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Helical Halbach undulators, formed by four alternately axially and radially magnetized rare-earth helices, produce a stronger field than two planar Halbach undulators with perpendicular polarization. Hybrid undulators of two longitudinally premagnetized rare-earth helices and two initially unmagnetized steel (or vanadium permendur) helices produce an equal or higher field and are easier to implement. We made sure that the required helices with a period of down to 3 mm or even less could be manufactured with high precision using Wire Electric Discharge Machining. According to the simulation results, when using identical electron bunches with moderate initial spreads in energy and angles, the gain in x-ray free electron laser (XFEL) power from replacing planar micro-undulators with the helical micro-undulators under consideration can be significantly greater than predicted by the simplest 1D theory with a cold electron beam. This makes such micro-undulators promising for creating compact XFELs of various types, both based on modern linear accelerators and plasma wakefield accelerators. [ABSTRACT FROM AUTHOR]

A novel design methodology based on a current-tunable magnetic focusing system that can flexibly switch between Brillouin and immersive flow focusing modes is presented in this paper. Such a magnetic system, which comprises cascaded solenoid coils and pole pieces, could be used in beam optics systems of vacuum tubes, particle accelerators, and free-electron lasers. Although the profile of this magnetic system is not brand new, the novel way proposed in this paper to establish and manipulate such a system to fit versatile purposes has never been reported in the literature to the best of the authors' knowledge. The specific structure of the magnetic system should be optimized, starting from the immersive flow focusing mode together with the electron gun design, which will be determined on successful optimization. According to our analysis, such systems could be transformed into the Brillouin bunching mode by simply adjusting the coil currents without modifying the hardware. To verify such an approach, single-beam and multi-beam optical systems are demonstrated in this paper. Moreover, the beam radius in such systems could also be conveniently adjusted via a similar technique. [ABSTRACT FROM AUTHOR]

The rapid advancement of x-ray free-electron laser technology facilitates the attainment of relativistic attosecond x-ray laser pulses in the future. It could serve as an ideal source to explore x-ray-driven high-energy density physics, such as particle acceleration and secondary radiation at solid densities. Here, utilizing particle-in-cell simulations, we systematically investigate high-order harmonic generation (HHG) based on intense x-ray lasers interacting with crystal-like structures. We find that the nonlinear interaction induces significant HHGs that are highly modulated due to the periodic target structure. By varying the spacing of the density spikes, the resonant conditions are tuned such that the efficiency of HHGs is drastically different. While each order of harmonic has multiple propagating directions, we find that the intensity can be enhanced significantly in the non-Bragg condition case than that in the Bragg case. The results indicate an approach to manipulate HHG emission driven by strong-field x-rays. [ABSTRACT FROM AUTHOR]

A model for a weakly ionized dusty plasma is proposed in which UV or x-ray radiation continuously creates free electrons at high energy, which then cool through collisions with a cold neutral gas before recombining. The transition of a free electron from high energy at birth to low energy at demise implies that the electron energy distribution is not the simple Maxwellian of an isolated system in thermal equilibrium, but instead has a high-energy tail that depends on the recombination time. This tail can have a major effect on dust grain charging because the flux of tail electrons can be substantial even if the density of tail electrons is small. Detailed analytic and numerical calculations of dust grain charging show that situations exist in which a small high-energy tail dominates charge behavior. This implies that dust grain charge in terrestrial and space dusty plasmas may be significantly underestimated if a Maxwellian distribution is assumed and the non-thermal dynamics are neglected. [ABSTRACT FROM AUTHOR]

Spatial distributions of electrons ionized and scattered from ultra-low-pressure gases are proposed and experimentally demonstrated as a method to directly measure the intensity of an ultra-high-intensity laser pulse. Analytic models relating the peak scattered electron energy to the peak laser intensity are derived and compared to paraxial Runge–Kutta simulations highlighting two models suitable for describing electrons scattered from weakly paraxial beams (f # > 5) for intensities in the range of 1 0 18 − 1 0 21 W cm−2. Scattering energies are shown to be dependent on gas species, emphasizing the need for specific gases for given intensity ranges. Direct measurements of the laser intensity at full power of two laser systems are demonstrated, both showing a good agreement between indirect methods of intensity measurement and the proposed method. One experiment exhibited the role of spatial aberrations in the scattered electron distribution, motivating a qualitative study on the effect. We propose the use of convolutional neural networks as a method for extracting quantitative information on the spatial structure of the laser at full power. We believe the presented technique to be a powerful tool that can be immediately implemented in many high-power laser facilities worldwide. [ABSTRACT FROM AUTHOR]

One of the most prominent applications of modern particle accelerators is the generation of radiation. In a synchrotron or an x-ray free electron laser (XFEL), high energy electrons oscillating in periodic magnetic structures emit bright x rays. In spite of their scientific appeal that will remain evident for many decades, one limitation of synchrotrons and XFELs is their typical mile-long size and their cost, which often limits access to the broader scientific community. This tutorial reviews the principles and prospects of using plasmas produced by intense lasers as particle accelerators and x-ray light sources, as well as some of the applications they enable. A plasma is an ionized medium that can sustain electrical fields many orders of magnitude higher than that in conventional radio frequency accelerator structures and can be used to accelerate electrons. When short, intense laser pulses are focused into a gas, it produces electron plasma waves in which electrons can be trapped and accelerated to GeV energies. This process, laser-wakefield acceleration (LWFA), is analogous to a surfer being propelled by an ocean wave. Many radiation sources, from THz to gamma-rays, can be produced by these relativistic electrons. This tutorial reviews several LWFA-driven sources in the keV-MeV photon energy range: betatron radiation, inverse Compton scattering, bremsstrahlung radiation, and undulator/XFEL radiation. X rays from laser plasma accelerators have many emerging applications. They can be used in innovative and flexible x-ray imaging and x-ray absorption spectroscopy configurations, for use in biology, industry, and high-energy density science. [ABSTRACT FROM AUTHOR]

