Free Electron Laser Intensity Research Articles

An experimental application of machine learning algorithms to optimize the FEL lasing via beam trajectory tuning at Dalian Coherent Light Source

The lasing optimization of Free-Electron Laser (FEL) facilities is a time-consuming and challenging task. Instead of operating manually by experienced operators, implementation of machine learning algorithms offers a rapid and adaptable approach for FEL lasing optimization. Recently, such an experiment has been conducted at the vacuum ultraviolet FEL facility - Dalian Coherent Light Source (DCLS). Four algorithms, namely the standard and the neural network-based genetic algorithms, the deep deterministic policy gradient and the soft actor critic reinforcement learning algorithms, have been employed to enhance the FEL intensity by optimizing the electron beam trajectory. These algorithms have shown notable efficacy in enhancing the FEL lasing, especially the reinforcement learning ones which achieved convergence within only approximately 400 iterations. This study demonstrates the validity of machine learning algorithms for FEL lasing optimization, providing a forward-looking perspective on the automatic operation of DCLS.

Koopman Operator and Path Integral of Quantum Free-Electron Laser Model

A quantum model of a free-electron laser (FEL) is considered. Two different approaches for the exploration of the the FEL system are considered. In the first case, the Heisenberg equations of motion are mapped on the basis of the initial wave functions, which consists of the photon coherent states and many-dimensional electron coherent states. This mapping is an exact procedure, which makes it possible to obtain an exact equation of motion for the intensity of the laser field in a closed form. The obtained equation is controlled by a Koopman operator. The analytical expression for the evolution of the FEL intensity is obtained in the framework of a perturbation theory, which is constructed for a small time scale. The second way of the consideration is based on the construction of the many-dimensional path integrals for the evolution of the wave function. This method also makes it possible to estimate the time evolution and the gain of the FEL intensity.

Intensity optimization of x-ray free-electron laser by using phase shifters

This paper presents a method that uses a gap scan of the phase shifter to optimize the intensity of a free-electron laser (FEL) by matching its phase with that of the electron beam. Phase shifters are essential instruments, especially for a long undulator line, which is segmented by drift sections. The phase-matched (in-phase) and the 180\ifmmode^\circ\else\textdegree\fi{} offset (out-of-phase) conditions are investigated using linear theory, FEL simulations, and experiments to understand how the phase shifter affects FEL amplification. We show that the FEL intensity is dominantly reduced by phase mismatch in the saturation region, where the microbunched electron beam is sufficiently developed, and that the difference of FEL intensity between the in-phase and out-of-phase conditions is an effect of evolution of the bunching factor. At the Pohang Accelerator Laboratory x-ray Free-Electron Laser (PAL-XFEL), the gap scan of the phase shifter at 9.7 keV increased FEL intensity by 4 times compared to the calculated gap of the phase shifter. This intensity increase was obtained dominantly in the saturation region.

Electron spectroscopic study of nanoplasma formation triggered by intense soft x-ray pulses.

Using electron spectroscopy, we investigated the nanoplasma formation process generated in xenon clusters by intense soft x-ray free electron laser (FEL) pulses. We found clear FEL intensity dependence of electron spectra. Multistep ionization and subsequent ionization frustration features are evident for the low FEL-intensity region, and the thermal electron emission emerges at the high FEL intensity. The present FEL intensity dependence of the electron spectra is well addressed by the frustration parameter introduced by Arbeiter and Fennel [New J. Phys. 13, 053022 (2011)].

Absolute laser-intensity measurement and online monitor calibration using a calorimeter at a soft X-ray free-electron laser beamline in SACLA

This paper reports measurement of the absolute intensity of free-electron laser (FEL) and calibration of online intensity monitors for a brand-new FEL beamline BL1 at SPring-8 Angstrom Compact free-electron LAser (SACLA) in Japan. To measure the absolute intensity of FEL, we used a room-temperature calorimeter originally developed for FELs in the hard X-ray range. By using the calorimeter, we calibrated online intensity monitors of BL1, gas monitors (GMs), based on the photoionization of argon gas, in the photon energy range from 25 eV to 150 eV. A good correlation between signals obtained from the calorimeter and GMs was observed in the pulse energy range from 1 μJ to 100 μJ, where the upper limit is nearly equal to the maximum pulse energy at BL1. Moreover, the calibration result of the GMs, measured in terms of the spectral responsivity, demonstrates a characteristic photon-energy dependence owing to the occurrence of the Cooper minimum in the total ionization cross-section of argon gas. These results validate the feasibility of employing the room-temperature calorimeter in the measurement of absolute intensity of FELs over the specified photon energy range.

