STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
STFC/DL/SRD, Daresbury, Warrington, Cheshire
An infra-red oscillator FEL was installed into the accelerator test facility, ALICE, at Daresbury Laboratory at the end of 2009. The FEL will be used to study energy recovery performance with a disrupted, large energy spread, beam and also to test novel FEL concepts. This paper will describe the installed hardware, the pre-alignment techniques that have been employed, the diagnostics that are being used to detect the infra-red output, and the progress with commissioning of the FEL itself.
RIKEN/SPring-8, Hyogo
The commissioning of the XFEL/SPring-8 facility is scheduled in the spring of 2011. Since the accelerator of XFEL/SPring-8 uses a thermionic gun with an 1 A initial beam current, the total bunch compression ratio reaches about 3000, which is one order higher than a photocathode system. For nonlinearity compensation in the bunch compression, two correction cavities are installed, which are operated at the same frequency as the linac and not at its higher-harmonic. A large compression ratio, particularly at the velocity bunching, results in larger projected parameters of the electron bunch compared to its slice values. The transverse optics of the accelerator is designed for the projected parameters using newly introduced linear formulation of the beam envelope including acceleration effects. The beam optics of the main linac and undulator sections are based on a FODO-like lattice and additional quadrupole magnets are installed at each chicane for dispersion correction. In this presentation, the XFEL/SPring-8 accelerator layout and its expected beam parameters are shown to achieve the 0.1 nm X-ray FEL.
DESY, Hamburg
Uni HH, Hamburg
The free-electron laser facility FLASH at DESY, Germany finished its very successful 2nd user period late summer 2009. Recently FLASH has been upgraded. The 3rd user period is scheduled to start late summer 2010. In many aspects the upgraded FLASH is an FEL with a new quality of performance. It can provide thousands of FEL pulses per second with wavelengths approaching the carbon 1s absorption edge and the water window. The extension of the photon wavelength range is realized by increasing the electron beam energy up to 1.2 GeV by adding a 7th superconducting accelerating module. The dynamics behavior of the electron beam is improved by installing 3rd harmonic superconducting RF cavities. In addition, an experiment for seeded FEL radiation, sFLASH, is integrated to the FLASH linac. Recently, FLASH achieved a beam energy above 1.2 GeV and lasing below 5 nm with a remarkably improved performance.
University of Pune, Pune
FZD, Dresden
A tapered undulator experiment was carried out at the Forschungszentrum Dresden-Rossendorf (ELBE) far-infrared FEL. The main motivation was to see whether the presence of a dispersive medium due to the partially waveguided resonator has any effect on the outcome. The FEL saturated power and the wavelength shifts have been measured as a function of both positive as well as negative undulator field amplitude tapering. In contrast to the typical high-gain FELs where positive tapering (i.e. a decrease of undulator field amplitude over the beam path) proves beneficial for the output power we observe an improvement of performance at negative taper. During the same experiments we studied the characteristics of the detuning curves. The width of the curves indicates a maximum small-signal gain for zero taper while the output peak power is highest for negative taper. Whereas the saturated power output and the detuning curve characteristics agree with the known theoretical predictions, the wavelength shifts showed deviations from the expected values. Details of the experiment are presented.
SOLEIL, Gif-sur-Yvette
PhLAM/CERCLA, Villeneuve d'Ascq Cedex
Nagoya University, Nagoya
UVSOR, Okazaki
JASRI/SPring-8, Hyogo-ken
In FEL oscillator, a desynchronisation between the electron-bunch passage frequency and the repetition rate of the laser can lead to instability, characterised by erratic longitudinal shape of the emitted light pulses. We show that this instability can be controlled using a simple feedback system which consist in re-injecting in the cavity a part of the emitted light. Analytical, numerical and experimental studies on the UVSOR-II storage ring have been performed, and show that the energy needed to achieved the control can be extremely weak, in practical higher than the noise level[1]. We also show that another important parameter is the phase of the re-injected signal with respect to the light in the cavity. Depending of the value of this phase, we can observe a shift of the emitted light wavelength, which can go with a modulation of the laser pulse envelop. Both of this two phenomenas are quantitatively analysed.
