Characterization and Optimization of Ultrashort-Laser-induced Al Kα source

Abstract

Conditions for producing an efficient Laser Induced X-ray Source (LIXS) were investigated through the exploration about optical characteristic and optimization of the laser induced Al Kα source. First, it was observed experimentally that Al Kα flux began to be generated at the intensity of laser is 1.63 × 1016 W/cm2 which is 20% higher than theoretical prediction. The difference between theoretical and experimental results had been interpreted by the short lifetime of plasma and difficulty of electron acceleration in low vacuum. Through interpretation about threshold region, that high vacuum is crucial for more efficient flux generation is suggested. Second, beyond threshold region, Al Kα flux was measured by increasing the intensity of laser. Al Kα flux increased as the laser intensity increased, and saturated when the intensity was 5.4×1016 W/cm2. The reason why the Al Kα flux was saturated was analyzed through laser-electron acceleration mechanisms and cross section for Al K-shell ionization by electron impact. S-polarized electric field cause wider distribution, (i.e., low density of electrons just in front of tape surface) by accelerating electrons in a direction parallel to the tape surface. Because wider distribution is inefficient for K-shell ionization. Also, Cross section of Al K-shell ionization decreases as kinetic energy of electrons increases. Due to inefficient spatial distribution of electrons and low cross section of Al K-shell ionization, Al Kα flux was saturated. For more efficient LIXS production, it is suggested that using p-polarized laser beam so that electrons just in front of the tape could not be dispersed by reducing electron velocity parallel to tape surface. Finally, the Al Kα flux was measured by changing the angle of incidence. The most optimized flux generation was observed when the angle of incidence was 25 degrees. This is well consistent with the priori results of other groups, and also with the prediction of the theory of resonance absorption. For optimum flux generation, adjusting the angle of incidence between 22 and 25 degrees for optimal flux generation is suggested where effect of resonance absorption is dominant for plasma model. As a result, optical characteristics exploration and optimization studies on Laser-induced Al Kα source were conducted, and optimization conditions and limitations were presented at the region where resonance absorption mechanism is dominant for laser-plasma interaction. This research will be a valuable asset as background knowledge for those planning to produce and optimize LIXS.