| dc.description.abstract | Laser plasma proton acceleration is an intensely researched topic in recent years. Radiation-Pressure Acceleration (RPA) is one of the most promising mechanisms. The first point of this paper is to discuss the detailed derivation of RPA theory. After that, we theoretically derived moving sheath acceleration (MSA), a new model that mixes RPA and Target Normal Sheath Acceleration (TNSA) through relativistic velocity superposition. Later, we use the plasma temperature obtained from the simulation results to incorporate the aforementioned theory, and then compare it with the results of our simulation through the Particle-in-cell (PIC) method, and obtain a similar trend of the theoretical and simulation results.
During the process, we also found that the optimal target thickness was underestimated in high-dimensional simulations, and we inferred from other studies that the source was high-dimensional instability and heating and deformation of the target. Through parameter scanning of different target thickness, we can still find a more suitable thickness.
Next, through the novel technology in PIC, we simulate the process of neutral gas ionization and the collision behavior of plasma particles with each other, expecting to discover some phenomenon when the ionization process and collision process should not be ignored with high-density targets added. Afterward, additional phenomena can be observed, and it is finally found that there is not much effect on the maximum energy and peak energy of the particle, and in this regard, the pre-ionized plasma is consistent with most of the simulations in the parameter space we have chosen. In other words, the ionization mechanism of the solid target with a plasma critical density of 200 times has little effect on the proton energy of RPA and MSA under certain laser parameters.
Next, we also compare the above-mentioned theory with the results of many laser plasma proton acceleration experiments, but the scope of application of our theory is limited by the plasma temperature parameter space, and it is impossible to predict the experimental results of long laser duration. Apart from this, the overall comparison is roughly consistent, the theoretical expectations are still partially higher than the experiments.
We also discussed the quality of the proton beam for different simulation parameters. The variable parameter space is the laser duration, the laser amplitude, the magnification of the optimal thickness of the target, and the ionization method. Different from the particle energy of the former, the effect of the ionization mechanism on the energy spread of the proton beam was found. Due to the ionization process, the particles experience a relatively concentrated laser field. In the region with high laser amplitude, we found that the PIC simulation considering the ionization process can show a lower energy spread, which is confirmed by the sampling of the data of the laser axis. The energy conversion efficiency of the laser and the proton beam and the charge of the proton beam has no significant relationship with the ionization mechanism, but are still related to the thickness of the target and the laser beam. It is related to the radiation intensity and duration. In terms of energy conversion efficiency, higher laser amplitude can obtain better energy conversion efficiency, which is the characteristic of PRA also found in other studies. The energy conversion efficiency when the irradiation duration is long and the target thickness is thicker, the beam charge of the proton beam is greatly improved, which is very important for the application of the proton beam requiring a high beam charge. Our research results are expected to provide a guideline for a neutral target experimental setup. | en_US |