Generation Of Quasi-monoenergetic Electron Beams Research Articles

Direct laser acceleration of electrons in a high-Z gas target and the effect of threshold plasma density on electron beam generation

An experimental study of laser driven electron acceleration in N2 and N2-He mixed gas-jet targets using laser pulses with durations of ∼60–70 fs is presented. Generation of relativistic electron beams with quasi-thermal spectra was observed at a threshold plasma density of ∼1.6 × 1018 cm−3 in the case of pure N2. The threshold density was found to increase with increasing doping concentrations of He. At an optimum fraction of 50% of He in N2, generation of quasi-monoenergetic electron beams was observed at a comparatively higher threshold density of ∼2 × 1018 cm−3, with an average peak energy of ∼168 MeV, average energy spread of ∼21%, and average total beam charge of ∼220 pC. The electron acceleration could be attributed to direct laser acceleration as well as the hybrid mechanism. The observation of an optimum fraction of He in N2 (in turn threshold plasma density) for comparatively better quality electron beam generation can be understood in terms of the plasma density dependent variation in the dephasing rate of electrons with respect to the transverse oscillating laser field. Results are also supported by 2D particle-in-cell simulations performed using the code EPOCH.

Laser wakefield acceleration in Kr–He plasmas and its application to positron beam generation

We present the generation of quasi-monoenergetic electron beams via ionization injection in a laser wakefield acceleration by using a krypton–helium plasma. We obtained higher-energy electron beams of low divergence as compared with beams from the nitrogen–helium plasma at low plasma densities. This is attributed to an immediate ionization, trapping and acceleration of many electrons from several inner shells of the krypton atoms. On the other hand, there are very high ionization potentials of the K-shell nitrogen electrons which require the pulse (in our laser-plasma parameters) to experience a self-focusing (after 1.6 mm of propagation) at first before any ionization injection and acceleration of those two electrons per atom can occur. Based on those high-quality electron beams from the Kr–He plasmas, we generated ultra-relativistic positron beams with energies up to 100 MeV by placing a lead slab converter with a thickness of the order of its radiation length in the electron beam path after the gas jet. Experimental and Mont-Carlo simulated energy spectra of the positron beams are compared for the energy range of 30 MeV ≤ Ee+ ≤ 100 MeV along with the positron yields. In addition, various parameters of the positrons are investigated against the primary electron beam parameters. Such electron–positron cascading experiment could be helpful in future development for the composition of an all-optical electron–positron collider.

Generation of electron beams from a laser wakefield acceleration in pure neon gas

We report on the generation of quasimonoenergetic electron beams by the laser wakefield acceleration of 17–50 TW, 30 fs laser pulses in pure neon gas jet. The generated beams have energies in the range 40–120 MeV and up to ∼430 pC of charge. At a relatively high density, we observed multiple electron beamlets which has been interpreted by simulations to be the result of breakup of the laser pulse into multiple filaments in the plasma. Each filament drives its own wakefield and generates its own electron beamlet.

Quasi-Monoenergetic Electron Beam Generation from Nanothickness Solid Foils Irradiated by Circular-Polarization Laser Pulses

Ultrashort 1022–1024 W/cm2 laser pulses will offer the possibility of producing monoenergetic electron beams in the GeV—TeV level in the future. A laser-electron acceleration scheme is proposed by the interaction between a thin solid foil and an ultra-intense laser pulse for α0 ≥ 800σ0, where α0 is the normalized laser field and σ0 is the normalized plasma surface density. The energy of the electrons as a function of time can be described by a simple model which indicates that an exponential relationship exists between the energy and the normalized time τ. A quasi-monoenergetic high density electron beam with 1.3 GeV energy and a 2.2% energy spread has been predicted for α0 = 223.5 by particle-in-cell simulations. Characteristics of the ultra-high density electron layer formed in the early period of the acceleration are discussed.

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.

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.

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.

