| The interaction of high intensity laser pulses with pre-formed and laser-produced plasmas is studied. Through experiments and simulations we have investigated stimulated Compton scattering in pre-formed plasmas and the plasma physics aspects of tunnel-ionized gases. A theoretical study is presented on the non-linear dynamics of relativistic plasma waves driven by colinear optical mixing.; The electron density-fluctuation spectra induced by stimulated Compton scattering have been directly observed for the first time. A CO{dollar}sb2{dollar} laser was focused into pre-formed plasmas with densities n{dollar}sb{lcub}rm e{rcub}{dollar} varied from 0.4-6 {dollar}times{dollar} 10{dollar}sp{lcub}16{rcub}{dollar} cm{dollar}sp3{dollar}. The fluctuations corresponding to backscatter were probed using Thomson scattering. At low n{dollar}sb{lcub}rm e{rcub}{dollar}, the scattered spectra peak at a frequency shift {dollar}Deltaomega{dollar} {dollar}approx{dollar} kv{dollar}sb{lcub}rm e{rcub}{dollar} and appears to be in a linear regime. At the highest n{dollar}sb{lcub}rm e{rcub}{dollar}, a nonlinear saturation of the SCS instability is observed due to a self-induced perturbation of the electron distribution function.; Tunnel-ionized plasmas have been studied through experiments and particle simulations. Experimentally, qualitative evidence for plasma temperature control by varying the laser polarization was obtained by the measurement of stimulated Compton scattering fluctuation spectra and X-ray emission from such plasmas. A higher parallel temperature than expected from the single-particle tunneling model was observed. Simulations indicate that stochastic heating and the Weibel instability play an important role in plasma heating in all directions and isotropization.; The non-linear dynamics associated with beatwave ({dollar}Deltaomega, Delta{dollar}k) excited long wavelength plasma waves in the presence of strong, short wavelength density ripple have been examined, using the relativistic Lagrangean oscillator model. This model shows period doubling that roughly follows Feigenbaum scaling, and a transition to chaos. However, wavebreaking is found to occur after the first bifurcation, thereby limiting the applicability of the Lagrangean model. It is found that the origin of this bifurcation is linked to the stability of an equivalent Mathieu equation to 1/2 subharmonic resonances. Finally, a PIC-code simulation shows spatial wavenumber peaks displaced {dollar}Delta{dollar}k/2 on both sides of the driver frequencies, giving support to the idea that the bifurcation behaviour may be experimentally observable. |
