SASE FEL Research Articles

Scheme for time-resolved experiments based on the use of statistical properties of the third harmonic of the SASE FEL radiation

A closer inspection of the statistical properties of the third-harmonic radiation from the SASE FEL reveals that it is possible to select single, temporary coherent radiation spikes by using a simple intensity trigger. A carefully designed optical system for splitting, delaying, filtering, and recombining the radiation would then allow time-resolved measurements with resolution down to the coherence time of the FEL, i.e. a few femtoseconds in the case of the TTF FEL.

Correlation function equation for the SASE FEL

In this paper, we develop a new approach to the SASE FEL theory. It is based on the well-known system of Klimontovich–Maxwell equations. The special choice of the independent variable makes it possible to eliminate both electromagnetic field and retardation. That allows us to write down the BBGKY hierarchy for the s-particle distribution functions. Due to the smallness of the higher-order correlations we consider only two equations from the beginning of this hierarchy.

Results of the VISA SASE FEL experiment at 840 nm

VISA (Visible to Infrared SASE Amplifier) is a high-gain self-amplified spontaneous emission FEL, which achieved saturation at 840nm within a single-pass 4-m undulator. A gain length shorter than 18cm has been obtained, yielding the gain of 2×108 at saturation. The FEL performance, including spectral, angular, and statistical properties of SASE radiation, has been characterized for different electron beam conditions. The results are compared to 3-D SASE FEL theory and start-to-end numerical simulations of the entire injector, transport, and FEL system. Detailed agreement between simulations and experimental results is obtained over the wide range of the electron beam parameters.

VUV FEL driven RF gun

In this paper we describe the regeneration of electron bunches from the RF gun by back-reflected radiation from VUV SASE FEL at the TESLA Test Facility (TTF) at DESY. The SASE FEL was running at the wavelength 96nm with 30–100fs pulses, SASE pulse energy was up to 20μJ. “Nominal” electron bunches for lasing were produced by RF gun with Cs2Te photocathode driven by UV quantum laser system. “Parasitic” bunches with a charge up to 1–1.5nC were extracted from the RF gun due to VUV FEL radiation reflected from the mirror (placed downstream of the undulator) to the cathode. Nontrivial dependence of a charge on a SASE pulse energy was found.

Observations of z-dependent microbunching harmonic intensities using COTR in a SASE FEL

The nonlinear generation of harmonics in a self-amplified spontaneous emission free-electron laser continues to be of interest. Complementary to such studies is the search for information on the electron beam microbunching harmonic components, which are revealed by coherent optical transition radiation experiments. An initial z-dependent set of data has been obtained with the fundamental at 530nm and the second harmonic at 265nm. The latter data were collected after every other undulator in a nine-undulator string. These results are compared to estimates based on GINGER and an analytical model for nonlinear harmonic generation.

Photon ring multi-user distribution system for soft X-ray SASE FEL laboratory

It is shown that although the soft X-ray SASE FEL photon beam from undulator is in principle a single user tool, just like an optical laser, the optical distribution system based on multifacet reflectors provide efficient ways to generate a multi-user facility—very similar to present-day synchrotron radiation facilities. Multifacet reflectors involve multiple reflections from a series of plane mirrors. These make use of the principle of total external reflectance. The candidate mirror materials include C, Mo, and Si which should yield a reflectance of 40–80% for an arc angle of 90° in the wavelength range 6–100nm. Multifacet reflector arcs can be repeated a number of times to form a complete ring. In order to make efficient use of the new SASE FEL source, it is proposed to install this photon ring distribution system in the experimental hall of soft X-ray FEL laboratory. A few tens beam lines with different experiments can be served by a single FEL source. Using movable mirrors in each photon ring cell, it is possible to quickly switch the FEL photon beam from one experiment to the other, thus providing simultaneous multi-user capability.

