion efficiency (&gt, 50%). These new facilities will be operated at high repetition rates (around 100 Hz) and only diode lasers are capable of delivering the necessary performance. Commercial (quasi-continuous wave, QCW) diode laser pulse-pump sources are, however, constructed as low-cost passively cooled stacked arrays that are limited either in brightness, efficiency or repetition rate. Commercial continuous wave diode laser pumps constructed using microchannel coolers (as used in high-value industrial machine tools) can fulfil all requirements, but are typically not preferred, due to their cost and complexity and the challenges of preventing cooler degradation. A custom solution is shown here to fill this gap, using advanced diode lasers in a novel passive side-cooling geometry to realize 100 &hellip, 200 Hz pump modules (10%&ndash, 20% duty cycle) that emit peak power of 6 kW at wavelength = 940 nm. The latest performance of these modules is summarized and compared to literature. We show that a brightness &gt, 1 MW/cm2/sr can be efficiently delivered across a wide range of laser pulse conditions with 10% duty cycle (pulse width: 100 &micro, s &hellip, 100 ms &hellip, cw, repetition rate up to 1 kHz). Furthermore, we describe how these pumps have been used to construct and reliably operate (&gt, 109 pulses without degradation) in high-energy-class regenerative and ring amplifiers at the Max-Born-Institut f&uuml, r Nichtlineare Optik und Kurzzeitspektroskopie (MBI). We also show first results on 100 Hz pumping of cryogenically cooled solid-state Yb:YAG slab amplifiers, as anticipated for use in the EuPRAXIA laser, and note that peak temperature is disproportionately increased, indicating that improved cooling and more detailed studies are needed.

A regenerative amplifier based on thin-disk technology has been upgraded and optimized. Within a CPA laser system chirped 1 ns pulses are amplified to more than 300 mJ pulse energy. In addition to the high pulse energy the amplifier shows a very good energy stability with 0.25% (rms) fluctuation as well as an excellent beam quality of M(2) = 1.04. The regenerative amplifier is equipped with an Yb:YAG thin-disk of 17 mm in diameter pumped with 1.75 kW peak power. It is operated at a repetition rate of 100 Hz. The optical-to-optical efficiency is better than 18%. The laser pulses are compressed to 1.8 ps pulse duration.

We describe the type of pulse shaper for production of flat-top pulses that is used at the Photo Injector Test Facility (PITZ) at DESY. This shaper consists of a two-stage birefringent filter. Using a simple model, this pulse shaper can be regarded as a system that splits the Gaussian input pulse into four mutually delayed replicas and stacks them in a phase-coherent way. Although a variety of different pulse shapes can be generated by this method, the system is particularly suited for the generation of flat-top output pulses with duration between 10 and 100 ps. When operating in combination with an actively modelocked Nd:YLF oscillator, it can shape picosecond pulses arranged in long trains, as required for photocathode lasers for the TESLA-type linear accelerators. Using trains of equally shaped picosecond pulses has an additional benefit: It permits measuring the shape of these pulses by cross-correlation in real-time using a so-called optical sampling system. Such a system is employed as a measurement tool for appropriate alignment of the individual components of the pulse shaper.

Most of the proposed electron accelerator projects for future FELs, ERLs or 4th generation light sources require electron beams with an unprecedented combination of high brightness, low emittance, and high average current. In all projects photoguns will be applied: DC-photoguns, normal conducting RF-photoguns (NC-guns), and superconducting RF photoguns (SRF-guns). While the concepts of DC- and NC-guns are well proofed, the SRF-gun development still possesses a high risk. Challenges are the design of the superconducting cavity, the choice of the photocathode type, its life time, a possible cavity contamination, the difficulty of coupling high average power into the gun, and, finally, the risk of beam excitation of higher-order cavity modes. In combination with SRF linacs, the SRF-guns seem to be the best solution for high average currents. Several R&D projects of SRF-gun have been launched. In this paper, we will give an overview of the progress of the SRF photoinjector development. In detail, the technical concept, the performance and the status of the Dresden Rossendorf SRF-gun project, a collaboration of BESSY, DESY, MBI and FZD, will be presented. The main design parameters of this SRF-gun are the final electron energy of 9.5 MeV, 1 mA average current, and transverse normalized emittances (rms) of 1 mm mrad at 77 pC and 2.5 mm mrad at 1 nC bunch charge. The 1.3 GHz cavity consists of three TESLA-shaped cells, a specially designed half-cell where the photocathode is placed and a choke filter in order to prevent RF losses at the cathode side. The normal-conducting photocathode with a Cs2Te photoemission layer is cooled by liquid nitrogen. The SRF-gun cryostat consists of a stainless steel vacuum vessel, a warm magnetic shield, a liquid nitrogen-cooled thermal shield and a titanium He tank with a two-phase supply tube. The 10 kW fundamental power coupler is adopted from the ELBE cryomodule. In a first commissioning and test period the gun will be operated in parallel to the accelerator. A diagnostic beamline will allow beam parameter measurement and further optimization of the SRF-gun. In a final step, the gun will be connected to the ELBE superconducting linear accelerator to deliver an improved electron beam to the user labs.

