SASE FEL Research Articles

First observations of electron-beam microbunching in the deep ultraviolet at 265 nm using COTR in a SASE FEL

We have recently extended our microbunching experiments in a self-amplified spontaneous emission free-electron laser using coherent optical transition radiation (COTR) to the deep ultraviolet wavelengths (265 nm) for the first time. These experiments were performed as a complement to the Advanced Photon Source SASE FEL project's thrust to shorter wavelengths. In order to do this, the optical diagnostics have been modified to include UV-sensitive cameras, and an optical transport has been installed that involves sets of mirrors and UV–visible lens pairs after each undulator. These optics provide transport to the in-tunnel Oriel UV–visible spectrometer. Since this is an imaging spectrometer, both spectral and x/ y-plane spatial information are simultaneously available by performing the projections of the images on the x- and y-axis. The initial angular distribution data and beam size data have been obtained in a z-dependent manner by sampling after every other undulator, or every 4.8 m at four z locations. Experiments with sampling of the COTR angular distribution and spectra after each of eight undulators are planned.

Transverse-emittance measurements on an S-band photocathode RF electron gun

Proposed fourth-generation light sources using SASE FELs to generate short pulse, coherent, X-rays require demonstration of high brightness electron sources. The gun test facility at SLAC was built to test high brightness sources for the proposed linac coherent light source at SLAC. The transverse-emittance measurements are made at nearly 30 MeV by measuring the spot size on a YAG screen using the quadrupole scan technique. The emittance was measured to vary from 1 to 3.5 mm mrad as the charge is increased from 50 to 350 pC using a laser pulse width of 2 ps FWHM. The measurements are in good agreement with simulation results using the LANL version of PARMELA.

Development of a femtosecond soft X-ray SASE FEL at DESY

In this paper we describe the extension of the soft X-ray SASE FEL at the TESLA Test Facility (TTF) at DESY for generation of femtosecond pulses. The proposed scheme operates as follows. The first stage is a conventional FEL amplifier seeded by 523 nm external laser. A zero area optical pulse (i.e. the pulse with zero value of optical field in the central area of the pulse) is timed to overlap with the electron bunch. Radiation power is amplified up to the saturation level. Following the first stage the electron beam enters the main 6 nm SASE undulator. Large energy spread is induced in the significant fraction of the electron beam due to the FEL interaction process, and only a small part of the electron bunch (near the center of zero area light pulse) is able to produce radiation in the 6 nm SASE FEL. The SASE FEL described in this paper will provide soft X-ray pulses with 30 fs (FWHM) duration. On the basis of the TTF parameters it should be possible to achieve an average brilliance of 10 22 photons/ s 1/ mrad 2/ mm 2/ (0.1% BW). The average number of photons can exceed 10 12 photon/pulse.

Statistical properties of SASE FEL radiation: experimental results from the VUV FEL at the TESLA test facility at DESY

This paper presents an experimental study of the statistical properties of the radiation from a SASE FEL. The experiments were performed at the TESLA Test Facility VUV SASE FEL at DESY operating in a high-gain linear regime with a gain of about 10 6. It is shown that fluctuations of the output radiation energy follows a gamma-distribution. We also measured for the first time the probability distribution of SASE radiation energy after a narrow-band monochromator. The experimental results are in good agreement with theoretical predictions, the energy fluctuations after the monochromator follow a negative exponential distribution.

Present status and recent results from the APS SASE FEL

The Low-Energy Undulator Test Line (LEUTL) at the Advanced Photon Source, Argonne National Laboratory, is intended to demonstrate the basic operation of a SASE-based free-electron laser. Goals include comparison of experimental results with theoretical predictions and scaling laws, identification of problems relevant to fourth-generation light source construction and operation and the means of addressing them, the development of operational and diagnostic techniques to optimize SASE FEL performance and increase repeatability from run to run, and performance of initial pioneering experiments capable of exploiting the unique properties of the laser. The basic layout and operational philosophy of the LEUTL experiment is presented. A summary of past results, including saturation, is reviewed, and a description of recent results is presented. We conclude with future plans, which include pressing to shorter wavelengths and incorporating user experiments into the LEUTL experimental program.

Statistical correlations and intensity spiking in the SASE FEL

In the linear regime before saturation, we describe the statistical correlations in the narrow band chaotic output of the self-amplified spontaneous emission free-electron laser. At a fixed position along the undulator axis, we derive joint probability distributions for the intensity in the output pulse to have values I 1 and I 2 at times t 1 and t 2, and for the spectral intensity to have values I ̃ 1 and I ̃ 2 at frequencies ω 1 and ω 2. Probability distributions for the peak values of intensity in the time and frequency domains are also determined.

