Objective In the research of inertial confinement fusion, the conventional center ignition scheme has very high requirements on the uniformity and symmetry of deuterium-tritium(D-T) pellet pellet compression and driver energy. To reduce these requirements, a fast ignition scheme has been proposed. In this scheme, the pellet would be first compressed to a state of high temperature and density, and then, the plasma would be driven by a short high-intensity laser pulse to produce several MeV magnitude hot electrons, which would then need to travel for a distance of dozens of micrometers to deposit their energy into a small volume of the pellet, igniting the pellet. Finally, fusion would be realized. Therefore, the fast ignition scheme needs a high beam quality of hot electrons. However, the Weibel, filament, and two-stream instabilities may decrease the beam quality of hot electrons during their transmission, which severely restricts the realization of the fast ignition scheme of inertial confinement fusion. In this study, a conical-entry-multilayer target is proposed to improve the collimation and transport efficiency of hot electrons. We hope that the model established in this study has a good significance for improving the beam quality of hot electrons in fast ignition.
Methods In this study, the generation and transport characteristics of hot electrons in the interaction between an intense laser and the conical-entry-multilayer target are investigated using the two-dimensional particle-in-cell simulation program Vorpal. We design two types of targets, namely, multilayer target and conical-entry-multilayer target. The multilayer target is mainly used as a reference to determine whether the conical-entry-multilayer target can improve the collimation and transport efficiency of hot electrons. By comparing the energy density distribution, longitudinal and transverse distribution, energy spectrum, and divergence angle distribution of hot electrons in the two targets, it is proved that the collimation and transport efficiency of hot electrons in the conical-entry-multilayer target is higher than that of hot electrons in the multilayer target. By analyzing the generation mechanism and transport process of hot electrons in the conical-entry-multilayer target, the reasons for the enhancement of collimation and transport efficiency of hot electrons in the conical-entry-multilayer targets are clarified.
Results and Discussions Compared with the multilayer target with no cone structure, in the conical-entry-multilayer target, the energy density of hot electrons near the y-axis in the region of x>12λ0 is increased (Fig. 2). In the multilayer target, the temperature of hot electrons behind the target is 0.73 MeV, and in the conical-entry-multilayer target, hot electrons behind the target can be divided into three parts, and their temperatures are 0.80, 1.31, and 13.17 MeV (Fig. 4). The results show that the temperatures of hot electrons in the conical-entry-multilayer target are higher. The divergence angle of hot electrons in both targets can be controlled mainly in the range of -38°--38°, but the energy of hot electrons with divergence angle between -38° and 38° in the conical-entry-multilayer target increases by ~0.6 times compared with that in the multilayer target (Fig. 4). The above results show that the collimation and transport efficiency of hot electrons in the conical-entry-multilayer target is higher than that of hot electrons in the multilayer target. There are three reasons for the efficiency enhancement: first, several hot electrons are pulled out from the cone wall using the laser (Fig. 5), and these hot electrons are guided to the rear of the target, which directly increases the hot electron current at the rear of the target; second, the focusing effect of the cone wall on the laser enhances the ponderomotive force near the cone tip (Fig. 6), and thus, the number and energy of hot electrons generated by the ponderomotive force acceleration near the cone tip increase significantly; third, a stronger self-generated magnetic field distribution is generated behind the cone tip (Fig. 8), and a stronger self-generated magnetic field distribution has a stronger binding effect on hot electrons.
Conclusions In this study, the collimation and transport characteristics of hot electrons generated in the interaction between an intense laser and the conical-entry-multilayer target are investigated. The results are compared with those in the multilayer target without a cone structure. It is demonstrated that hot electrons in the conical-entry-multilayer target are more numerous, more energetic, and more spatially concentrated than those in the multilayer target, the energy of hot electrons with divergence angle between -38° and 38° increases by ~0.6 times, and the collimation and transport efficiency of hot electrons can be improved using the conical-entry-multilayer target. The main reasons for the enhancement of the collimation and transport efficiency of hot electrons in the conical-entry-multilayer target are as follows: first, the laser pulls a large number of hot electrons out of the cone wall; second, the focusing effect of the cone wall on the laser enhances its ponderomotive force; and third, a stronger self-generated magnetic field distribution is generated behind the cone tip.
