Quasi-monoenergetic Electron Research Articles

Investigation by simulation of X-ray beams produced via interactions of suitable quasi-monoenergetic electrons with solid targets

X-ray generation by interactions of quasi-monoenergetic electrons with solid lead targets is studied. Various electron energy spectra generated by a laser-plasma accelerator are employed to generate Bremsstrahlung and characteristic X-rays. These X-ray photons are simulated by using the Monte Carlo N-Particle Transport Code (MCNP). The present work is mainly focused on exploring the influence of two parameters of the electron profile, the peak energy (E p ) and the pulse width (ΔE), on X-ray generation. If an electron source with higher E p and smaller ΔE is used, the efficiency of X-ray creation is found to be increased in thick targets. However, in thin samples, the electron profile with lower E p and larger ΔE dominates.

Generation of quasi-monoenergetic electron beams with small normalized divergences angle from a 2 TW laser facility.

We report the generation of a 6 pC, 23 MeV electron bunch with the energy spread ± 3.5% by using 2 TW, 80 fs high contrast laser pulses interacting with helium gas targets. Within the optimized experimental condition, we obtained quasi-monoenergetic electron beam with an ultra-small normalized divergence angle of 92 mrad, which is at least 5 times smaller than the previous LPA-produced bunches. We suggest the significant decrease of the normalized divergence angles is due to smooth transfer from SM-LWFA to LWFA. Since the beam size in LPA is typically small, this observation may explore a simple way to generate ultralow normalized emittance electron bunches by using small-power but high-repetition-rate laser facilities.

Quasi monoenergetic electron bunch generation by frequency variation laser pulse in magnetized plasma

The effect of the frequency variation of laser pulse in uniform magnetized plasma is considered in one-dimensional laser wakefield acceleration and carried out particle simulation. It is shown that wakefield amplitude is increased threefold for the negative Gaussian chirped laser pulse in the magnetized plasma. In our simulation, electrons with initial energy about 0.3 MeV with initial energy spread about 10 % were trapped, effectively compressed in longitudinal direction and accelerated to ultra-relativistic energy about 1.3 GeV with final energy spread about 6 %.

X-ray pulse generation by laser Compton scattering using a high-charge, laser-accelerated, quasi-monoenergetic electron beam

X-ray pulse generation is demonstrated by laser Compton scattering using a laser-accelerated quasi-monoenergetic electron (QME) beam containing 70 pC electrons with an energy of 60 MeV in the monoenergetic peak. A well-collimated X-ray beam with a divergence angle of approximately 5 mrad is produced. The X-ray photon number is estimated to be 2 × 107 per pulse, which is 10 thousand times larger than that achieved in a previous study using a laser-accelerated QME beam. The peak energy of the X-rays is estimated to be 60 keV by a numerical simulation using the measured characteristics of the QME beam.

Effect of chirp on self-modulation and laser wakefield electron acceleration in the regime of quasimonoenergetic electron beam generation

Strongly self-modulated regime of laser wakefield electron acceleration has been experimentally studied using positively/negatively chirped Ti:sapphire laser pulses with duration $\ensuremath{\ge}45\text{ }\text{ }\mathrm{fs}$, and intensity $\ensuremath{\le}1.2\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ interacting with a plasma having density $>5\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$. The total charge of the high energy electrons produced from self-injection and acceleration in the wakefield was found to depend strongly and asymmetrically on the magnitude and sign of the chirp, which was also corroborated by the forward Raman scattering signals. Generation of low divergence ($\ensuremath{\le}10\text{ }\text{ }\mathrm{mrad}$), quasimonoenergetic electron beam was observed with a maximum energy of $\ensuremath{\sim}28\text{ }\text{ }\mathrm{MeV}$ for a positively chirped laser pulse of duration $\ensuremath{\sim}70\text{ }\text{ }\mathrm{fs}$. The observations are explained considering faster rise time associated with a positive chirped laser pulse which generates intense seed for stronger modulation of the laser pulse and consequently stronger wakefield excitation leading to higher electron beam charge and energy.

