| dc.description.abstract | 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. | en_US |