Electron wakefield acceleration by intense subpicosecond laser radiation[
High Power Laser Science and Engineering, Volume. 1, Issue 2, 02000080(2013)
Collimated quasi-monochromatic beams of accelerated electrons in the interaction of a weak-contrast intense femtosecond laser pulse with a metal foil
We demonstrated experimentally the formation of monoenergetic beams of accelerated electrons by focusing femtosecond laser radiation with an intensity of onto the edge of an aluminum foil. The electrons had energy distributions peaking in the 0.2–0.8 MeV range with energy spread less than 20%. The acceleration mechanism related to the generation of a plasma wave as a result of self-modulation instability of a laser pulse in a dense plasma formed by a prepulse (arriving 12 ns before the main pulse) is considered. One-dimensional and two-dimensional Particle in Cell (PIC) simulations of the laser–plasma interaction showed that effective excitation of a plasma wave as well as trapping and acceleration of an electron beam with an energy on the order of 1 MeV may occur in the presence of sharp gradients in plasma density and in the temporal shape of the pulse.
1. Introduction
Electron wakefield acceleration by intense subpicosecond laser radiation[
Trapping of electrons by a plasma wake is one of the key problems for laser–plasma acceleration. The initial momentum of the electrons has to be sufficient for them to stay in an accelerating phase of the plasma wave traveling with speed close to the speed of light[
In the regime of linear plasma wave excitation which produces a stable plasma wave but with low amplitude, the problem of electron injection is especially acute. Although the excitation of weakly linear plasma waves at long paths, which are necessary to accelerate particles to several hundreds of MeV, was demonstrated in Refs. [
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In some studies, attempts were made to inject electrons using an external high-frequency accelerator. However, this scheme has difficulties with synchronizing the electron beam and the laser pulse, and a relatively large length of the electron bunches. Currently, an injector based on a radio frequency (RF) accelerator, the photocathode of which is irradiated by a femtosecond pulse exactly synchronized with the strong pulse exciting a plasma wave[
An alternative way to realize a synchronized electron injection is to focus high-intensity radiation on a solid target. The emission of electron beams in the specular direction at focusing laser radiation on the solid was observed in Refs. [
The goal of the present paper is theoretical and experimental investigation of the generation of collimated quasi-monoenergetic beams of accelerated electrons in plasma with sharp spatial gradients by intense laser radiation. To achieve this goal in experiments, laser radiation was focused on the edge of an aluminum foil perpendicular to its plane, so that half of the laser beam bypassed the foil. We observed generation of highly collimated electron beams propagating in the direction of the laser pulse and characterized by a narrow energy distribution peaking in the 0.2–0.8 MeV range. The electron-beam charge ranges from 1 to 10 pC. In the theoretical consideration, 1D and 2D PIC simulations were performed on the propagation of laser pulses with different envelopes in inhomogeneous plasma, which revealed plasma wave excitation as a result of self-modulation laser pulse instability with consecutive electron acceleration due to self-injection into the wakefield plasma wave on longitudinal plasma inhomogeneities. The role of the transverse plasma inhomogeneity is discussed on the base of the obtained 2D modeling results.
2. Theoretical description
One of the possible mechanisms of formation of accelerated electrons with a quasi-monochromatic spectrum is acceleration in the field of the plasma wave generated in the prepulse-induced dense plasma due to the self-modulation instability of the laser pulse[
To analyze the wakefield acceleration of electron bunches, we first performed a pilot PIC simulation in one-dimensional inhomogeneous (1D3V) plasma. The simulation was done using the 1D version of the VLPL code[
The calculations performed with a Gaussian time profile of the laser pulse showed the following: for the plasma and pulse parameters under consideration, when the pulse had a smooth envelope and relatively low intensity, the ‘seed’ plasma wave amplitude (generated by a smooth Gaussian front of the laser pulse, which is longer than the plasma wavelength) is insufficiently large to develop a self-modulation instability and generate an accelerating wake field.