Induction of electron injection and betatron oscillation in a plasma-waveguide-based laser wakefield accelerator by modification of waveguide structure

By adding a transverse heater pulse into the axicon ignitor-heater scheme for producing a plasma waveguide, a variable three-dimensionally structured plasma waveguide can be fabricated. With this technique, electron injection in a plasma-waveguide-based laser wakefield accelerator was achieved and resulted in production of a quasi-monoenergetic electron beam. The injection was correlated with a section of expanding cross-section in the plasma waveguide. Moreover, the intensity of the X-ray beam produced by the electron bunch in betatron oscillation was greatly enhanced with a transversely shifted section in the plasma waveguide. The technique opens a route to a compact hard-X-ray pulse source.

High-quality electron beam from laser wake-field acceleration in laser produced plasma plumes

Generation of highly collimated (θdiv ∼10 mrad), quasi-monoenergetic electron beam with peak energy 12 MeV and charge ∼50 pC has been experimentally demonstrated from self-guided laser wake-field acceleration (LWFA) in a plasma plume produced by laser ablation of solid nylon (C12H22N2O2)n target. A 7 TW, 45 fs Ti:sapphire laser system was used for LWFA, and the plasma plume forming pulse was derived from the Nd:YAG pump laser of the same system. The results show that a reproducible, high quality electron beam could be produced from this scheme which is simple, low cost and has the capability for high repetition rate operation.

Collimated quasi-monoenergetic electron beam generation from intense laser solid interaction

A quasi-monoenergetic electron beam with divergence of 3° and energy peak of 1 MeV is observed along the target surface from interaction of a bulk Cu target and an intense relativistic laser pulse of 1 TW and 70 fs at a grazing incident angle. A preplasma formed by high-contrast picosecond prepulse plays a crucial role. Particle-in-cell simulations broadly reproduce the result and show that a preplasma with the proper density and a large angle of incidence is required. The preplasma sets up a static electric field along the surface can accelerate electrons. The static electric field is formed just after the passage of the laser. This approach can be extended to higher intensities to generate higher energy beams.

Stable, tunable, quasimonoenergetic electron beams produced in a laser wakefield near the threshold for self-injection

Stable operation of a laser-plasma accelerator near the threshold for electron self-injection in the blowout regime has been demonstrated with 25--60 TW, 30 fs laser pulses focused into a 3--4 millimeter length gas jet. Nearly Gaussian shape and high nanosecond contrast of the focused pulse appear to be critically important for controllable, tunable generation of 250--430 MeV electron bunches with a low-energy spread, $\ensuremath{\sim}10\text{ }\text{ }\mathrm{pC}$ charge, a few-mrad divergence and pointing stability, and a vanishingly small low-energy background. The physical nature of the near-threshold behavior is examined using three-dimensional particle-in-cell simulations. Simulations indicate that properly locating the nonlinear focus of the laser pulse within the plasma suppresses continuous injection, thus reducing the low-energy tail of the electron beam.

Direct Electron Acceleration with Radially Polarized Laser Beams

In the past years, there has been a growing interest in innovative applications of radially polarized laser beams. Among them, the particular field of laser-driven electron acceleration has received much attention. Recent developments in high-power infrared laser sources at the INRS Advanced Laser Light Source (Varennes, Qc, Canada) allowed the experimental observation of a quasi-monoenergetic 23-keV electron beam produced by a radially polarized laser pulse tightly focused into a low density gas. Theoretical analyses suggest that the production of collimated attosecond electron pulses is within reach of the actual technology. Such an ultrashort electron pulse source would be a unique tool for fundamental and applied research. In this paper, we propose an overview of this emerging topic and expose some of the challenges to meet in the future.

