Structure and Expansion Characteristics of Laser Ablation Tin Plasma into a Vacuum

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The internal structure and expansion characteristics of a laser ablation tin plasma into a vacuum have been investigated. A Q-switched Nd:YAG laser with the power density of $10^{11}$ W/cm2 at the focal spot on the tin bulk target was employed to create the ablation plasma. The ion velocity distribution calculated from the time-of-flight measurements displays a multimodal structure; shifted-Maxwell–Boltzmann fitting indicates ions with multiple charge states exist in the ablation plume, and the temperature of the Knudsen layer was estimated to be ${\sim}4.0\times 10^{5}$ and ${\sim}8.3\times 10^{5}$ K for the laser energy of 77 and 129 mJ, respectively. Particle acceleration mechanisms were discussed according to the time resolved, two-dimensional images of the Sn I and Sn II plasma plume. The unstable adiabatic expansion with the formation of Knudsen layer was found dominates the expansion behavior of Sn I, and the Knudsen layer temperature was calculated to be $6.68\times 10^{5}$ K according to the front edge velocity of the Sn I plume.
2010-05-25