FEL Gain Research Articles

THz SASE FEL at PITZ: lasing at a wavelength of 100 μm

Development of an accelerator-based tunable THz source prototype for pump-probe experiments at the European XFEL is ongoing at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The proof-of-principle experiments on the THz SASE FEL are performed utilizing the LCLS-I undulator (on loan from SLAC) installed in the PITZ beamline. The first lasing at a center wavelength of 100 μm was observed in the summer of 2022. The lasing of the narrowband THz source was achieved using an electron beam with an energy of ∼17 MeV and a bunch charge up to several nC. Optimization of beam transport and matching resulted in the measurement of THz radiation with a pulse energy of tens of μJ, measured with pyroelectric detectors. The THz FEL gain curves were measured by means of specially designed short coils along the undulator. The results of the first characterization of the THz source at PITZ will be presented.

Reconstruction of x-ray free-electron laser pulse duration and energy chirp from spectral intensity fluctuations

Accurate measurements of the x-ray pulse duration produced by x-ray free-electron lasers (XFELs) typically rely on longitudinal electron beam phase space diagnostics, e.g., an x-band transverse deflecting cavity (XTCAV). An alternative group of methods is based on measurements of radiation statistics and spectral correlations. In this paper, we present a statistical analysis of FEL radiation which extends the previous technique by Lutman et al. [Phys. Rev. ST Accel. Beams 15, 030705 (2012)], including the electron beam energy chirp in the FEL gain model. In doing so, we show that with measurements of the average spectrum and the spectral intensity correlations, one can reconstruct the x-ray pulse length, e-beam chirp, and corresponding spectrometer resolution. We validate our approach with 1D and 3D simulations before moving to an experimental demonstration in which our method finds results similar to those obtained from XTCAV measurements.

Misalignment effects on the performance and stability of x-ray free electron laser oscillator

We report on the effects of transverse spatial misalignment on the performance and stability of x-ray free electron laser oscillators (XFELOs). For this study, we adopt the FEL driven paraxial resonator model in which the transverse profile of the radiation field is represented by Gauss-Hermite mode expansion. Then, we apply Gaussian optics for transporting the radiation field along with the misalignment in the optical cavity. We divide the misalignment into relevant categories of static and periodic types to understand their effects on FEL gain, lasing, and saturation. After that, we associate the first order moments of the radiation field to that of driven simple harmonic oscillator models to identify regions of instability. Throughout this report, we validate our models and analyses via theoretical calculations, analytical approximations, and simulations.

Investigation of electron energy spread effects on the intracavity MIR-FEL power at the PBP-CMU electron linac laboratory

At the PBP-CMU Electron Linac Laboratory, the existing linear accelerator system is under developing to produce electron beams with suitable properties for generation of oscillator mid-infrared free-electron laser (MIR-FEL) and super-radiant terahertz (THz) undulator radiation. This work focuses on the simulation of the oscillator MIR-FEL with expected electron beam energy of 20 – 25 MeV to evaluate the required electron beam parameters. To achieve FEL saturation, the single-pass FEL simulation using the code GENESIS 1.3 was performed to estimate the laser wavelength, FEL gain and electron macro-pulse duration. In the simulation, electron beams with energy spread of 0, 0.5 and 1.0 %, and the electron peak current of 25 and 50 A were considered. As a result, the electron beam with energy of 20 – 25 MeV can produce the FEL wavelengths of 13.5 – 21.2 µm using a 1.6-m undulator with maximum undulator parameter of 1.07. The largest FEL gain is 216% when using electron beam energy and peak current of 20 ± 0% MeV and 50 A, which requires the electron macro-pulse duration of 1.6 µs to saturate FEL. Due to the possible macro-pulse duration at our facility of 6 µs, the operation of electron beam with energy spread of 0.5% and peak current of higher than 25 A is required to saturate FEL. Results from this single-pass simulation can be used as a based information for further practical consideration with multi-pass simulation.

