Abstract

Plasma created by ultra-short-pulse lasers has much interest of radiation emission. For example, low temperature properties are suitable for an excellent laser medium, or non-thermal ionization process such as inner-shell ionization achieves the efficient production of highly charge-state ions. To study such kind of plasmas, radiation and interaction properties have to be measured with various irradiation conditions. In this paper, ultra-short KrF laser, in which the prepulse energy could be controlled with the density of saturable absorber, was used for producing high density plasma. CF₂ coated Al targets were irradiated and K-shell line emissions were observed by x-ray spectroscopic methods. At the moderate intensity of 10¹⁴ W/cm² and 10⁻⁶ contrast prepulse condition, maximum absorption was obtained with 60 degree of incident angle. From the theory of resonant absorption, the plasma scale length was estimated to be 0.12λ. Because it was shorter than calculated value from thermal expansion velocity and pulse duration, there was steep gradient in the interaction region. The electron density estimated from Stark broadening of He-like line was 4 × 10²² cm⁻³, which was larger than the critical density of KrF laser light. In addition the strong Al Ka line was also measured, while He and H-like lines were not observed. It was considered that a large amount of energy was transported into high density region via hot electrons and the ionization of K-shell ion was occurred mainly due to their impact. The intercombination lines which had intensity larger than resonance one were observed in ultra-short pulse laser irradiation, while they had smaller intensity in longer pulse. It also denoted that there was anisotropy of the electron distribution function in the case of ultra-short pulse interaction.

© 1995 IEEE