Observation of time-domain modulation of free-electron-laser pulses by multipeaked electron-energy spectrum

Advanced Stabilization Methods of Plasma Devices for Plasma-Based Acceleration

Towards the next generation of compact plasma-based accelerators, useful in several fields, such as basic research, medicine and industrial applications, a great effort is required to control the plasma creation, the necessity of producing a time-jitter free channel, and its stability namely uniformity and reproducibility. In this Letter, we describe an experimental campaign adopting a gas-filled discharge-capillary where the plasma and its generation are stabilized by triggering its ignition with an external laser pulse or an innovative technique based on the primary dark current (DC) in the accelerating structure of a linear accelerator (LINAC). The results show an efficient stabilization of the discharge pulse and plasma density with both pre-ionizing methods turning the plasma device into a symmetrical stable accelerating environment, especially when the external voltage is lowered near the breakdown value of the gas. The development of tens of centimeter long capillaries is enabled and, in turn, longer acceleration lengths can be adopted in a wide range of plasma-based acceleration experiments.

ARIA—A VUV Beamline for EuPRAXIA@SPARC_LAB

EuPRAXIA@SPARC_LAB is a new Free Electron Laser (FEL) facility that is currently under construction at the Laboratori Nazionali di Frascati of the INFN. The electron beam driving the FEL will be delivered by an X-band normal conducting LINAC followed by a plasma wakefield acceleration stage. It will be characterized by a small footprint and will deliver ultra-bright photon pulses for experiments in the water window to the user community. In addition to the soft-X-rays beamline already planned in the project, we propose the installation of a second photon beamline with seeded FEL pulses in the range between 50 and 180 nm. Here, we will present the FEL generation scheme, the layout of the dedicated beamline and the potential applications of the FEL radiation source in this low energy range.

Generating three-color pulses in high-gain harmonic-generation free-electron lasers with a tilted electron bunch

A multi-color light source is a significant tool for nonlinear optics experiments, pump-dump/repump-probe experiments and in other fields. Here, a novel method is proposed to create three-color pulses based on a high-gain harmonic-generation (HGHG) free-electron laser with a tilted electron bunch. In this method, the initial bunch tilt is created by transverse wakefields after the bunch passes through a corrugated structure with an off-axis orbit, and is further enlarged in a following drift section. Then the tilted bunch experiences the off-axis field of a quadrupole magnet to cool down the large transverse velocity induced before. After that, it enters an HGHG configuration adopting a transverse gradient undulator (TGU) as the radiator, where only three separated fractions of the tilted bunch will resonate at three adjacent harmonics of the seed wavelength and are enabled to emit three-color pulses simultaneously. In addition, the use of the natural transverse gradient of a normal planar undulator instead of the TGU radiator to emit three-color pulses is also studied in detail. Numerical simulations including the generation of the tilted bunch and the free-electron laser radiation confirm the validity and feasibility of this scheme both for the TGU radiator and the natural gradient in the extreme-ultraviolet waveband.

Longitudinal Phase-Space Manipulation with Beam-Driven Plasma Wakefields

The development of compact accelerator facilities providing high-brightness beams is one of the most challenging tasks in the field of next-generation compact and cost affordable particle accelerators, to be used in many fields for industrial, medical, and research applications. The ability to shape the beam longitudinal phase space, in particular, plays a key role in achieving high-peak brightness. Here we present a new approach that allows us to tune the longitudinal phase space of a high-brightness beam by means of plasma wakefields. The electron beam passing through the plasma drives large wakefields that are used to manipulate the time-energy correlation of particles along the beam itself. We experimentally demonstrate that such a solution is highly tunable by simply adjusting the density of the plasma and can be used to imprint or remove any correlation onto the beam. This is a fundamental requirement when dealing with largely time-energy correlated beams coming from future plasma accelerators.

Focusing of High-Brightness Electron Beams with Active-Plasma Lenses

Plasma-based technology promises a tremendous reduction in size of accelerators used for research, medical, and industrial applications, making it possible to develop tabletop machines accessible for a broader scientific community. By overcoming current limits of conventional accelerators and pushing particles to larger and larger energies, the availability of strong and tunable focusing optics is mandatory also because plasma-accelerated beams usually have large angular divergences. In this regard, active-plasma lenses represent a compact and affordable tool to generate radially symmetric magnetic fields several orders of magnitude larger than conventional quadrupoles and solenoids. However, it has been recently proved that the focusing can be highly nonlinear and induce a dramatic emittance growth. Here, we present experimental results showing how these nonlinearities can be minimized and lensing improved. These achievements represent a major breakthrough toward the miniaturization of next-generation focusing devices.

