| 摘要: | 雷射尾流場加速器可以提供超高的加速梯度以生成極短且高能的電子束，其主導著未來雷射驅動電子加速器的發展。迄今，相關實驗大多是使用高尖峰功率且高能的摻鈦藍寶石雷射進行。然而這些800奈米脈衝通常都操作在相對低的重覆率(<10赫茲)，其會限制需要高輻射通量的相關應用，如先進放射線照相。因此，本論文主要為探討使用相對低功率雷射驅動先進雷射電漿電子加速器之研究。
準相位匹配的直接雷射電子加速器可以透過將徑向極化雷射脈衝注入週期密度變化之電漿波導，使其軸向電場可以直接加速電子。然而，當使用至數兆瓦雷射脈衝時，雷射有質動力會擾動電漿電子且引發靜電場使得加速電子束散焦及降低其加速效益。為了要提升加速效果，前導電子束的被引入此加速機制中。三維粒子式模擬的結果呈現了當前導電子束被引入後，加速電子束的橫向性質能顯著的改善。前導電子束的橫向大小、電量及與加速電子的距離皆被探討。當前導電子束提供了合適的離子聚焦力，加速電子可以在相對高功率(2兆瓦)脈衝的作用下，達到優化的加速效果。
除了直接雷射加速器外，雷射尾流場加速器可以操作在將次兆瓦雷射脈衝注入高密度氣體。如此一來，自聚焦及自耦合效應可以有效的增強脈衝強度，使其可以驅動非線性電漿波來加速電子。我們透過模擬來探討0.6兆瓦、800奈米的脈衝與高密度的氫電漿之作用。氣體靶材被設定為高斯密度分佈或平頂式分佈以用來探討其實驗操作在氣體噴嘴或氣囊的情況下。當尖峰氣體密度使雷射功率約等於2倍自聚焦功率時，其為合適的操作條件。否則過高的氣體密度會引起不必要的絲狀效應，其會快速的瓦解雷射脈衝且破壞電漿波產生的過程。平頂式密度分佈的平台區使自聚焦且自耦合的脈衝產生電漿波來生成加速電子。由於電子注入的機制非常複雜，因此，增加平台區的長度可以視為直接的方法來達到電子注入與加速。如此一來，有效的次兆瓦雷射尾流場加速器機制得以實現。
除了使用800奈米的脈衝外，使用1030奈米的二極體激升雷射與平頂式密度分佈的氣體作用也透過模擬來探討。因為1030奈米雷射的脈衝寬度通常>200飛秒，所以展頻技術需要被用來壓縮脈衝寬度，使其能夠達到更強的有質動力來驅動電子加速機制。為了瞭解脈寬的效應，我們進行了一系列脈寬從200飛秒至10飛秒的0.5兆瓦、1030奈米脈衝的模擬。其結果呈現出50飛秒的脈衝可以達到優化的加速效果，其為較弱的有質動力(脈寬>100飛秒)與衰減效應(脈寬<25飛秒)的折中取捨。當使用0.25兆瓦的脈衝，衰減效應會變的顯著且降低加速效果。因此，使用較長波長(>2微米)的脈衝為可行的方法來達到操作低雷射功率(0.25兆瓦)脈衝以達到有效的雷射尾流場加速機制，其較佳的操作條件為當氣體密度使雷射功率約為1.25倍自聚焦功率下。;Laser wakefield acceleration (LWFA) has been a well-recognized technique to generate a huge acceleration gradient to produce ultrashort and energetic electron bunches that leads to the development of future laser-driven electron accelerators. To date, most of the LWFA experiments are conducted with high-peak-power, high-energy Ti:Sapphire-based lasers. However, these 800-nm pulses of requisite pulse energies are typically operated at a relatively low repetition rate <= 10 Hz, which can inhibit the attractiveness of LWFA when applying it in advanced x-ray radiography in which a sufficiently high radiation flux is desired. Therefore, this thesis reports our efforts and accomplishments on the development of novel laser-driven acceleration methods that can be conducted with a relatively low laser peak power.
One of the promising methods is quasi-phase matched direct laser acceleration (DLA) of electrons, which can be achieved by utilizing the axial field of a well-guided, radially polarized laser pulse in a density-modulated plasma waveguide. When a laser pulse of a few TW peak power is applied, however, the laser ponderomotive force perturbs plasma electrons to concentrate in the center, such that the generated electrostatic fields can significantly defocus the externally injected electron witness bunch and considerably deteriorate the acceleration efficiency. To improve the performance of DLA, a leading electron bunch, which acts as a precursor, can be introduced in DLA to effectively confine the witness bunch. Three-dimensional particle-in-cell simulations have been conducted to demonstrate that the transverse properties of the witness bunch can be significantly improved when a precursor bunch is used. Selected bunch transverse sizes, bunch charges, and axial separation from the witness bunch have been assigned to the precursor in a series of DLA simulations. Since a favorable ion-focusing force is provided by the precursor, the transverse properties of witness bunch can be maintained when a relatively high-power (~2 TW) laser pulse is used in DLA, and an improved overall acceleration efficiency can be achieved.
In addition to DLA, LWFA can be accomplished by introducing a sub-terawatt (TW) laser pulse into a thin, high-density gas target. In this way, the self-focusing effect and the self-modulation happened on the laser pulse produce a greatly enhanced laser peak intensity that can drive a nonlinear plasma wave to accelerate electrons. A particle-in-cell model is developed to study the sub-TW LWFA when a 0.6-TW, 800-nm laser pulse interacts with a dense hydrogen plasma. Gas targets having a Gaussian density profile or a flat-top distribution are defined for investigating the properties of sub-TW LWFA when conducting with a gas jet or a gas cell. The peak density which allows the laser peak power PL~2Pcr of self-focusing critical power is favourable for conducting sub-TW LWFA. Otherwise, an excessively high peak density can induce undesired filament effect which rapidly disintegrates the laser field envelope and violates the process of plasma wave excitation. The plateau region of a flat-top density distribution allows the self-focusing and the self-modulation of the laser pulse to develop, from which well-established plasma bubbles can be produced to accelerate electrons. The process of the electron injection is complicated in such a high-density plasma condition; however, increasing the length of the plateau region represents a straightforward method to realize the injection and acceleration of electrons within the first bubble, such that an improved LWFA performance can be accomplished.
In addition to using 800-nm laser pulses, simulations are performed to study the scheme in which 1030-nm pulses produced from a diode-pumped laser system are introduced into a gas cell with a flat-top density profile, allowing the LWFA to be operated at high frequencies. Because 1030-nm lasers are typically produced with a long duration > 200 fs, a spectral broadening technique can be applied to reduce the pulse duration, from which a greater ponderomotive force is acquired to drive LWFA. To understand the dependence of LWFA performance on the driving pulse duration, selected durations, ranging from 200 fs to 10 fs, are assigned for 0.5-TW, 1030-nm pulses in a series of simulations. Results show that a duration around 50 fs can provide the optimal LWFA results, as a compromise between the weak ponderomotive force available from a long pulse > 100 fs and the depletion effect which can rapidly diminish a short pulse < 25 fs in a dense plasma. When a low laser peak power of 0.25-TW is available, the pulse depletion can be significant at a high target density and render LWFA ineffective. Using a laser pulse with a longer wavelength > 2 micrometer represents a viable route to realize the LWFA with a low laser peak power; in this way, an appropriately selected target density which allows the laser peak power PL ~ 1.25 Pcr of self-focusing critical power is favourable for realizing an efficient LWFA process. |
