Laser-electron Interaction Research Articles

Evolution of density-modulated electron beams in drift sections

Initiating the amplification process in a short-wavelength free-electron laser (FEL) by external seed laser pulses results in radiation with a high degree of longitudinal and transverse coherence. The basic layout in seeded harmonic generation involves a periodic electron energy modulation by laser-electron interaction in a short undulator (the “modulator”), which is converted into a density modulation in a dispersive section immediately followed by a long FEL undulator (the “radiator”) tuned to a harmonic of the seed laser wavelength. With the advent of more complex seeding schemes, density-modulated beams may need to be transported in drift sections before entering the radiator. Long FEL undulators may also contain several drift spaces to accommodate focusing elements and diagnostics. Therefore, it is of general interest to study the evolution of density-modulated electron beams in drift sections under the influence of repulsive Coulomb forces. At FERMI, a seeded FEL user facility in Trieste, Italy, systematic studies of the impact of varying drift length on coherent harmonic emission were undertaken. In order to make the underlying physics transparent, the emphasis of this paper is on reproducing the experimental findings with analytical estimates and a simple one-dimensional numerical model. Furthermore, the Coulomb forces in a drift section may be employed to enhance the laser-induced energy modulation and yield an improved density modulation before entering the FEL radiator. Published by the American Physical Society 2024

Subrelativistic Alternating Phase Focusing Dielectric Laser Accelerators.

We demonstrate a silicon-based electron accelerator that uses laser optical near fields to both accelerate and confine electrons over extended distances. Two dielectric laser accelerator (DLA) designs were tested, each consisting of two arrays of silicon pillars pumped symmetrically by pulse front tilted laser beams, designed for average acceleration gradients 35 and 50 MeV/m, respectively. The DLAs are designed to act as alternating phase focusing (APF) lattices, where electrons, depending on the electron-laser interaction phase, will alternate between opposing longitudinal and transverse focusing and defocusing forces. By incorporating fractional period drift sections that alter the synchronous phase between ±60° off crest, electrons captured in the designed acceleration bucket experience half the peak gradient as average gradient while also experiencing strong confinement forces that enable long interaction lengths. We demonstrate APF accelerators with interaction lengths up to 708 μm and energy gains up to 23.7±1.07 keV FWHM, a 25% increase from starting energy, demonstrating the ability to achieve substantial energy gains with subrelativistic DLA.

Time evolution of temporal, spatial and spectral distribution of high-energy electron radiation in intense laser pulses

To study the time evolution of high-energy electron radiation in circularly polarized intense laser pulses in detail, a model of the interaction between the high-energy single electron and intense laser pulses is constructed based on the Lagrangian equation and the electron energy equation. Through simulation, this article vividly displays the evolution process of radiation in the spatial, frequency and time domain. By modulating the interaction time between the laser and electron and referring to the spatial distribution image of energy, the value and direction of the maximum radiation energy per unit solid angle are calculated. In addition, in specific directions, this paper discusses the effects of interaction duration on the energy frequency distribution and the power variation pattern. The results prove that the maximum radiation energy per unit solid angle will appear when the interaction time comes to about 450 fs, which is also the boundary moment when the frequency and time spectrum no longer change obviously. Therefore, by modifying the duration of the electron–laser interaction, it is possible to produce the electron radiation with desired characteristics more precisely.

Radially Polarized Laser-Induced Electron Acceleration in Vacuum

Enhanced electron acceleration due to tightly focused (TF) short-intense radially polarized (RP) Gaussian laser pulse in vacuum has been investigated. An axicon optical lens is utilized to accomplish a TF laser beam. The TF-RP laser fields are truncated to fifth order in diffraction angle. The innate and attractive symmetry of the tightly focused RP laser pulse imposes the trapping of electrons in the route of the laser propagation throughout laser–electron interaction. The applied axial magnetic field further ensures the enhancement in ponderomotive strength, pushing the electron in the expediting phase up to longer distances. Utilizing the optimum parameters of RP laser and magnetic field, noteworthy augmentation in the electron acceleration up to GeV range is obtained. The electron energy gain variations are plotted and analyzed for initial laser intensity, initial phase, electron initial energy and applied magnetic field parameters.