Double-shot MeV electron diffraction and microscopy

In this paper, we study by numerical simulations a time-resolved MeV electron scattering mode where two consecutive electron pulses are used to capture the evolution of a material sample on 10 ps time scales. The two electron pulses are generated by illuminating a photocathode in a radiofrequency photogun by two short laser pulses with adjustable delay. A streak camera/deflecting cavity is used after the sample to project the two electron bunches on two well separated regions of the detector screen. By using sufficiently short pulses, the 2D spatial information from each snapshot can be preserved. This "double-shot" technique enables the efficient capture of irreversible dynamics in both diffraction and imaging modes. In this work, we demonstrate both modes in start-to-end simulations of the UCLA Pegasus MeV microscope column.

Free-electron laser multiplex driven by a superconducting linear accelerator

Free-electron lasers (FELs) generate femtosecond XUV and X-ray pulses at peak powers in the gigawatt range. The FEL user facility FLASH at DESY (Hamburg, Germany) is driven by a superconducting linear accelerator with up to 8000 pulses per second. Since 2014, two parallel undulator beamlines, FLASH1 and FLASH2, have been in operation. In addition to the main undulator, the FLASH1 beamline is equipped with an undulator section, sFLASH, dedicated to research and development of fully coherent extreme ultraviolet photon pulses using external seed lasers. In this contribution, the first simultaneous lasing of the three FELs at 13.4 nm, 20 nm and 38.8 nm is presented.

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.

Generation of Phase-Locked Pulses from a Seeded Free-Electron Laser

In a coherent control experiment, light pulses are used to guide the real-time evolution of a quantum system. This requires the coherence and the control of the pulses' electric-field carrier waves. In this work, we use frequency-domain interferometry to demonstrate the mutual coherence of time-delayed pulses generated by an extreme ultraviolet seeded free-electron laser. Furthermore, we use the driving seed laser to lock and precisely control the relative phase between the two free-electron laser pulses. This new capability opens the way to a multitude of coherent control experiments, which will take advantage of the high intensity, short wavelength, and short duration of the pulses generated by seeded free-electron lasers.

Two-Color Radiation Generated in a Seeded Free-Electron Laser with Two Electron Beams

We present the experimental evidence of the generation of coherent and statistically stable two-color free-electron laser radiation obtained by seeding an electron beam double peaked in energy with a laser pulse single spiked in frequency. The radiation presents two neat spectral lines, with time delay, frequency separation, and relative intensity that can be accurately controlled. The analysis of the emitted radiation shows a temporal coherence and a shot-to-shot regularity in frequency significantly enhanced with respect to the self-amplified spontaneous emission.

Novel schemes for the optimization of the SPARC narrow band THz source

A pulsed, tunable, narrow band radiation source with frequency in the THz region can be obtained collecting the coherent transition radiation produced by a train of ultra-short electron bunches having picosecond scale inter-distance. In this paper, we review the techniques feasible at the SPARC_LAB test facility to produce and manipulate the requested train of electron bunches and we examine the dynamics of their acceleration and compression. In addition, we show how the performances of the train compression and the radiation intensity and bandwidth can be significantly improved through the insertion of a fourth order harmonic cavity, working in the X-band and acting as a longitudinal phase space linearizer.

High-intensity double-pulse X-ray free-electron laser

The X-ray free-electron laser has opened a new era for photon science, improving the X-ray brightness by ten orders of magnitude over previously available sources. Similar to an optical laser, the spectral and temporal structure of the radiation pulses can be tailored to the specific needs of many experiments by accurately manipulating the lasing medium, that is, the electron beam. Here we report the generation of mJ-level two-colour hard X-ray pulses of few femtoseconds duration with an XFEL driven by twin electron bunches at the Linac Coherent Light Source. This performance represents an improvement of over an order of magnitude in peak power over state-of-the-art two-colour XFELs. The unprecedented intensity and temporal coherence of this new two-colour X-ray free-electron laser enable an entirely new set of scientific applications, ranging from X-ray pump/X-ray probe experiments to the imaging of complex biological samples with multiple wavelength anomalous dispersion.

Two-colour X-ray pulses from free-electron lasers can be used to probe ultrafast dynamics, but the total power is a fraction of the saturation power. Here, Marinelli et al. use twin electron bunches to reach full saturation power and increase the two-colour intensity by an order of magnitude at hard-X-ray energies.

Demonstration of single-crystal self-seeded two-color x-ray free-electron lasers

A scheme for generating two simultaneous hard-x-ray free-electron laser pulses with a controllable difference in photon energy is described and then demonstrated using the self-seeding setup at the Linac Coherent Light Source (LCLS). The scheme takes advantage of the existing LCLS equipment, which allows two independent rotations of the self-seeding diamond crystal. The two degrees of freedom are used to select two nearby crystal reflections, causing two wavelengths to be present in the forward transmitted seeding x-ray pulse. The free-electron laser system must support amplification at both desired wavelengths.

Two color free-electron laser and frequency beating

We review the theory of two color high gain free-electron laser emission, derive the integral equation characterizing the evolution of the optical intensities, and provide a description of the relevant dynamics. The characteristic feature of this regime is the existence of a mutual bunching, whose origin and role are discussed.