Investigation of Electron Beam Parameter in Seeded THz-FEL Amplifier using Photocathode RF Gun

Simple, compact, and powerful seeded terahertz-free electron laser (THz-FEL) which consists of a photocathode radio-frequency gun, a focusing solenoid, an undulator and a THz wave parametric generator has been designed. A start-to-end calculation has been performed by using the particle tracking code PARMELA and the FEL gain simulation code GENESIS1.3. To obtain a higher FEL gain, we searched a better condition of the electron beam parameter and the seed light. In the present design scheme, the FEL peak power was calculated to be 3kW at the FEL resonant wavelength of 179 m. Since the power of the seed light was 0.20W, the light power was amplified almost 104 times.

Generation of doublet spectral lines at self-seeded X-ray FELs

Self-seeding schemes, consisting of two undulators with a monochromator in between, aim to reduce the bandwidth of SASE X-ray FELs. We recently proposed to use a new method of monochromatization exploiting a single crystal in Bragg transmission geometry for self-seeding in the hard X-ray range. The obvious and technically possible extension is to use such kind of monochromator setup with two (or more) crystals arranged in a series to spectrally filter the SASE radiation at two (or more) closely-spaced wavelengths within the FEL gain band. This allows for the production of doublet (or multiplet) spectral lines. Exploitations of such mode of operation involve any situation where there is a large change in cross-section over a narrow wavelength range. In this paper we consider the simultaneous operation of the LCLS hard X-ray FEL at two closely spaced wavelengths. We present simulation results for the LCLS baseline, and we show that this method can produce fully coherent radiation shared between two longitudinal modes. Mode spacing can be easily tuned within the FEL gain band, i.e. within 10 eV. An interesting aspect of the proposed scheme is a way of modulating the electron bunch at optical frequencies without a seed quantum laser. In fact, the XFEL output intensity contains an oscillating “mode-beat” component whose frequency is related to the frequency difference between the pair of longitudinal modes considered. Thus, at saturation one obtains FEL-induced modulations of energy loss and energy spread in the electron bunch at optical frequency. These modulations can be converted into density modulation at the same optical frequency with the help of a weak chicane installed behind the baseline undulator. Powerful coherent radiation can then be generated with the help of an optical transition radiation (OTR) station, which have important applications. In this paper we briefly consider how the doublet structure of the XFEL generation spectra can be monitored by an optical spectrometer. Furthermore, the OTR coherent radiation pulse is naturally synchronized with the X-ray pulses, and can be used for timing the XFEL to high power conventional lasers with femtosecond accuracy for pump-probe applications.

Lasing at 12 µm Mid-Infrared Free-Electron Laser in Kyoto University

Laser amplification using a 12 µm mid-infrared free-electron laser (MIR-FEL) was observed at the Institute of Advanced Energy (IAE), Kyoto University. A 25 MeV electron beam of 17 A peak current was used for the lasing experiment. A beam loading compensation method with an RF amplitude control in the thermionic RF gun was used to extend the macropulse duration against the backbombardment effect in the thermionic RF gun. As a result, an electron beam with a 4 µs duration was generated. A laser output with an intensity 50 times as high as the spontaneous emission intensity was observed. FEL gain was estimated to be 16% from the exponential growth of the laser output signal, and a cavity loss of 2.8% was estimated from the decay of the laser output signal. Three-dimensional (3D) FEL simulation was also performed to achieve the gain saturation in our FEL device.

Extension in far-infrared of the CLIO free-electron laser

The infrared free-electron laser offers the advantage of a potential large tunability since the FEL gain itself remains substantially high throughout the infrared spectral range, and the reflectivity of metal mirrors used in the optical cavity remains also close to unity. The main limitation comes from the diffraction of the optical beam due to the finite size of the vacuum chamber of the undulator and other optical cavity elements. At CLIO, we have optimized the accelerator from 50 to 8 MeV, with a satisfactorily high electron current. By using toroidal mirrors, we take advantage of optical guiding at long wavelengths in the undulator vacuum chamber. We report here the results of the first measurements with this new configuration, leading to an FEL tunable from 3 to 120 μm.