Bunching and exotic undulator configurations in SASE FELs

We discuss non-conventional undulator configurations for SASE FEL devices aimed at enhancing the emission on higher harmonics, reducing the saturation length and operating in less sensitive conditions for the electron beam qualities.

Coherence properties of the radiation from SASE FEL

We present a comprehensive analysis of coherence properties of radiation from a SASE FEL. The effective transverse correlation function is calculated by means of numerical simulations with the code FAST. This allows us to calculate the area and degree of transverse coherence. Evolution of these parameters is traced from the beginning of the undulator up to the deep nonlinear regime. It is shown that the degree of transverse coherence reaches maximum value at the saturation point. Then, it drops drastically because of poor transverse coherence of the radiation produced in the nonlinear regime.

Statistical properties of radiation from SASE FEL driven by short electron bunches

We analyze statistical properties of the radiation from a SASE FEL driven by short electron bunches. In the linear regime the radiation from a SASE FEL is a Gaussian random process. When approaching saturation point, statistical properties of the radiation change drastically on a scale of one field gain length. Particular attention is devoted to the analysis of fluctuations of total energy in the radiation pulse and after a narrow-band monochromator. It was found that fluctuations at saturation are significantly suppressed when electron pulse length becomes comparable with cooperation length.

Spectral measurements of the second harmonic of the SASE FEL radiation at APS

We present the z-dependent spectral measurement results for the second harmonic of the SASE FEL radiation before and after saturation. The measurements were performed at the Advanced Photon Source FEL with the fundamental wavelength of 530nm. The spectral properties of the second harmonic are compared with those of the fundamental and with the theoretical expectations.

New results and prospects for harmonic generation in storage ring FELs

In this paper we review new results and prospects for harmonic generation in storage ring (SR) FELs. Traditionally, high harmonics were generated in SR FELs using conventional (IR or visible) pulsed lasers as a driver. Discovery of super-pulses with GW levels of intra-cavity peak power provided for new ways of generating coherent harmonics in the VUV and X-ray ranges. Super-pulses are generated in the OK-4/Duke SR FEL using gain modulation technique. As we reported at the previous FEL conference, the intra-cavity power on the OK-4 FEL was sufficient for effective generation of third and higher harmonics in the fundamental FEL wavelength. Recently, the OK-4/Duke SR FEL generated second, third, fourth, fifth and seventh coherent harmonics in the range from 37 to 135nm. The tunability of the FEL wavelength provided for natural wavelength tunability of the harmonic radiation.Recent experimental results of the UV and VUV harmonic generation in the OK-4/Duke SR FEL demonstrated peak spectral brightness comparable with VUV SASE FELs. We discuss the prospects of the intra-cavity short-wavelength harmonic generation in SR FEL.

Fundamental and harmonic microbunching in a high-gain self-amplified spontaneous-emission free-electron laser.

Electron beam microbunching in both the fundamental and second harmonic in a high-gain self-amplified spontaneous emission free-electron laser (SASE FEL) was experimentally characterized using coherent transition radiation. The microbunching factors for both modes (b(1) and b(2)) approach unity, an indication of FEL saturation. These measurements are compared to the predictions of FEL simulations. The simultaneous capture of the microbunching and SASE radiation for individual micropulses correlate the longitudinal electron beam structure with the FEL gain.

Scheme for time-resolved experiments based on the generation of femtosecond pulses by a sideband-seeded soft X-Ray SASE FEL

In this paper, we describe the extention of the soft X-ray SASE FEL facility for two-color time-resolved experiments. A new type of pump–probe technique is proposed which is based on the use of soft X-ray SASE FEL combined with an 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 time-compensated monochromator is used to distinguish the fs pulses from the sub-ps intense SASE pulses. Finally, the fs soft X-ray pulses are combined with optical pulses generated in the seed laser system. In the proposed scheme for pump–probe experiments, fs soft X-ray pulse is naturally synchronized with his fs optical pulse and cancel jitter.

