On the generation of high flux densities within absorbing plasmas due to self-focussing of a laser beam

Evidence of filamentation (self-focusing) of a laser beam propagating in a laser-produced aluminium plasma

“…If the collapse of the beam occurs in a relatively short time (for example < I ns), shock waves are generated in the surrounding region as the plasma is ejected from the beam centre. In such a case the plasma density profile would take on a shape similar to that given by Siegrist (1977). This is shown in figure 12 together with an indication of the x-ray emission intensity resulting from such a density variation.…”

“…The TRSF code also includes a second self-focusing mechanism which is important for very high laser intensities and occurs due to the relativistic increase in the mass of the clectrons driven into oscillatory motion by the laser field (Max and Arons 1974, Hora 1975, Siegrist 1976). Our calculations have shown that in most situations we have considered, even for pulses whose durations are as short as 20 ps, the ponderomotive force self-focusing mechanism gives thresholds which are lower than those due to the relativistic mechanism and hence the latter can be neglected in further discussion (Del Pizzo 1977, 1978.…”

“…Our theoretical treatment of self-focusing is based upon a computational model, TRSF, which has been described in a number of previous papers (Siegrist et al 1976, Siegrist 1977, Del Pizzo et a1 1977, 1978. For the present work we have concentrated on selffocusing of a nanosecond duration laser pulse in an initially uniform plasma slab up to 180 pm thick and with a density (N) between 0.05 and 0.95 Ncr (Nc, being the critical density for neodymium laser radiation: Ncr= 1021 cm-3).…”