Free Electron Laser Applications Research Articles

Ensemble inequivalence in systems with wave-particle interaction.

The classical wave-particle Hamiltonian is considered in its generalized version, where two modes are assumed to interact with the coevolving charged particles. The equilibrium statistical mechanics solution of the model is worked out analytically, both in the canonical and the microcanonical ensembles. The competition between the two modes is shown to yield ensemble inequivalence, at variance with the standard scenario where just one wave is allowed to develop. As a consequence, both temperature jumps and negative specific heat can show up in the microcanonical ensemble. The relevance of these findings for both plasma physics and free electron laser applications is discussed.

A polygon laser scanning micrometer for magnet size measurement studies

In this paper, we describe the design and development of a polygon laserscanner for undulator magnet size measurement studies. In the laser scanmicrometer, a laser light source is focused on to the facets of a rotatingpolygon mirror that reflects the laser beam. The parallel light emergingfrom the polygon surface falls on the object through a F-theta lens. Theobject obstructs the light. A lens at the opposite side of the objectcollects and focuses the light onto a detector. The shadow caused by theobstruction results in an output voltage for a period of time that isproportional to the size of the object. The method is used for undulatormagnet size measurements for free electron laser applications.

Direct Laser Acceleration of 28 keV Electrons at a Single Dielectric Grating

Direct laser acceleration exploiting the large optical field strength of short laser pulses and the proximity of a dielectric structure can support high acceleration gradients and may therefore lead to much smaller accelerators, with potential application in table-top free electron lasers. We report a proof-of-concept experiment demonstrating direct laser acceleration of non-relativistic 28 keV electrons derived from a conventional scanning electron microscope column at a single fused-silica grating. The electrons pass the grating as closely as 50 nm and interact with the third spatial harmonic, which is excited by 100 fs Titanium:sapphire laser pulses with a peak electric field of 2.85 GV/m. The observed maximum acceleration gradient of 25 MeV/m is already comparable to state- of-the-art RF structures. This work represents the first demonstration of scalable laser acceleration and of the inverse Smith-Purcell effect in the optical regime.

Demonstration of harmonic interaction in an undulator up to the 15th order

We report on experimental studies on the harmonic interaction between an optical laser and a relativistic electron beam in an undulator up to the 15th order. In this experiment, a significant energy modulation is imprinted on the beam longitudinal phase space through the electron-laser interaction when the laser frequency is the 3rd, 5th, 7th, 9th or 15th harmonic of the fundamental resonant frequency of the undulator. The experimental results are in good agreement with theory, and indicate that high harmonic interactions in undulators with large $K$ values and small phase errors can be quite efficient. The results confirm the basic physics of harmonic interaction with a goal toward ushering forward the development of many high harmonic based applications in free-electron lasers.

Dielectric laser acceleration of 28 keV electrons with the inverse Smith–Purcell effect

Dielectric laser acceleration exploiting the large optical field strength of short laser pulses and the proximity of a dielectric structure can support high acceleration gradients may therefore lead to much smaller accelerators, with future potential application in table-top free electron lasers. We report a proof-of-concept experiment demonstrating dielectric laser acceleration of non-relativistic 28keV electrons derived from a conventional scanning electron microscope column at a single fused-silica grating. The electrons pass the grating as closely as 50nm and interact with the third spatial harmonic, which is excited by 110fs long 800nm laser pulses with a peak electric field of 2.85GV/m. The observed maximum acceleration gradient of 25MeV/m is already comparable to state-of-the-art radio-frequency structures. This work thus represents a demonstration of scalable laser acceleration and of the inverse Smith–Purcell effect in the optical regime. For relativistic electrons and otherwise identical conditions up to two orders of magnitude larger acceleration gradients are expected.