Slow Interatomic Coulombic Decay of Multiply Excited Neon Clusters.

Ne clusters (∼5000 atoms) were resonantly excited (2p→3s) by intense free electron laser (FEL) radiation at FERMI. Such multiply excited clusters can decay nonradiatively via energy exchange between at least two neighboring excited atoms. Benefiting from the precise tunability and narrow bandwidth of seeded FEL radiation, specific sites of the Ne clusters were probed. We found that the relaxation of cluster surface atoms proceeds via a sequence of interatomic or intermolecular Coulombic decay (ICD) processes while ICD of bulk atoms is additionally affected by the surrounding excited medium via inelastic electron scattering. For both cases, cluster excitations relax to atomic states prior to ICD, showing that this kind of ICD is rather slow (picosecond range). Controlling the average number of excitations per cluster via the FEL intensity allows a coarse tuning of the ICD rate.

Impact of Non-Gaussian Electron Energy Heating upon the Performance of a Seeded Free-Electron Laser

Laser-heater systems have been demonstrated to be an important component for the accelerators that drive high gain free electron laser (FEL) facilities. These heater systems suppress longitudinal microbunching instabilities by inducing a small and controllable slice energy spread to the electron beam. For transversely uniform heating, the energy spread augmentation is characterized by a non-Gaussian distribution. In this Letter, we demonstrate experimentally that in addition to suppression of the microbunching instability, the laser heater-induced energy distribution can be preserved to the FEL undulator entrance, significantly impacting the performance of high-gain harmonic generation (HGHG) FELs, especially at soft x-ray wavelengths. In particular, we show that the FEL intensity has several local maxima as a function of the induced heating caused by the non-Gaussian energy distribution together with a strong enhancement of the power at high harmonics relative to that expected for an electron beam with an equivalent Gaussian energy spread at an undulator entrance. These results suggest that a single stage HGHG FEL can produce scientifically interesting power levels at harmonic numbers m ≥ 25 and with current seed laser technology could reach output photon energies above 100 eV or greater.

Analytical solution for phase space evolution of electrons operating in a self-amplified spontaneous emission free electron laser

I present an analytical solution for the phase space evolution of electrons in a self-amplified spontaneous emission (SASE) free-electron laser (FEL) operating in the linear regime before saturation in the resonant case by solving the one dimensional FEL equation together with the solution of the cubic equation, which represents the evolution of the SASE FEL field. The electrons are shown to be bunched around $\ensuremath{\pi}/6$ ahead of a resonant electron every resonant FEL wavelength in the high gain regime. The phase relation is similar to that in a low gain FEL where an electron beam above resonance is injected, explaining the positive FEL gain. The analytical solutions agree well with numerical simulations and are applied to obtain the coherent optical transition radiation (OTR) intensity produced from electron microbunching at FEL wavelength. The coherent OTR intensity is shown to be proportional to FEL intensity.

Intensity saturation mechanism in free-electron lasers

It is shown that the free-electron laser (FEL) exhibits a scaling of the gain saturation versus the intensity analogous to that of conventional lasers. The concept of FEL intensity saturation is introduced and its link to the maximum output intensity is discussed. >

Cavity length adjustment and output FEL intensity optimization

The authors reconsider in more quantitative detail the possibility of enhancing the output intensity of a free electron laser (FEL) oscillator operating with bunched electron beam and finite macropulse duration, adjusting the cavity length while the signal grows. In a forthcoming publication G. Dattoli et al. point out that the output power can be enhanced by moving the cavity mirrors while the intercavity signal grows in such a way that the system is operating at the value of the cavity length providing the maximum gain. Practical solutions to realize the mirror movements are also discussed. The authors complete the analysis to be published by presenting simple formulas which allow a straightforward evaluation of the power enhancement.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>