Self-amplified Spontaneous Emission Scheme Research Articles

Experimental Demonstration of Mode-Coupled and High-Brightness Self-Amplified Spontaneous Emission in an X-Ray Free-Electron Laser

X-ray free-electron lasers (FELs) are modern research tools with applications in multiple scientific fields. Standard x-ray FEL pulses are produced by the self-amplified spontaneous emission (SASE) mechanism. SASE-FEL pulses have high power, short duration, and excellent transverse coherence but exhibit poor temporal coherence with power and spectral profiles consisting of multiple randomly distributed spikes. Here, we present the demonstration of two modes that enhance the temporal coherence of SASE-FEL radiation: mode-coupled and high-brightness SASE. Both schemes are based on delaying the electron beam with magnetic chicanes placed between the undulator modules of the FEL facility. First, we show the generation of frequency combs with tunable peak separation via the mode-coupled SASE scheme. Second, we present a proof-of-principle demonstration of the high-brightness SASE mechanism, producing FEL pulses with a bandwidth reduction up to a factor of 3 with respect to standard SASE pulses. The demonstration was done at Athos, the soft x-ray beamline of SwissFEL, for photon energies between 500 and 600 eV. Our work will benefit current applications and may open up new research areas requiring frequency combs or narrow bandwidths. Published by the American Physical Society 2024

Photon Diagnostics for the High-Gain THz FEL at PITZ

Research and development of an accelerator-based THz source prototype for pump-probe experiments at the European XFEL are ongoing at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). Proof-of-principle experiments have been performed to generate a high-gain THz Free-electron Laser (FEL) based on the Self-Amplified Spontaneous Emission scheme. The FEL radiation pulses with a central wavelength of about 100 μm (3 THz) and single pulse energy of several tens μm can be generated. In this paper, we present and discuss the photon diagnostic setup for the THz FEL together with examples of diagnostic results, including pulse energy and an FEL gain curve. The upgraded photon diagnostic setup, capable of measuring pulse energy, transverse distribution, and spectral distribution, is expected to be operational in the spring of 2023.

High field X-ray laser physics

High field X-ray laser physics

A simple and compact scheme to enhance the brightness of self-amplified spontaneous emission free-electron-lasers.

A simple and compact scheme that enhances the brightness of self-amplified spontaneous-emission (SASE) free-electron lasers is presented. The method combines the high-brightness SASE scheme and the optical klystron concept to increase the temporal coherence of the produced radiation and to reduce the required length of the undulator beamline at the same time. The scheme is very simple and only requires compact chicanes between the modules of the undulator beamline. Simulations show that, in comparison with SASE, the brightness can be improved by up to a factor of ten and the required length to achieve saturation can be reduced by 20% or more.

Accelerator-based X-ray sources: synchrotron radiation, X-ray free electron lasers and beyond.

The evolution of synchrotron radiation (SR) sources and related sciences is discussed to explain the 'generation' of the SR sources. Most of the contemporary SR sources belong to the third generation, where the storage rings are optimized for the use of undulator radiation. The undulator development allowed to reduction of the electron energy of the storage ring necessary for delivering 10 keV X-rays from the initial 6-8 GeV to the current 3 Gev. Now is the transitional period from the double-bend-achromat lattice-based storage ring to the multi-bend-achromat lattice to achieve much smaller electron beam emittance. Free electron lasers are the other important accelerator-based light sources which recently reached hard X-ray regime by using self-amplified spontaneous emission scheme. Future accelerator-based X-ray sources should be continuous wave X-ray free electron lasers and pulsed X-ray free electron lasers. Some pathways to reach the future case are discussed. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

Attosecond Hard X-ray Free Electron Laser

In this paper, several schemes of soft X-ray and hard X-ray free electron lasers (XFEL) and their progress are reviewed. Self-amplified spontaneous emission (SASE) schemes, the high gain harmonic generation (HGHG) scheme and various enhancement schemes through seeding and beam manipulations are discussed, especially in view of the generation of attosecond X-ray pulses. Our recent work on the generation of attosecond hard X-ray pulses is also discussed. In our study, the enhanced SASE scheme is utilized, using electron beam parameters of an XFEL under construction at Pohang Accelerator Laboratory (PAL). Laser, chicane and electron beam parameters are optimized to generate an isolated attosecond hard X-ray pulse at 0.1 nm (12.4 keV). The simulations show that the manipulation of electron energy beam profile may lead to the generation of an isolated attosecond hard X-ray of 150 attosecond pulse at 0.1 nm.

Spontaneous emission high-gain harmonic generation free-electron laser

A scheme, spontaneous emission high-gain harmonic generation (SEHG) free-electron laser (FEL), is proposed and analyzed for generating the X-ray FEL. The SEHG scheme works in a similar mechanism as high-gain harmonic generation (HGHG), but without the need for a seed laser. The scheme requires two undulators. The 1st undulator must be sufficiently long so that the energies of electrons are modulated within the bunch, but still away from saturation. A dispersion section is followed to transfer energy modulation into density modulation. The 2nd undulator simply serves as a radiator. A simple, one-dimensional, analytical estimation of SEHG is given to show the process of energy modulation and optimize the system parameters. The three-dimensional FEL simulation code, GENESIS, has been used to simulate, verify, and optimize the SEHG scheme for the soft X-ray free-electron laser (SXFEL) project in China. The simulation results are presented in comparison with the self-amplified spontaneous emission (SASE) and HGHG schemes. At 9 nm radiation wavelength, up to 120 MW of output power can be achieved by the SEHG scheme, with a total length of 47.3 m long undulators. Though the undulator length is comparable with the SASE scheme, the output bandwidth of the SEHG scheme is smaller. Moreover, it is tunable and does not require a seed laser. The SEHG scheme offers an attractive alternative option for the X-ray FEL.

Coherent hard X-ray production by cascading stages of High Gain Harmonic Generation

We study a new approach to produce coherent hard X-rays by cascading several stages of High Gain Harmonic Generation (HGHG). Our calculation shows that such a scheme is feasible within the present technology. Compared with the Self-Amplified Spontaneous Emission (SASE) scheme for the Linac Coherent Light Source (LCLS), the HGHG scheme can provide radiation with full longitudinal coherence, narrower bandwidth, more stable central wavelength, more controllable pulse length, and much higher performance stability. The electron bunch parameters used in our calculation are those of the DESY TTF and SLAC LCLS projects. To achieve similar output power, i.e. about 15 GW , the total length of the HGHG scheme will be 88 m , while that of SASE scheme will be around 100 m .

Generation of ultrahigh harmonics with a two-stage free electron laser and a seed laser

We consider the possibility to premodulate an ultrarelativistic electron beam on the nanometer length scale, so that it can produce coherent spontaneous radiation in the x-ray range. The scheme that uses the same basic elements as the high gain harmonic generation (HGHG) scheme, two wigglers and a conventional seed laser, is shown to suit this task if the process of premodulation occurs at zero net gain of the seed-laser wave. Operation with zero gain provides the opportunity to reach much higher harmonics than in the original HGHG technique. The output radiation is tunable between discrete harmonics of the seed frequency. The total length of the periodic magnetic structure is shown to be of the order of several meters, an order of magnitude shorter than that required for self-amplified spontaneous emission (SASE). For a beam quality similar to that in the SASE scheme and with realistic seed-laser parameters, the efficiency of the beam premodulation at the 50th harmonic (5 nm for a seed wavelength of 250 nm) is shown to be as high as 15%.