F. Q. Shao

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.

Propagation of attosecond electron bunches along the cone-and-channel target

Generation and propagation of attosecond electron bunches along a cone-and-channel target are investigated by particle-in-cell simulation. The target electrons are pulled out by the oscillating electric field of an intense laser pulse irradiating a cone target and accelerated forward along the cone walls. It is shown that the energetic electrons can be further guided and confined by a channel attached to the cone tip. The propagation of these electrons along the channel induces a strong quasistatic magnetic field as well as a sheathelectric field since a part of the energetic electrons expands into the surrounding vacuum. The electromagnetic field in turn confines the surface currents. With the cone-and-channel target the energetic electrons can be much better collimated and propagate much farther than that from the classical cone target.

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.

Enhanced laser-radiation-pressure-driven proton acceleration by moving focusing electric-fields in a foil-in-cone target

A foil-in-cone target is proposed to enhance stable laser-radiation-pressure-driven proton acceleration by avoiding the beam degradation in whole stage of acceleration. Two and three-dimensional particle-in-cell simulations demonstrate that the guiding cone can substantially improve the spectral and spatial properties of the ion beam and lead to better preservation of the beam quality. This can be attributed to the focusing effect of the radial sheath electric fields formed on the inner walls of the cone, which co-move with the accelerated foil and effectively suppress the undesirable transverse explosion of the foil. It is shown that, by using a transversely Gaussian laser pulse with intensity of ∼2.74 × 1022 W∕cm2, a quasi-monoenergetic proton beam with a peak energy of ∼1.5 GeV/u, density ∼10nc, and transverse size ∼1λ0 can be obtained.

High-energy-density electron jet generation from an opening gold cone filled with near-critical-density plasma

By using two-dimensional particle-in-cell simulations, we propose a scheme for strong coupling of a petawatt laser with an opening gold cone filled with near-critical-density plasmas. When relevant parameters are properly chosen, most laser energy can be fully deposited inside the cone with only 10% leaving the tip opening. Due to the asymmetric ponderomotive acceleration by the strongly decayed laser pulse, high-energy-density electrons with net laser energy gain are accumulated inside the cone, which then stream out of the tip opening continuously, like a jet. The jet electrons are fully relativistic, with speeds around 0.98−0.998 c and densities at 1020/cm3 level. The jet can keep for a long time over 200 fs, which may have diverse applications in practice.

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.

Generation of high-energy-density ion bunches by ultraintense laser-cone-target interaction

A scheme in which carbon ion bunches are accelerated to a high energy and density by a laser pulse (∼1021 W/cm2) irradiating cone targets is proposed and investigated using particle-in-cell simulations. The laser pulse is focused by the cone and drives forward an ultrathin foil located at the cones tip. In the course of the work, best results were obtained employing target configurations combining a low-Z cone with a multispecies foil transversely shaped to match the laser intensity profile.

Collimated proton beam generation from ultraintense laser-irradiated hole target

Collimated proton beams from laser interaction with a slab having a hole on its backside are investigated using particle-in-cell simulation. The hot target electrons driven by the laser expand rapidly into the hole. However, at the holes corners the electrons are strongly compressed and an intense electron jet is emitted from each corner, tightly followed by the ions. The plasma jets focus and collimate along the axis of the hole and can propagate without divergence within the hole. The effect of the hole diameter on the collimated proton beam is considered.