[1] C. Evain, C. Szwaj, S. Bielawski, M. Hosaka, A. Mochihashi, M. Katoh, and M.-E. Couprie, Phys. Rev. Lett. {10}2, 134501 (2009)
BARC, Mumbai
Mode stability can restrict the tuning range of FEL oscillators. We investigate the stability of FEL oscillators as a function of wavelength as well as size of the coupling hole. We show that concentric configurations are preferred to confocal ones. We study mode-stability using multi-particle simulations, for both, symmetric as well as asymmetric modes.
AIST, Tsukuba, Ibaraki
Inverse Compton X-rays were obtained during lasing of the NIJI-IV free electron laser (FEL) in the infrared range. The position of the Compton collisions between electron bunches and laser pulses inside the FEL oscillator strongly influenced the energy spectrum of the Compton X-rays. Collisions outside the undulator magnetic field led to a sharp and clear Compton edge, while collisions inside the undulator magnetic field made it quite obscure. The position of collisions can be determined by changing the bunch-filling pattern on the 16 RF successive buckets. In this experiment two or three bunches selectively remained in suitable RF buckets. The maximum X-ray energies were 0.7-2.1 MeV for the laser wavelengths of 2.6 μm - 0.88 μm with a fixed e-beam energy of 310 MeV. Relative energy width of the 1.2 MeV Compton X-rays was observed to be 11 % with a lead collimator of 10 mm in diameter. The maximum X-ray yield was of the order of 106 photons/sec in a three-bunch operation mode. The energy spectra and X-ray yields were investigated under various Compton collision conditions. The results will be discussed in the conference.
JAEA/ERL, Ibaraki
JAEA, Ibaraki-ken
Hybrid K-edge densitometer (HKED) is used for concentration measurement of U, Pu and minor actinides in liquid solution samples. In the HKED, the concentration of the most-abundant element is determined by K-edge densitometer and concentrations of other elements are derived from XRF signals. We propose a multi-turn small-size energy-recovery linac (ERL) to produce laser-Compton scattered X-rays for the HKED. The X-rays with good monochromaticity and energy tunability allow measurement of actinides with much better resolution than the existing HKED systems based on X-ray tubes. The ERL energy is 85 MeV to produce 130-keV X-rays. In the present design, we adopt a racetrack configuration, in which electrons are accelerated six times by L-band superconducting linac and decelerated six times for the energy recovery. Design and expected performance of the ERL-HKED are presented.
LBNL, Berkeley, California
UCB, Berkeley, California
ANL, Argonne
A concept for a tunable soft x-ray free electron laser (FEL) oscillator is proposed and studied numerically. It is based on the idea of echo enabled harmonic generation [1] and takes advantage of the oscillator’s ability to start up from spontaneous emission, thereby eliminating the need for optical lasers. In the proposed concept, harmonic tunability is accomplished through beam manipulations using magnetic chicanes and a tunable radiator while two FEL oscillators remain at a fixed frequency. An additional advantage of the proposed technique is the possibility to utilize multilayer x-ray mirrors with a high backward reflectivity of the order of 70%, allowing the initial beam manipulation to be accomplished at a short wavelength, close to the final soft x-ray output. The high repetition rate soft x-ray output is expected to have longitudinal coherence and a narrow bandwidth.
[1] G. Stupakov, PRL, 2009
JLAB, Newport News, Virginia
ATF, Boulder
Mesa+, Enschede
We report on the design, and broadly tunable operation, of a high average power free-electron laser using edge-outcoupling. For this type of outcoupling, the cavity mode has a larger area than the mirror diameter, and the mode ‘spills” around it. While used in positive branch unstable resonators, in this case, the resonator was in a stable configuration. Using an edge-outcoupler composed of an aluminum-coated sapphire substrate, the IR Upgrade FEL at Jefferson Lab achieved a maximum power of 260W at 3.87 microns, with an output power of 20 W or higher from 0.8 to 4.2 microns. Measurements of gain, loss, and output mode are compared with our models.
