Tong-Pu Yu

Stable Laser-Driven Proton Beam Acceleration from a Two-Ion-Species Ultrathin Foil

By using multidimensional particle-in-cell simulations, we present a new regime of stable proton beam acceleration which takes place when a two-ion-species shaped foil is illuminated by a circularly polarized laser pulse. In the simulations, the lighter protons are nearly instantaneously separated from the heavier carbon ions due to the charge-to-mass ratio difference. The heavy ion layer expands in space and acts to buffer the proton layer from the Rayleigh-Taylor-like (RT) instability that would have otherwise degraded the proton beam acceleration. A simple three-interface model is formulated to explain qualitatively the stable acceleration of the light ions. In the absence of the RT instability, the high quality monoenergetic proton bunch persists even after the laser-foil interaction ends.

Radiation reaction effects on ion acceleration in laser foil interaction

The radiation reaction effects on ion acceleration in laser foil interaction are investigated via analytical modeling and multi-dimensional particle-in-cell simulations. We find that the radiation effects are important in the area where some electrons move backward due to a static charge separation field at a laser intensity of 1022 W cm−2. Radiation reaction tends to impede these backward motions. In the optical transparency region, ion acceleration is enhanced when the radiation effects are considered.

High quality GeV proton beams from a density-modulated foil target

We study proton acceleration from a foil target with a transversely varying density using multi-dimensional Particle-in-Cell (PIC) simulations. In order to reduce electron heating and deformation of the target, circularly polarized Gaussian laser pulses at intensities of the order of 1022Wcm-2 are used. It is shown that when the target density distribution fits that of the laser intensity profile, protons accelerated from the center part of the target have quasi-monoenergetic spectra and are well collimated. In our two-dimensional PIC simulations, the final peak energy can be up to 1.4 GeV with the full-width of half maximum divergence cone of less than 4o. We observe highly efficient energy conversion from the laser to the protons in the simulations.

Dense GeV electron–positron pairs generated by lasers in near-critical-density plasmas

Pair production can be triggered by high-intensity lasers via the Breit–Wheeler process. However, the straightforward laser–laser colliding for copious numbers of pair creation requires light intensities several orders of magnitude higher than possible with the ongoing laser facilities. Despite the numerous proposed approaches, creating high-energy-density pair plasmas in laboratories is still challenging. Here we present an all-optical scheme for overdense pair production by two counter-propagating lasers irradiating near-critical-density plasmas at only ∼1022 W cm−2. In this scheme, bright γ-rays are generated by radiation-trapped electrons oscillating in the laser fields. The dense γ-photons then collide with the focused counter-propagating lasers to initiate the multi-photon Breit–Wheeler process. Particle-in-cell simulations indicate that one may generate a high-yield (1.05 × 1011) overdense (4 × 1022 cm−3) GeV positron beam using 10 PW scale lasers. Such a bright pair source has many practical applications and could be basis for future compact high-luminosity electron–positron colliders.

Enhanced dense attosecond electron bunch generation by irradiating an intense laser on a cone target

By using two-dimensional particle-in-cell simulations, we demonstrate enhanced spatially periodic attosecond electron bunches generation with an average density of about 10nc and cut-off energy up to 380 MeV. These bunches are acquired from the interaction of an ultra-short ultra-intense laser pulse with a cone target. The laser oscillating field pulls out the cone surface electrons periodically and accelerates them forward via laser pondermotive force. The inner cone wall can effectively guide these bunches and lead to their stable propagation in the cone, resulting in overdense energetic attosecond electron generation. We also consider the influence of laser and cone target parameters on the bunch properties. It indicates that the attosecond electron bunch acceleration and propagation could be significantly enhanced without evident divergency by attaching a plasma capillary to the original cone tip.

Enhanced electron trapping and γ ray emission by ultra-intense laser irradiating a near-critical-density plasma filled gold cone

The radiation trapping effect (RTE) of electrons in the interaction of an ultra-intense laser and a near-critical-density plasma-filled gold cone is numerically investigated by using the particle-in-cell code EPOCH. It is found that, by using the cone, the threshold laser intensity for electron trapping can be significantly decreased. The trapped electrons located behind the laser front and confined near the laser axis oscillate significantly in the transverse direction and emit high-energy photons in the forward direction. With parameters optimized, a narrow photon angular distribution and a high-energy conversion efficiency from the laser to the photons can be obtained. The proposed scheme may offer possibilities to demonstrate the RTE of electrons in experiments at approachable laser intensities and serve as a novel table-top ray source.

Bright tunable femtosecond x-ray emission from laser irradiated micro-droplets

It is demonstrated that bright femtosecond X-rays can be obtained by irradiating a moderate laser onto a helium micro-droplet. The laser ponderomotive force continuously sweeps electrons from the droplets and accelerates them forward. The electrons exposed in the outrunning laser field oscillate transversely and emit photons in the forward direction. The total flux of photons with energies above 1 keV is as high as 109/shot which is about 10-fold enhancement compared with betatron oscillation under similar laser conditions. The maximum achieved peak brightness is up to 1021 photons/s/mm2/mrad2/0.1%BW. By adjusting laser and droplet parameters, we can get tunable X-rays with required brightness and energy.

Stabilized radiation pressure dominated ion acceleration from surface modulated thin-foil targets

We study transverse and longitudinal electron heating effects on the target stability and the ion spectra in the radiation pressure dominated regime of ion acceleration by means of multi dimensional particle-in-cell (PIC) simulations. Efficient ion acceleration occurs when the longitudinal electron temperature is kept as low as possible. However, tailoring of the transverse electron temperature is required in view of suppressing the transverse instability, which can keep the target structure intact for longer duration during the acceleration stage. We suggest using the surface erosion of the target to increase the transverse temperature, which improves both the final peak energy and the spectral quality of the ions in comparison with a normal flat target.

Target shape effects on monoenergetic GeV proton acceleration

When a circularly polarized laser pulse interacts with a foil target, there are three stages: pre-hole-boring, hole-boring and light sail acceleration. We study the electron and ion dynamics in the first stage and find the minimum foil thickness requirement for a given laser intensity. Based on this analysis, we propose using a shaped foil for ion acceleration, whose thickness varies transversely to match the laser intensity. Then, the target evolves into three regions: the acceleration, transparency and deformation regions. In the acceleration region, the target can be uniformly accelerated producing a mono-energetic and spatially collimated ion beam. Detailed numerical simulations are performed to check the feasibility and robustness of this scheme, such as the influence of shape factors and surface roughness. A GeV mono-energetic proton beam is observed in three-dimensional particle-in-cell simulations when a laser pulse with a focus intensity of 1022 W cm−2 is used. The energy conversion efficiency of the laser pulse to the accelerated proton beam with the simulation parameters is more than 23%.

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.