Nonlinear spectral features of the relativistic interaction between electron and ultrashort laser pulse

Features of the radiation spectra are investigated with the quantum electrodynamic theory for the nonlinear scattering interaction between relativistic electron and few-cycle linearly polarized laser pulse. The angle between the moving direction of the electron and that of the laser pulse affects the scattered radiation characteristics. When the electron has a head-on collision with the laser pulse having a carrier envelope phase as zero, the angular distribution of the scattered radiation spectrum is symmetrical with respect to the laser pulse propagation direction. Such a symmetry disappears when the electron collides with the laser obliquely and the direction of most of the radiation energy shifts at an acute angle towards the electron direction. The CEP has significant effects on the spectral angular distribution. The CEP influence is determined by the exertion of the electromagnetic field on the motion of the electron, whose trajectory overlap in phase space leads to the interference between scattered radiation in different intervals of the interaction process. The supercontinuum radiation could be produced under certain CEPs. An external electrostatic field applied as the environment for the laser-electron interaction is another important factor affecting the scattered radiation features by modulating the harmonic components and their angular distribution. This study may help the analysis of the spectral data from relativistic laser-solid target experiments.

Deformation of an electron bunch caused by free-electron lasers

This study reported the longitudinal compression and extension of an electron bunch by free electron laser (FEL) interaction. By observing coherent edge radiation generated by an FEL facility at Kyoto University, which has the highest extraction efficiency of an FEL oscillation, we found that an electron bunch interacting with a cavity-type linac FEL elongated when a detuning length of an optical cavity was positive. The transition from compression to extension of the electron bunch was bounded by a perfect synchronism condition between the electron bunch and the FEL pulse, and the change rate of the root-mean-square bunch length was as high as 14%. Therefore, it was experimentally demonstrated that a slight temporal difference between the electron bunch and the optical pulse dramatically changed not only the FEL power but also the outline of electron distribution in the bunch.

The Synthetic Hilbert Space of Laser-Driven Free-Electrons

Recent advances in laser interactions with coherent free electrons have enabled to shape the electron's quantum state. Each electron becomes a superposition of energy levels on an infinite quantized ladder, shown to contain up to thousands of energy levels. We propose to utilize the quantum nature of such laser-driven free electrons as a "synthetic Hilbert space" in which we construct and control qudits (quantum digits). The question that motivates our work is what qudit states can be accessed using electron-laser interactions, and whether it is possible to implement any arbitrary quantum gate. We find how to encode and manipulate free-electron qudit states, focusing on dimensions which are powers of 2, where the qudit represents multiple qubits implemented on the same single electron – algebraically separated, but physically joined. As an example, we prove the possibility to fully control a 4-dimenisonal qudit, and reveal the steps required for full control over any arbitrary dimension. Our work enriches the range of applications of free electrons in microscopy and spectroscopy, offering a new platform for continuous-variable quantum information.

Generalized nondipole strong-field approximation of high-order harmonic generation

The strong-field approximation (SFA) is a widely used theoretical framework that describes the process of high-order harmonic generation of atoms and molecules. Here, we propose a generalization of the dipole SFA towards weakly relativistic contributions to the laser-electron interaction. These weakly relativistic contributions are closely related to the spatial structure of the light field and imply a correction of the relativistic order $1/c$. Within this generalized nondipole SFA (GN-SFA), we demonstrate how to obtain explicit results and discuss their physical aspects. This approach enables one to investigate the nondipole effects of linear polarized plane waves as well as the influence of structured light fields, such as twisted light, that have not yet been captured by the currently available models. In order to utilize our generalized nondipole SFA, we consider a linearly polarized plane wave and demonstrate the decrease of the harmonic yield that is directly related to the nondipole effects of the laser field. Furthermore, we discuss the GN-SFA with regard to other nondipole SFA approaches by determining their physical and mathematical context. Finally, the GN-SFA is a powerful theoretical framework that extends the nonrelativistic SFA rigorously to the weakly relativistic regime and therefore will be a useful model for further theoretical investigations.

GPU accelerated Monte Carlo simulation of high-intensity pulsed laser-electron interaction

We present a new Monte Carlo code for simulating strong field QED effects in high intensity Laser-Electron collisions, accelerated by massively parallel computations on GPUs. The numerical simulation tool is paired with a separate code which solves the Stratton-Chu vector diffraction integrals given the incoming beam on the focusing mirror, to more accurately describe the electromagnetic field at the focus of the mirror. Program summaryProgram title: SFQED codeCPC Library link to program files:https://doi.org/10.17632/bd5m7tf5yr.1Licensing provisions: MIT licenseProgramming language: C++ and CUDANature of problem: Evaluating the electromagnetic field, in the focus of a laser pulse with an arbitrary wavefront and transverse intensity profile focused by a parabolic mirror, is analytically not possible without performing several approximations along the way. It may prove vital in upcoming strong field QED experiments to know the field of an imperfect focused laser pulse exactly when laser intensities become large, as the interaction between high energy electrons and the focused laser pulse is numerically modeled when comparing to experiments.Solution method: We propose to numerically evaluate the Stratton-Chu vector diffraction integrals, given the wavefront and spatial intensity profile of the laser on the focusing mirror, through CUDA, and evaluate the electromagnetic field on a spatial grid around the focus of the laser pulse. A Monte-Carlo code that simulates the interaction between high energy particles and an electromagnetic field, will then use the evaluated focused laser field through trilinear interpolation to find the resulting electric field at the location of the particle in each time step.