Z-dependent spectral measurements of SASE FEL radiation at APS

We report on the first measurements of the dependence on the axial distance z of the spectrum of self-amplified spontaneous emission free-electron laser (SASE FEL) radiation. The measurements are performed under different FEL conditions in the wavelength range from 265 to 530 nm. Spectral measurements of the radiation at FEL saturation are included. The z-dependence of the radiation spectrum is compared with theory and simulations.

The BESSY FEL project

Berliner Elektronenspeicherring-Gesellschaft für 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 (<20 fs RMS) optical pulses.

Measurements of nonlinear harmonic generation at the Advanced Photon Source's SASE FEL

SASE saturation was recently achieved at the Advanced Photon Source's SASE FEL in the low-energy undulator test line at 530 nm and 385 nm. The electron beam microbunching becomes more and more prominent until saturation is achieved. This bunching causes nonlinear harmonic emission that extends the usefulness of a SASE system in achieving shorter FEL wavelengths for the same electron beam energy. We have investigated the intensity of the fundamental and second harmonic undulator radiation as a function of distance along the undulator line and present the experimental results and compare them to numerical simulations. In addition, we have measured the single-shot second harmonic spectra as well as the simultaneous fundamental and second harmonic spectra and present the experimental results.

Undulator systems for the TESLA X-FEL

A large X-ray FEL lab is under consideration within the TESLA project and is supposed to be operated in parallel with the TESLA linear collider. There will be five SASE FELs and five conventional spontaneous undulators. A conceptual design study has been made for the undulator systems for these X-FELs. It includes segmentation into 6.1 m long undulator ‘cells’. Each consists of a 5 m long undulator ‘segment’, a separate quadrupole, one horizontal and one vertical corrector, and a phase shifter. These items are presented and discussed.

Photon statistics of the SASE FEL

The photon statistics of the free-electron laser radiation are examined. The photon number moments are calculated for the multimode self-amplified spontaneous emission including the effects of the Gaussian free-electron laser spectral distribution. It is shown that, in the parameter regime where the pulse duration is of the order of the coherence time, the expectation value of the photon number moments deviate from those predicted by a negative-binomial probability distribution.

High-efficiency ultrashort pulse generation in a high-gain FEL oscillator near the perfect synchronism

It has been verified experimentally in JAERI-FEL that a high-gain FEL oscillator has the maximum extraction efficiency at the perfect synchronism of optical-cavity length. The simultaneous measurement of FEL efficiency and absolute cavity length has clearly shown that a sharp peak of detuning curve at the perfect synchronism appears in high-gain and low-loss regime and the FEL extraction efficiency exceeds the scaling law of short-bunch FEL oscillators. A numerical analysis indicates that lasing at the perfect synchronism is quasi-stationary superradiance with random fluctuations, which is analogous to a SASE FEL. Second-order autocorrelation measurements show that FEL pulses shorter than four optical cycles are generated successively for a number of round trips at the perfect synchronism, which is consistent with numerical results.

Alignment of the optical feedback system of VUV regenerative FEL amplifier at the TESLA test facility at DESY

In this paper, we describe optical feedback system of VUV Regenerative FEL Amplifier (RAFEL) at the TESLA test facility at DESY. The aim of the RAFEL experiment is to construct fully coherent, tunable VUV radiation source by means of applying narrow-band optical feedback in the VUV SASE FEL operating currently at DESY. One of the problem of the realization of the RAFEL is severe requirements for the angular stability of the optical elements (about few microradians). This problem has been solved by means of installation of active alignment system with reference laser. Another problem is alignment of optical elements separated by 65 m within complicated experimental conditions connected with aperture limitations (down to 6 mm) . This problem has been solved in two steps. Preliminary alignment with an accuracy of about 80 μrad has been performed with laser alignment system and OTR screens used at the TTF accelerator for electron beam diagnostics. Final alignment has been performed with VUV SASE FEL radiation. Measured feedback coefficient is about 1 percent and is in agreement with the designed value.