Self-amplified spontaneous emission FEL with energy-chirped electron beam and its application for generation of attosecond x-ray pulses

Influence of a linear energy chirp in the electron beam on a SASE FEL operation is studied analytically and numerically using 1-D model. Explicit expressions for Green's functions and for output power of a SASE FEL are obtained for high-gain linear regime in the limits of small and large energy chirp parameter. Saturation length and power versus energy chirp parameter are calculated numerically. It is shown that the effect of linear energy chirp on FEL gain is equivalent to the linear undulator tapering (or linear energy variation along the undulator). A consequence of this fact is a possibility to perfectly compensate FEL gain degradation, caused by the energy chirp, by means of the undulator tapering independently of the value of the energy chirp parameter. An application of this effect for generation of attosecond pulses from a hard X-ray FEL is proposed. Strong energy modulation within a short slice of an electron bunch is produced by few-cycle optical laser pulse in a short undulator, placed in front of the main undulator. Gain degradation within this slice is compensated by an appropriate undulator taper while the rest of the bunch suffers from this taper and does not lase. Three-dimensional simulations predict that short (200 attoseconds) high-power (up to 100 GW) pulses can be produced in Angstroem wavelength range with a high degree of contrast. A possibility to reduce pulse duration to sub-100 attosecond scale is discussed.

Observation of Anomalously Large Spectral Bandwidth in a High-Gain Self-Amplified Spontaneous Emission Free-Electron Laser

Observation of ultrawide bandwidth, up to 15% full-width, high-gain operation of a self-amplified spontaneous emission free-election laser (SASE FEL) is reported. This type of lasing is obtained with a strongly chirped beam (deltaE/E approximately 1.7%) emitted from the accelerator. Because of nonlinear pulse compression during transport, a short, high current bunch with strong mismatch errors is injected into the undulator, giving high FEL gain. Start-to-end simulations reproduce key features of the measurements and provide insight into mechanisms, such as angular spread in emitted photon and electron trajectory distributions, which yield novel features in the radiation spectrum.

Adding of microtron electron bunches for improving of far infrared FEL performance

The method of microtron electron bunch charge accumulation for a far infrared free electron laser operation is studied. Microtron electron bunches are added by the achromatic and isochronous round-trip magnetic system. The electron bunch peak current will grow proportionally to the number of bunches added. The growth of the small signal FEL gain is estimated.

Femtosecond X-ray pulses from a spatially chirped electron bunch in a SASE FEL

We propose a simple method to produce short X-ray pulses using a spatially chirped electron bunch in a SASE FEL. The spatial chirp is generated using an RF deflector which produces a transverse offset (in y and/or y′) correlated with the longitudinal bunch position. Since the FEL gain is very sensitive to an initial offset in the transverse phase space at the entrance of the undulator, only a small portion of the electron bunch with relatively small transverse offset will interact significantly with the radiation, resulting in an X-ray pulse length much shorter than the electron bunch length. The X-ray pulse is also naturally phase locked to the RF deflector and so allows high precision timing synchronization. We discuss the generation and transport of such a spatially chirped electron beam and show that tens of femotsecond long pulse can be generated for the linac coherent light source.

Characteristics of the fundamental FEL and the higher harmonic generation at LEBRA

The FEL system of Laboratory for Electron Beam Research and Application (LEBRA) at Nihon University has provided relatively high FEL gain around 10% at the wavelength of 1.5μm. The numerical estimation of the FEL gain suggests that LEBRA-FEL system has a sufficient performance on the electron beam bunching.In addition, the amplification of the third harmonics accompanying the intense fundamental FEL has been observed and the spectra of which have been measured. The experimental result indicates that the phenomenon is a kind of nonlinear harmonic generation.

