In this paper, the modelling and manufacturing of a two-stage supersonic gas jet nozzle enabling the formation of adaptive plasma concentration profiles for injection and acceleration of electrons using few-cycle laser beams are presented. The stages are modelled using the rhoSimpleFoam algorithm of the OpenFOAM computational fluid dynamics software. The first 200–300 ${\rm \mu}$m diameter nozzle stage is dedicated to 1 % N2 + He gas jet formation and electron injection. By varying the pressure between the first and second stages of the injectors, the electron injection location could be adjusted, and the maximum acceleration distance could be ensured. By changing the concentration of the nitrogen in the gas mixture, the charge of the accelerated electrons could be controlled. The second nozzle stage is designed for acceleration in fully ionised He or hydrogen gas and forms the optimal plasma concentration for bubble formation depending on the laser pulse energy, duration and focused beam diameter. In order to reduce the diameter of the plasma profile formed by the first nozzle and the concentration drop gap between the two nozzles, a one-side straight section was introduced in the first nozzle. The shock wave reflected from the straight section of the wall propagates parallel to the shock wave of the intersecting supersonic jets and ensures a minimal gap between the jets. The second-stage longitudinal plasma concentration profile could have an increasing gas density gradient to compensate for dephasing between the electron bunch and the plasma wave due to wave shortening with increasing plasma concentration.

In this research, the process of electron acceleration and wakefield generation by Gaussian-like (GL), super-Gaussian (SG) and Bessel–Gaussian (BG) laser pulses through cold collisionless plasma in the presence of a planar magnetostatic wiggler are studied. Three different types of laser spatial profiles, GL, SG and BG, are considered. Additionally, using the hydrodynamics fluid equations, Maxwell's equations as well as the perturbation technique for GL, SG and BG laser pulses in the weakly nonlinear regime and in the presence of a planar magnetostatic wiggler, governing equations for analysing the laser wakefield and electron acceleration have been derived and compared correspondingly. In addition, the effect of some important factors, including the wiggler field strengths, laser intensity, pulse length, plasma electron density and laser frequency on the wakefield and the electron energy gain, have been investigated. Numerical results show that enhancing the wiggler magnetic field results in an increase in the amplitude of the wakefield. Furthermore, it is observed that in comparison with the wakefield amplitude excited by SG and GL laser pulses, the amplitude of the wakefield excited by BG laser pulse is larger when the wiggler field is enhanced. Moreover, it is realized that the type of the laser profile, selected laser parameters and wiggler magnetic field are the most decisive and effective factors in the wakefield amplitude and shape of wakefield generation through cold collisionless plasma. Also, it is seen that as the pulse length declines, the amplitude of the wakefield increases, and correspondingly the resonance positions shift to higher ${({\varOmega _w}/{\varOmega _p})_{max}}$ values.

Electron dynamics in an axially localized large amplitude electron Bernstein mode in a magnetic mirror is studied. The mode is localized due to plasma density and magnetic field profiles and could be driven by an electron cyclotron wave, launched from outside, via linear mode conversion. Energetic electrons of finite gyro-radius resonantly interact with the mode and gain primarily transverse energy favoring stronger mirror confinement. At Bernstein wave normalized amplitude of A00 = 0.01 and for other normalized parameters Zn0 = 40, k⊥c/ω = 10, ${L}^{\prime}_m = 215$, ωc0/ω = 0.9, ψn0 = 3π/2, the electrons can gain energy in the hundreds of keV range.

Efficient electron acceleration by a linearly chirped ultrashort laser pulse in vacuum is investigated using the particle swarm optimization method. By applying this method for optimizing the initial parameters of the laser pulse, a pronounced increase in final energy gain of the electron is obtained compared to that expected from the successive optimization method. Our results also suggest that the value of the optimal chirp parameter is independent of laser polarization and the energy gain could be insensitive to the sign of this parameter when the initial phase is optimally adjusted. In addition, utilizing the chirped laser pulse with optimized conditions for acceleration of an electron bunch reveals that the energy spectrum is shifted to considerably higher energies and the spatial distribution is significantly improved in a polarization-dependent manner.

Hermite–Gaussian (HG) laser beam with transverse electromagnetic (TEM) mode indices (m, n) of distinct values (0, 1), (0, 2), (0, 3), and (0, 4) has been analyzed theoretically for direct laser acceleration (DLA) of electron under the influence of an externally applied axial magnetic field. The propagation characteristics of a TEM HG beam in vacuum control the dynamics of electron during laser–electron interaction. The applied magnetic field strengthens the $\vec v \times \vec B$ force component of the fields acting on electron for the occurrence of strong betatron resonance. An axially confined enhanced acceleration is observed due to axial magnetic field. The electron energy gain is sensitive not only to mode indices of TEM HG laser beam but also to applied magnetic field. Higher energy gain in GeV range is seen with higher mode indices in the presence of applied magnetic field. The obtained results with distinct TEM modes would be helpful in the development of better table top accelerators of diverse needs.