We report on a CPA system providing two synchronized pulses with more than 500 mJ, each. We also report on a special thin-disk ring amplifier that delivers 1 J pulse energy.

This paper presents results of the photocathode cooling system test of the 3 1 2 cell SRF gun at the Forschungszentrum Rossendorf. The SRF gun will produce short electron pulses with high bunch charges and low transverse emittance. The requirement for the superconducting electron linear accelerator in Rossendorf (ELBE) is to provide a low emittance electron beam up to 1 mA current and 9.5 MeV energy. Additionally, it will easily operate in continuous wave (cw) mode because of the low RF power losses in the superconducting material. Therefore, the normal conducting copper cathode must be cooled by liquid nitrogen in order to preserve the temperature of the cavity at 2.2 K. The estimated power input from the RF field into the cathode could be more than 10 W [P. vom Stein, Thesis, TU-Dresden, 1998]. First results of temperature measurements of the photocathode, respectively, from the cooling system at a heat load up to 30 W are presented.

This paper describes the photocathode laser of the TESLA Test Facility (TTF). The laser system generates trains of ultraviolet picosecond pulses with 262 nm wavelength. Special measures were required to allow for stable and reliable generation of these pulse trains that are up to 800 μs long and can contain up to 800 micropulses at 1 MHz repetition rate, alternatively, up to 2400 micropulses at 3 MHz repetition rate within the train. The duration of the individual micropulses is 11 ps FWHM ( σ ∼4.5 ps). In the 1 MHz mode the micropulse energy at 262 nm wavelength reaches 54 μJ. Laser pulses are generated synchronously to the RF system of the TTF-2 linear accelerators (linac) with synchronization accuracy better than 1 ps. The laser is able to run at 10 Hz repetition rate with full remote control. It illuminates the photocathode of an RF gun to generate the electron bunches which are subsequently accelerated in the TTF linac.

The Photo Injector Test Facility at DESY Zeuthen (PITZ) was built to test and optimize electron sources for Free Electron Lasers and future linear colliders. The focus is on the production of intense electron beams with minimum transverse emittance and short bunch length as required for FEL operation. The experimental setup includes a 1.5 cell L-band gun cavity with coaxial RF coupler, a solenoid for space charge compensation, a laser capable to generate long pulse trains with variable temporal and spatial pulse shape, an UHV photo cathode exchange system, and an extensive diagnostics section. This contribution will give an overview on the facility and will mainly discuss the measurements of the electron beam transverse phase space. This will include measurements of the transverse and longitudinal laser profile, beam charge as a function of RF phase, and transverse emittance as a function of different parameters. The corresponding measurements of momentum and momentum spread as well as the RF commissioning results will be summarized. As a first application of the PITZ electron source it will be installed at the TESLA Test Facility Free Electron Laser at DESY Hamburg in autumn 2003.

We studied extreme ultraviolet (EUV) emission from Xe cluster jet targets irradiated with high repetitive ps and ns laser pulses in the intensity range between 10(11) and 10(15) W/cm(2). It was fou ...

