| 摘要: | X光在生物醫學研究、物理學和材料科學中具有廣泛的應用，例如觀察昆蟲的骨骼結構。強大的X 光源通常由儲存環同步加速器和自由電子雷射（free-electron laser，FEL）產生，但這些系統佔用大量空間。相較之下，雷射尾場加速器（Laser wakefield acceleration，LWFA）具有佔地面積小的優勢，且技術已相對成熟。然而，LWFA 產生的單一高能量電子的X光強度仍有改進空間。
我們在實驗中使用了中央大學強場物理實驗室的100TW系統作為雷射光源。當雷射擊中氣體靶材時，由於有質動力作用，雷射會吹出電子，而正離子則留在原處。被推開的電子圍繞著帶正電的離子形成一個空腔，稱為泡泡（bubble）。當電子進入泡泡時，它們在強大的加速場和聚焦場中被加速和振盪。其振盪軌跡類似於簡諧振動，在速度變化最大和轉折點發出X光輻射，稱為Betatron輻射。
控制電子進入泡泡的方式稱為注入機制，對產生的電子表現有很大影響。我們採用震波前註入法，使電子進入加速場。此機制是在氣體噴嘴上方插入刀片，噴射時產生的震波前方是短距離、高梯度的密度分佈。當雷射經過震波前時，泡泡會瞬間拉長，使原本在泡泡後面的電子被注入。由於注射發生的時間很短，產生的電子能量集中，Betatron 輻射的頻譜較為穩定，但電子數量較少。
為了提高震波前註入法產生的Betatron 輻射強度，我們透過旋轉刀片改變了空間上氣體密度的分佈，破壞了注入的對稱性，增加了單側電子在泡泡中的振幅，從而提高Betatron 輻射的強度。
最終，我們透過震波前註入法產生了相對單能電子，其峰值能量落在 120~180 MeV。透過旋轉刀片，在刀片角度為 20 度時，輻射強度增幅最高，達到 146$\%$。然而，臨界能量只有 3.56 keV~3.32 keV，距離硬 X 光的 5 keV 還有一段距離。;X-rays have numerous applications in biomedical research, physics, and materials science, such as observing the skeletal structures of insects. Powerful X-ray sources are typically generated from storage-ring synchrotrons and free-electron laser (FEL), but these systems occupy extensive space. Laser wakefield accelerators (LWFA), with their smaller footprint, offer a promising alternative. Although the technology has matured, there is still room for improvement, such as enhancing the X-ray intensity from single high-energy electrons.
In our experiment, we used the 100TW system from the High-Field Physics Laboratory at National Central University as the laser source. When the laser hits the gas target, the ponderomotive force blows out electrons while positive ions remain in place. The displaced electrons surround the positively charged ions, forming a cavity called a bubble. When electrons enter the bubble, they are accelerated and oscillated by a strong acceleration field and focusing field. Their oscillation trajectory is similar to simple harmonic motion, emitting X-ray radiation at the turning points where their speed changes the most, known as Betatron radiation.
The method of controlling electron injection into the bubble, known as the injection mechanism, significantly impacts the produced electron characteristics. We used the shock-front injection method to inject electrons into the acceleration field. This mechanism involves inserting a blade above the gas nozzle to create a shock front with a short-range, high-gradient density distribution during gas flow. When the laser passes through this shock front, the bubble elongates momentarily, allowing electrons initially behind the bubble to be injected. Due to the brief injection period, the generated electron energy is concentrated, resulting in a stable Betatron radiation spectrum, though the electron quantity is relatively low.
To enhance the Betatron radiation intensity produced by the shock-front injection method, we rotated the blade to alter the spatial gas density distribution, disrupting the symmetry of the injection and increasing the amplitude of electron oscillation within the bubble, thereby boosting Betatron radiation intensity.
Ultimately, we generated relatively monoenergetic electrons with peak energy at 150 MeV by using the shock-front injection method. By rotating the blade to an angle of 20 degrees, the radiation intensity achieved a maximum increase of 146$\%$. However, the critical energy was between 3.56 keV and 3.32 keV, still short of the 5 keV required for hard X-rays. |