Generation of high power femtosecond pulses by a sideband-seeded X-ray FEL

New proposal of a fs X-ray facility, which is described in this paper, is based on the use of X-ray SASE FEL combined with a fs quantum laser. An ultrashort laser pulse is used for modulation of the energy and density of the electrons within a slice of the electron bunch at a frequency ω opt. The density modulation exiting the modulator (energy-modulation undulator and dispersion section) is about 10%. Following the modulator the beam enters an X-ray SASE FEL undulator, and is bunched at a frequency ω 0. This leads to an amplitude modulation of the beam density at the sidebands ω 0± ω opt. The sideband density modulation takes place at the part of the electron pulse defined by the duration of the seed laser pulse that is much shorter than the electron pulse. Following the SASE FEL undulator the beam and SASE radiation enter undulator section (radiator) which is resonant at the frequency ω 0− ω opt. Because the beam has a large component of bunching at the sideband, coherent emission is copiously produced within fs slice of the electron bunch. Separation of the sideband frequency from the central frequency by a monochromator is used to distinguish the fs pulses from the sub-ps intense SASE pulses.

Laser driven attosecond SASE X-ray FEL

Modern lasers with emission wavelengths ∼1 μm can be used as electromagnetic wigglers and have sufficient output power to obtain wiggler parameters K (normalized vector potential) close to unity. With this kind of wiggler, in order to make a 10-keV X-ray SASE FEL, electron beam energies of ∼25 MeV are sufficient. In this presentation, scaling laws for the operation of FELs will be discussed. Using these scaling laws, requirements for electron sources will be obtained. The required electron beams must be about six orders of magnitude brighter than those available with existing sources. We propose a novel source based on vacuum laser pondermotive acceleration that will produce an electron beam meeting these requirements. This opens up a realistic possibility of realizing the dream of a “table top” X-ray SASE FEL.

Initial gain measurements of an 800 nm SASE FEL, VISA

The Visible to Infrared SASE Amplifier (VISA) FEL is designed to obtain high gain at a radiation wavelength of 800 nm. The FEL uses the high brightness electron beam of the Accelerator Test Facility (ATF), with energy of 72 MeV. VISA uses a novel, 4 m long, strong focusing undulator with a gap of 6 mm and a period of 1.8 cm. To obtain large gain the beam and undulator axis have to be aligned to better than 5 μm. Results from initial measurements on the alignment, gain, and spectrum will be presented and compared to theoretical calculations and simulations.

Proposal for a IR waveguide SASE FEL at the PEGASUS injector

Free Electron Lasers up to the visible regime are dominated by diffraction effects, resulting in a radiation size much larger than the electron beam. Thus the effective field amplitude at the location of the electron beam, driving the FEL process, is reduced. By using a waveguide, the radiation field is confined within a smaller aperture and an enhancement of the FEL performance can be expected. The PEGASUS injector at UCLA will be capable to provide the brilliance needed for an IR SASE FEL. The experiment Power Enhanced Radiation Source Experiment Using Structures (PERSEUS) is proposed to study the physics of a waveguide SASE FEL in a quasi 1D environment, where diffraction effects are strongly reduced as it is the case only for future FELs operating in the VUV and X-ray regime. The expected FEL performance is given by this presentation.

Development of a coherent transition radiation-based bunch length monitor with application to the APS RF thermionic gun beam optimization

We report further development of an EPICS-compatible bunch length monitor based on the autocorrelation of coherent transition radiation (CTR). In this case the monitor was used to optimize the beam from the S-band thermionic RF gun on the Advanced Photon Source (APS) linac. Bunch lengths of 400–500 fs (FWHM) were measured in the core of the beam, which corresponded to about 100-A peak current in each micropulse. The dependence of the CTR signal on the square of the beam charge for the beam core was demonstrated. We also report the first use of the beam accelerated to 217 MeV for successful visible wavelength SASE FEL experiments.

Different focusing solutions for the TTF-FEL undulator

While aiming at shorter wavelengths, the SASE-FEL undulators become longer. Therefore, maintaining the small electron beam sizes required, the question of what kind of focusing structures should be used for optimum performance becomes of increasing interest. Present SASE FEL undulators that have been developed and proposed for future FELS both include integrated focusing (TTF-FEL), separate function quadrupoles combined with the natural undulator focusing (LEUTL) and triplet or FODO structures. In this contribution, the consequence of the different undulator designs in terms of alignment (tolerances) and flexibility will be discussed.

Development of a pump-probe facility with sub-picosecond time resolution combining a high-power ultraviolet regenerative FEL amplifier and a soft X-ray SASE FEL

This paper presents the conceptual design of a high power radiation source with laser-like characteristics in the ultraviolet spectral range at the TESLA Test Facility (TTF). The concept is based on the generation of radiation in a regenerative FEL amplifier (RAFEL). The RAFEL described in this paper covers a wavelength range of 200–400 nm and provides 200 fs pulses with 2 mJ of optical energy per pulse. The linac operates at 1% duty factor and the average output radiation power exceeds 100 W. The RAFEL will be driven by the spent electron beam leaving the soft X-ray FEL, thus providing minimal interference between these two devices. The RAFEL output radiation has the same time structure as the X-ray FEL and the UV pulses are naturally synchronized with the soft X-ray pulses from the TTF FEL. Therefore, it should be possible to achieve synchronization close to the duration of the radiation pulses (200 fs) for pump-probe techniques using either an UV pulse as a pump and soft X-ray pulse as a probe, or vice versa.