Electron bunch lengths and time jitters have been measured with sub-ps resolution at the TESLA Test Facility at DESY. Synchrotron radiation emitted from a dipole magnet reflects the longitudinal charge distribution of the electron bunch and can also be used as a timing reference of the radiation pulses produced by the free-electron laser. For sub-ps time resolution, dispersion effects of the spectrally broad-band synchrotron radiation have to be suppressed. The phase stability of the FEL pulses with respect to the radio-frequency master oscillator has been studied with a streak camera operating in synchroscan mode.

The Photo Injector Test facility at DESY Zeuthen (PITZ) was built to develop electron sources for the TESLA Test Facility Free Electron Laser and future linear colliders. The main goal is to study the production of minimum transverse emittance beams with short bunch length at medium charge (∼1 nC). The facility includes a 1.5 cell L-band cavity with coaxial RF coupler, a solenoid for space charge compensation, a laser capable to generate long pulse trains, an UHV photo cathode exchange system, and different diagnostics tools. Besides an overview of the facility, its main components and their commissioning, this contribution will concentrate on the first measurements at PITZ with photoelectrons. This will include measurements of the transverse and longitudinal laser profile, charge and quantum efficiency, momentum and momentum spread, transverse electron beam profiles at different locations and first results on transverse emittance.

Berliner Elektronenspeicherring-Gesellschaft fur Synchrotronstrahlung (BESSY) plans to construct a linac-based single-pass FEL as an addition to its existing third generation storage-ring-based light-source. The project aims to obtain an FEL-based user-facility that covers the VUV and soft X-ray spectral range (20 eV⩽ℏ ω ⩽1 keV). At present, the design stage is funded as a collaboration between BESSY, DESY, the Hahn–Meitner-Institute in Berlin, and the Max–Born-Institute in Berlin. This stage focuses on optimization of the FEL as a user light-source, both with respect to its capabilities and in its performance. Important issues are: stability, seeding options of the SASE FEL, wavelength-tunability, synchronization with external laser sources and, on a longer time-scale, the generation of ultra-short (

We present a concept of a laser system for a photon collider at the TESLA linac. It is based on an external optical ring cavity which is pumped by a short-pulse laser. A detailed discussion of the geometry of the external cavity is given.

While huge progress has been achieved for EUVL system design and multilayer optics during the last years the decision on the best suited source is still open. In a German basic research cooperation on short-wavelength plasma based sources some key issues also relevant for EUVL are addressed: the comparison of existing sources, investigations on the scalability of source concepts and the demonstration of key features. Preliminary results of investigations on parameters for best conversion efficiency of laser produced plasmas, concepts for high-power lasers and scalability of gas discharge based sources are presented. Comparability of results is assured by calibrated metrology tools which are cross checked with ASML’s flying circus.

We describe the development of a table top soft X-ray source based on a laser produced plasma. The plasma is produced by a Nd:YLF pulse-train laser which delivers pulse trains of up to 400 pulses each of about 25 ps duration. Number and energy of the micropulses are fully computer controlled adjustable for an optimum interaction with a rotating solid target. Spectral, spatial and temporal characteristics of the X-ray emission from Mylar and copper targets have been studied. A calibrated back-illuminated CCD camera together with a slit grating spectrograph were utilized for the measurement of the absolute soft X-ray photon fluxes. The temporal measurements were performed using a streak camera while the plasma size was measured with a zone plate. An application of the system in a X-ray absorption experiment is shown. The advantages of the system in view of possible optical pump and soft X-ray probe experiments will be discussed.

Photocathodes are a limiting factor for the next generation of ultrahigh brightness photoinjectors. We studied the behavior of a superconducting Pb cathode in the cryogenic environment of a superconducting rf gun cavity to measure the quantum efficiency, its spatial distribution, and the work function. We will also discuss how the cathode surface contaminants modify the performance of the photocathode as well as the gun cavity and we discuss the possibilities to remove these contaminants.

The parameters of a superconducting RF gun cavity have been measured in a test cryostat of DESY. A fieldstrength of 31.8 MV/m at the cathode has been obtained.