This article establishes a very important part of the transmission line (TL) design of the novel 6 MeV compact particle accelerator. Practically, the entire new array of particle accelerators has been designed and is in the final stages of construction at the Ariel University. Except for a novel hybrid photo-cathode electron gun, which has been developed and built in University of California, Los Angeles's Laboratory, it was commissioned at The Schlesinger Family Center for Compact Accelerators, Radiation Sources and Applications (free electron laser) as a part of an ongoing collaboration between the two universities. In this work, we have built a complete simulator for the analysis and propagation of terahertz pulses through the TL, which meets our specific requirements with a high level of flexibility. To simulate the propagation of an output wave packet of the source through the TL quasi-optical system, the original wave packet is decomposed in terms of free-space Hermite–Gaussian basis while the following transformations of the wave packet is described by the transfer matrices technique. Total radiation losses and spectral properties of the TL transfer are analyzed. [ABSTRACT FROM AUTHOR]

Operation of a Quantum Free Electron Laser (QFEL) could provide a compact and fully coherent source of X- and γ-rays. Imperative to experimental realization is allowing for decoherence effects of either spontaneous emission or space-charge to take place, having opposing constraints. Here, for the first time, we present a one-dimensional QFEL Wigner model that includes longitudinal space-charge effects by quantizing the periodic potential derived from the Fourier components of the longitudinal electron beam density. The model is used to investigate steady-state QFEL gain and momentum state dynamics for a variety of space-charge regimes. We find increased saturation lengths and lower saturation intensity as a result of attenuated transitions in the two-level quantum system. In addition, we characterize a space-charge regime where specific transitions outside the QFEL bandwidth are targeted, such that the conventional description breaks down. These findings serve as a consistent theoretical extension of existing QFEL models. [ABSTRACT FROM AUTHOR]

A 6 MeV hybrid RF e-gun is currently driving a THz FEL in Ariel University. Ultra-fast electron diffraction (UED) experimental plans are currently in progress in the center, and therefore, a secondary parallel beam line is required. The addition of a secondary beamline using a single e-gun can be achieved using a dogleg section. However, the high-quality beam parameters such as emittance and electron bunch duration are significantly distorted after passing a dispersive section, such as a dogleg section, making the e-beam quality insufficient for UED experiments. In this paper, we suggest an optimization method, for the reconstruction of the beam quality after the dogleg using realistic quad-fields and sextupoles. Full 3D general particle tracer simulations of this secondary beamline were used in the optimization procedure using realistic field-maps and fringe fields of the quadrupoles, which were designed in-house, and their 3D field-maps were exported using computer simulation technology. Significantly improved beam parameters were obtained using the real quadrupoles field profile combined with an optimization procedure using a large number of electron optical optimization variables. [ABSTRACT FROM AUTHOR]

The potential for laser-produced plasmas to yield fundamental insights into high energy density physics (HEDP) and deliver other useful applications can sometimes be frustrated by uncertainties in modeling the properties and behavior of these plasmas using radiation-hydrodynamics codes. In an effort to overcome this and to corroborate the accuracy of the HEDP capabilities in the publicly available FLASH radiation-hydrodynamics code, we present detailed code-to-code comparisons between FLASH and the HYDRA code developed at Lawrence Livermore National Laboratory using previously published HYDRA simulations from Grava et al. [Phys. Rev. E 78, 016403 (2008)]. That study describes a laser experiment that produced a jet-like feature that the authors compare to astrophysical jets. Importantly, the Grava et al. [Phys. Rev. E 78, 016403 (2008)] experiment included detailed x-ray interferometric measurements of electron number densities and a time-integrated measurement of the soft x-ray spectrum. Despite markedly different methods for treating the computational mesh, and different equations of state and opacity models, the FLASH results resemble the results from HYDRA and, most importantly, the experimental measurements of electron density. Having validated the FLASH code in this way, we use the code to further investigate and understand the formation of the jet seen in the Grava et al. [Phys. Rev. E 78, 016403 (2008)] experiment and discuss its relation to the Wan et al. [Phys. Rev. E 55, 6293 (1997)] experiment at the NOVA laser. [ABSTRACT FROM AUTHOR]

Cherenkov interaction between a wave pulse and a flow of electrons possessing a very wide (on the scale of the characteristic band of the resonant electron–wave interaction) velocity spread is considered. We show that if the wave pulse is short enough, and its group velocity is close to the phase velocity, then the effect of the slippage of the resonant electrons with respect to the wave pulse leads to the transformation of an inert electronic medium into an active one (absorbing or amplifying the wave pulse, depending on the slippage sign). This can be a mechanism of formation of short powerful electromagnetic pulses as a result of amplification of short-pulse weak noises by electron flows which, due to natural reasons, have a large velocity spread, namely, electron flows in the magnetosphere of planets, in the plasma envelope of brown dwarfs and neutron stars, as well as in electron masers with weak electron–wave interaction (including ultra-relativistic electron beams used in free-electron lasers). [ABSTRACT FROM AUTHOR]