FOM Rijnhuizen, Nieuwegein
Several of the short-pulse FELs that are operated in a wavelength range starting well below and ending well above 100 microns make use of a partial waveguide in the resonator and a central hole in one of the mirrors for outcoupling. The purpose of the waveguide is to confine the optical mode, in particular within the gap of the undulator. Experimentally, it was found that these FELs suffer from one or a number of tuning 'gaps': narrow wavelength windows within the tuning range where the output is strongly reduced or where the laser even does not turn on. Recently, Prazeres et al.[1] , using a simulation model, were able to reproduce some of the main features of the tuning curve and showed that the cavity outcoupling and losses change abruptly across a tuning gap. In this contribution we will present experimental results for the gain, cavity loss, saturated power and spectral intensity across one of the most prominent gaps in the tuning curve of the FELIX long-wavelength FEL. Both for the normal case and for the case where a slit is used to limit the optical mode extent on the free-space mirror.
[1]. R. Prazeres, F. Glotin, and J.-M. Ortega, PRST-AB, 12 (2009) 010701
Fermilab, Batavia
It is recognized that beam manipulations such as a flat beam transformation followed by an emittance exchange (EEX) could support a high gain free-electron laser (FEL) push for shorter wavelengths. An ongoing program on demonstrating the exchange of transverse horizontal and longitudinal emittances at the Fermilab A0 photoinjector (A0PI) has benefited recently from the upgrade of several of the key diagnostics stations. The use of an array of 50-micron wide slits to sample the phase spaces to measure divergences of less than 100 microradians resulted in 20 times smaller images with positions distributed over several mm. Improvements in the screen resolution term and reduction of the system depth-of-focus impact by using YAG:Ce single crystals normal to the beam direction will be described. On the longitudinal side, the requirements to measure small energy spreads (<10 keV) in the spectrometer and bunch lengths less than 500 fs dictated specifications. Upgrades to the Hamamatsu C5680 streak camera and the addition of the Martin-Puplett interferometer addressed the short bunch lengths. An example of the EEX tables will be presented.
DESY Zeuthen, Zeuthen
DESY, Hamburg
YerPhI, Yerevan
Chiang Mai University, Chiang Mai
A new photocathode electron gun is about to be characterized at PITZ*. It is an L-band normal conducting 1.6 copper cell cavity with improved cooling system. It has the same design as the previously installed gun, characterized at PITZ during the run period 2008/9**. Due to the particle-free surface cleaning method utilizing dry ice, a significant reduction of the dark current was achieved in case of the previously tested cavity. This effect is also expected for the new gun. To improve the accuracy of the RF power measurement and control, a new in-vacuum directional coupler was installed between the T-combiner combining the two 5 MW arms of the RF source and the input coaxial coupler. The new in-vacuum coupler will provide much more accurate information about the RF power in the gun and will allow applying appropriate control feedback. Consequently improved stability of the gun operation is expected. Tuning and conditioning results of this new gun cavity will be presented as well as the results of the measurements of the gradient and the gun phase measurements using this new coupler.
* Photoinjector Test Facility at Zeuthen
** S. Rimjaem et al., EPAC 2008, Genoa, Italy.
RIKEN/SPring-8, Hyogo
JASRI/SPring-8, Hyogo-ken
To measure the femtosecond bunch length (10 - {10}00 fs) of the XFEL facility at SPring-8, we developed a coherent synchrotron radiation (CSR) monitor and a streak camera system. A pyro-electric detector was employed to measure the CSR intensity, since the CSR frequency region is THz or far infra-red. The CSR source is a dipole magnet of a chicane section. For the streak camera, we used Hamamatsu FESCA200, which has 200 fs resolution. The temporal structure of the optical transition radiation (OTR) from a metal mirror is observed by this camera. By using these monitors, the bunch length dependence was measured as a function of the rf phase of an S-band accelerator upstream of the bunch compressor at the SCSS test accelerator. A strong correlation between the CSR intensity and the S-band phase was observed. The CSR intensity was small at a debunching phase and the intensity increased as the rf phase was shifted to the bunching direction. Finally, it decreased in the over-bunching region. The bunch length data from the streak camera also had the same tendency. Thus, the bunch compression characteristics were appropriately measured and were consistent with our simulation results.
