Theoretical And Numerical Studies On Stimulated Raman Scattering By A Chirped Laser Pulse

Stimulated Raman Scattering (SRS) occurs during the interaction of a laser pulse with an underdense plasma with electron density n₀≤n_cr/4, where n_cr is the plasma critical density. SRS is an instability whereby an incident laser wave decays into a scattered electromagnetic wave and a plasma wave. Raman instability plays an important role in laser-driven inertial confinement fusion scheme, because electrons may be accelerated to a high energy by the plasma wave. And these suprathermal electrons may preheat the fusion fuel to decrease compression efficiency. In addition, the scattered electromagnetic wave may be lost from the plasma and cause a decreasing in absorption efficiency of the incident laser. On the other hand, in the laser-plasma accelerator, SRS can be used to drive a large amplitude plasma wave , which can accelerate electrons to MeV energy.Because of the important relevance of SRS to these fields, a means to control its growth by adjustment of laser frequency chirp has been proposed recently. In this thesis, we analyze the effect of a small-bandwidth linear frequency chirp on the growth of Raman instability by using the coupled relativistic Maxwell-fluid equations. It is shown that the threshold of Raman backward scattering is increased by frequency chirp with no dependence on the sign of chirp. And the growth rate of Raman forward scattering increases (decreases) for a positive (negative) chirp. But the influence is minor for the experimental parameters. For instance, the growth rates are changed by about 1% for the frequency chirp with bandwidth of 2%. It must be pointed that when a large-bandwidth frequency chirp (about 20%) is introduced, the effects of group velocity dispersion and finite-bandwidth of instability should be taken into account. These two effects couple and result in that the Raman forward scattering is sufficiently enhanced (inhibited) by a positive (negative) chirp with large-bandwidth. And 1D3V particle-in-cell simulations are performed, which show an agreement with theoretical analysis.