Improvement of KHI FEL device at FEL-SUT

The FEL energy of the KHI FEL device was improved to 65mJ from 40mJ by tuning of the vertical position (the same direction as the electron beam from the cathode) of the RF-gun cathode. This is due to the improvement of the beam emittance. In order to evaluate the FEL gain, the emittance is measured. The FEL gain which is calculated by using the measured parameters such as the emittance agrees with the value estimated from the rising time.

Design studies of IR-FEL system at IAE, Kyoto University

An infrared FEL facility for bio/chemical energy research is under construction at the Institute of Advanced Energy, Kyoto University. The electron beam of 25–40MeV with macropulse duration up to 3μs will be generated by an S-band linac with a thermionic RF gun. Numerical studies to estimate the electron beam parameters and expected FEL gain of the present system have been performed to settle the goal for the system commissioning and the first operational condition. The results show that the present system can provide lasing in the wavelength from 4 to 13μm by using a 180° arc. However, the macropulse duration is too short to deliver stable FEL for a practical usage.

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.

Systematic measurement of maximum efficiencies and detuning lengths at the JAERI free-electron laser

We made a systematic measurement of efficiency detuning curves at several gain and loss parameters. The absolute detuning length (δ L) of an optical cavity was measured within an accuracy of 0.1 μm around the maximum efficiency by a pulse-stacking method using an external laser. The FEL gain was controlled by the undulator gap instead of bunch charge, because we can change the gain rapidly while maintaining constant electron bunch conditions. For the high-gain and low-loss regions, the maximum efficiency is obtained at δL=0 μm and is larger than the value derived from the theoretical scaling law in the superradiant regime, while for the low-gain region the maximum efficiency is obtained for δ L shorter than 0 μm and is similar to the scaling law.

Study of expected performance of the hard X-ray beam for the FEL-X project

We advance the FEL-X project and plan to install a new optical klystron for the infrared FELs within next year. The expected FEL gain at a wavelength of 10.6 μm is over 2% with the present electron-beam qualities in the storage ring NIJI-IV. Infrared FELs based on the fundamental and higher harmonics from the new optical klystron will generate hard X-ray beams with an energy of 0.1–2 MeV by the FEL-Compton backscattering process. Conventional light sources cannot provide sufficient amounts of photons in this energy region. The expected yield of the hard X-ray beam is 10 5–10 6 per second with an energy spread of 3%.

Power limitations in the OK-4/Duke storage ring FEL

In this paper, we present results of our experimental and theoretical studies of average power in the OK-4/Duke storage ring FEL. Our theoretical studies are based on the 3D FEL macro-particle model, which includes the local interactions, diffusion, radiation damping and spontaneous radiation. The OK-4/Duke storage ring FEL is operational since 1996 and demonstrated lasing in a wavelength range from 193.7 to 730 nm using electron beam energies from 220 to 800 MeV. It operated in both CW and giant pulse modes. During this period of time we collected substantial amounts of data regarding the FEL power and electron beam dynamics. We compare selected results on CW lasing with our theoretical predictions based on the rigorous numerical model. We also discuss a number of simplified scaling laws for the FEL gain and power as functions of electron beam energy and current, as well as, the cavity losses.

Operation of the OK-4/Duke storage ring FEL below 200 nm

For a number of years the wavelength of 200 nm was a psychological barrier for FEL oscillators. The progress towards short wavelength was marginal since the OK-4/VEPP-3 storage ring FEL lased at 240 nm in 1988. After 10 years, in 1998, the OK-4/Duke FEL and the NIJI-IV FEL group moved the limit to 217 and 212 nm, respectively. Improvements of the OK-4/Duke storage ring FEL gain above 10% and the use of custom manufactured mirror coatings brought the success in August 1999. The OK-4 FEL lased in the range from 193.7 to 209.8 nm using electron energies from 500 to 800 MeV. In this paper, we present the description of the OK-4/Duke FEL up-grades and the lasing results below 200 nm obtained in August and October of 1999.