Yan-Yun Ma

Electron injection and trapping in a laser wakefield by field ionization to high-charge states of gases

A scheme for electron injection into a laser wakefield is presented, which makes use of two orthogonally directed laser pulses and a gaseous medium with a moderate or high atomic number such as neon. A pump laser pulse ionizes the medium to its midcharge states to form underdense plasma and meanwhile excites a high amplitude wakefield firstly. Another ultrashort laser pulse with higher intensity is then injected transversely, which further ionizes the medium to high-charge states to produce new free electrons with certain energy. Part of these new-born electrons can be trapped and accelerated by the laser wakefield to high energies. Numerical simulations using a particle-in-cell code with field ionization included are conducted to verify the scheme.

Dense quasi-monoenergetic attosecond electron bunches from laser interaction with wire and slice targets

A scheme is proposed to produce high-quality quasi-monoenergetic attosecond electron bunches based on laser ponderomotive-force acceleration along the surface of wire or slice targets. Two- and three-dimensional particle-in-cell simulations demonstrate that the electron energy depends weakly on the target density. A simple analytical model shows that the electron energy scales linearly with the laser field amplitude, in good agreement with the simulation results. Electron bunches produced by this scheme are suitable for applications such as coherent x-ray radiation, radiography, and injectors in accelerators, etc.

Surface electron acceleration in relativistic laser-solid interactions

Under the grazing incidence of a relativistic intense laser pulse onto a solid target, two-dimensional particle-in-cell simulations show that intense quasistatic magnetic and electric fields are generated near the front target surface during the interaction. Some electrons are confined in these quasistatic fields and move along the target surface with betatron oscillations. When this oscillating frequency is close to the laser frequency in the particle frame, these electrons can be accelerated significantly in the reflected laser field, similar to the inverse free-electron-laser acceleration. An analytical model for this surface betatron acceleration is proposed.

Quasimonoenergetic proton beam from ultraintense-laser irradiation of a target with holed backside

A scheme for producing quasimonoenergetic proton beams is presented. In the scheme, a thin solid target with a tiny hole on its backside is employed. The optimal hole diameter is of the order of the laser spot size so that a localized uniform sheath field can be generated in the hole. Particle-in-cell simulations demonstrate that the highly localized uniform fields can produce monoenergetic target normal sheath acceleration protons in the hole. The transverse electric fields can well suppress the transverse divergence of the protons. The dependence of the proton beam quality on the focal radius and hole structure is also investigated. This special tailed target together with optimized laser parameters can serve as sources for collimated as well as quasimonoenergetic proton beams.

High-quality MeV protons from laser interaction with umbrellalike cavity target

A scheme for producing high-intensity collimated MeV protons from laser interaction with an umbrellalike (cone cavity with an axial filament stemming from the apex) target back side is investigated by two- and three-dimensional particle-in-cell simulations. The process is based on target-normal sheath acceleration. The characteristics of the proton beam are analyzed and compared to that from the recently proposed cone-shaped back side target. It is found that instead of diverging after first focusing, as in the cone-shaped target, the protons produced from the umbrellalike target are well collimated. The large transverse sheath electric field at TV/m level and the self-generated quasistatic magnetic field at hundreds of megagauss level around the filament play important roles in the collimation of the protons.

Dense electron-positron plasmas and gamma-ray bursts generation by counter-propagating quantum electrodynamics-strong laser interaction with solid targets

We use quantum electrodynamics (QED) particle-in-cell simulations to investigate and compare the generation of dense electron-positron plasmas and intense γ-ray bursts in the case of counter-propagating laser solid interaction (two-side irradiation) and single laser solid interaction (one-side irradiation). In the case of counter-propagating linearly polarized laser pulses irradiating a thin aluminum foil with each pulse peak power of 12.5 PW (Iu2009=u20094u2009×u20091023u2009W/cm2), we calculate that about 20% of the laser energy is converted into a burst of γ-rays with flux exceeding 1014u2009s.−1 This would be one of the most intense γ-ray sources among those currently available in laboratories. The γ-ray conversion efficiency in the case of two-side irradiation is three times higher than in the case of one-side irradiation using a single 12.5 PW laser. Dense electron-positron plasma with a maximum density of 6u2009×u20091027u2009m−3 are generated simultaneously during the two-side irradiation which is eightfold denser compared to the on...

