Influence of absorption and refraction on laser interaction studies in preformed plasmas

We report on a numerical and experimental study of the propagation of a laser beam in a performed plasma. The physical parameters concerning the laser and the plasma are those of an experiment described in the Proc. 16 European Conf. on Controlled fusion and plasma physics, 1989, p. 837. The stationary electron density profile is provided by hydrodynamic simulations. We show that refraction is important for plasma densities related to a critical density (n_e/n_c) higher than 0.1. The main consequence is the variation of the spatial laser intensity distribution inside the plasma and the lowering of the interaction length which can, consequently, modify thresholds and growth rates of parametric instabilities. We also show that a transverse depression in the plasma density, which could be due to filamentation of the interaction beam, can modify the angular diagram of the transmission.