Dongxiao Liu

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.

Fast ignition by a laser-accelerated deuteron beam

Fast ignition (FI) of a conically guided DT assembly by a laser-accelerated deuteron beam is proposed. The uniformly pre-compressed fuel of 300 g cm−3 is heated by the deuteron beam of a Maxwellian energy distribution with a temperature of 3 MeV. This scheme makes full use of the deposited energy of the alpha particles produced by the athermal nuclear reactions and can save about 4.5% ion-beam energy compared with the FI by fast proton or carbon ion beams. The ignition energy delivered by the external beam can be reduced appreciably.

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.

High direct drive illumination uniformity achieved by multi-parameter optimization approach: a case study of Shenguang III laser facility

The uniformity of the compression driver is of fundamental importance for inertial confinement fusion (ICF). In this paper, the illumination uniformity on a spherical capsule during the initial imprinting phase directly driven by laser beams has been considered. We aim to explore methods to achieve high direct drive illumination uniformity on laser facilities designed for indirect drive ICF. There are many parameters that would affect the irradiation uniformity, such as Polar Direct Drive displacement quantity, capsule radius, laser spot size and intensity distribution within a laser beam. A novel approach to reduce the root mean square illumination non-uniformity based on multi-parameter optimizing approach (particle swarm optimization) is proposed, which enables us to obtain a set of optimal parameters over a large parameter space. Finally, this method is applied to improve the direct drive illumination uniformity provided by Shenguang III laser facility and the illumination non-uniformity is reduced from 5.62% to 0.23% for perfectly balanced beams. Moreover, beam errors (power imbalance and pointing error) are taken into account to provide a more practical solution and results show that this multi-parameter optimization approach is effective.

Note: Tandem Kirkpatrick–Baez microscope with sixteen channels for high-resolution laser-plasma diagnostics

Multi-channel Kirkpatrick-Baez (KB) microscopes, which have better resolution and collection efficiency than pinhole cameras, have been widely used in laser inertial confinement fusion to diagnose time evolution of the target implosion. In this study, a tandem multi-channel KB microscope was developed to have sixteen imaging channels with the precise control of spatial resolution and image intervals. This precise control was created using a coarse assembly of mirror pairs with high-accuracy optical prisms, followed by precise adjustment in real-time x-ray imaging experiments. The multilayers coated on the KB mirrors were designed to have substantially the same reflectivity to obtain a uniform brightness of different images for laser-plasma temperature analysis. The study provides a practicable method to achieve the optimum performance of the microscope for future high-resolution applications in inertial confinement fusion experiments.

Efficient production of strong magnetic fields from ultraintense ultrashort laser pulse with capacitor-coil target

The ion beam bunching in a cascaded target normal sheath acceleration is investigated by theoretical analysis and particle-in-cell simulations. It is found that a proton beam can be accelerated and bunched simultaneously by injecting it into the rising sheath field at the rear side of a laser-irradiated foil target. In the rising sheath field, the ion phase rotation may take place since the back-end protons of the beam feels a stronger field than the front-end protons. Consequently, the injected proton beam can be compressed in the longitudinal direction. At last, the vital role of the ion beam bunching is illustrated by the integrated simulations of two successive stages in a cascaded acceleration.An ultraintense femtosecond laser pulse was used, for the first time, to produce a strong magnetic field with controlled shapes by interactions with a capacitor-coil target with high efficiency. The temporal evolution of the strong magnetic field was obtained by the time-gated proton radiography method. A comparison of high-resolution radiographic images of proton deflection and particle-track simulations indicates a peak magnetic field of ∼20 T. The energy conversion efficiency from the ultraintense laser pulse to the magnetic field is as high as ∼10%. A simple model of the ultraintense laser-driven capacitor-coil target gives a relationship between the magnetic field strength and the electron temperature produced by the laser. Our results indicate that magnetic fields of tens of tesla could be stably produced by most of the existing ultraintense laser facilities. It potentially opens new frontiers in basic physics which require strong magnetic field environments.An ultraintense femtosecond laser pulse was used, for the first time, to produce a strong magnetic field with controlled shapes by interactions with a capacitor-coil target with high efficiency. The temporal evolution of the strong magnetic field was obtained by the time-gated proton radiography method. A comparison of high-resolution radiographic images of proton deflection and particle-track simulations indicates a peak magnetic field of ∼20 T. The energy conversion efficiency from the ultraintense laser pulse to the magnetic field is as high as ∼10%. A simple model of the ultraintense laser-driven capacitor-coil target gives a relationship between the magnetic field strength and the electron temperature produced by the laser. Our results indicate that magnetic fields of tens of tesla could be stably produced by most of the existing ultraintense laser facilities. It potentially opens new frontiers in basic physics which require strong magnetic field environments.

Optimization of direct drive irradiation uniformity of cylindrical target

The irradiation uniformity of a cylindrical target directly driven by laser beams has been considered, which is relevant for fast ignition electron-transport experiments. The laser intensity distribution on the cylindrical target surface is analyzed and optimized by applying the polar direct drive technique and adjusting the laser beam parameters. Moreover, the rotation of laser spot around its propagation axis is taken into consideration. A case study based on the SG-III prototype laser configuration is presented to demonstrate the optimization approach. The irradiation uniformity is reduced from 10% to 1.6% for perfectly balanced beams, and the effects of uncertainties in beam errors (power imbalance and pointing error) are also studied. Furthermore, differences in laser absorption with different incident angles are taken into account and the results show that highly uniform energy deposition can be achieved.

A simple method to prevent hard X-ray-induced preheating effects inside the cone tip in indirect-drive fast ignition implosions

During fast-ignition implosions, preheating of inside the cone tip caused by hard X-rays can strongly affect the generation and transport of hot electrons in the cone. Although indirect-drive implosions have a higher implosion symmetry, they cause stronger preheating effects than direct-drive implosions. To control the preheating of the cone tip, we propose the use of indirect-drive fast-ignition targets with thicker tips. Experiments carried out at the ShenGuang-III prototype laser facility confirmed that thicker tips are effective for controlling preheating. Moreover, these results were consistent with those of 1D radiation hydrodynamic simulations.

Optimizing design of irradiation uniformity of direct drive cone-in-shell target for fast ignition

The irradiation uniformity of a cone-in-shell target directly driven by laser beams has been considered. First, a model is established to include the influence of the cone on laser beam propagation. Then, the irradiation uniformity on the target surface outside the cone during the initial imprinting phase is analyzed, and highly uniform irradiation on the target surface outside the cone is achieved by optimizing the intensity distribution within laser beams, as well as the polar direct drive displacement. As an illustrative example, direct drive irradiation uniformity of a typical cone-in-shell target is improved for Shenguang III laser facility, the illumination non-uniformity is reduced from 5.8% to 1.1%. Irradiation on the cone surface outside the target is also analyzed, and it is found that for the laser-target configuration considered in this work, a gold cone thicker than 50μm is needed to avoid shock breakout. Moreover, sensitivity to beam uncertainties (power imbalance and pointing error) is anal...