Quasi-monoenergetic electron beam generation by laser wakefield acceleration for all-optical ultrashort X-ray sources

Quasi-monoenergetic electron beam generation by laser wakefield acceleration for all-optical ultrashort X-ray sources

Generation and analysis of quasimonoenergetic electron beams by laser-plasma interaction in transitional region from the self-modulated laser wakefield to bubble acceleration regime

Generation of quasimonoenergetic electron beams in a transitional region from the self-modulated laser wakefield to bubble acceleration regime is reported. Quasimonoenergetic electron beams containing more than 3×108 electrons in the monoenergetic peak with energies of 40–60 MeV have been obtained from a plasma with an electron density of 1.6×1019 cm−3 produced by an 8 TW, 50 fs laser pulse. The generation of quasimonoenergetic electron beams is investigated by two-dimensional particle-in-cell simulations. Few periods of the plasma wave are located inside the laser pulse, because the laser pulse duration is longer than the wavelength of the plasma wave. Electrons trapped in the first period of the plasma wave can form the monoenergetic bunch, even though the trapped electrons interact directly with the laser field. The quasimonoenergetic electron beam can be obtained due to the small contribution of the direct acceleration by the laser field. This type of monoenergetic electron acceleration is different from that of both the self-modulated laser wakefield and bubble acceleration regimes, in which the trapped electrons in the plasma wave are located behind the laser pulse due to the pulse compression or fragmentation and free from the laser electric field. This result suggests a new regime for the quasimonoenergetic electron acceleration in the region between the self-modulation and bubble regime.

Quasi-monoenergetic Electron Acceleration for All-optical Ultrashort X-ray Source

In laser-driven plasma acceleration, which is charged particle acceleration via the interaction of an intense laser pulse with a plasma, the accelerating electric field is a thousand times higher than that of conventional radio-frequency accelerators. Such high accelerating field enables a compact accelerator. Recently, the generation of quasi-monoenergetic electron beams has been demonstrated and the way toward a practical, compact electron accelerator is opened. Furthermore, the electron pulse duration is extremely short, of the order of femtosecond in laser-driven plasma acceleration. This set of unique characteristics opens the way toward a novel, compact, all-optical, ultrashort X-ray source based on such as laser Compton scattering scheme. The present status of the quasi-monoenergetic electron acceleration and prospect for all-optical ultrashort X-ray sources are reviewed.

Stable generation of GeV-class electron beams from self-guided laser–plasma channels

Table-top laser-driven plasma accelerators are gaining attention for their potential use in miniaturizing future high-energy accelerators. By irradiating gas jet targets with ultrashort intense laser pulses, the generation of quasimonoenergetic electron beams was recently observed. Currently, the stability of beam generation and the ability to scale to higher electron beam energies are critical issues for practical laser acceleration. Here, we demonstrate the first generation of stable GeV-class electron beams from stable few-millimetre-long plasma channels in a self-guided wakefield acceleration process. As primary evidence of the laser wakefield acceleration in a bubble regime, we observed a boost of both the electron beam energy and quality by reducing the plasma density and increasing the plasma length in a 1-cm-long gas jet. Subsequent three-dimensional simulations show the possibility of achieving even higher electron beam energies by minimizing plasma bubble elongation, and we anticipate dramatic increases in beam energy and quality in the near future. This will pave the way towards ultracompact, all-optical electron beam accelerators and their applications in science, technology and medicine.

Measurement of stability of electron beam generated by laser-driven plasma-based accelerator

Quasi-monoenergetic electron beams with the energy of 30-80 MeV and large number of electrons more than 108 were produced by focusing a 8TW, 50 fs Ti:sapphire laser pulse onto 1.6−1.9 × 1019 cm−3 plasmas. Stability of the quasi-monoenergetic electron beam generation was evaluated using an in-situ observation system for the electron beam diagnostics.

Laser-Driven Acceleration of Electrons in a Partially Ionized Plasma Channel

The generation of quasimonoenergetic electron beams, with energies up to 200 MeV, by a laser-plasma accelerator driven in a hydrogen-filled capillary discharge waveguide is investigated. Injection and acceleration of electrons is found to depend sensitively on the delay between the onset of the discharge current and the arrival of the laser pulse. A comparison of spectroscopic and interferometric measurements suggests that injection is assisted by laser ionization of atoms or ions within the channel.

Dependence of the electron beam parameters on the stability of laser propagation in a laser wakefield accelerator

Characteristics of electron beams produced by the laser wakefield acceleration are presented. The dependence of the electron beam parameters on the laser focal spot size is investigated. The experimental result shows the generation of quasimonoenergetic electron beam although the laser spot size was several times larger than the plasma wavelength. Stable electron beam generation at large laser spots was owing to the stable laser propagation in plasma channels. At a small laser spot, the beam quality is poor and this is attributed to the the filamentation instability of the laser beam.