Bandgap characteristics of one-dimensional plasma photonic crystal

When two pump laser pulses intersect in an underdense plasma, plasma Bragg grating (PBG) is induced by the slow-varying ponderomotive force [Z. M. Sheng et al., Appl. Phys. B: Lasers Opt. 77, 673 (2003)]. Such a PBG can be considered as a one-dimensional (1D) plasma photonic crystal (PPC). Here the bandgap characteristic of 1D PPC composed of plasma layers of different densities is investigated theoretically and numerically. It is found that when the maximum density is lower than the critical density of the pump laser, there is only one normal-incidence bandgap. When the maximum density is higher than the critical density of the pump laser, high-order bandgaps are found. The theoretical results are verified by 1D particle-in-cell simulations.

Acceleration and evolution of a hollow electron beam in wakefields driven by a Laguerre-Gaussian laser pulse

We show that a ring-shaped hollow electron beam can be injected and accelerated by using a Laguerre-Gaussian laser pulse and ionization-induced injection in a laser wakefield accelerator. The acceleration and evolution of such a hollow, relativistic electron beam are investigated through three-dimensional particle-in-cell simulations. We find that both the ring size and the beam thickness oscillate during the acceleration. The beam azimuthal shape is angularly dependent and evolves during the acceleration. The beam ellipticity changes resulting from the electron angular momenta obtained from the drive laser pulse and the focusing forces from the wakefield. The dependence of beam ring radius on the laser-plasma parameters (e.g., laser intensity, focal size, and plasma density) is studied. Such a hollow electron beam may have potential applications for accelerating and collimating positively charged particles.

High-flux low-divergence positron beam generation from ultra-intense laser irradiated a tapered hollow target

By using two-dimensional particle-in-cell simulations, we demonstrate high-flux dense positrons generation by irradiating an ultra-intense laser pulse onto a tapered hollow target. By using a laser with an intensity of 4u2009×u20091023u2009W/cm2, it is shown that the Breit-Wheeler process dominates the positron production during the laser-target interaction and a positron beam with a total number >1015 is obtained, which is increased by five orders of magnitude than in the previous work at the same laser intensity. Due to the focusing effect of the transverse electric fields formed in the hollow cone wall, the divergence angle of the positron beam effectively decreases to ∼15° with an effective temperature of ∼674u2009MeV. When the laser intensity is doubled, both the positron flux (>1016) and temperature (963u2009MeV) increase, while the divergence angle gets smaller (∼13°). The obtained high-flux low-divergence positron beam may have diverse applications in science, medicine, and engineering.

Enhanced electron–positron pair production by ultra intense laser irradiating a compound target

High-energy-density electron–positron pairs play an increasingly important role in many potential applications. Here, we propose a scheme for enhanced positron production by an ultra intense laser irradiating a gas-Al compound target via the multi-photon Breit–Wheeler (BW) process. The laser pulse first ionizes the gas and interacts with a near-critical-density plasma, forming an electron bubble behind the laser pulse. A great deal of electrons are trapped and accelerated in the bubble, while the laser front hole-bores the Al target and deforms its front surface. A part of the laser wave is thus reflected by the inner curved target surface and collides with the accelerated electron bunch. Finally, a large number of γ photons are emitted in the forward direction via the Compton back-scattering process and the BW process is initiated. Dense electron–positron pairs are produced with a maximum density of m−3. Simulation results show that the positron generation is greatly enhanced in the compound target, where the positron yield is two orders of magnitude greater than that in only the solid slab case. The influences of the laser intensity, gas density and length on the positron beam quality are also discussed, which demonstrates the feasibility of the scheme in practice.