We propose a multi-particle self-consistent Hamiltonian (derived from an N-body description) that is applicable for periodic structures, such as traveling-wave tubes (TWTs), gyrotrons, free-electron lasers, or particle accelerators. We build a one-dimension symplectic multi-particle algorithm to simulate the nonlinear wave–particle interaction in the time domain occurring in an experimental 3-m-long helix TWT. Our algorithm is efficient thanks to a drastic reduction model. A 3D helix version of our reduction model is provided. Finally, we establish an explicit expression of the electromagnetic power in the time domain and in non-monochromatic (non-"continuous waveform") regime. [ABSTRACT FROM AUTHOR]

We present a proof-of-principle study demonstrating x-ray Raman Spectroscopy (XRS) from carbon samples at ambient conditions in conjunction with other common diagnostics to study warm dense matter, performed at the high energy density scientific instrument of the European x-ray Free Electron Laser (European XFEL). We obtain sufficient spectral resolution to identify the local structure and chemical bonding of diamond and graphite samples, using highly annealed pyrolytic graphite spectrometers. Due to the high crystal reflectivity and XFEL brightness, we obtain signal strengths that will enable accurate XRS measurements in upcoming pump–probe experiments with a high repetition-rate, where the samples will be pumped with high-power lasers. Molecular dynamics simulations based on density functional theory together with XRS simulations demonstrate the potential of this technique and show predictions for high-energy-density conditions. Our setup allows simultaneous implementation of several different diagnostic methods to reduce ambiguities in the analysis of the experimental results, which, for warm dense matter, often relies on simplifying model assumptions. The promising capabilities demonstrated here provide unprecedented insights into chemical and structural dynamics in warm dense matter states of light elements, including conditions similar to the interiors of planets, low-mass stars, and other celestial bodies. [ABSTRACT FROM AUTHOR]

We report on time-resolved measurements of electron number density by continuous-wave laser absorption in a low-energy nanosecond-scale laser-produced spark in atmospheric pressure air. Laser absorption is a result of free-free and bound-free electron excitation, with the absorption coefficient modeled and evaluated using estimates of the time-variation in electron temperature and probe laser absorption path length. Plasma electron number densities are determined to be as high as n e = 7 × 10 19 cm−3 and decay to 1 / e of their peak values over a period of about 50 ns following plasma formation using a 20 mJ, 10 ns pulse width frequency-doubled Nd:YAG laser. The measured plasma densities at later times are shown to be in reasonable agreement with Stark broadening measurements of the 3s[ 5 S o ]-3p[ 5 P ] electronic transition in atomic oxygen at 777 nm. This study provides support for the use of such continuous wave laser absorption for time resolved electron density measurements in low energy spark discharges in air, provided that an estimate of the electron temperature and laser path length can be made by accompanying diagnostics. [ABSTRACT FROM AUTHOR]

X-ray Thomson scattering (XRTS) is a powerful diagnostic technique that involves an x-ray source interacting with a dense plasma sample, resulting in a spectrum of elastically and inelastically scattered x-rays. Depending on the plasma conditions, one can measure a range of parameters from the resulting spectrum, including plasma temperature, electron density, and ionization state. To achieve sensitivity to collective electron oscillations, XRTS measurements require limited momentum transfer where the spectral separation of elastic and inelastic scattering is small. Such measurements require an x-ray probe source with a narrow bandwidth in order to reduce the spectral overlap between scattering contributions, allowing for the different features to be more precisely deconvolved. In this investigation, we discuss the theory behind how the bandwidth for a common XRTS probe, Zn He-α emission at 9 keV, can be reduced using a Cu K-edge filter. Proof-of-principle experiments conducted at the OMEGA laser facility confirm that this is an effective method for attenuating the higher energy He-α peak in the Zn emission spectrum. Calibration measurements at the National Ignition Facility show a reduction in spectral bandwidth from 87 eV to 48 eV when using the Cu filter, which will be important to improve the spectral resolution of future XRTS measurements that will probe plasmon oscillations in strongly compressed plasmas of low-Z materials at densities of tens of g/cm3. [ABSTRACT FROM AUTHOR]

The studies of incoherent single-particle and collective multi-particle effects are, in general, separated in accelerator beam dynamics in that the two dynamical phenomena involve quite different time scales. Recent experimental measurements indicate that in some parameter regime, the small-angle, multiple scattering effects within a high-brightness electron beam can have a strong influence on microbunched beam dynamics. In this paper, we apply our recently developed semi-analytical kinetic analysis to investigate the collective phase space microbunched dynamics in the presence of incoherent single-particle effects. Particular emphasis will be placed on evaluation of the intrinsic or slice energy spread. An example of two linear beam transport lines, followed by two identical, interleaving bunch compressor chicanes, is then presented. The semi-analytical calculations are consistent with particle tracking simulations. Moreover, the threshold condition is derived, indicating the relation among relevant physical quantities. At threshold, the incoherent effect can be beneficial for effective suppression of the collective microbunching instability. We expect that this work could shed light on high-brightness electron transport beamline design to improve short-wavelength free-electron laser performance. [ABSTRACT FROM AUTHOR]

