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We study the angular distribution of relativistic electrons generated through laser-plasma interaction with pulse intensity varying from 10(18) W/cm2 up to 10(21) W/cm2 and plasma density ranging from 10 times up to 160 times critical density with the help of 2D and 3D particle-in-cell simulations. This study gives clear evidence that the divergence of the beam is an intrinsic property of the interaction of a laser pulse with a sharp density gradient. It is entirely due to the excitation of large static magnetic fields in the layer of interaction. The energy deposited in this layer increases drastically the temperature of the plasma independently of the initial temperature. This makes the plasma locally collisionless and the simulation relevant for the current experiments.

The interaction of ultraintense laser beams with underdense plasma slabs has been investigated with two-dimensional particle-in-cell numerical simulations, showing a strong absorption and a correlatively low transmission. Energetic electrons in the multi-MeV range are produced. At very high intensities the plasma transparency is recovered. These results are interpreted in terms of the development of electron parametric instabilities in the self-consistently heated plasma.

Thomson scattering measurements are presented which demonstrate conclusively the occurrence of the Langmuir decay instability (LDI) in a laser-produced plasma experiment. Both products of the instability, the ion acoustic wave and the electron plasma wave, were simultaneously observed and identified with their spectral characteristics. The secondary decay of the LDI-generated electron plasma wave, into another Langmuir wave and an ion acoustic wave, has been observed for the first time. The connection with growth and saturation of the stimulated Raman instability is discussed.