UMD, College Park, Maryland
NPS, Monterey, California
NRL, Washington, DC
Photocathodes are a promising electron source for future high average current FELs, with ps response, kA/cm2 peak and A/cm2 average current, but will require delicate cesium-based coatings to achieve requisite quantum efficiency (QE). The UMD dispenser photocathode replenishes cesium from a subsurface reservoir, extending lifetime [1]. Recesiation has been shown to reverse oxidizer-induced QE loss [2]. Optimization of pore size and spacing will enable uniform recesiation without emitting excess cesium into the cavity. We here quantify for the first time cesium emission from active dispenser photocathodes and summarize status of experimental and modeling efforts.
[1] N.A. Moody et.al., Appl. Phys. Lett. 90, 114108 (2007).
[2] E.J. Montgomery et al., AIP Conf. Proc. {10}86, 599 (2009).
HZDR, Dresden
HZB, Berlin
MBI, Berlin
The superconducting RF photo-injector (SRF gun) at FZD is the first operating electron injector of its kind. The gun with a 3½-cell cavity and a frequency of 1.3 GHz produces an electron beam of 3 MeV with a maximum bunch charge of about 400 pC. Also the design values for the acceleration gradient could not be reached with the cavity which is in use at present the SRF gun will improve the beam quality for ELBE users. End of 2009 the beamline was installed which connects the SRF gun with the ELBE accelerator. We will report on the first test and on the progress in applying the SRF gun for user operation.
NPS, Monterey, California
Niowave, Inc., Lansing, Michigan
Boeing Company, Seattle, Washington State
Stanford University, Stanford, California
The Naval Postgraduate School Beam Physics Laboratory, Niowave, Inc., and The Boeing Company have completed construction of a superconducting 500 MHz quarter-wave gun and photocathode drive laser system. This prototype gun went from conception to initial operation in just under one calendar year. Such rapid progress is due in part to the decision to develop the gun as a prototype, deliberately omitting some features, such as tuners and cathode loadlocks, desired for a linac beam source. This will enable validation of the basic concept for the gun, including high-charge bunch dynamics, as rapidly as possible, with lessons learned applied to the next generation gun. This paper presents results from initial testing of the gun, technical challenges of the prototype design, and improvements that would enhance capabilities in future versions of this novel design.
LBNL, Berkeley, California
LLNL, Livermore, California
POSTECH, Pohang, Kyungbuk
The fabrication and installation at the Lawrence Berkeley National Laboratory of a high-brightness high-repetition rate photo-gun, based on a normal conducting 187 MHz (VHF) RF cavity operating in CW mode, is in its final phase. The cavity will generate an electric field at the cathode plane of ~20 MV/m to accelerate the electron bunches up to ~750 keV, with peak current, energy spread and transverse emittance suitable for FEL and ERL applications. The gun vacuum system has been designed for pressures compatible with high quantum efficiency but "delicate" semiconductor cathodes to generate up to a nC bunches at MHz repetition rate with present laser technology. Several photo-cathode/laser systems are under consideration, and in particular photo-cathodes based on K2CsSb are being developed and have already achieved a QE of 8% at 532 nm wavelength, or close to 20% including the Schottky barrier lowering. The cathode will be operated by a microjoule fiber laser in conjunction with refractive optics to create a flat top transverse profile, as well as a birefringent pulse stacker to create a flat top temporal profile. The present status and the plan for future activities are presented.
