Zongqing Zhao

Acceleration and guiding of fast electrons by a nanobrush target

Laser interaction with a nanobrush target plasma is investigated at the SILEX-I laser facility [X. F. Wei et al., J. Phys. Conf. Ser. 112, 032010 (2008)] with a laser of intensity 7.9×1018 W/cm2. Highly collimated fast electron beams with yields of more than three times higher than that from the planar target can be produced. Two-dimensional particle-in-cell simulation confirms that a layered surface structure can increase the efficiency of laser energy absorption, and the resulting fast electrons are tightly collimated and guided by the plasma layers to a cross section of about the laser spot size.

Enhancement of monoenergetic proton beams via cone substrate in high intensity laser pulse-double layer target interactions

A scheme capable of enhancing the energy of monoenergetic protons in high intensity laser-plasma interactions is proposed and demonstrated by two dimensional particle-in-cell simulations. The focusing of laser light pulse and the guiding of surface current via the high Z material cone-shaped substrate increase the temperature of hot electrons, which are responsible for the electrostatic field accelerating protons. Moreover, the sub-micron proton layer coated on the cone-shaped substrate makes the total proton beam experience the same accelerating field, thus the monochromaticity is maintained. Compared to the normal film double layer target, the energy of monoenergetic proton beams can be improved about three times.

Elimination of higher-order diffraction using zigzag transmission grating in soft x-ray region

We present a realization of the sinusoidal transmission function using a series of zigzag-profiled strips where the transmission takes on the binary values 0 and 1 in a two-dimensional distribution. A zigzag transmission grating of 1000 line/mm has been fabricated and demonstrated on the soft x-ray beam of synchrotron radiation. The axial single-order diffraction indicates that the zigzag transmission grating is adequate for spectroscopic application.

Monte Carlo simulation study of positron generation in ultra-intense laser-solid interactions

The Monte Carlotransport code Geant4 has been used to study positron production in the transport of laser-produced hot electrons in solid targets. The dependence of the positron yield on target parameters and the hot-electron temperature has been investigated in thick targets (mm-scale), where only the Bethe-Heitler process is considered. The results show that Au is the best target material, and an optimal target thickness exists for generating abundant positrons at a given hot-electron temperature. The positronangular distributions and energy spectra for different hot electron temperatures were studied without considering the sheath field on the back of the target. The effect of the target rear sheath field for positron acceleration was studied by numerical simulation while including an electrostatic field in the Monte Carlo model. It shows that the positron energy can be enhanced and quasi-monoenergetic positrons are observed owing to the effect of the sheath field.

Fractional spiral zone plates

In this paper, we generalize the concept of classical spiral zone plates (SZPs) to fractional spiral zone plates (FSZPs). By using an SZP with a fractional topological charge and controlling the starting orientation, we can break down the symmetry of the focusing process to give orientation-selective anisotropic vortex foci. Numerical results show that its binary structure gives additional high-order foci on the optical axis and the intensities in the foci can be controlled by properly choosing the fractional topological charge. Our study reveals the feasibility to control the intensity in the foci by means of FSZPs.

Enhancement in coupling efficiency from laser to forward hot electrons by conical nanolayered targets

To improve the energy coupling efficiency from laser to forward hot electrons, we propose a conical nanolayered target (CNT) and investigate by two-dimensional particle-in-cell simulations. Compared with nanolayered target, the energy coupling efficiency is enhanced from 34% to more than 68%. Detailed simulations indicate that this enhancement is attributed to both oblique incidence and focusing of the conical target. Moreover, CNT collimates the hot electrons better. The proposed target may serve as a new method for enhancing laser to forward hot electrons energy coupling efficiency.

Strong magnetic fields generated with a simple open-ended coil irradiated by high power laser pulses

A simple scheme to produce strong magnetic fields due to cold electron flow in an open-ended coil heated by high power laser pulses is proposed. It differs from previous generation of magnetic fields driven by fast electron current in a capacitor-coil target [S. Fujioka et al., Sci. Rep. 3, 1170 (2013)]. The fields in our experiments are measured by B-dot detectors and proton radiography, respectively. A 205 T strong magnetic field at the center of the coil target is generated in the free space at Iλ2 of 6.85 × 1014 W cm−2 μm2, where I is the laser intensity, and λ is the laser wavelength. The magnetic field strength is proportional to Iλ2. Compared with the capacitor-coil target, the generation mechanism of the magnetic field is straightforward and the coil is easy to be fabricated.

Realizing a Gabor zone plate with quasi-random distributed hexagon dots.

We propose a quasi-random-dot-array binary Gabor zone plate (QBGZP) with focusing properties of single order foci only. These features are verified with simulations and experiments in the visible light region. Moreover, we find that the performance of QBGZP, which is composed of hexagon patterns, is determined by the ratio of hexagon circumcircle diameter to the outermost zone width. The QBGZP offers a potential alternative for focusing and imaging in the soft x-ray and extreme ultraviolet region.

Transport of fast electrons in a nanowire array with collisional effects included

The transport of picosecond laser generated fast electrons in a nanowire array is studied with two-dimensional particle-in-cell simulations. Our simulations show that a fast electron beam is initially guided and collimated by strong magnetic filaments in the array. Subsequently, after the decomposition of the structure of nanowire array due to plasma expansion, the beam is still collimated by the resistive magnetic field. An analytical model is established to give a criterion for long-term beam collimation in a nanowire array; it indicates that the nanowire cell should be wide enough to keep the beam collimated in picosecond scale.

Effect of inside diameter of tip on proton beam produced by intense laser pulse on double-layer cone targets

The laser-driven acceleration of proton beams from a double-layer cone target, comprised of a cone shaped high-Z material target with a low density proton layer, is investigated via two-dimensional fully relativistic electro-magnetic particle-in-cell simulations. The dependence of the inside diameter (ID) of the tip size of a double-layer cone target on proton beam characteristics is demonstrated. Our results show that the peak energy of proton beams significantly increases and the divergence angle decreases with decreasing ID size. This can be explained by the combined effects of a stronger laser field that is focused inside the cone target and a larger laser interaction area by reducing the ID size.