Transverse dynamics and energy tuning of fast electrons generated in sub-relativistic intensity laser pulse interaction with plasmas

The regimes of quasi-monoenergetic electron beam generation were experimentally studied in the sub-relativistic intensity laser plasma interaction. The observed electron acceleration regime is unfolded with two-dimensional-particle-in-cell simulations of laser-wakefield generation in the self-modulation regime.

Observation of strong correlation between quasimonoenergetic electron beam generation by laser wakefield and laser guiding inside a preplasma cavity

We use a one-shot measurement technique to study effects of laser prepulses on the electron laser wakefield acceleration driven by relativistically intense laser pulses (lambda=790 nm, 11 TW, 37 fs) in dense helium gas jets. A quasimonoenergetic electron bunch with an energy peak approximately 11.5 MeV[DeltaE/E approximately 10% (FWHM)] and with a narrow-cone angle (0.04pi mm mrad) of ejection is detected at a plasma density of 8 x 10(19) cm(-3). A strong correlation between the generation of monoenergetic electrons and optical guiding of the pulse in a thin channel produced by picosecond laser prepulses is observed. This generation mechanism is well corroborated by two-dimensional particle-in-cell simulations.

Quasimonoenergetic electron beam generation by using a pinholelike collimator in a self-modulated laser wakefield acceleration

A relativistic electron bunch with a large charge (>2 nC) was produced from a self-modulated laser wakefield acceleration configuration. For this experiment, an intense laser beam with a peak power of 2 TW and a duration of 700 fs was focused in a supersonic He gas jet, and relativistic high-energy electrons were observed from the strong laser-plasma interaction. By passing the electron bunch through a small pinholelike collimator, we could generate a quasimonoenergetic high-energy electron beam, in which electrons within a cone angle of 0.25 mrad (f/70) were selected. The beam clearly showed a narrow-energy-spread behavior with a central energy of 4.3 MeV and a charge of 200 pC. The acceleration gradient was estimated to be about 30 GeV/m. Particle-in-cell simulations were performed for comparison study and the result shows that both the experimental and simulation results are in good agreement and the electron trapping is initiated by the slow beat wave of the Raman backward wave and the incident laser pulse.

Quasi-monoenergetic electron beam generation during laser pulse interaction with very low density plasmas

The results of experiments are presented for the single laser pulse interaction with a very low density gas target, under the conditions when the generated wake wave is below the wave-breaking threshold and the laser pulse power is lower than the critical power for relativistic self-focusing. A quasi-monoenergetic electron beam is found to be stably generated for various laser pulse intensity values by controlling the acceleration length. The results of two-dimensional particle-in-cell simulations show that for the electron acceleration an additional mechanism of electron injection into the acceleration phase is required. It is demonstrated that the longitudinal inhomogeneity of the plasma density leads to the electron injection.

Near-GeV electron beam from a laser wakefield accelerator in the bubble regime

The bubble regime of interaction of an ultra-short petawatt-class laser pulse with an underdense plasma is investigated with two-dimensional (2D) particle-in-cell (PIC) simulations. The primary interest is in generation of high-energy electron beams in GeV energy level with good quality (high charge, relatively small energy spread and short pulse length). A wide range of parameters in plasma density, laser pulse duration, and laser focal spot size were used in the simulations. It is shown that quality of the high-energy part of the electron beam substantially depends on the laser focal spot size. The simulation studies imply that an optimum condition exists for generation of quasi-monoenergetic electron beams near the GeV-level and the combination of a laser spot size and plasma density is a dominant factor.

Demonstration of quasi-monoenergetic electron-beam generation in laser-driven plasma acceleration

The generation of a quasi-monoenergetic electron beam in laser-driven plasma acceleration is reported. A monoenergetic electron beam with an energy of 7 MeV was emitted from a high-density plasma (electron density >1020cm−3) produced by a 2 TW 50 fs laser pulse. The divergence of the monoenergetic beam was ±1.2°. The first Stokes satellite peak of stimulated forward Raman scattering was observed in the spectrum of the light transmitted through the plasma. The plasma wave was excited in the region of which electron density was around 1.3×1020cm−3. The acceleration length was estimated to be 500μm from the length of the side-scattered light image. It is considered that the monoenergetic beam generation is due to the matching of the acceleration length to the dephasing length determined by the velocity difference between the accelerated electrons and the plasma wave.