ELETTRA, Basovizza
INFN/LNF, Frascati (Roma)
Rome University La Sapienza, Roma
DEEI, Trieste
A RF deflector is a useful tool to completely characterize the beam phase space by means of measurements of the bunch length and the transverse slice emittance. At FERMI@Elettra, a soft X-ray next-generation light source under development at the Sincrotrone Trieste laboratory in Trieste, Italy, we are installing low-energy and high-energy deflectors. In particular, two deflecting cavities will be positioned at two points in the linac. One will be placed at 1.2 GeV (high energy), just before the FEL process starts; the other at 250 MeV (low energy), after the first bunch compressor (BC1). This paper concerns only the low-energy deflector. The latter was built over the past year in collaboration with the SPARC project team at INFN-LNF-Frascati, Italy and the University of Rome. In this paper we will describe the RF measurements performed to characterize the standing wave cavity before the installation in the FERMI@Elettra linac, and we will compare them with the simulations done using the electromagnetic code HFSS.
Kyoto IAE, Kyoto
A triode rf gun has been developed aiming at drastic reduction of back-streaming electrons at the thermionic cathode. Thermionic rf gun shows some advantages over photocathode gun such as low cost, easy operation and high average current, which are suitable for oscillator FELs. However, use of thermionic rf gun leads to inherent back-bombardment effect, which not only limits the macro-pulse duration, but also degrades the electron beam quality. In order to reduce the back-streaming electrons, we developed a thermionic triode rf gun which employs coaxial rf cavity much shorter than rf wavelength as the first cell. The phase and amplitude of the electric field for the first cell are independently controlled from successive cells. The results from simulations showed that the back-bombardment power was expected to be reduced by more than 80% without loss of beam brightness. The coaxial rf cavity to be installed in the rf gun for KU-FEL has been developed and a cold test has been performed. In this paper, we will report on the cold test results and comparison of them with the designed performance as well.
ANL, Argonne
We review the development of pulse shaping techniques for high brightness beam generation. A scheme of generating a uniform ellipsoidal laser pulse for s is discussed. The scheme is based on the chromatic aberration of a dispersive lens. Fourier optics simulation reveals the interplay of group velocity delay and dispersion in the scheme, as well as diffractions. Particle tracking simulation shows that the beam generated by such a laser pulse approaches the performance of that by an ideal ellipsoidal laser pulse and represents a significant improvement from the traditionally proposed cylindrical beam geometry. The scheme is tested in an 800-nm, optical proof-of-principle experiment at lower peak power with excellent agreement between the measurement and simulation.
RIKEN/SPring-8, Hyogo
JASRI/SPring-8, Hyogo-ken
An 8-GeV C-band (5712 MHz) accelerator is employed as a main accelerator for XFEL/SPring-8. Since a C-band accelerating structure generates a high accelerating gradient of higher than 35 MV/m, the total length of the accelerator fits within 400 m, including the injector and three bunch compressors. We use 64 C-band rf units, which consist of 128 accelerating structures, 64 rf pulse compressors and waveguide components, 64 klystrons and modulators, etc. Mass-production of the C-band rf components has been done by several Japanese manufacturers. The components reliability has been improved during the production, and all the components finally have excellent quality. The production quality was also confirmed by a high power rf test. We achieved the accelerating gradient of 40 MV/m without any problem. Since XFEL realizes high bunch compression with precise control of the energy chirp, the rf should be quite stable. We developed a high precision high voltage charger combined with a low-noise klystron modulator. The pulse-to-pulse stability of the PFN voltage was less than 0.01%. Installation of the components started in August 2009 and was now almost completed on schedule.
LBNL, Berkeley, California
SLAC, Menlo Park, California
The scientific potential of femtosecond x-ray pulses at linac-driven FELs such as the LCLS is tremendous. Time-resolved pump-probe experiments require a measure of the relative arrival time of each x-ray pulse with respect to the experimental pump laser. To achieve this, precise synchronization is required between the arrival time diagnostic and the laser which are often separated by hundreds of meters. For seeded FELs, synchronization is necessary between the seed and pump laser. We describe an optical timing system based on stabilized fiber links which has been developed for the LCLS. Preliminary results show stability of the timing distribution at the sub-20 fsec level. We present details of the results measured during LCLS operation for the first pump-probe experiment in October 2009 and the present user run starting in April 2010. We conclude with a discussion of potential for development.