By sophisticated application of particle-in-cell simulations, we demonstrate the ultimate role of non-linear pulse evolutions in accelerating electrons during the entrance of an intense laser pulse into a preformed density profile. As a key point in our discussions, the non-linear pulse evolutions are found to be very fast even at very low plasma densities, provided that the pulse length exceeds the local plasma wavelength. Therefore, these evolutions are sufficiently developed during the propagation of typical short density scale lengths occurred at high contrast ratios of the pulse, and lead to plasma heating via stochastic acceleration in multi-waves. Further analysis of simulation data at different physical parameters indicates that the rate of evolutions increases with the plasma density leading to higher plasma heating and overgrown energetic electrons. In the same way, shortening the density scale length results into increase in the evolution rate and, simultaneously, decrease in the interaction time. This behavior can describe the observed optimum value of pre-plasma scale length for the maximum electron heating.

Resonant acceleration of electrons by a laser in the background of an extra longitudinal magnetic field is investigated analytically and numerically. The resonant condition is independent of laser intensity, and when satisfied, the energy gain is proportional to $a_0^2 $ and the square of phase difference. This process is mainly limited by the magnitude and spatial size of the extra magnetic field. Under the laboratory conditions, simulation results show that a monoenergetic and collimated electron bunch can still be obtained in ~ GV/cm scale, which sheds a light on the vacuum table-top laser-driven electron accelerators.

The improved laser-to-pedestal contrast ratio enabled by current high-power laser pulse cleaning techniques allows the fine features of the target survive before the main laser pulse arrives. We propose to introduce the nano-fabrication technologies into laser–plasma interaction to explore the novel effects of micro-structures. We found out that not only laser-driven particle sources but also the laser pulse itself can be manipulated by specifically designed micro-cylinder and -tube targets, respectively. The proposal was supported by full-3D particle-in-cell simulations and successful proof-of-principle experiments for the first time. We believe this would open a way to manipulate relativistic laser–plasma interaction at the micro-size level.

The acceleration of an electron by the ponderomotive force of a Gaussian whistler pulse in a magnetized high-density quantum plasma obeying Fermi–Dirac distribution is studied using the recently developed quantum hydrodynamic model. Effective acceleration takes place when the peak whistler amplitude exceeds a threshold value, and the whistler frequency is greater than the cyclotron frequency. The threshold amplitude decreases with ratio of plasma frequency to electron cyclotron frequency. The electron is accelerated at velocities of about twice the group velocity of the whistler.

The energy spectra of a sparse ensemble of electrons scattered by relativistically intense laser pulses are studied numerically by solving the relativistic Newton equations with the Lorentz force generated by an electromagnetic envelope in vacuum. The expressions for the envelope describe focused optical fields, include significant short-pulse corrections, and afford the representation of laser radiation with various types of transverse distributions of amplitude. The dependence of the character of the electron energy spectra on the type of the transverse distribution of laser amplitude is explored. For Gaussian pulses, the electron energy spectra within specific angular ranges tend to either include a relativistic maximum while being localized around it or to have the shapes of evanescent distributions dominated by the cold component. Conversely, the energy spectra of electrons ejected into certain angular ranges by laser pulses having first-order Laguerre profiles combine pronounced cold components and structured strongly relativistic features. The presumed laser pulse transverse structure and the shapes of the calculated electron energy spectra for first-order Laguerre amplitude distributions are shown to match, qualitatively, those reported in a recent experimental study by Kalashnikov et al. in 2015, which revealed the electron energy spectra spanning both the sub-relativistic and the markedly relativistic energy domains.

The acceleration of an electron beam by surface plasma waves (SPW), in the presence of external magnetic field parallel to surface and perpendicular to direction of propagation of SPW has been studied. This wave propagating along the $\hat z$-axis is excited using Kretschmann geometry, having maximum amplitude at the metal–vacuum interface. Equations of motion have been solved for electron energy and trajectory. The electron gains and retains energy in the form of cyclotron oscillations due to the combined effect of the static magnetic field and SPW field. The energy gained by the beam increases with the strength of magnetic field and laser intensity. In the present scheme, electron beams can achieve ~15 KeV energy for the SPW amplitude A1 = 1.6 × 1011 V/m, plasma frequency ωp = 1.3 × 1016 rad/s and cyclotron frequency ωc/ωp = 0.003.

Electron acceleration by a circularly polarized Hermite–Gaussian (HG) laser beam in the plasma has been investigated theoretically for the different transverse electromagnetic (TEM) mode indices (m, n) as (0, 1), (0, 2), (0, 3), and (0, 4). HG laser beam possesses higher trapping force compared with a standard Gaussian beam owing to its propagation characteristics during laser–electron interaction. A single-particle simulation indicates a resonant enhancement in the electron acceleration with HG laser beam. We present the intensity distribution for different TEM modes. We also analyze the dependence of beam width parameter on electron acceleration distance, which effectively influences the electron dynamics. Electron acceleration up to longer distance is observed with the lower modes. However, the higher electron energy gain is observed with higher modes at shorter distance of propagation.

