The effects of optical smoothing techniques on filamentation in laser plasmas

The effect of the induced spatial incoherence (ISI) and the random phase screen (RPS) optical smoothing techniques on the filamentation instability in laser plasmas has been investigated numerically and analytically. A two-dimensional time-dependent laser–plasma propagation code, including both ponderomotive and thermal-conduction dominated filamentation, is used to simulate the laser–plasma interaction. The results of these simulations are compared to the predictions of a simple theory that describes the filamentation of both coherent light and spatially and temporally incoherent light. It is shown that filaments driven by the thermal mechanism tend to cluster together and produce greater large scale nonuniformities in the laser illumination than the ponderomotively driven filaments. The RPS optical smoothing technique is found to reduce filamentation only if fast focusing optics (f/♯≲5) are used. The ISI smoothing method suppresses filamentation for fast or slow focusing optics, and requires only moderate laser bandwidth (Δω/ω≂0.1%). In general, the ISI smoothing method provides the best suppression of filamentation. Under common laboratory conditions, filamentation is pronounced at longer laser wavelengths (1.06 μm–0.53 μm), suggesting that current experiments may be dominated by filamentation effects. Optical smoothing methods can suppress, but not eradicate, filamentation effects at these wavelengths. The optical smoothing methods are most effective in short-wavelength (0.25 μm) laser-driven plasmas. At 0.25 μm laser wavelength, ISI is found to completely eliminate filamentation effects in both time-averaged and instantaneous intensity distributions.