USTRAT/SUPA, Glasgow
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
Chicanes placed between undulator modules in a high-gain FEL amplifier have been shown to generate a set of axial modes that may be locked to generate attosecond pulse trains in the x-ray [1]. Using numerical simulations, it is shown in this paper that a similar system of undulator/chicane modules may be used in a low-gain FEL cavity resonator to generate a equally spaced set of frequency modes with a spacing much greater than those of the cavity. As with the high-gain FEL amplifier case, these mode can lock to generate a pulse train.
[1] N.R. Thompson & B.W.J. McNeil, Phys. Rev. Lett. 100, 203901 (2008)
Mesa+, Enschede
Compact, slow-wave, low energy electron beam radiation sources, like Cerenkov free-electron lasers (FELs), emit high power microwaves. However, they seriously degrade in output power, when scaled towards the THz range (0.1-10 THz). This prevents industry from applying THz radiation, although it would allow many new applications, like chemical selective security surveillance. The photonic free-electron laser (pFEL) is a promising concept for a handheld, tunable and Watt-level THz laser. In a pFEL several electron beams stream through a photonic crystal (PhC) leading to the emission of coherent Cerenkov radiation. The beams emit phase-locked due to the transverse scattering inside the PhC, which allows increasing the output power by increasing the number of beams streaming through the PhC. Therefore, scaling the pFEL’s operating frequency towards THz frequencies can be done without loss in output power. Furthermore, compact, low energy electron sources (< 15 keV) can drive the laser, due to the strong deceleration of the light by PhC’s. As a proof of principle, we developed the setup for a pFEL operating at 20 GHz to study the interaction between a single electron beam and the PhC.
ELETTRA, Basovizza
DEEI, Trieste
The cavity Beam Position Monitor (BPM) is a fundamental beam diagnostic instrument for a seeded FEL, like FERMI@Elettra. It allows the measurements of the electron beam trajectory in a non-destructive way and with sub-micron resolution. The high resolution cavity BPM relies on the excitation of the dipole mode that is originated when the bunch passes off axis in the cavity. In this paper we present the prototype of cavity BPM developed for the FERMI@Elettra facility. The RF parameters of the cavities have been determined by means of Ansoft HFSS; while using the CST Particle Studio the level of the output signals from the cavities have been also estimated. Furthermore, the design of the RF frontend for the acquisition and conditioning of the signals from the BPM cavities is presented as well. The prototype has been succesfully installed in the FERMI Linac during the last commissioning phase and preliminary results with the electron beam are also presented.
ANL, Argonne
European XFEL GmbH, Hamburg
Free-electron lasers for hard x-rays can be constructed in oscillator (XFELO) configuration, providing ultra-high spectral purity and brightness [1]. The average brightness is expected to be several orders of magnitude higher than, and peak brightness comparable to that of SASE XFELs. XFELOs can enable revolutionary scientific opportunities as well as drastically improve experimental techniques developed at third-generation x-ray facilities. Low-loss x-ray crystal cavity and ultra-low-emittance electron beams are two major technical challenges in the realization of XFELOs. The requirements to x-ray cavity components are demanding: diamond crystals and curved grazing incidence mirrors must have near-perfect reflectivity, negligible wave-front distortions, and are subject to very tight tolerances on angular, spatial, and thermal stability under high heat load of the XFELO radiation. This paper gives an overview on the recent progress [2-4] and future plans in the R&D on the feasibility of x-ray cavities for XFELOs. The experimental and simulation studies results provide strong evidence for the feasibility of the x-ray cavities.
1. K-J. Kim, et al, PRL 100 (2008) 244802
2. Yu. Shvyd'ko, et al, Nature Phys. 6 (2010) 196
3. S. Stoupin, Yu. Shvyd'ko, PRL 104 (2010) 085901
4. S. Stoupin et al, Rev. Sci. Instr. 81 (2010) 055108