An investigation of self-focussing of a laser pulse in a laser-produced plasma

We have investigated self-focussing of a laser beam in a laser-produced plasma, exploring the possibility of using the phenomenon as a technique to generate very high laser flux densities within a plasma. Using a modified version of Siegrist's TRSF computer program we have studied the variation of the self-focussing threshold as a function of several laser and plasma parameters particularly those affecting the peak intensity achievable in the self-focussing filament, namely, laser pulse duration, plasma temperature, ionic charge, plasma density and size. We describe the design and operation of a Q-switched Nd-Yag oscillator which, used in conjunction with the ANU SPL laser system, provides a laser pulse meeting the requirements indicated by the computer simulations. We have carried out experiments in which the plasma parameters closely matched those which, according to our calculations, resulted in a low threshold for self-focussing. When the predicted threshold was exceeded we have observed an increase in the divergence of the light transmitted through the plasma and filamentary structures in the X-ray emission from the plasma. Both effects can be explained as the result of self-focussing