To implement an efficient source of coherent radiation with negative mass longitudinal stabilization, the methods of the formation and injection of a dense electron bunch onto a stationary helical trajectory in a combined helical undulator and a strong uniform magnetic field are studied. Using a magnetic or electric lens permits sending particles almost along the converging lines of the magnetic field (magnetic following) and obtaining a nearly rectilinearly moving compressed bunch inside a solenoid. After that, the bunch can be injected into an adiabatically increasing field of the helical undulator. In this way, it is possible to excite operating undulator oscillations of particles, significantly mitigating the effects of destructive bunch expansion, the excitation of parasitic cyclotron oscillations, and velocity spread, thereby providing stabilization and terahertz radiation of a dense bunch. Due to a significant mode selection for an axis-encircling bunch, as well as due to the long-term interaction of the particles with a dominant mode, which is closest to the group synchronism conditions, radiation with a relatively narrow spectrum and high efficiency can be obtained even in a strongly oversized waveguide. An additional efficiency enhancement can be obtained due to the reduction of the velocity spread caused by the mutual Coulomb repulsion of electrons during injection due to the initial energy chirp of the bunch. [ABSTRACT FROM AUTHOR]

Laser produced plasma (LPP) soft x-ray and extreme ultraviolet sources utilize various types of targets. Some of them are based on gaseous targets. The most important disadvantage of such targets is the very limited number of elements that can be used in the gaseous form under normal conditions, including chemical compounds in the gaseous state. In this paper, the authors propose a new type of target, constituting an extension of the gaseous target, namely aerosol target, based on solutions of solid chemical compounds in liquids. Unlike previous attempts to produce aerosol targets from overheated liquids, in our system, the target is created by injection of a mist, preformed in a dedicated cell, into the vacuum chamber. Such targets have many advantages: the emission spectra can be adjusted by choosing proper chemical compounds, the aerosol target is elongated, hence, the laser focus can be positioned at a relatively long distance from the nozzle outlet, its average density can be higher compared to gaseous targets. In this work, such a target and its usefulness for the LPP soft x-ray source was demonstrated. The target was investigated by the soft x-ray backlighting method. Its density distribution at the position of the laser focus was determined. The emission spectra from the soft x-ray range, corresponding to multiple charged ions, originating from the solvent and the diluted salt were measured. Based on numerical simulations of the spectra, an ionic composition of the resulting plasmas was determined. [ABSTRACT FROM AUTHOR]

This article first gives the authors' perspectives on how the field of plasma-based acceleration (PBA) developed and how the current experiments, theory, and simulations are motivated by long term applications of PBA to a future linear collider and an x-ray free electron laser. We then focus on some early applications that will likely emerge from PBA research such as electron beam radiotherapy, directional but incoherent x-ray beams for science and technology, near single cycle continuously tunable infrared pulses for spectroscopy, and non-perturbative quantum electrodynamics enabled by PBA electron beams. In our opinion, these near term applications could be developed within the next decade with a concerted effort by the community. [ABSTRACT FROM AUTHOR]

X-ray free electron laser (XFEL) interaction with solids has been simulated to resolve simultaneously variable XFEL photon energy and x-ray spectral distribution of the target emission (2D-maps). It is discovered that the highly transient charge state distribution exhibits a characteristic target response due to the action of the sharply rising radiation field. Finally, we identify advantageous features for studies of dense plasma effects of two K-shell vacancy hollow ion x-ray emission excited via resonance excitation. These features and characteristics permit the global study of dense plasma effects via the simulation of the time-integrated joint distribution of pumped and fluorescence energies. It is shown that the simulation of these specific 2D-maps offers a global vision of the complex interplay between different processes or phenomena such as photoionization, resonance excitation, or ionization potential depression. [ABSTRACT FROM AUTHOR]

The use of electron–wave interaction systems consisting of several tapered sections is considered as a method of efficiency enhancement of free-electron lasers (FELs) operating in different regimes of emission of a short wave pulse from a short electron bunch. These regimes are the principally multi-frequency self-amplified spontaneous emission regime traditionally used in short-wavelength FELs and the regime of electron–wave group synchronism, in which super-radiation of a quasi-monochromatic wave packet propagating together with the electron bunch occurs. In both the cases, the use of multi-stage trapping of electrons in the bunch by the radiated wave provides a significant (at least by an order of magnitude) increase in efficiency as compared to the saturated-stage efficiency in regular systems. [ABSTRACT FROM AUTHOR]

The chirped-pulse inverse free electron laser (CIFEL) interaction has been investigated by considering a tightly focused chirped Gaussian laser pulse and a helical undulator in vacuum. We consider the more realistic description of a laser, i.e., seventh-order calculation in the spatial term and first-order calculation in the temporal term. Numerical calculations show that better electron acceleration can be achieved using a tightly focused chirped Gaussian laser pulse in the CIFEL scheme. The investigation shows that a 10 MeV electron can be accelerated up to a few giga-electron volts energy with the present available laser parameters. [ABSTRACT FROM AUTHOR]

The production of intense x-ray and particle sources is one of the most remarkable aspects of high energy laser interaction with a solid target. Wide application of these laser-driven secondary sources requires a high yield, which is partially limited by the amount of laser energy absorbed by the target. Here, we report on the enhancement of laser absorption and x-ray and particle flux by target surface modifications. In comparison to targets with flat front surfaces, our experiments show exceptional laser-to-target performance for our novel cone-shaped silicon microstructures. The structures are manufactured via laser-induced surface structuring. Spectral and spatial studies of reflectance and x-ray generation reveal significant increases of the silicon Kα line and a boost of the overall x-ray intensity, while the amount of reflected light decreases. Also, the proton and electron yields are enhanced, but both temperatures remain comparable to those of flat foil targets. We support the experimental findings with 2D particle in cell simulations to identify the mechanisms responsible for the strong enhancement. Our results demonstrate how custom surface structures can be used to engineer high power laser-plasma sources for future applications. [ABSTRACT FROM AUTHOR]