The situation was different in the case of a sharp leading edge of a laser pulse (rectangular or hyper-Gaussian), when there occurred generation of a wake wave and acceleration of electrons. Note that the pulse leading edge may be sharpened under experimental conditions due to the ionization nonlinearity[
However, the phase space images showed that the dynamics of a small part of the electrons in the region of the sharp change in the spatial distribution of the plasma density at the layer input (in the case of a linear profile) radically differed from the case of a smooth Gaussian density profile passing to a homogeneous layer without jumps in the derivative. For a smooth Gaussian density profile, no electrons were injected into the wakefield and accelerated (Figures
The corresponding electron energy spectrum is shown in Figure
The simulation results (Figures
The reason for our pilot 1D PIC simulation is the fact that under experimental conditions we expect a relatively large transverse length of plasma inhomogeneity in comparison with the radius of the laser beam (which determines the characteristic transverse size of the plasma wave responsible for electron acceleration). Note that the applicability of this approach needs further investigation. Moreover, the discussed mechanism based on the laser pulse self-modulation is not the only mechanism that can lead to collimated electron acceleration. A direct laser acceleration mechanism based on a steep perpendicular density gradient was considered in some publications, but usually it requires relativistic[
To analyze the role of 2D plasma density inhomogeneity and finite laser pulse spot size, modeling was performed by a modified LSP 2D PIC code[
For the above-described 1D inhomogeneous plasma density with linear profile at the layer input (the dashed line in Figure
To investigate the influence of transversal (relative to the direction of laser pulse propagation, -axis) inhomogeneity of plasma (along the -axis), the ion density profile was described as
The electron energy distribution for the electrons moving in the forward direction (angle to the -axis ) is presented in Figure
Thus, from our 2D simulations, we can conclude that significant electron acceleration is determined by three scales of inhomogeneity: plasma density gradients along two axes and laser pulse steepness. In the case of a smooth density gradient in the transversal direction, a sharp gradient along the laser propagation axis and steep laser pulse time envelope for acceleration of electrons are demanded. For acceleration by a smooth (Gaussian) laser pulse time envelope, a sharp density gradient in the transversal direction in respect to laser beam propagation is required. The mechanism of laser electron acceleration in plasma with sharp transverse density gradients will be a subject of future investigations.
3. Experimental technique
A Ti:sapphire laser generating 60 fs pulses with maximum energy up to 100 mJ and central wavelength 800 nm was used in the experiment. A spherical mirror with focal length focused the laser beam on the edge of an aluminum foil ( thick), normally to its surface, placed in a vacuum camera (Figure
To diagnose accelerated electrons, we recorded the luminescence of a Lanex scintillation screen with known sensitivity[
The laser beam passed through the interaction region was monitored by another CCD camera. The electron energy was measured using a magnetic spectrometer 5 cm in diameter based on a pair of NdFeB magnets that provided a 0.2 kG uniform magnetic field. A metal slit was placed at the spectrometer input to collimate the electron beam. The electron energy spectra were reconstructed by luminescence data from the luminescence screen.
A laser pulse was focused by a spherical mirror on the aluminum foil edge so that approximately half of the laser beam fell on the foil and the other half passed by. The foil plane was oriented normally to the laser beam direction. A pair of motorized translators was used to adjust the transverse foil position.
4. Experimental results
The spatial distribution of accelerated electrons was studied in the absence of the magnetic spectrometer. The same area of the foil was successively irradiated by several laser pulses. The spatial distribution of luminescence on the scintillation screen was controlled in each laser pulse. After four or five laser pulses, the screen luminescence disappeared. Visual analysis by means of an optical microscope showed the formation of a diameter hole in a foil at the interaction point, as a result of which the laser beam passed through without distortions. Then the foil was shifted to a new place and the procedure was repeated. The spatial distribution of luminescence for the majority of the laser pulses revealed the formation of highly collimated electron beams with an angular divergence of about 10–20 mrad propagating close to the laser beam direction (Figure
To measure the energy spectrum of spatially collimated beams, we place a magnetic spectrometer with an input slit in front of the scintillation screen. An example of the Lanex screen luminescence after the spectrometer for a single laser pulse is presented in Figure
5. Conclusions
In this paper, we report the results of experimental and theoretical studies of a source of accelerated electrons that can be used for injection in schemes of laser–plasma acceleration of electrons in a plasma wave[
The experiment on irradiation of an aluminum foil edge by a focused high-intensity femtosecond laser pulse showed generation of highly collimated quasi-monochromatic electron beams whose direction coincided with the laser beam propagation direction. The electron energy in the energy distribution peak was 0.2–0.8 MeV, with peak width smaller than 20%; the beam divergence was . The high directionality of the electron beam and its narrow energy spectrum can be explained by the acceleration of electrons in the plasma wave generated as a result of self-modulation instability of an intense laser pulse in the plasma formed by the prepulse arriving at the target about 10 ns before the main pulse.
The results of one (1D3V) and two (LSP 2D) PIC code simulations of the interaction of femtosecond laser radiation with dense plasma showed that effective excitation of a plasma wave as well as trapping and acceleration of electron beams to the energies of about 1 MeV may occur in the presence of plasma inhomogeneities at the plasma boundary, sharp transversal density gradients, and laser pulse time envelope steepness. Under our experimental conditions, the inhomogeneities in the temporal envelope of the laser pulse may be caused by ionization nonlinearity of the plasma formed by the prepulse, and the spatial density inhomogeneities may be due to the sharp boundaries of the foil and the complex configuration of the plasma spread.
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Yu.A. Malkov, A.N. Stepanov, D.A. Yashunin, L.P. Pugachev, P.R. Levashov, N.E. Andreev, K.Yu. Platonov, A.A. Andreev. Collimated quasi-monochromatic beams of accelerated electrons in the interaction of a weak-contrast intense femtosecond laser pulse with a metal foil[J]. High Power Laser Science and Engineering, 2013, 1(2): 02000080
Received: May. 28, 2013
Accepted: Jun. 9, 2013
Published Online: Nov. 19, 2018
The Author Email: Yu.A. Malkov (yurymalkov@mail.ru)