During the run 1998/1999, the new injector based on a laser driven RF gun was brought into operation at the TESLA Test Facility Linac (TTFL) at DESY. A key element of the injector is the laser system to illuminate the RF gun cathode to produce short (ps) electron bunches of high charge (nC). This electron beam is used to perform various experiments for the future TESLA linear collider, and to drive the free electron laser TTF-FEL. The laser design is challenged by the unusual requirement of providing synchronized ps UV pulses in 0.8 ms long trains with ambitious stability requirements. The design was also driven by the requirement to have an operational system with a high reliability. The system is based on a mode locked solid-state (Nd:YLF) pulse train oscillator followed by a linear ampli"er chain. In a "rst phase, a laser pulse rate of 1 MHz within the train has been realized, 2.25 MHz and 9 are in preparation. Performance and running experiences with the laser system during the last TTF run are reported. ( 2000 Elsevier Science B.V. All rights reserved.

We describe the amplifier chain of the photocathode laser of the TESLA Test Facility. This chain is optimized for amplification of trains of picosecond pulses. At its output, pulse trains with a rectangularly shaped envelope of 0.7% flatness and a long-term stability of the energy in the individual picosecond pulses in excess of 0.9% are obtained. This stability is achieved by using a computerized feedback control system, which we describe in detail together with the versatile feedback algorithm applied. The feedback loop performs a tight adjustment of both the shape and magnitude of the flashlamp pulses as well as the starting time of the pump with respect to the pulse train. High-current insulated gate bipolar transistors are used to directly manipulate the flashlamp current in a microsecond time scale.

A novel two-step excitation scheme for an efficient table-top x-ray laser has been realized for the first time. A nanosecond pulse creates a plasma of neonlike ions of titanium, followed by a subpicosecond pulse which excites a nonstationary population inversion. With only a few joules of pump energy, a compact x-ray laser at 32.6 nm with a very high gain coefficient of $g\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}19{\mathrm{cm}}^{\ensuremath{-}1}$ and a gain-length product of $gL\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9.5$ was achieved.

A hybrid Ti:Sa Nd:glass laser is described delivering two synchronized laser pulses with duration of 0.7 ps and 1.2 ns and output energies of 4 J in the short pulse and 8 J in the long pulse. The system was successfully used to drive efficiently a transient inversion XUV laser on [Ne]-like ions [see Nickles et al., SPIE Vol. 2520].

We present a compact FROG system for determination of both pulse shape and phase of sub-picosecond pulses in a single-shot measurement. The system utilizes third-order auto-correlation due to the induced optical Kerr effect in heavy flint glass (SF6). The optical layout allows to use single Calcite wedges as high-contrast polarizers. Instead of using a separate spectrgraph, we incorporated a grating and appropriate imaging optics directly into the optical scheme. The FROG system has proven to be a convenient measurement tool to optimally align the grating compressor of CPA laser systems.

A CPA laser system for high pulse energy at high average power has been developed. The system is based on Yb:YAG thin-disk technology. It provides two laser beams with more than 500 mJ pulse energy each at 100 Hz repetition rate and 2 ps pulse duration. The system consists of a common oscillator, a grating stretcher and compressor and two identical amplifier chains that are both equipped with a regenerative amplifier and a ring amplifier. The compressor supports an individual alignment of the dispersion for the two laser channels.

High brightness electron sources for linac based free-electron lasers (FELs) are being developed at the Photo Injector Test facility at DESY, Zeuthen site (PITZ). Production of electron bunches with extremely small transverse emittance is the focus of the PITZ scientific program. The photoinjector optimization in 2008–2009 for a bunch charge of 1, 0.5, 0.25, and 0.1 nC resulted in measured emittance values which are beyond the requirements of the European XFEL [S. Rimjaem et al., Nucl. Instrum. Methods Phys. Res., Sect. A 671, 62 (2012)NIMAER0168-900210.1016/j.nima.2011.12.101]. Several essential modifications were commissioned in 2010–2011 at PITZ, resulting in further improvement of the photoinjector performance. Significant improvement of the rf gun phase stability is a major contribution in the reduction of the measured transverse emittance. The old TESLA prototype booster was replaced by a new cut disk structure cavity. This allows acceleration of the electron beam to higher energies and supports much higher flexibility for stable booster operation as well as for longer rf pulses which is of vital importance especially for the emittance optimization of low charge bunches. The transverse phase space of the electron beam was optimized at PITZ for bunch charges in the range between 0.02 and 2 nC, where the quality of the beam measurements was preserved by utilizing long pulse train operation. The experimental optimization yielded worldwide unprecedented low normalized emittance beams in the whole charge range studied.