Warm dense matter (WDM)—an exotic state of highly compressed matter—has attracted increased interest in recent years in astrophysics and for dense laboratory systems. At the same time, this state is extremely difficult to treat theoretically. This is due to the simultaneous appearance of quantum degeneracy, Coulomb correlations, and thermal effects, as well as the overlap of plasma and condensed phases. Recent breakthroughs are due to the successful application of density functional theory (DFT) methods which, however, often lack the necessary accuracy and predictive capability for WDM applications. The situation has changed with the availability of the first ab initio data for the exchange–correlation free energy of the warm dense uniform electron gas (UEG) that were obtained by quantum Monte Carlo (QMC) simulations; for recent reviews, see Dornheim et al., Phys. Plasmas 24, 056303 (2017) and Phys. Rep. 744, 1–86 (2018). In the present article, we review recent further progress in QMC simulations of the warm dense UEG: namely, ab initio results for the static local field correction G(q) and for the dynamic structure factor S (q , ω). These data are of key relevance for comparison with x-ray scattering experiments at free electron laser facilities and for the improvement of theoretical models. In the second part of this paper, we discuss the simulations of WDM out of equilibrium. The theoretical approaches include Born-Oppenheimer molecular dynamics, quantum kinetic theory, time-dependent DFT, and hydrodynamics. Here, we analyze the strengths and limitations of these methods and argue that progress in WDM simulations will require a suitable combination of all methods. A particular role might be played by quantum hydrodynamics, and we concentrate on problems, recent progress, and possible improvements of this method. [ABSTRACT FROM AUTHOR]

We demonstrate a novel approach to the generation of femtosecond electron bunch trains via laser-driven wakefield acceleration. We use two independent high-intensity laser pulses, a drive, and an injector, each creating their own plasma wakes. The interaction of the laser pulses and their wakes results in a periodic injection of free electrons in the drive plasma wake via several mechanisms, including ponderomotive drift, wake-wake interference, and pre-acceleration of electrons directly by strong laser fields. Electron trains were generated with up to four quasi-monoenergetic bunches, each separated in time by a plasma period. The time profile of the generated trains is deduced from an analysis of beam loading and confirmed using 2D particle-in-cell simulations. [ABSTRACT FROM AUTHOR]

We study the equation of state of the non-relativistic free-electron gas in a constant magnetic field at arbitrary degeneracy based on the seminal work of Biswas et al. [Phys. Plasmas 20, 052503 (2013)]. The approach naturally unifies the Pauli paramagnetism, the Landau diamagnetism, and the de Haas–van Alphen effect. We consider the magnetization and the susceptibility as well as various thermodynamic quantities. In particular, the specific heats at constant volume and constant pressure are calculated, from which the adiabatic index is obtained. Weak and strong field limits are examined in detail. It is shown that the various quantities of interest saturate at strong magnetic field. Results are consistent with previous calculations performed at zero magnetic field. The polylogarithms are more adapted than the Fermi–Dirac integrals to describe the present system. The de Haas–van Alphen effect is not restricted to the magnetization and susceptibility but can be seen for other thermodynamic quantities. [ABSTRACT FROM AUTHOR]

We present a relativistic quantum average-atom model where bound and free electrons are treated self-consistently. We propose a relativistic exchange potential that generalizes the nonrelativistic Kohn-Sham exchange potential. The relativistic exchange potential is considered in detail. We interpolate smoothly between the nonrelativistic and ultrarelativistic limits. We neglect the correlation effect. Numerical examples are given. [ABSTRACT FROM AUTHOR]

In the present analysis, we study the dynamics of charged particles under the action of slowly modulated electromagnetic carrier waves. With the use of a high-frequency laser mode along with a modulated static magnetic wiggler, we show that the ensuing total field effectively acts as a slowly modulated high-frequency beat-wave field typical of inverse free-electron laser schemes. This effective resulting field is capable of accelerating particles in much the same way as space-charge wake fields do in plasma accelerators, with the advantage of being more stable than plasma related methods. Acceleration occurs as particles transition from ponderomotive to resonant regimes, so we develop the ponderomotive formalism needed to examine this problem. The ponderomotive formalism includes terms that, although not discussed in the usual applications of the approximation, are nevertheless of crucial importance in the vicinity of resonant capture. The role of these terms is also briefly discussed in the context of generic laser-plasma interactions. [ABSTRACT FROM AUTHOR]

A solid density target irradiated by a high-intensity laser pulse can become relativistically transparent, which then allows it to sustain an extremely strong laser-driven longitudinal electron current. The current generates a filament with a slowly varying MT-level azimuthal magnetic field that has been shown to prompt efficient emission of multi-MeV photons in the form of a collimated beam required for multiple applications. This work examines the feasibility of using an x-ray beam from the European x-ray free electron laser for the detection of the magnetic field via the Faraday rotation. Post-processed three dimensional particle-in-cell simulations show that, even though the relativistic transparency dramatically reduces the rotation in a uniform target, the detrimental effect can be successfully reversed by employing a structured target containing a channel to achieve a rotation angle of 10−4 rad. The channel must be relativistically transparent with an electron density that is lower than the near-solid density in the bulk. The detection setup has been optimized by varying the channel radius and focusing the laser pulse driving the magnetic field. We predict that the Faraday rotation can produce 103 photons with polarization orthogonal to the polarization of the incoming 100 fs long probe beam with 5 × 1012 x-ray photons. Based on the calculated rotation angle, the polarization purity must be much better than 10−8 in order to detect the signal above the noise level. [ABSTRACT FROM AUTHOR]