Generation of hot electrons at grazing incidence of a subpicosecond relativistic-intense laser pulse onto a plane solid target is analyzed for the parameters of petawatt class laser systems. We study preplasma formation on the surface of solid aluminum targets produced by laser prepulses with a different time structure. For modeling of the preplasma dynamics, we use a wide-range two-temperature hydrodynamic model. As a result of simulations, the preplasma expansion under the action of the laser prepulse and the plasma density profiles for different contrast ratios of the nanosecond pedestal are found. These density profiles are used as the initial density distributions in three-dimensional particle-in-cell simulations of electron acceleration by the main P-polarized laser pulse. Results of modeling demonstrate a substantial increase of the characteristic energy and number of accelerated electrons for the grazing incidence of a subpicosecond intense laser pulse in comparison with the ponderomotive scaling of laser–target interaction.

We report on an experimental study on the interaction of a high-contrast 40 fs duration 2 TW laser pulse with an argon-cluster target. A high-charge, homogeneous, large divergence electron beam with moderate kinetic energy (~2 MeV) is observed in the forward direction. The results show that an electron beam with a charge as high as 12 nC can be obtained using a table-top laser system. It was demonstrated that the accelerated electron beam is suitable for a variety of applications such as micro-radiography of thin samples in a wide field of view. It can also be applied for in vitro dosimetry studies.

An effective theory of laser–plasma-based particle acceleration is presented. Here we treated the plasma as a continuous medium with an index of refraction nm in which a single electron propagates. Because of the simplicity of this model, we did not perform particle-in-cell (PIC) simulations in order to study the properties of the electron acceleration. We studied the properties of the electron motion due to the Lorentz force and the relativistic equations of motion were numerically solved and analyzed. We compared our results with PIC simulations and experimental data.

Electron acceleration is studied during the resonant interaction of launched electron beam with the surface plasma wave (SPW) in the presence of static magnetic field. A configuration of two parallel metal sheets separated by a vacuum region supports the SPW of amplitude maximum on the two parallel interfaces and minimum in the middle. Kretschmann geometry is used to excite surface plasma mode by shinning laser on a glass prism. Dispersion relation of SPW is established in the presence of magnetic field and smaller cut-off frequencies are observed as compared with that of without magnetic field. An electron beam launched in the middle region, experiences a longitudinal ponderomotive force due to SPWs and gets accelerated to the velocity of the order of phase velocity of the surface wave. The energy gained by electron is higher in the presence of magnetic field as compared with zero magnetic field. The electron energy and trajectory are also presented for varying parameters such as amplitude of SPW and magnetic field strength. In the present scheme, electron beams can achieve maximum 550 KeV energy for the SPW amplitude ESP = 1.2 × 1011 V/m, plasma frequency ωp = 1.3 × 1016 rad/s, and cyclotron frequency ωc/ωp = 0.05.

The bubble regime acceleration of electrons by a short pulse laser in a carbon nanotube (CNT) embedded plasma is investigated, employing two-dimensional Particle-in-Cell simulations. The laser converts the CNT placed on the laser axis into dense plasma and expels the electrons out, to form a co-moving positive charged sheet inside the bubble. The additional field generated due to sheet enhances the energy of the monoenergetic bunch by about 5% and their number by 5–20%. For a typical 40 fs, 7.5 × 1019 Wcm−2 pulse in an underdense plasma of density n0, CNT of thickness 25 nm and electron density 30n0, produces a monoenergetic bunch of 115 MeV with 5% energy spread. When CNT density is raised to 90n0, the energy gain, energy spread and accelerated charge increases further. The analytical framework supports these features.

A new two-step approach for frequency conversion of laser radiation towards hard X-rays is developed and examined experimentally. Fast electrons are produced in a form of thin jets at the first stage, as an intense femtosecond laser pulses impinges on a micrometer water target. In the second stage the accelerated electrons hit a secondary metal target and generate characteristic K-shell radiation with a duration down to sub-femtosecond. It is shown that counter propagating laser radiation experiences very strong up-shift with up to 6 × 103 times of fundamental frequency by reflection from the electron jets.

Evolution of longitudinal electrostatic wakefields, due to the propagation of a linearly polarized super-Gaussian laser pulse through homogeneous plasma has been presented via two-dimensional particle-in-cell simulations. The wakes generated are compared with those generated by a Gaussian laser pulse in the relativistic regime. Further, one-dimensional numerical model has been used to validate the generated wakefields via simulation studies. Separatrix curves are plotted to study the trapping and energy gain of an externally injected test electron, due to the generated electrostatic wakefields. An enhancement in the peak energy of an externally injected electron accelerated by wakes generated by super-Gaussian pulse as compared to Gaussian pulse case has been observed.