Fast synchrotron and FEL beam monitors based on single-crystal diamond detectors and InGaAs/InAlAs quantum well devices

Simultaneous photon-beam position and intensity monitoring is becoming of increasing importance for new-generation synchrotron radiation sources and free-electron lasers (FEL). Thus, novel concepts of beam diagnostics are required in order to keep such beams under control. From this perspective diamond is a promising material for the production of semitransparent in situ photon beam monitors, which can withstand the high dose rates occurring in such radiation facilities. Here, we report on the development of freestanding, single-crystal chemical-vapor-deposited diamond detectors with segmented electrodes. Due to their direct, low-energy band gap, InGaAs quantum well devices operated at room temperature may also be used as fast detectors for photons ranging from visible to X-ray. These features are valuable in low-energy and time-resolved FEL applications. In particular, a novel segmented InGaAs/InAlAs device has been developed and will be discussed. Dedicated measurements carried out on both these devices at the Elettra Synchrotron show their capability to monitor the position and the intensity of the photon beam with bunch-by-bunch temporal performances. Furthermore, preliminary tests have been performed on diamond detectors at the Fermi FEL, extracting quantitative intensity and position information for 100-fs-wide FEL pulses with a photon energy of 28.8eV.

Time-of-flight measurement of ionic species generated during ablation for optimization of focusing condition at free-electron laser beamline

Optimization of focusing conditions is important in free-electron laser applications. A time-of-flight mass analyzer has been designed and constructed for this purpose. The time-of-flight spectra of ionic species evolved from laser ablation of gold were measured. The yields of ionic species showed strong correlations with free-electron-laser intensity. This method conveniently allows for direct estimation of laser intensity on sample and determination of focusing position.

Single-Shot Coherent Diffraction Imaging of Microbunched Relativistic Electron Beams for Free-Electron Laser Applications

With the advent of coherent x rays provided by the x-ray free-electron laser (FEL), strong interest has been kindled in sophisticated diffraction imaging techniques. In this Letter, we exploit such techniques for the diagnosis of the density distribution of the intense electron beams typically utilized in an x-ray FEL itself. We have implemented this method by analyzing the far-field coherent transition radiation emitted by an inverse-FEL microbunched electron beam. This analysis utilizes an oversampling phase retrieval method on the transition radiation angular spectrum to reconstruct the transverse spatial distribution of the electron beam. This application of diffraction imaging represents a significant advance in electron beam physics, having critical applications to the diagnosis of high-brightness beams, as well as the collective microbunching instabilities afflicting these systems.

Ultrashort pulses and free-electron lasers applications

This topical issue of the Central European Journal of Physics (CEJP) highlights the emerging field of ultrashort pulses and free-electron lasers applications. The systematic application of lasers in physics, chemistry, biology and medicine has been one the most important scientific revolutions of the past century. Their intensity, coherence and, more recently, short pulse duration has made it possible to go beyond traditional single-photon transitions, revealing novel aspects of atomic and molecular structure and dynamics. Current developments of coherent light sources in the x-ray regime promise to revolutionize even more these fields.

Experimental verification of the 3-step model of photoemission for energy spread and emittance measurements of copper and CsBr-coated copper photocathodes suitable for free electron laser applications

This paper presents measurements and analysis of the quantum efficiency (QE) and intrinsic emittance of Cu and CsBr coated Cu photocathodes. The data analysis uses expressions for the quantum efficiency and the intrinsic emittance for metal cathodes previously derived from Spicer's three-step model of photoemission. Data taken with a 257 nm CW laser on (100) Cu crystals indicate an emittance of 0.77 (μm/mm-rms) for CsBr coated and 0.42 (μm/mm-rms) for uncoated cathodes. The high quantum efficiency and low emittance observed for CsBr coated cathodes have applications in free electron laser and other devices requiring high brightness electron beams.