To meet the requirements of high power and broad-tunable terahertz sources, we proposed and investigated a modified Smith-Purcell free-electron laser (SP-FEL) driven by periodic electron-bunches (PEBs). We first designed an electron-gun with a photocathode, which generates a train of PEBs with tunable macro-bunching frequency. These PEBs then interact with surface waves on a grating and are further micro-bunched. The macro-bunching frequency together with its harmonics is superimposed on the micro-bunching one such that the multi-color superradiant Smith-Purcell radiation is achieved at the superimposed harmonics. By adjusting the macro-bunching frequency, the superradiant frequency can be tuned from 0.5 to 1 THz—a region hard to reach by conventional SP-FELs and vacuum electron devices. Compared to DC-beams, the PEBs have higher peak current density and generate higher peak power. Thus, the proposed scheme can be promisingly developed as high power and broad-tunable terahertz sources. [ABSTRACT FROM AUTHOR]

In this study, the free electron laser (FEL) with a planar wiggler and an ion-channel in a rectangular waveguide is studied for TE waves with numerical simulation. The output power of FEL is obtained self-consistently from the equation of motion and Maxwell's equations. Numerical analysis shows that using the ion-channel, in spite of the magnetic field case [Sakamoto et al., J. Appl. Phys. 75, 36 (1994)], can improve output power in addition to guiding the electrons into the center and prevent dissipation of the beam. [ABSTRACT FROM AUTHOR]

X-ray free-electron lasers (XFELs) provide unique opportunities to generate and investigate dense plasmas. The absorption and transmission properties of x-ray photons in dense plasmas are important in characterizing the state of the plasmas. Experimental evidence shows that the transmission of x-ray photons through dense plasmas depends greatly on the incident XFEL intensity. Here, we present a detailed analysis of intensity-dependent x-ray transmission in solid-density aluminum using collisional-radiative population kinetics calculations. Reverse saturable absorption (RSA), i.e., an increase in x-ray absorption with intensity has been observed for photon energies below the K-absorption edge and in the intensity range of 1016–1017 W/cm2 for XFEL photons with 1487 eV. At higher intensities, a transition from RSA to saturable absorption (SA) is predicted; thus, the x-ray absorption decreases with intensity above a threshold value. For XFEL photon energies of 1501 eV and 1515 eV, the transition from RSA to SA occurs at XFEL intensities between 1017–1018 W/cm2. Electron temperatures are predicted to be in the range of 30–50 eV for the given experimental conditions. Detailed population kinetics of the charge states explains the intensity-dependent absorption of x-ray photons and the fast modulation of XFEL pulses for both RSA and SA. [ABSTRACT FROM AUTHOR]

The injection physics in a shock-induced density down-ramp injector was characterized, demonstrating precise control of a laser-plasma accelerator (LPA). Using a jet-blade assembly, experiments systematically varied the shock injector profile, including shock angle, shock position, up-ramp width, and acceleration length. Our work demonstrates that beam energy, energy spread, and pointing can be controlled by adjusting these parameters. As a result, an electron beam that was highly tunable from 25 to 300 MeV with 8% energy spread (ΔEFWHM/E), 1.5 mrad divergence, and 0.35 mrad pointing fluctuation was produced. Particle-in-cell simulation characterized how variation in the shock angle and up-ramp width impacted the injection process. This highly controllable LPA represents a suitable, compact electron beam source for LPA applications such as Thomson sources and free-electron lasers. [ABSTRACT FROM AUTHOR]

We show that a short relativistic electron beam propagating in a plasma with a density gradient perpendicular to the direction of motion generates a wakefield in which a witness bunch experiences a transverse force. A density gradient oscillating along the beam path would create a periodically varying force--an undulator, with an estimated strength of the equivalent magnetic field more than 10 T. This opens an avenue for the creation of a high-strength, short-period undulator, which eventually may lead to all-plasma, free electron lasers where a plasma wakefield acceleration is naturally combined with a plasma undulator in a unifying, compact setup. [ABSTRACT FROM AUTHOR]

We proposed and investigated a novel terahertz free-electron laser, which is based on two gratings driven by two electron-beams. Two gratings are symmetrically arranged to form an open-cavity. Two electron-beams generate special Smith-Purcell radiations, respectively, from two gratings. When radiation interferes constructively, operation modes of the open-cavity are excited and then amplified by beam-wave interactions. By means of particle-in-cell simulations, we have shown that, with compact equipments and available electron-beams, this scheme can generate radiation with power and efficiency being higher than those of majority radiation sources in the vicinity region of 1 THz. It can promisingly be developed as a high-power, high-efficiency, and compact terahertz source for practice. [ABSTRACT FROM AUTHOR]

