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A thermal model has been developed for laser welding which describes the heat input in terms of point and line sources. The model was used to generate weld profiles which closely matched those found by experiment. Outputs of the model (the thermal gradient GL and the growth rate R) were used to describe the macroscopic grain structure found along the weld centreline. Columnar structures were predicted at low welding speeds (high GL/R ratio) and equiaxed structures at high welding speeds (low GL/R ratio). Using the thermal model, cooling rates of ∼1500 K s–1 were estimated for the lowest welding speed, which increased by an order of magnitude for the highest welding speed considered. There was excellent agreement between the dendrite secondary arm spacings measured by experiment and those predicted using the thermal model.

The occurrence of stimulated Raman scattering from within localized density cavities that may be present in many laser-produced plasmas is studied. Then scattering in both the backward and forward directions from a single cavity becomes absolutely unstable, with a common growth rate and zero inhomogeneous plasma thresholds whenever electron plasma waves are totally trapped within the cavity. Both back- and forward scattering mechanisms contribute to this common growth rate. A cavity therefore equalizes the growth of emission in backward and forward directions. Forward scattering in its own right gives rise to absolute growth. The threshold for absolute growth is then set by damping, and by Landau damping of the plasma wave in particular. Only very modest density depressions are needed to trap the long-wavelength weakly damped plasma waves. Absolutely unstable back- and forward scattering from within shallow cavities in the underdense coronal plasmas of laser fusion targets may prove to be commonplace rather than the exception, although growth rates are generally slow and sensitive to the cavity density and scale.

Convective gain and absolute growth of stimulated Raman scattering (SRS) are determined for a hot inhomogeneous collisional plasma using an integrated global model that includes the effects of wave propagation and both collisional and Landau damping in a density ramp. This unifies the time asymptotic theory of SRS occurring near the quarter-critical density of the laser light and that occurring at lower densities. The level of collisions has a striking effect on the convective behavior of the instability. When collisions are weak, no significant regime of convective gain exists. In consequence, absolute resonances are very sensitive and susceptible to detuning, particularly so near the quarter-critical density. This is shown to be a key factor in understanding the Raman ‘‘gap’’ seen in experiments. No such gap appears in highly collisional regimes in which the absolute resonances are insensitive and significant convective gain occurs over the whole range of wavelengths.