- 學位論文
利用兆瓦級雷射脈衝在次毫米尺度之稠密氮氣氣靶實現之雷射尾流場加速
Laser wakefield acceleration using terrawatt-level laser pulses and sub-millimeter dense nitrogen gas target
摘要
利用高強度（> 5x1018 W/cm2）飛秒（fs）的雷射脈衝聚焦到高密度氣體（> 1018 cm-3）氣靶中來實現雷射尾流場加速（LWFA）。在該機制作用下，雷射脈衝的有質動力激發非線性電漿波產生巨大的加速場，該加速場可以上升至〜100 GV/m來加速電子。為了有效激發非線性電漿波並實現電漿空泡加速機制，當前的大多數實驗都使用Ti:sapphire雷射系統生成800 nm的10s-100 TW峰值功率的脈衝來驅動LWFA。但是，這些> 10 TW脈衝通常以≤ 10 Hz的低重複頻率生成，這最終限制了經由LWFA所產生之電子束的平均電流，並限制了LWFA在 X射線成像或電子治療等領域需要需要高輻射通量之應用。隨著新型二極體飛秒雷射系統的快速發展，近年可預期該系統可輸出頻率為kHz之TW級雷射脈衝，輸出脈衝可用於驅動高重複頻率的LWFA，並產生10s-MeV電子，平均電流顯著增加> 10 nA。因此，本研究的重點是開發由時寬約40 fs和能量<150 mJ雷射脈衝所驅動之few TW LWFA機制。通過直徑≤ 200 μm孔徑之噴嘴和寬度≤ 400 μm的氣室所產生高密度次毫米級的氮氣氣靶，當< 4 TW脈衝聚焦在電漿密度> 2.5 x 1019 cm-3的稠密氮靶上時，透過自聚焦效應和自調製效應可以將脈衝強度大大提高到能夠激發電漿空泡加速機制的水平。當將40 fs，3.2-TW，810 nm的雷射脈衝聚焦到由直徑為152μm孔徑之噴嘴以600 psi的背壓所產生的瞬態、自由流動的氮氣噴流時，會產生能量峰值為~11 MeV、電荷量約為23 pC（> 5 MeV）的電子束。在將2-TW脈衝引入400μm寬度之氣室，並使用背壓20 psi所產生氮氣氣靶之實驗案例中，可產生具有高達40 MeV連續分佈能譜的電子束，其電荷為23.5 pC（> 5 MeV）。
並列摘要
Laser wake-field acceleration (LWFA) can be achieved by focusing an intense (> 5x 1018 W/cm2), femtosecond (fs) laser pulse into a high-density gas target. In this mechanism, the ponderomotive force of the laser pulse excites nonlinear plasma waves and results in huge accelerating fields that can rise to ~ 100 GV/m for accelerating electrons. To effectively excite strong plasma waves and realize the plasma cavitation, most of the current experiments use Ti:sapphire laser systems to generate 800-nm pulses of 10s-100 TW peak power to drive LWFA. However, these >10 TW pulses are typically generated with a low repetition rate ≤10 Hz that ultimately limits the average current of electrons from LWFA and inhibits their applications into X-ray imaging or electron therapy that requires a high radiation flux. It is expected that TW-level pulses produced from novel diode-pump fs lasers at kHz frequencies can be used to drive high-repetition-rate LWFA and generate 10s-MeV electrons with a considerably increased average current > 10 nA. Therefore, this research focuses on developing the scheme of few-TW LWFA driven by laser pulse with a duration ~ 40 fs and an energy < 150 mJ. High-density, sub-mm nitrogen gas targets are produced from gas jets with an orifice of a diameter ≤ 200 μm or from gas cells having a width ≤ 400 μm. When a < 4 TW pulse is focused onto a dense nitrogen target with a plasma density > 2.5 x 1019 cm-3, the induced self-focusing and self-modulation effects can greatly enhance the pulse intensity to a level capable of exciting plasma bubbles. When 40-fs, 3.2-TW, 810-nm pulses are focused onto transient, free- flowing nitrogen jets produced from a 152- μm diameter orifice with a backing pressure of 600 psi, electrons are generated with an energy spectrum peaks at 11 MeV and the charge ~ 23 pC (> 5 MeV). In the case that 2 –TW pulses are introduced into nitrogen targets produced with a 400-μm wide gas cell and 20-psi backing pressure, electrons having a continuously distributed spectrum up to 40 MeV are generated with a charge of 23.5 pC (>5 MeV).