Radiation reaction enhancement in flying focus pulses

Radiation reaction (RR) is the oldest still-unsolved problem in electrodynamics. In addition to conceptual difficulties in its theoretical formulation, the requirement of exceedingly large charge accelerations has thus far prevented its unambiguous experimental identification. Here, we show how measurable RR effects in a laser-electron interaction can be achieved through the use of flying focus pulses (FFPs). By allowing the focus to counterpropagate with respect to the pulse phase velocity, a FFP overcomes the intrinsic limitation of a conventional laser Gaussian pulse (GP) that limits its focus to a Rayleigh range. For an electron initially also counterpropagating with respect to the pulse phase velocity, an extended interaction length with the laser peak intensity is achieved in a FFP. As a result, the same RR deceleration factors are obtained, but at FFP laser powers orders of magnitude lower than for ultrashort GPs with the same energy. This renders the proposed setup much more stable than those using GPs and allows for more accurate \emph{in situ} diagnostics. Using the Landau-Lifshitz equation of motion, we show numerically and analytically that the capability of emerging laser systems to deliver focused FFPs will allow for a clear experimental identification of RR.

Narrowband Terahertz Emission With Tunable Orbital Angular Momentum by Vortex Laser-Beam Interaction

Terahertz vortex beams need to be developed for supporting new applications in fundamental sciences. In this paper, a novel and feasible scheme is presented for generation of helically microbunched electron beam for the emission of terahertz (THz) vortex pulses. In the scheme, a seed laser pulse with longitudinal and azimuthal modulation is firstly produced by optical beating of dual-vortex pulses. Through the laser-electron interaction, such a complex profile of laser can be converted to a periodically azimuth-dependence energy modulation in the electron beam, which finally forms a helical microbunching structure in the beam after a dispersion section. Then, this electron beam is applied to emit THz vortex beams and its quantum number can be continuously tuned by changing the topological charges of the optical laser pulses. This approach is suited to conventional accelerator system consisted of a modulator and a chicane. We expect this presentation can access to new researches in THz science.

Tuning electron bunch with a longitudinally shaped laser to generate half-cycle terahertz radiation pulse

We present a new scheme for the realization of ultrashort terahertz (THz) radiation pulses by electron beam with a frequency-chirped microbunching structure travelling in a tapered undulator. In this proposal, a longitudinally modulated laser pulse with a chirp rate in the instantaneous modulation frequency is obtained through the optical beating of dual-chirp pulses; then the chirped micro-structure is converted to a chirped density modulation on electron beams by the laser-electron interaction and dispersion effect; finally by sending this electron beam into a designed undulator, ultrashort pulse and broad-band THz radiation can be produced. Considering the fact that the density modulation of the electron beam is directly inherited from the shaped laser, the central frequency and chirp rate can be easily tuned by the optical elements. Numerical simulations confirm that radiation pulses with ultrashort duration can be generated from such a microbunched electron beam. This technique may open new research avenues for strong-field few-cycle THz photonics.

Spectral shape of quasi-monochromatic radiation from electron colliding with tightly focused laser pulses

We theoretically investigate spectra of emitted light in nonlinear Thomson backscattering, which is generated during the head-on collision of the electron and the tightly focused circularly polarized laser pulse. With numerical analysis, a novel approach of generating quasi-monochromatic x-rays is obtained. By properly setting the electron–laser interaction point, limiting the laser intensity, and increasing pulse duration, the emission spectral bandwidth would be effectively narrowed without reducing the radiant energy of the electrons. In the end, the optimal situations for obtaining the quasi-monochromatic x-ray source are presented.

Coherent terahertz radiation with orbital angular momentum by helically microbunched electron beam

A scheme to generate coherent terahertz (THz) radiation carrying orbital angular momentum (OAM) through down-frequency conversion of optical lasers and higher order mode coupling is proposed and analyzed. Based on the laser–electron interaction in the undulator under a linear regime, the scheme utilizes dual-frequency lasers resonating at the harmonics of two consecutive undulators and a dispersion section to impose a THz helical microbunching in the electron beam. By choosing appropriate parameters of lasers and dispersion section, the density modulation can be down-converted to a THz spectral-domain and the central frequency can be extensively tuned together with the helical mode number. Then, the helically microbunched beam can be used as a source of THz OAM radiation in a downstream radiator. We expect that the intense vortex beam in the terahertz band will bring new opportunities for the research of quantum topological materials.