High brightness electron sources for linac based short-wavelength free-electron lasers are developed and optimized for small transverse projected emittance at the photo-injector test facility at DESY, location Zeuthen (PITZ). A major part of the measurement program at PITZ is dedicated to transverse phase space optimization in order to fulfill the requirements of the European X-ray free-electron laser (European XFEL). A laser-driven RF-gun, treated with a dry-ice sublimation-impulse cleaning technique, a new photocathode laser system allowing short rise and fall times of the flat-top temporal distribution as well as several new diagnostic components have been installed at PITZ in 2008. The electrons generated via the photo-effect at a cesium telluride (Cs2Te) cathode are accelerated by a 1.6 cell L-band RF-gun cavity with a maximum accelerating gradient at the cathode of about 60 MV/m. The transverse projected emittance is measured using a single slit scan technique. In the 2008–2009 run period, a detailed characterization of the projected transverse emittance was performed at different operating conditions. Optimizations and measurement results as well as simulation predictions of the transverse projected emittance for bunch charges of 1, 0.5, 0.25 and 0.1 nC are presented and discussed in this paper. The geometric mean of the normalized projected rms emittance in both transverse directions for an electron bunch charge of 1 nC was measured to be

We report on a compact, all-glass laser using a fiberless CPA technique as a potential system for the generation of extreme high-contrast picosecond pulses in the terawatt power range. New exciting experiments in the field of laser-matter interaction at extremely high intensities require both a high output energy and a very high peak to background intensity ratio (contrast). At present the system delivers 1.5 ps-pulses of 1.5-J energy with an intensity contrast of 4.109.

The interaction of 2 ps FWHM laser pulses with solid targets at intensities ${10}^{16}$-${10}^{18}$ W/${\mathrm{cm}}^{2}$ is studied for two essentially different cases of laser front steepness resulting in preplasma and preplasma-free interaction. Red and blue frequency shifts of a backscattered fundamental wave and its second harmonic depending on the incident laser pulse shape are observed. They are associated with Doppler shifts corresponding to inward or outward movement of the critical density surface. A model including ponderomotive force was developed to explain the experimental results.

Thin disk technology has the potential for providing laser pulses with high pulse energy and high repetition rate. Large laser projects like ELI or HiPER benefit from this technology as well as stand alone systems like e.g. laser driven x-ray sources

It is shown that a compact, all glass laser using fiberless CPA technique is a potential system for generation of extreme high contrast picosecond pulses in the terawatt power range. At present the system delivers 1.5 ps pulses of 1.5 J energy with an intensity contrast of 4×109.

Extreme ultraviolet (EUV) emission in the 11–15 nm wavelength range from a thin liquid water jet target under illumination with a high repetition rate, high average power laser (Nd-YLF) has been studied. To find the optimum conversion efficiency of laser light into EUV radiation, different laser parameters were applied. The laser intensity was varied between 1011 and 1015 W/cm2, and pulse duration in the range from 30 ps to 3 ns. A maximum conversion efficiency of 0.12% in 2.2% bandwidth and 4π steradian at 13 nm was achieved at a repetion rate of 250 kHz, and a strong dependence of the conversion efficiency on both laser intensity and pulse duration was found.

The photoinjector test facility at DESY, Zeuthen site (PITZ), was built to develop and optimize photoelectron sources for superconducting linacs for high-brilliance, short-wavelength free-electron laser (FEL) applications like the free-electron laser in Hamburg (FLASH) and the European x-ray free-electron laser (XFEL). In this paper, the detailed characterization of two laser-driven rf guns with different operating conditions is described. One experimental optimization of the beam parameters was performed at an accelerating gradient of about 43 MV/m at the photocathode and the other at about 60 MV/m. In both cases, electron beams with very high phase-space density have been demonstrated at a bunch charge of 1 nC and are compared with corresponding simulations. The rf gun optimized for the lower gradient has surpassed all the FLASH requirements on beam quality and rf parameters (gradient, rf pulse length, repetition rate) and serves as a spare gun for this facility. The rf gun studied with increased accelerating gradient at the cathode produced beams with even higher brightness, yielding the first demonstration of the beam quality required for driving the European XFEL: The geometric mean of the normalized projected rms emittance in the two transverse directions was measured to be 1.26±0.13 mm mrad for a 1-nC electron bunch. When a 10% charge cut is applied excluding electrons from those phase-space regions where the measured phase-space density is below a certain level and which are not expected to contribute to the lasing process, the normalized projected rms emittance is about 0.9 mm mrad.

