| 摘要: | 自由電子雷射是一種以高品質電子束作為介質，讓接近光速的電子束在周期性磁 場中受到激發且放大電磁輻射的新型雷射光源，這需要高品質和高穩定的電子束作為 光源，開發加速器便成為開發自由電子雷射不可或缺的環節，雷射電漿交互作用中電 子的加速已經進行了二十多年的實驗研究，這種加速器中的加速電場是傳統射頻加速 器的 1000 倍，能夠在數釐米內將電子加速至數 GeV，這意味著能將過去數公里長的傳 統直線加速器縮短近 1000 倍，被廣泛認為是能夠替代傳統射頻加速器的方案。目前在 厘米等級的雷射電漿波電子加速器，已經被證實可以將電子加速至十億電子伏特，並 具備發散角小穩定性高的電子脈衝，有很大的潛力投入自由電子雷射應用。本論文所 呈現的是透過二維粒子式模擬(Particle-In-Cell simulation)來研究電漿尾流場加速衝擊 波注入的物理機制。當前電漿源設計方面有著許多不同的課題。為達到自由電子雷射 所要求的電子能量擴散低於 0.1%，本論文的重點是控制注入以及優化加速電子的品質， 以及研究此種注入的各項特徵。第一部分是利用稱為衝擊波前沿的超音速現象來刺激 瞬間注入。我們調整雷射電漿參數將加速電子優化至單能，模擬中加入高密度區間來 微幅調整雷射強度，取得最佳化的參數，並討論各項參數如何有效的降低加速電子束 之能量擴散。
第二部分，我們發現特定條件下的注入方式具有不同於多數研究的衝擊波注入方 式，也就是能夠捕捉行經電漿波邊界的電子，且這些電子初始位置分布也更密集，這 意味著能量擴散也更小。為了比較這兩種機制，我們將重現普遍研究常見的衝擊波注入，並且透過軌跡追蹤比較兩種注入的加速電子性質差異。利用這樣的注入機制配合 傾斜角度的衝擊波能產生明顯的不對稱注入，還能夠微幅度的提升電子在加速器中的 振幅，有望成為提升 betatron radiaton 的一種方法。本研究結果顯示，經優化的電子束 成功在 640 MeV 的峰值能量下，具備小於 1% 的能量擴散，相比優化前下降了 80%。
;Free electron laser (FEL) is a new type of laser light source that uses high-quality electron beams as the medium to stimulate and amplify electromagnetic radiation in a periodic magnetic field. FEL requires high-quality and high-stable electron beams as driving sources, and the de- velopment of accelerators has become an indispensable part in the development of free electron lasers. Laser wakefield acceleration has been studied for more than 20 years. The electric accel- eration field in this accelerator is 1000 times that of the traditional RF cavity, being capable of accelerating electrons to several GeV within a few centimeters. This implies that conventional linear accelerators that few kilometers can be shortened nearly 1,000 times. Therefore, LWFA is widely believed to be a solution that can replace conventional RF accelerators. At present, laser- plasma accelerators at the centimeter level have been proven to accelerate electrons to 1 billion electron volts with small divergent angle and high stability, with great potential to be applied in free electron laser applications.This thesis presents the physical mechanism of accelerating shock wave injection in the laser plasma wakefield through two-dimensional Particle-In-Cell simulation.
In order to achieve the electron energy spread required by free electron lasers below 0.1 %, this thesis focuses on controlling the injection and optimizing the quality of accelerated electrons and study the characteristics of this injection. The first part is using the supersonic phenomenon, the front of the shock wave, to stimulate instantaneous injection.
The accelerated electrons are optimized to a monoenergeitc beam by adjusting the laser plasma parameters, and a high-density region is added to the simulation to fine-tune the laser intensity to obtain the optimized parameters, then discuss how each parameter can effectively reduce the energy spread of the accelerated electron beam.
In the second part, it is found that the injection method under certain conditions is different from the shock front injection method of most studies, that is, it can capture electrons passing through the plasma wave boundary, and the initial position distribution of these electrons is also more concentrated, which means that the energy spread is also smaller. In order to compare these two injections, we will reproduce the commonly studied shock front injection, and compare the difference in the accelerated electron properties of the two injections through trajectory tracing. Utilizing such an injection mechanism with shock waves with a tilted angle can produce obvious asymmetric injection, and can also slightly increase the amplitude of electrons in the accelerator, which is expected to become a method to enhance betatron radiation.
The result shows that the optimized electron beam at dephasing has an energy spread of less than 1%, which is a decrease of 80% compared with before optimization. In addition, using this injection mechanism combined with the shock front at a tilted angle can produce apparent asymmetric injection, which may make the betatron radiation polarized. |
