Multiply charged ion emission from laser produced tungsten plasma

Plasma was generated by focusing 10 ns Nd:YAG (lambda = 1064 nm) laser pulse on the thick tungsten target. The laser fluence at the target was varied in the range of 3.57-10.97 J/cm(2). The ion emission from the expanding tungsten plasma was analyzed with the help of an ion collector and time-of-flight electrostatic ion energy analyzer. About 44 times rise in the ion charge per laser shot was observed in the investigated laser fluence range. The measured threshold fluence for onset of the tungsten plasma was 3.27 J/cm(2). The estimated plume expansion coefficient Z(inf)/X-inf = 2.5 +/- 0.2 was in agreement with the previous experimental studies and the predictions of self-similar plume expansion model. The electrostatic ion energy analyzer study showed that charge state of the W ions increases with the laser fluence and maximum ion charge state was 5+. It was observed that threshold fluence for appearance of a specific charge state can be measured. A clear correlation between the relative abundances of W(n-1)+, Wn+, and W(n+1)+ indicates that higher order charge states are most probably produced by stepwise ionization process.