We investigate experimentally the plasma dynamics of a centimeter-scale, laser filamentation-guided spark discharge. Using electrical and optical diagnostics to study monopolar discharges with varying current pulses, we show that plasma decay is dominated by free electron recombination if the current decay time is shorter than the recombination characteristic time. In the opposite case, the plasma electron density closely follows the current evolution. We demonstrate that this criterion holds true in the case of damped alternating current sparks, and that alternative current is the best option to achieve a long plasma lifetime for a given peak current. [ABSTRACT FROM AUTHOR]

Detuning of the fundamental is a way to enhance harmonic generation. By this method, the wiggler is composed of two segments in such a way that the fundamental resonance of the second segment to coincide with the third harmonic of the first segment of the wiggler to generate extreme ultraviolet radiation and x-ray emission. A set of coupled, nonlinear, and first-order differential equations in three dimensions describing the evolution of the electron trajectories and the radiation field with warm beam is solved numerically by CYRUS 3D code in the steady-state for two models (1) seeded free electron laser (FEL) and (2) shot noise on the electron beam (self-amplified spontaneous emission FEL). Thermal effects in the form of longitudinal velocity spread are considered. Three-dimensional simulation describes self-consistently the longitudinal spatial dependence of radiation waists, curvatures, and amplitudes together with the evaluation of the electron beam. The evolutions of the transverse modes are investigated for the fundamental resonance and the third harmonic. Also, the effective modes of the third harmonic are studied. In this paper, we found that detuning of the fundamental with shot noise gives more optimistic result than the seeded FEL. [ABSTRACT FROM AUTHOR]

The possibility of creating hot-dense plasma samples by isochoric heating of solid targets with highintensity femtosecond X-ray lasers has opened up new opportunities in the experimental study of such systems. A study of the X-ray spectra emitted from solid density plasmas has provided significant insight into the X-ray absorption mechanisms, subsequent target heating, and the conditions of temperature, electron density, and ionization stages produced (Vinko et al., Nature 482, 59-62 (2012)). Furthermore, detailed analysis of the spectra has provided new information on the degree of ionization potential depression in these strongly coupled plasmas (Ciricosta et al., Phys. Rev. Lett. 109, 065002 (2012)). Excellent agreement between experimental and simulated spectra has been obtained, but a full outline of the procedure by which this has been achieved has yet to be documented. We present here the details and approximations concerning the modelling of the experiment described in the above referenced work. We show that it is crucial to take into account the spatial and temporal gradients in simulating the overall emission spectra, and discuss how aspects of the model used affect the interpretation of the data in terms of charge-resolved measurements of the ionization potential depression. [ABSTRACT FROM AUTHOR]

In this paper, an analysis of equilibrium orbits for electrons by a simultaneous solution of the equation of motion and the dispersion relation for electromagnetic wave wiggler in a free-electron laser (FEL) with ion-channel guiding has been presented. A fluid model has been used to investigate interactions among all possible waves. The dispersion relation has been derived for electrostatic and electromagnetic waves with all relativistic effects included. This dispersion relation has been solved numerically. For group I and II orbits, when the transverse velocity is small, only the FEL instability is found. In group I and II orbits with relatively large transverse velocity, new couplings between other modes are found. [ABSTRACT FROM AUTHOR]

The irradiation of an atomic cluster with a femtosecond x-ray free-electron laser pulse results in a nanoplasma formation. This typically occurs within a few hundred femtoseconds. By this time the x-ray pulse is over, and the direct photoinduced processes no longer contributing. All created electrons within the nanoplasma are thermalized. The nanoplasma thus formed is a mixture of atoms, electrons, and ions of various charges. While expanding, it is undergoing electron impact ionization and three-body recombination. Below we present a hydrodynamic model to describe the dynamics of such multi-component nanoplasmas. The model equations are derived by taking the moments of the corresponding Boltzmann kinetic equations. We include the equations obtained, together with the source terms due to electron impact ionization and three-body recombination, in our hydrodynamic solver. Model predictions for a test case, expanding spherical Ar nanoplasma, are obtained. With this model, we complete the two-step approach to simulate x-ray created nanoplasmas, enabling computationally efficient simulations of their picosecond dynamics. Moreover, the hydrodynamic framework including collisional processes can be easily extended for other source terms and then applied to follow relaxation of any finite non-isothermal multi-component nanoplasma with its components relaxed into local thermodynamic equilibrium. [ABSTRACT FROM AUTHOR]

The direct and secondary nuclear excitation produced by an x-ray free electron laser when interacting with a solid-state nuclear target is investigated theoretically. When driven at the resonance energy, the x-ray free electron laser can produce direct photoexcitation. However, the dominant process in that interaction is the photoelectric effect producing a cold and very dense plasma in which also secondary processes such as nuclear excitation by electron capture may occur. We develop a realistic theoretical model to quantify the temporal dynamics of the plasma and the magnitude of the secondary excitation therein. Numerical results show that depending on the nuclear transition energy and the temperature and charge states reached in the plasma, secondary nuclear excitation by electron capture may dominate the direct photoexcitation by several orders of magnitude, as it is the case for the 4.8 keV transition from the isomeric state of 93Mo, or it can be negligible, as it is the case for the 14.4 keV Mössbauer transition in 57Fe. These findings are most relevant for future nuclear quantum optics experiments at x-ray free electron laser facilities. [ABSTRACT FROM AUTHOR]