Self-consistent numerical approach to track particles in free electron laser interaction with electromagnetic field modes

In this paper we present a novel approach to FEL simulations based on the decomposition of the electromagnetic field in a finite number of radiation modes. The evolution of each mode amplitude is simply determined by energy conservation. The code is developed as an expansion of the General Particle Tracer framework and adds important capabilities to the suite of well-established numerical simulations already available to the FEL community. The approach is not based on the period average approximation and can handle long-wavelength waveguide FELs as it is possible to include the dispersion effects of the boundaries. Futhermore, it correctly simulates lower charge systems where both transverse and longitudinal space charge forces still play a significant role in the dynamics. For free-space FEL interactions, a source dependent expansion approximation can be used to limit the number of transverse modes required to model the field profile and speed up the calculation of the system evolution. Three examples are studied in detail including a single pass FEL amplifier, the high efficiency TESSA266 scenario and a THz waveguide FEL operating in the zero-slippage regime.

Theory of Shaping Electron Wavepackets with Light

We present the quantum theory governing the interaction between short laser pulses and relativistic free electrons and reveal intrinsic conservation laws for such quantum interactions. Through the judicious design of the amplitude and phase of a laser pulse, we propose the complete shaping of the electron temporal wavepacket and energy spectrum. To exemplify the prospects and limitations of the shaping process, we optimize laser pulses that can reduce the energy spread of an electron beam. We also find attosecond laser pulse trains that generate electron combs with record short attosecond features that push the lowest limits of electron temporal duration. We propose to utilize the quantum shaping of the electron energy spectrum for a new kind of light–matter interaction: Matching the shaped electron energy shift to a target material excitation. This way we can transfer the electron coherent state into a new coherent material excitation, allowing access to states that are forbidden with optical excitations. Our formalism generalizes all the previous theoretical models in the area of photon-induced nearfield electron microscopy (PINEM); our formalism can fit experiments in the field that cannot be modeled by the conventional PINEM theory. Altogether, our new formalism of shaping quantum electron-laser interactions in the dimension of time/energy has prospects for novel phenomena in attosecond science, for new techniques in electron microscopy, and for their potential marriage in ultrafast quantum electron optics.

Simulation of XFEL induced fluorescence spectra of hollow ions and studies of dense plasma effects

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.

Observation of the Relativistic Reversal of the Ponderomotive Potential.

The secular dynamics of a nonrelativistic charged particle in an electromagnetic wave can be described by the ponderomotive potential. Although ponderomotive electron-laser interactions at relativistic velocities are important for emerging technologies from laser-based particle accelerators to laser-enhanced electron microscopy, the effects of special relativity on the interaction have only been studied theoretically. Here, we use a transmission electron microscope to measure the position-dependent phase shift imparted to a relativistic electron wave function when it traverses a standing laser wave. The kinetic energy of the electrons is varied between 80 and 300keV, and the laser standing wave has a continuous-wave intensity of 175 GW/cm^{2}. In contrast to the nonrelativistic case, we demonstrate that the phase shift depends on both the electron velocity and the wave polarization, confirming the predictions of a quasiclassical theory of the interaction. Remarkably, if the electron's speed is greater than 1/sqrt[2] of the speed of light, the phase shift at the electric field nodes of the wave can exceed that at the antinodes. In this case there exists a polarization such that the phase shifts at the nodes and antinodes are equal, and the electron does not experience Kapitza-Dirac diffraction. Our results thus provide new capabilities for coherent electron beam manipulation.

Effect of a tightly focused chirped Gaussian laser pulse on electron acceleration in helical undulator

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.

Enhancement of laser-driven ion acceleration in non-periodic nanostructured targets

Using particle-in-cell simulations, we demonstrate an improvement of the target-normal-sheath acceleration (TNSA) of protons in non-periodically nanostructured targets with micron-scale thickness. Compared to standard flat foils, an increase in the proton cutoff energy by up to a factor of two is observed in foils coated with nanocones or perforated with nanoholes. The latter nano-perforated foils yield the highest enhancement, which we show to be robust over a broad range of foil thicknesses and hole diameters. The improvement of TNSA performance results from more efficient hot-electron generation, caused by a more complex laser–electron interaction geometry and increased effective interaction area and duration. We show that TNSA is optimized for a nanohole distribution of relatively low areal density and that is not required to be periodic, thus relaxing the manufacturing constraints.