Spectra of highly charged Fe ions excited by electron collisions: Dependence on electron energy and density

We present the extreme ultraviolet to soft X-ray spectra of highly charged iron ions observed with an electron beam ion trap (EBIT). An EBIT represents a simple plasma which consists of a quasi-monoenergetic electron beam and trapped ions; it is thus useful for testing the atomic models used in the spectroscopic diagnostics of astrophysical plasmas. The dependence on electron energy and density is compared with model calculations.

Optimum conditions of the plasma density for the generation of the maximum bunch charge in a laser wakefield accelerator

The effects of plasma density on the bunch charge in a laser wakefield accelerator were investigated by using experiments and simulations. We used an energy spectrometer and Integrating Current Transformer (ICT) to measure the energy and the charge of a quasi-monoenergetic electron bunch. Also, the plasma density was measured using a biprism interferometer. The measured electron energy showed that the electron energy decreased with increasing plasma density due to the dephasing effect. The results of the bunch charge measurement showed that the effect of the plasma density on the bunch charge could be separated into two different density regions. In the low-density region (cτ/λp 2), the bunch charge decreased with increasing plasma density. Thus, an optimum plasma density to generate a high bunch charge exists. Particle-in-cell simulations show that multi-cavitation of the plasma wake wave results in maximum generation of the bunch charge in a laser wakefield accelerator.

Electron Emission at Locked Phases from the Laser-Driven Surface Plasma Wave

By irradiating a flat Al target with femtosecond laser pulses at moderate intensities of ∼10(17) W/cm(2), we obtained stable collimated quasimonoenergetic electrons in the specular direction but deviated somewhat toward the target normal. An associated local minimum located on the other side of the specular direction seems to indicate that the peak actually results from the deflection of the collimated electrons from their initial ejection direction. We have proposed a two-step model in which some laser-accelerated electrons are able to leave the plasma in a narrow phase-locked window of the moving wave interference pattern, and are then steered toward the target normal by the ponderomotive force of the interference field. The periodic repetition of the electron emission leads to a pulse train of collimated quasimonoenergetic electrons with subcycle duration.

Quasi-monoenergetic electron beams production in a sharp density transition

Using a laser plasma accelerator, experiments with a 80 TW and 30 fs laser pulse demonstrated quasi-monoenergetic electron spectra with maximum energy over 0.4 GeV. This is achieved using a supersonic He gas jet and a sharp density ramp generated by a high intensity laser crossing pre-pulse focused 3 ns before the main laser pulse. By adjusting this crossing pre-pulse position inside the gas jet, among the laser shots with electron injection, more than 40% can produce quasi-monoenergetic spectra. This could become a relatively straight forward technique to control laser wakefield electron beams parameters.

Tuning the electron energy by controlling the density perturbation position in laser plasma accelerators

A density perturbation in an underdense plasma was used to improve the quality of electron bunches produced in the laser-plasma wakefield acceleration scheme. Quasi-monoenergetic electrons were generated by controlled injection in the longitudinal density gradients of the density perturbation. By tuning the position of the density perturbation along the laser propagation axis, a fine control of the electron energy from a mean value of 60 MeV to 120 MeV has been demonstrated with a relative energy-spread of 15 ± 3.6%, divergence of 4 ± 0.8 mrad, and charge of 6 ± 1.8 pC.

Generation of tunable, 100–800 MeV quasi-monoenergetic electron beams from a laser-wakefield accelerator in the blowout regime

In this paper, we present results on a scalable high-energy electron source based on laser wakefield acceleration. The electron accelerator using 30–80 TW, 30 fs laser pulses, operates in the blowout regime, and produces high-quality, quasi-monoenergetic electron beams in the range 100–800 MeV. These beams have angular divergence of 1–4 mrad, and 5%–25% energy spread, with a resulting brightness 1011 electrons mm−2 MeV−1 mrad−2. The beam parameters can be tuned by varying the laser and plasma conditions. The use of a high-quality laser pulse and appropriate target conditions enables optimization of beam quality, concentrating a significant fraction of the accelerated charge into the quasi-monoenergetic component.