We present an analysis of a Cherenkov free-electron laser based on a single slab made from negative-index materials. In this system, a flat electron beam with finite thickness travelling close to the surface of the slab interacts with the copropagating electromagnetic surface mode. The dispersion equation for a finitely thick slab is worked out and solved numerically to study the dispersion relation of surface modes supported by negative-index materials, and the calculations are in good agreement with the simulation results from a finite difference time domain code. We find that under suitable conditions there is inherent feedback in such a scheme due to the characteristics of negative-index materials, which means that the system can oscillate without external reflectors when the beam current exceeds a threshold value, i.e., start current. Using the hydrodynamic approach, we setup coupled equations for this system, and solve these equations analytically in the small signal regime to obtain formulas for the spatial growth rate and start current. [ABSTRACT FROM AUTHOR]

Super-radiant regimes in electron oscillators can be attractive for applications requiring powerful and relatively short pulses of microwave radiation, since the peak power of the super-radiant pulse can exceed the power of the operating electron beam. In this paper, possibilities for realization of the super-radiant regimes are studied in various schemes of electron oscillators based on excitation of near-cutoff operating waves (gyrotron and orotron). [ABSTRACT FROM AUTHOR]

A nonlinear model and simulations of the output power of an ion-channel guiding free-electron laser (FEL) are presented in this paper. Results show that the nonlinear output power of an ionchannel guiding FEL is comparable to that of an axial guide magnetic field FEL. Compared to an axial guide magnetic field FEL, an ion-channel guiding FEL substantially weakens the negative effect of the electron-beam energy spread on the output power due to its advantageous focusing mechanism on the electron motion. [ABSTRACT FROM AUTHOR]

The harmonic lasing free-electron laser amplifier, in which two wigglers is employed in order for the fundamental resonance of the second wiggler to coincide with the third harmonic of the first wiggler to generate ultraviolet radiation, is studied. A set of coupled nonlinear first-order differential equations describing the nonlinear evolution of the system, for a long electron bunch, is solved numerically by CYRUS code. Solutions for the non-averaged and averaged equations are compared. Remarkable agreement is found between the averaged and non-averaged simulations for the evolution of the third harmonic. Thermal effects in the form of longitudinal velocity spread are also investigated. For efficiency enhancement, the second wiggler field is set to decrease linearly and nonlinearly at the point where the radiation of the third harmonic saturates. The optimum starting point and the slope of the tapering of the amplitude of the wiggler are found by a successive run of the code. It is found that tapering can increase the saturated power of the third harmonic considerably. In order to reduce the length of the wiggler, the prebunched electron beam is considered. [ABSTRACT FROM AUTHOR]

The efficiency and uniformity of heating induced by hard x-ray free-electron laser pulse is investigated for 0.5 μm silver foils using the X-ray Pump Probe instrument at the Linac Coherent Light Source facility. Intense 8.9 keV x-ray pulses of 60 fs duration deposit energy predominantly via inner-shell ionization to create a non-equilibrium Ag solid density plasma. The x-ray pulses are focused to 14 × 17 μm² by means of beryllium lenses and by varying the total beam energy, the energy deposition is varied over a range of irradiances from 4.4 to 6.5 × 1015 W/cm². Two time-and-space resolved interferometers simultaneously probed the expansion of the front and rear sample surfaces and find evidence of a nearly symmetric expansion pointing to the uniformity of energy deposition over the full target thickness. The experimental results are compared with two different hydrodynamic simulations of the sample expansion. The agreement between experimental and theoretical results yields an estimate of the temperature evolution as a function of x-ray irradiance that varies from 8 to 10 eV for the x-ray irradiances studied. [ABSTRACT FROM AUTHOR]

Time of flight dynamics of slow neutrals, fast neutrals, and ions from a nanosecond laser produced nickel (Ni) plasma are investigated. Species arrival times confirm the hypothesis that fast neutrals are formed by the recombination of fast ions with free electrons. Both neutrals and ions are found to accelerate for a short interval immediately after ablation, which is attributed to internal Coulomb forces which create electrostatic potentials resulting in the charged particle acceleration. This process is further enhanced by laser-plasma energy coupling. Emission from neutrals could be measured for longer axial distances in the plume compared to that of ions confirming that the ions recombine to form neutrals as they move away from the target surface. [ABSTRACT FROM AUTHOR]

In the present work, we extend results of a previous paper [Peter et al., Phys. Plasmas 20,12 3104 (2013)] and develop a semi-analytical model to account for thermal effects on the nonlinear dynamics of the electron beam in free-electron lasers. We relax the condition of a cold electron beam but still use the concept of compressibility, now associated with a warm beam model, to evaluate the time scale for saturation and the peak laser intensity in high-gain regimes. Although vanishing compressibilites and the associated divergent densities are absent in warm models, a series of discontinuities in the electron density precede the saturation process. We show that full wave-particle simulations agree well with the predictions of the model. [ABSTRACT FROM AUTHOR]

In this study, the method of perturbation has been applied to obtain the dispersion relation (DR) of a two-stream free-electron laser (FEL) with a helical wiggler and an ion-channel with all relativistic effects on waves. This DR has been solved numerically to find the unstable modes and their growth rate. Numerical solutions of DR show that the growth rate is considerably enhanced in comparison with single-stream free-electron laser. In group II orbits, with relatively large wiggler induced velocities, new couplings are found. The effect of the velocity difference of the two electron beams on the instabilities has also been investigated in this study. Moreover, the effect of the ion-channel density on the maximum growth rate of FEL resonance has been analyzed. [ABSTRACT FROM AUTHOR]