Experimental study of the modification of parametric instabilities in multiple plasmas

The presence of several plasmas along the path of an energetic laser beam is likely to produce noticeable changes in the behaviour of parametric instabilities compared to the case of interaction with single plasma. In order to identify the different effects that can come into play in this kind of situation, a tunable double target system has been designed, allowing controlled variation in the distance between the two targets. The first target was low density foam while the second one was a thin plastic foil. Part of this thesis is devoted to characterization of laser-plasma interaction into the foams since this subject has been not much explored yet for such low densities. In the cas of interaction with two plasmas, three effects have been identified and analysed: hydrodynamic coupling, the effect of laser-induced incoherence and electromagnetic coupling of the plasmas. For the first time it has been evidenced that laser-induced incoherence can lead to the suppression of the stimulated Brillouin backscattering generated into separate plasma. On the other hand, laser -induced incoherence benefits are lost when hydrodynamic effects occur due to the collision of the two plasmas if not enough distance is put between the targets. Stimulated Raman scattering is more sensitive to the electromagnetic coupling between the plasmas. The latter occurs when the backscattered light from the foil target propagates through the foam plasma. In this case, a re-amplification of the scattered light can be observed leading to an increasing in reflectivity levels. This phenomenon is particularly desired for plasma amplification of short pulses. It at also been observed that in the low density region of the foam, backscattered light from the foil leads to Raman amplification in the kinetic regime. (author)