Advanced photoinjector experiment photogun commissioning results

The Advanced Photoinjector Experiment (APEX) at the Lawrence Berkeley National Laboratory is dedicated to the development of a high-brightness high-repetition rate (MHz-class) electron injector for x-ray free-electron laser (FEL) and other applications where high repetition rates and high brightness are simultaneously required. The injector is based on a new concept rf gun utilizing a normal-conducting (NC) cavity resonating in the VHF band at 186 MHz, and operating in continuous wave (cw) mode in conjunction with high quantum efficiency photocathodes capable of delivering the required charge at MHz repetition rates with available laser technology. The APEX activities are staged in three phases. In phase 0, the NC cw gun is built and tested to demonstrate the major milestones to validate the gun design and performance. Also, starting in phase 0 and continuing in phase I, different photocathodes are tested at the gun energy and at full repetition rate for validating candidate materials to operate in a high-repetition rate FEL. In phase II, a room-temperature pulsed linac is added for accelerating the beam at several tens of MeV to reduce space charge effects and allow the measurement of the brightness of the beam from the gun when integrated in an injector scheme. The installation of the phase 0 beam line and the commissioning of the VHF gun are completed, phase I components are under fabrication, and initial design and specification of components and layout for phase II are under way. This paper presents the phase 0 commissioning results with emphasis on the experimental milestones that have successfully demonstrated the APEX gun capability of operating at the required performance.

X-ray pump optical probe cross-correlation study of GaAs

Ultrafast dynamics in atomic, molecular and condensed-matter systems are increasingly being studied using optical-pump, X-ray probe techniques where subpicosecond laser pulses excite the system and X-rays detect changes in absorption spectra and local atomic structure(1-3). New opportunities are appearing as a result of improved synchrotron capabilities and the advent of X-ray free-electron lasers(4,5). These source improvements also allow for the reverse measurement: X-ray pump followed by optical probe. We describe here how an X-ray pump beam transforms a thin GaAs specimen from a strong absorber into a nearly transparent window in less than 100 ps, for laser photon energies just above the bandgap. We find the opposite effect-X-ray induced optical opacity-for photon energies just below the bandgap. This raises interesting questions about the ultrafast many-body response of semiconductors to X-ray absorption, and provides a new approach for an X-ray/optical cross-correlator for synchrotron and X-ray free-electron laser applications.

The BTFEL, an infrared free-electron laser amplifier based on a new-design short-period superconducting tape undulator

Abstract The development of undulator technologies capable of generating sub-cm undulator periods is assuming an increasing importance in X-ray free electron laser (FEL) applications. Indeed, such devices jointly with the high brightness electron beams already demonstrated at operating facilities would allow for lower energy, more compact electron linacs with a beneficial impact on the size and cost of X-ray FEL facilities. A novel design super-conducting undulator is being developed at the Lawrence Berkeley National Laboratory (LBNL) with the potential of sub-cm periods with reasonably large undulator parameter and gap. The potential and capability of such undulator technology need to be experimentally demonstrated. In this paper, the possibility of constructing an infrared FEL by combining the new undulator with the high brightness beam from the APEX injector facility at LBNL is investigated. Calculations show that the resulting FEL, when operated in self-amplified-spontaneous-emission mode, is expected to deliver a saturated power of almost a MW within a ∼ 4 m undulator length, in a single-spike of coherent radiation at ∼ 2 μ m wavelength. It will be also shown that the small-period of the undulator associated with the relatively low energy of the APEX beam, forces the FEL to operate in a regime with unusual and interesting characteristics. The alternative option of laser seeding the FEL is also briefly examined, showing the potential to reduce the saturation length even further.

Design of a high repetition rate S-band photocathode gun

Photocathode RF guns have been developed in many laboratories for generating high quality electron beams for free-electron lasers based on linear accelerators. Such guns can generate electron beams with an exceptionally high peak current as well as a small transverse emittance. Their applications have been recently expanded for ultrafast electron diffraction, coherent terahertz radiation, and X-ray or γ ‐ ray radiation by Compton scattering. In this paper, we design an S-band normal-conducting gun with capabilities of high quality beam generation and high repetition rate operation. The RF design and thermal analysis of the gun cavity and coupler are introduced. Optimal position of the gun focusing solenoid for low emittance beam generation is found by performing particle tracking simulations. Then, the gun system is designed to be able to afford the optimal solenoid position. The cooling-water channel surrounding the gun cavity and coupler is designed and analyzed numerically. The pressure in the gun is simulated with a vacuum model containing the detailed inner structure of the gun. An injector for a free-electron laser application is designed by using this gun and the beam dynamics simulation is shown. A cold test with a prototype gun for confirmation of the RF design is reported.

