ANALYTICAL CALCULATION OF THE AMPLIFICATION AND ANGULAR DIVERGENCE OFTHE STIMULATED BACKSCATTERED LIGHT FROM A GAUSSIAN HOT-SPOT

The problem of backscattering instabilities from a single laser hot sp ot is considered analytically in the frame of the paraxial approximati on. An analytical calculation of the convective gain and of the far-fi eld angular divergence of the backscattered light is presented for bot h homogeneous and inhomogeneous plasmas. It is shown that the far-fiel d angular divergence is determined by the propagation of the backscatt ered light not only within the amplification region but also outside i t, where the coupling is very weak. For homogeneous plasmas, the far-f ield angular divergence computed at the end of the interaction region is always less than what it would be if it were computed at the end of the amplification region. For inhomogeneous plasmas, both the power a nd the far-field angular divergence of the backscattered beam depend o n the sign of the inhomogeneity gradient.