透過高頻電磁波加速電子來間接加速質子的數值模擬研究 ; A Numerical Simulation Study of Proton Acceleration Through the Acceleration of Electrons by High-Frequency Electromagnetic Waves

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题名:	透過高頻電磁波加速電子來間接加速質子的數值模擬研究;A Numerical Simulation Study of Proton Acceleration Through the Acceleration of Electrons by High-Frequency Electromagnetic Waves
作者:	劉恩旖;En-yi Liu
贡献者:	太空科學研究所
关键词:	高頻電磁波;質子加速;Proton Acceleration;High-Frequency Electromagnetic Waves
日期:	2010-06-28
上传时间:	2010-12-09 10:50:09 (UTC+8)
出版者:	國立中央大學
摘要:	太空與天文電漿中有很多不同時空尺度的波動現象，我們很難用解析的方式研究空間尺度與入射波波長相近的電漿區域中的入射波與電漿交互作用過程。數值模擬則不失為一個比較好的研究工具。本研究用particle-in-cell方法來模擬大振幅之高頻電磁波，撞擊厚度約等於或小於電磁波波長之電漿靶材。由於電漿中的正離子的質量比較大，因此通常認為只有低頻的電磁波可以與它們交互作用來加速這些正離子。然而，最近的雷射電漿研究顯示質子可以被高頻電磁波間接加速。在本論文中，我們研究正離子被高頻的電磁波間接加速的過程。從模擬結果可知電子首先被高頻震盪之電磁力的時間平均有效力 (electromagnetic pondermotive force, EMPF) 加速並與質子形成靜電場，隨之質子被靜電場持續加速，形成高能質子束。若給一個雷射脈衝持續時間是，入射波電場強度為，則質子能量可達到400MeV，其中是一個雷射週期，。本研究也為超新星爆炸時所產生出來宇宙射線(高能正離子)之加速機制，提供一個與傳統理論不同的想法。另外，本研究比較兩組電漿靶材厚度略長於入射波波長的實驗。這種實驗中高頻電磁波推動電子後，也會間接產生靜電場來加速質子。但因為是分段加速，使得質子獲得的能量遠低於薄電漿靶材的實驗。Waves found in the space plasma and the astrophysical plasma cover a wide range of frequency and wavelength. Analytical analysis becomes impossible when the wavelength of the incident wave is comparable or less than the thickness of the target plasma. Numerical simulations provide a possible solution for studying the wave-particle interactions in these events. In this study, a particle-in-cell simulation is carried out to study the wave-particle interactions when a high frequency electromagnetic wave bombarded a finite-extended layer of dense plasma with thickness equal or less than the wavelength of the incident wave. Owing to large difference between ion and electron inertia, it is commonly believed that only the low frequency electromagnetic waves can accelerate ions to higher energy. However, recent studies of laser-plasma interactions indicated that ions can be accelerated indirectly by high frequency electromagnetic waves. The ions acceleration process in the laser-plasma interactions are studied in this thesis. Our simulation results indicate that if the thickness of the target plasma is less than the wavelength of the laser wave the electrons will be accelerated by the electromagnetic ponderomotive force (EMPF). An electrostatic electric field is established between the electrons and protons to balance the EMPF on the electrons and to continuously accelerate the protons until the laser wave fields are completely reflected by the target plasma. The proton energy obtained from the wave-particle interactions increases with increasing the time of interactions. For a given laser pulse with pulse length of 100TL and electric field wave amplitude ，the protons can be accelerated up to 400 MeV, where TL is the laser period and . The proton acceleration process shown in this thesis study provides a non-conventional mechanism for the acceleration of cosmic ray during the supernova explosion. Studies of laser-plasma interactions with target plasma thickness slightly greater than the laser wavelength are also carried out. In these events, the ions cannot be accelerated continuously, thus efficiency of proton acceleration is much less than it in the thin-plasma-target experiment.
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