THEORY OF STIMULATED RAMAN-SCATTERING FOR RELATIVISTICALLY STRONG LASER-RADIATION IN PLASMA

For an arbitrary field amplitude of high-frequency radiation that is r elativistically strong and circularly polarized, the growth rates of f orward and backward stimulated Raman scattering in both underdense and dense plasmas are considered neglecting thermal effects. For the case of an underdense (n(o) much less than n(cr) = m(e) omega(o)(2)/4 pi e (2)) plasma, it is shown that both the maximum instability growth rate s of forward and backward Raman instabilities [Gamma(fmax) similar or equal to omega(p)(2)/4 omega(o) and Gamma(bmax) similar or equal to 2( -2/3)(omega(p)(2) omega(o))(1/3)] and also the maximum instability ban dwidth in wavenumber space correspond to the pump field amplitude E(o) of several units of m(e) omega(o)c/e, that is, the field comparable w ith the laser fields in recent laser-plasma experiments. For greater v alues of E(o), our theory predicts a decrease in both the instability growth rate and the half-width of the unstable region in the wavenumbe r space. It is shown that the instability regions associated with forw ard and backward Raman scattering merge as the plasma density increase s up to the value n(o) similar or equal to 0.2n(cr). This occurs initi ally at \A(o)\ = eE(o)/m(e) omega(o)c similar or equal to 1, that is, at the amplitude corresponding to the maximum growth rate; for even gr eater values of plasma density (at n(o) = n(cr)/4), these regions merg e even at \A(o)\ much less than 1. Also, if the field exceeds some thr eshold value, it is shown that the Raman instability can occur in a pl asma with density n(o) > n(cr)/4. For n(o) greater than or similar to n(cr), the maximum value of the growth rate approaches the limiting va lue Gamma(max) similar or equal to 0.3 omega(o) as the plasma density increases. Consequently, in the case of nonlinear transparency induced by laser radiation in an overdense plasma [n(o) > n(cr) and (eE(o)/m( e) omega(o)c)(2)/2 > n(o)(2)/n(cr)(2) - 1], there are extremely high g rowth rates of the Raman instability. As a result, even for the intens ity of a relativistically strong laser radiation, which is high enough to provide an induced plasma transparency, a deep penetration into th e dense plasma is hardly possible because, in this case, Raman scatter ing is extremely strong.