Quasimonoenergetic electron beams from laser wakefield acceleration in pure nitrogen

Quasimonoenergetic electron beams with maximum energy >0.5 GeV and 2 mrad divergence have been generated in pure nitrogen gas via wakefield acceleration with 80 TW, 30 fs laser pulses. Long low energy tail features were typically observed due to continuous ionization injection. The measured peak electron energy decreased with the plasma density, agreeing with the predicted scaling for electrons. The experiments showed a threshold electron density of 3×1018cm-3 for self-trapping. Our experiments suggest that pure Nitrogen is a potential candidate gas to achieve GeV monoenergetic electrons using the ionization induced injection scheme for laser wakefield acceleration.

Simulation of enhanced characteristic x rays from a 40-MeV electron beam laser accelerated in plasma

Simulation of x-ray generation from bombardment of various solid targets by quasimonoenergetic electrons is considered. The electron bunches are accelerated in a plasma produced by interaction of 500 mJ, 30 femtosecond laser pulses with a helium gas jet. These relativistic electrons propagate in the ion channel generated in the wake of the laser pulse. A beam of MeV electrons can interact with targets to generate x-ray radiation with keV energy. The MCNP-4C code based on Monte Carlo simulation is employed to compare the production of bremsstrahlung and characteristic x rays between 10 and 100 keV by using two quasi-Maxwellian and quasimonoenergetic energy distributions of electrons. For a specific electron spectrum and a definite sample, the maximum x-ray flux varies with the target thickness. Besides, by increasing the target atomic number, the maximum x-ray flux is increased and shifted towards a higher energy level. It is shown that by using the quasimonoenergetic electron profile, a more intense x ray can be produced relative to the quasi-Maxwellian profile (with the same total energy), representing up to 77% flux enhancement at ${K}_{\ensuremath{\alpha}}$ energy.

Characterization of transverse beam emittance of electrons from a laser-plasma wakefield accelerator in the bubble regime using betatron x-ray radiation

We propose and use a technique to measure the transverse emittance of a laser-wakefield accelerated beam of relativistic electrons. The technique is based on the simultaneous measurements of the electron beam divergence given by $v_{\perp}/v_{\parallel}$, the measured longitudinal spectrum $\gamma_\parallel$ and the transverse electron bunch size in the bubble $r_{\perp}$. The latter is obtained via the measurement of the source size of the x-rays emitted by the accelerating electron bunch in the bubble. We measure a \textit{normalised} RMS beam transverse emittance $<0.5$ $\pi$ mm$\:$mrad as an upper limit for a spatially gaussian, spectrally quasi-monoenergetic electron beam with 230 MeV energy in agreement with numerical modeling and analytic theory in the bubble regime.

Radiative diagnostics for sub-Larmor scale magnetic turbulence

Radiative diagnostics of high-energy density plasmas is addressed in this paper. We propose that the radiation produced by energetic particles in small-scale magnetic field turbulence, which can occur in laser-plasma experiments, collisionless shocks, and during magnetic reconnection, can be used to deduce some properties of the turbulent magnetic field. Particles propagating through such turbulence encounter locally strong magnetic fields but over lengths much shorter than a particle gyroradius (that is λB≪γmν/q⟨B⊥⟩). Consequently, the particle does not gyrate but undergoes a series of transverse accelerations reflective of the magnetic field variations it encounters.In the limit of small deflection angle, the particle is accelerated but never substantially deviated from a straight line path. We develop the general jitter radiation solutions for this case and show that the resulting radiation is directly dependent upon the spectral distribution of the magnetic field through which the particle propagates. We demonstrate the power of this approach in considering the radiation produced by particles moving through a region in which a (Weibel-like) filamentation instability grows magnetic fields randomly oriented in a plane transverse to counterstreaming particle populations. We calculate the spectrum as would be seen from the original particle population and as could be seen by using a quasi-monoenergetic electron beam to probe the turbulent region at various angles to the filamentation axis.