Eduard Prat

Generation of ultra-large-bandwidth X-ray free-electron-laser pulses with a transverse-gradient undulator

A novel, flexible yet simple method to generate gigawatt X-ray free-electron-laser radiation with unprecedented spectral bandwidth above the 10% level is presented. Such broadband radiation will improve substantially the efficiency of techniques like X-ray crystallography and spectroscopy, paving the way for outstanding progress in fields like biology and material science.

Two-color operation of a free-electron laser with a tilted beam.

With the successful operation of free-electron lasers (FELs) as user facilities there has been a growing demand for experiments with two photon pulses with variable photon energy and time separation. A configuration of an undulator with variable-gap control and a delaying chicane in the middle of the beamline is proposed. An injected electron beam with a transverse tilt will only yield FEL radiation for the parts which are close to the undulator axis. This allows, after re-aligning and delaying the electron beam, a different part of the bunch to be used to produce a second FEL pulse. This method offers independent control in photon energy and delay. For the parameters of the soft X-ray beamline Athos at the SwissFEL facility the photon energy tuning range is a factor of five with an adjustable delay between the two pulses from -50 to 950 fs.

Transverse gradient in Apple-type undulators

This theory demonstrates the existence of a variable transverse K gradient in Apple-type undulators. It is supported by numerical simulations to assess the limits of this analytical approach.

Undulator beamline optimization with integrated chicanes for X-ray free-electron-laser facilities.

An optimization of the undulator layout of X-ray free-electron-laser (FEL) facilities based on placing small chicanes between the undulator modules is presented. The installation of magnetic chicanes offers the following benefits with respect to state-of-the-art FEL facilities: reduction of the required undulator length to achieve FEL saturation, improvement of the longitudinal coherence of the FEL pulses, and the ability to produce shorter FEL pulses with higher power levels. Numerical simulations performed for the soft X-ray beamline of the SwissFEL facility show that optimizing the advantages of the layout requires shorter undulator modules than the standard ones. This proposal allows a very compact undulator beamline that produces fully coherent FEL pulses and it makes possible new kinds of experiments that require very short and high-power FEL pulses.

Simulation of FEL pulse length calculation with THz streaking method

Simulation of THz streaking of photoelectrons created by X-ray pulses from a free-electron laser and reconstruction of the free-electron laser pulse lengths.

Using irregularly spaced current peaks to generate an isolated attosecond X-ray pulse in free-electron lasers

A method is proposed to generate an isolated attosecond X-ray free-electron laser pulse with the peak power beyond 1 TW.

Compact coherence enhancement by subharmonic self-seeding in X-ray free-electron laser facilities

A simple method to significantly enhance the coherence of X-ray free-electron laser (FEL) pulses by combining self-seeding and the harmonic generation mechanism is presented. This work is a fundamental step towards obtaining fully coherent FEL pulses, solving in this way a critical problem in the FEL field and its multidisciplinary scientific applications. In particular, the coherence improvement will be extremely beneficial for many photon science applications requiring a small bandwidth such as resonant inelastic X-ray scattering, therefore opening new areas of research and playing a crucial role in the future of all scientific users of the FEL facilities.

Generation and measurement of sub-micrometer relativistic electron beams

The generation of low-emittance electron beams has received significant interest in recent years. Driven by the requirements of X-ray-free electron lasers, the emittance of photocathode injectors has been reduced significantly, with corresponding increase in beam brightness. This has put increasingly stringent requirements on the instrumentation to measure the beam size. These requirements are even more stringent for novel accelerator developments, such as laser-driven accelerators based on dielectric structures or on a plasma. We present here the generation and measurement of a sub-micrometer electron beam, at a particle energy of 330 MeV, and a bunch charge below 1 pC. An electron-beam optics with a vertical β-function of a few millimeters has been setup. The beam is characterized through a wire scanner that employs a 1 μm wide metallic structure fabricated using the electron-beam lithography. The smallest (rms) transverse beam size presented here is <500 nm.The continue advancement in accelerators instrumentation is placing increasingly stringent requirements on the measure of beam sizes. The paper discusses the development of an electron beam diagnostics for measuring sub-micron beam sizes, opening the window to sub-micrometre resolution.

Energy efficiency studies for dual-grating dielectric laser-driven accelerators

Abstract Dielectric laser-driven accelerators (DLAs) can provide high accelerating gradients in the GV/m range due to their having higher breakdown thresholds than metals, which opens the way for the miniaturization of the next generation of particle accelerator facilities. Two kinds of scheme, the addition of a Bragg reflector and the use of pulse-front-tilted (PFT) laser illumination, have been studied separately to improve the energy efficiency for dual-grating DLAs. The Bragg reflector enhances the accelerating gradient of the structure, while the PFT increases the effective interaction length. In this paper, we investigate numerically the advantages of using the two schemes in conjunction. Our calculations show that, for a 100-period structure with a period of 2 μ m, such a design effectively increases the energy gain by more than 100 % when compared to employing the Bragg reflector with a normal laser, and by about 50 % when using standard structures with a PFT laser. A total energy gain of as much as 2.6 MeV can be obtained for a PFT laser beam when illuminating a 2000-period dual-grating structure with a Bragg reflector.