雷射電漿質子加速機制之比較研究;Comparative Study of the Laser Plasma Proton Acceleration Mechanisms

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题名:	雷射電漿質子加速機制之比較研究;Comparative Study of the Laser Plasma Proton Acceleration Mechanisms
作者:	陳漢偉;Chen, Han-Wei
贡献者:	物理學系
关键词:	雷射電漿質子加速;粒子網格點法;電漿模擬;輻射光壓加速;靶背法向鞘場加速;移動鞘場加速;Laser Plasma Proton Acceleration;particle in cell (PIC) method;Plasma Simulation;Radiation pressure acceleration (RPA);Target normal sheath acceleration (TNSA);(moving sheath acceleration, MSA)
日期:	2022-09-27
上传时间:	2022-10-04 11:17:43 (UTC+8)
出版者:	國立中央大學
摘要:	雷射電漿質子加速是近年來被熱烈研究的議題，其中輻射光壓加速(Radiation-Pressure Acceleration, RPA)是最被看好的機制之一，本論文中第一要點是討論RPA理論的詳細推導。其後，理論推導出前人提出的移動鞘場加速機制(moving sheath acceleration, MSA)，一種透過相對論性速度疊加來混合RPA以及靶背法向鞘場加速(Target Normal Sheath Acceleration, TNSA)的新模型，並且運用模擬結果所得到之電漿溫度與前述理論融合，再將其與我們透過粒子網格點法(Particle-in-cell, PIC)模擬之結果相互比較，得到理論以及模擬結果之趨勢相互吻合的結論。過程之中，我們也發現了最佳靶材厚度於高維度模擬時的低估，我們根據其他研究推斷其來源為高維度之不穩定性以及靶材的加熱與變形。透過不同靶材厚度的參數掃描，我們也得以找到較為合適的厚度。接著，我們透過PIC中的新穎技術，模擬中性氣體游離之過程，以及電漿粒子彼此之間的碰撞行為，期待能夠發現在添加了高密度靶材時，理應不可忽略的游離過程以及碰撞過程後，可以觀察到額外的現象，最終發現在粒子最高能量以及峰值能量方面，沒有太大之影響，就該方面而言，預游離之電漿即符合我們所選用的參數空間中的大部分模擬，換句話說，200倍電漿臨界密度之固態靶材的游離機制在特定的雷射參數之下，對RPA及MSA在質子能量方面的影響不大。接著，我們也將前述之理論與許多雷射電漿質子加速實驗之結果相互比較，但我們的理論之適用範圍，受到電漿溫度參數空間之侷限，無法預測雷射持續時間較長之實驗結果，處此之外，整體雖大致相符，但理論預期仍部份高於實驗。我們也對不同的模擬參數進行了質子束品質的討論，變動的參數空間為雷射持續時間、雷射振幅、靶材最佳厚度之倍率，以及游離方式，與前者的粒子能量不同，我們確實發現了游離機制對於質子束之能散所造成的影響，由於游離過程使粒子體驗到較為集中的雷射場，在雷射振幅較高的區域，我們發現考慮游離過程之PIC模擬能夠呈現較低的能散，這點透過雷射軸心位置的取樣繪圖加以證實，而雷射與質子束的能量轉換效率以及質子束之帶電量則與游離機制沒有顯著的關係，卻仍然與靶材厚度以及雷射強度、持續時間有關，在能量轉換效率方面，較高的雷射振幅可以得到較好的能量轉換效率，這是其他研究中也有發現的PRA之特性，除此之外，厚度增加可以增加雷射持續時間較長時的能量轉換效率，並且大幅提高質子束的帶電量，對於需要高帶電量的質子束之應用至關重要，我們的研究成果，期望能提供一個在實驗中架設中性靶材時的依據。 ;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.
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