Light source performance achievements

More and more third generation light sources (3GLS) have come into operation. All of them were successful in reaching their performances. Some of them, like the ESRF, achieved brilliances up to two orders of magnitude higher than their initial target. Experience with the operation of existing machines indicates some imperfections and possibilities for new projects to integrate possible corrections in their design. These include: operation at the diffraction limit with even higher brilliances, higher position stability, enhanced Touschek lifetimes with larger momentum acceptances combined with low chromaticity and feedback of coupled bunch transverse resistive wall instability, efficient control of the longitudinal coupled bunch instability, higher currents in single bunch, better signal to noise ratio by localising losses at selected places in the ring and avoiding the emission of high energy bremsstrahlung photons in the direction of beamlines, possibilities of permanent injection, production of focused photon beams, high quality insertion devices, etc. Over the last 5 years, intensive R&D was dedicated to linac driven SASE FELs. They are considered as genuine 4th generation light sources. Results are encouraging, but there is still a certain way to go before planning an X-ray FEL facility. Therefore, rings are likely to remain for a long time the sources of excellence for users.

Diffraction effects in the self-amplified spontaneous emission FEL

In this paper we present a systematic approach for analytical description of SASE FEL (SASE: self-amplified spontaneous emission) in the linear mode. We calculate the average radiation power, radiation spectrum envelope, angular distribution of the radiation intensity in far zone, longitudinal and transverse correlation functions, degree of transverse coherence etc. Using the results of analytical calculations presented in reduced form, we analyze various features of the SASE FEL in the linear mode. The general result is applied to the special case of an electron beam having Gaussian profile and Gaussian energy distribution. These analytical results can serve as a primary standard for testing the codes. In this paper we present numerical study of the process of amplification in the SASE FEL using three-dimension time-dependent code FAST. Comparison with analytical results shows that in the high-gain linear limit there is good agreement between the numerical and analytical results. It has been found that even after finishing the transverse mode selection process the degree of transverse coherence of the radiation from SASE FEL visibly differs from unity. This is consequence of the interdependence of the longitudinal and transverse coherence. The SASE FEL has poor longitudinal coherence which develops slowly with the undulator length thus preventing a full transverse coherence.

Undulators for SASE FELs

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

Numerical research on SASE FEL using a multi-frequency SDE code

A multi-frequency 2D SASE FEL simulation code has been developed using the Source Dependent Expansion (SDE) method. With this code we calculated the evolution of the radiation amplitude and the spectrum of a SASE FEL using the beam parameters of FELI and the micro-wiggler developed by ILT/ILE/OSU. The effect of beam energy spread is also clarified.

Status and initial commissioning of a high gain 800 nm SASE FEL

We describe the status and initial commissioning of the Visible to Infrared SASE Amplifier (VISA) experiment. VISA uses a strong focusing 4 m undulator, the Brookhaven National Laboratory ATF linac with an energy of 72 MeV, and a photoinjector electron source. The VISA fundamental radiation wavelength is near 800 nm and the power expected at saturation is near 60 MW. Power, angular and spectral measurements are planned for the VISA radiation and these results will be analyzed and compared with SASE FEL theory and computer simulation. In addition, the induced electron beam micro-bunching will be measured using coherent transition radiation.

An RF-gun-driven recirculated linac as injector and FEL driver

A new pre-injector for the MAX-Laboratory is under design and construction. A thermionic RF gun, designed to operate at medium currents with low back bombardment power, is under construction. The gun will, via a magnetic compressor and energy filter, feed a recirculated linac consisting of two SLED-equipped structures giving 125 MeV each. The first will be delivered in 1999. The system is aimed as a pre-injector for the existing storage rings at MAX-Lab, but will also open up possibilities for a SASE FEL in the UV reaching above 100 MW below 100 nm.

First operation of cesium telluride photocathodes in the TTF injector RF gun

During the run 1998/1999 a new injector based on a laser-driven RF gun was brought in operation at the TESLA Test Facility (TTF) linac at DESY, in order to produce the beam structure and quality required either by TeV collider and SASE FEL experiments. High quantum efficiency cesium telluride photocathodes, prepared at Milano and transferred to DESY, have been successfully operated in the RF gun. A bunch charge of 50 nC, only limited by space charge effects, was achieved. The photocathodes have shown an operative lifetime of several months. A new cathode surface finishing has showed a promising decrease of the photocathode dark current. Measurements of dark current, quantum efficiency and lifetime are reported.