Diffraction before destruction

Two recent reports demonstrate the first applications of X-ray free-electron lasers to look at biological structures.

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Application of X-ray Free Electron Laser in Biological Science

Application of X-ray Free Electron Laser in Biological Science

Ionization heating in rare-gas clusters under intense XUV laser pulses

The interaction of intense extreme ultraviolet (XUV) laser pulses ($\ensuremath{\lambda}=32 \mathrm{nm}$, $I={10}^{11}--{10}^{14}$ W/cm${}^{2}$) with small rare-gas clusters (${\mathrm{Ar}}_{147}$) is studied by quasiclassical molecular dynamics simulations. Our analysis supports a very general picture of the charging and heating dynamics in finite samples under short-wavelength radiation that is of relevance for several applications of free-electron lasers. First, up to a certain photon flux, ionization proceeds as a series of direct photoemission events producing a jellium-like cluster potential and a characteristic plateau in the photoelectron spectrum as observed in Bostedt et al. [Phys. Rev. Lett. 100, 133401 (2008)]. Second, beyond the onset of photoelectron trapping, nanoplasma formation leads to evaporative electron emission with a characteristic thermal tail in the electron spectrum. A detailed analysis of this transition is presented. Third, in contrast to the behavior in the infrared or low vacuum ultraviolet range, the nanoplasma energy capture proceeds via ionization heating, i.e., inner photoionization of localized electrons, whereas collisional heating of conduction electrons is negligible up to high laser intensities. A direct consequence of the latter is a surprising evolution of the mean energy of emitted electrons as function of laser intensity.

Detailed characterization of electron sources yielding first demonstration of European X-ray Free-Electron Laser beam quality

The photoinjector test facility at DESY, Zeuthen site (PITZ), was built to develop and optimize photoelectron sources for superconducting linacs for high-brilliance, short-wavelength free-electron laser (FEL) applications like the free-electron laser in Hamburg (FLASH) and the European x-ray free-electron laser (XFEL). In this paper, the detailed characterization of two laser-driven rf guns with different operating conditions is described. One experimental optimization of the beam parameters was performed at an accelerating gradient of about 43 MV=m at the photocathode and the other at about 60 MV=m. In both cases, electron beams with very high phase-space density have been demonstrated at a bunch charge of 1 nC and are compared with corresponding simulations. The rf gun optimized for the lower gradient has surpassed all the FLASH requirements on beam quality and rf parameters (gradient, rf pulse length, repetition rate) and serves as a spare gun for this facility. The rf gun studied with increased accelerating gradient at the cathode produced beams with even higher brightness, yielding the first demonstration of the beam quality required for driving the European XFEL: The geometric mean of the normalized projected rms emittance in the two transverse directions was measured to be 1:26 ` 0:13 mm mrad for a 1-nC electron bunch. When a 10% charge cut is applied excluding electrons from those phase-space regions where the measured phase-space density is below a certain level and which are not expected to contribute to the lasing process, the normalized projected rms emittance is about 0.9 mm mrad.

Applications of Free Electron Lasers in Biology and Medicine

The advent of free electron lasers opens up new opportunities to probe the dynamics of ultrafast processes and the structure of matter with unprecedented spatial and temporal resolution. New methods inaccessible with other known types of radiation sources can be developed, resulting in a breakthrough in deep understanding the fundamentals of life as well as in numerous medical and biological applications. In the present work the properties of free electron laser radiation that make the sources excellent for probing biological matter at an arbitrary wavelength, in a wide range of intensities and pulse durations are briefly discussed. A number of biophysical and biomedical applications of the new sources, currently considered among the most promising in the field, are presented.