We report on a specially designed actively-passively modelocked Nd-glass laser. Negative feedback control and frequency tuning by a Lyot filter allows the generation of subpisecond pulses if the generated wavelength is shifted to the red side of the fluorescence spectrum of the active material. Experimental results and possible explanations for the observed spectral broadening and pulse shortening are given and discussed in the approximation of a linear amplification model. The oscillator should be useful for the chirped pulse amplification experiments.

This paper deals with the pulse-shaping properties of birefringent filters that feature an optical layout similar to a Solc fan filter. A simple computational model is given that explains the pulse-shaping process in the fan filter in two steps: First, the input pulse is split into several mutually delayed replicas due to the birefringence of the crystals. Second, these replicas interfere at the output polarizer of the filter and form the shaped output pulse. Fine-tuning of the phases of the replicas of the input pulse is permitted by tuning the temperature of the crystals. A birefringent pulse shaper containing ten birefringent crystals was investigated experimentally. The shape of the output pulses was measured by means of a special cross-correlation technique. Although a variety of pulse shapes can be generated with the described filter, it is particularly well suited for generation of flattop pulses featuring a 20-ps-long plateau and rising and falling edges shorter than 2 ps.

A superconducting radio frequency (SRF) photoelectron injector is currently under construction by a collaboration of BESSY, DESY, FZD, and MBI. The project aims at the design and setup of a continuous-wave SRF injector including a diagnostics beamline for the ELBE free electron laser (FEL) and to address R&D issues on low emittance injectors for future light sources such as the BESSY FEL. Of critical importance for the injector performance is the control of the electron beam parameters. For this reason a compact diagnostics beamline is under development, serving a multitude of operation settings. In this paper the layout and the rationale of the diagnostics beamline are described. Furthermore detailed information on specific components is given, together with results from laboratory tests and data taking.

A superconducting radio frequency (RF) photoelectron injector (SRF gun) is under development at the Research Center Dresden–Rossendorf. This project aims mainly at replacing the present thermionic gun of the superconducting electron linac ELBE. Thereby the beam quality is greatly improved. Especially, the normalized transverse emittance can be reduced by up to one order of magnitude depending on the operating conditions. The length of the electron bunches will be shortened by about two orders of magnitude making the present bunchers in the injection beam line dispensable. The maximum obtainable bunch charge of the present thermionic gun amounts to 80 pC. The SRF gun is designed to deliver also higher bunch charge values up to 2.5 nC. Therefore, this gun can be used also for advanced facilities such as energy recovery linacs (ERLs) and soft X-ray FELs. The SRF gun is designed as a 3 1 2 cell cavity structure with three cells basically TESLA cells supplemented by a newly developed gun cell and a choke filter. The exit energy is projected to be 9.5 MeV. In this paper, we present a description of the design of the SRF gun with special emphasis on the physical and technical problems arising from the necessity of integrating a photocathode into the superconducting cavity structure. Preparation, transfer, cooling and alignment of the photocathode are discussed. In designing the SRF gun cryostat for most components wherever possible the technical solutions were adapted from the ELBE cryostat in some cases with major modifications. As concerns the status of the project the design is finished, most parts are manufactured and the gun is being assembled. Some of the key components are tested in special test arrangements such as cavity warm tuning, cathode cooling, the mechanical behavior of the tuners and the effectiveness of the magnetic screening of the cavity.

In the paper, we report on the status and progress of the superconducting RF gun project in Rossendorf. The gun is designed for cw operation mode with 1 mA current and 9.5 MeV electron energy, and it will be installed at the ELBE superconducting electron linear accelerator. The gun will have a 3½ cell niobium cavity operating at 1.3 GHz. The cavity consists of three cells with TESLA geometry and a specially designed half-cell in which the photocathode will be placed. The production of two Nb cavities, with RRR 300 and 40 respectively, has be finished at the beginning of 2005. After delivery, the RF tests will be performed and the preparation of the cavities will be started. At the same time, the design of the cryostat and the fabrication of its components are already finished. Further activities are the design of the diagnostic beam line, the testing of the new photocathode preparation system, and the upgrade of the 262 nm driver laser system.

The ability to produce high brightness electron beams as required for modern Free Electron Lasers (FELs) has been demonstrated during the first stage of the Photo Injector Test Facility at DESY Zeuthen (PITZ1). The electron source optimization at PITZ1 was successfully completed, resulting in the installation of the PITZ rf gun at the VUV-FEL (DESY, Hamburg). One of the main goals of the second stage of PITZ (PITZ2) is to apply higher gradients in the rf gun cavity in order to obtain smaller beam emittance by faster acceleration of the space charge dominated beams. In order to reach the required gradients a 10 MW klystron has to be installed and the gun cavity has to be conditioned for higher peak power. Another important goal of PITZ2 is a detailed study of the emittance conservation principle by using proper electron beam acceleration with a booster. Further photo injector optimization, including update of the photocathode laser and diagnostic tools, is foreseen as well. Recent progress on the PITZ developments will be reported.

Hard photons well above 100 GeV have to be generated in a future photon-collider which essentially will be based on the infrastructure of the planned International Linear Collider (ILC). The energy of near-infrared laser photons will be boosted by Compton backscattering against a high energy relativistic electron beam. For high effectiveness, a very powerful lasersystem is required that exceeds todays state-of-the-art capabilities. In this paper a design of an auxiliary passive cavity is discussed that resonantly enhances the peak-power of the laser. The properties and prospects of such a cavity are addressed on the basis of the specifications for the European TeV Energy Superconducting Linear Accelerator (TESLA) proposal. Those of the ILC are expected to be similar., Comment: V2: Version accepted by NIM

Published online 16 November 2005 Groupe SERA; Many scientific disciplines ranging from physics, chemistry and biology to material sciences, geophysics and medical diagnostics need a powerful X-ray source with pulse lengths in the femtosecond range [1-4]. This would allow, for example, time-resolved observation of chemical reactions with atomic resolution. Such radiation of extreme intensity, and tunable over a wide range of wavelengths, can be accomplished using high-gain free-electron lasers (FEL) [5-10]. Here we present results of the first successful operation of an FEL at a wavelength of 32 nm, with ultra-short pulses (25 fs FWHM), a peak power at the Gigawatt level, and a high degree of transverse and longitudinal coherence. The experimental data are in full agreement with theory. This is the shortest wavelength achieved with an FEL to date and an important milestone towards a user facility designed for wavelengths down to 6 nm. With a peak brilliance exceeding the state-of-the-art of synchrotron radiation sources [4] by seven orders of magnitude, this device opens a new field of experiments, and it paves the way towards sources with even shorter wavelengths, such as the Linac Coherent Light Source [3] at Stanford, USA, and the European X-ray Free Electron Laser Facility [4] in Hamburg, Germany.

The generation of femtosecond X-ray pulses with circular polarization is planned at BESSY II. The paper describes the underlying principle ("femtoslicing"), its technical implementation, and the expected output, based on simulations and measurements.

This paper provides an overview of the advantages and problems of superconducting RF guns. The results of the Rossendorf experiments are presented here. These results are integrated in the design of a new 3.4 cell superconducting RF gun. The beam parameters of this gun correspond to the demands for the new generation of high current, high brightness injectors.

The Drossel collaboration was established for the development a low emittance, high average current, cw electron injector suitable for the ELBE project. The injector is based on a photocathode RF gun with a superconducting cavity. A half-cell 1.3 GHz superconducting niobium cavity incorporated with a normal conducting cathode unit was designed and produced to check the basic design concepts. Cold RF tests of the cavity at T=2 K were successfully carried out at DESY TTF. Now preparation of the necessary components for the beam tests is in progress.