Studies Of Laser Bandwidth Effects On Suppression Of Parametric Instabilities In Laser Plasma Interactions

Parametric instabilities excited in laser plasma interactions represent a critical obstacle to be solved in laser confined fusion.Since the 1960s,several parametric instability mechanisms have been discovered and investigated widely,such as stimulated Raman scattering?SRS?,stim-ulated Brillouin scattering?SBS?,two plasmon decay instability?TPD?,and so on.SRS is known to occur below the quarter critical density,which reduces the laser energy coupling to the tar-get and meanwhile produces harmful hot electrons.SBS develops below the critical density,which not only reduces the laser energy coupling efficiency but also causes asymmetric target compression due to the crossing beam interactions.TPD develops mainly near the quarter crit-ical density,where it produces a large number of hot electrons.These three instabilities are the most important parametric mechanisms in inertial confinement fusion,which widely exist in the direct-ignition and indirect-ignition.The experiments performed in the last few years on NIF in LLNL have indicated that the parametric instabilities in laser plasma interactions are still not totally understood,and the control of parametric instabilities is far from satisfaction.This thesis focuses on the physical mechanisms of nonlinear saturation and possible schemes to suppress the instabilities either at the linear or nonlinear stage.In particular,we concentrate on the laser bandwidth effects on the suppression of the parametric instabilities.The major four achievements of this thesis are described as follows.Firstly,we studied the parametric instabilities in relativistic-bot temperature plasma.The dispersion relation has been derived,which can be applied to relativistic-high laser intensity and relativistic-hot temperature plasma.In relativistic hot plasma,the critical density for incident laser is modified by electron temperature modulation.We have obtained the maximum growth rate and instability region for SRS and SBS,which agree well with the numerical solutions.The relativistic temperature can reduce the instability regions of SRS and SBS.With the increase of plasma temperature,the unstable wave-vector is reduced,and the frequency of backscattering light appears red shift.Particle-in-cell?PIC?simulations indicate that the long time interactions between laser and plasma can heat the electrons to relativistic temperature,and then suppress the instabilities.Secondly,we have studied the nonlinear coupling of SRS and SBS.Based upon the nonlin-ear coupling equations,we obtained the plasma-density-perturbation threshold for the inhibition of SRS.When the ion density perturbations are above this threshold,the SRS instability mode is suppressed.In the fluid regime,SBS can develop a large amplitude of ion density perturbations,which always satisfies the threshold.Therefore,the inhibition of SRS due to SBS is always found in the fluid regime.Considering the heating effects of SRS,we introduced a damping term of Langmuir waves in the coupling equations.When SRS is suppressed,Langmuir wave loses its energy to electrons by the damping.The matching conditions are satisfied in the local region for inhomogeneous plasmas,therefore we can extend this threshold to the inhomogeneous plasma,which agrees well with the numerical simulations.Thirdly,we have studied the suppression of SRS by laser bandwidth via PIC simulations.According to the theoretical calculation,the bandwidth for the suppression of SRS under ICF conditions should be larger than 10^-2??5%?.Here we adopted the sinusoidal-frequency-modulation laser model.PIC simulations indicate that the larger the laser bandwidth is,the stronger the suppression effect is.This bandwidth suppression is only effective in the linear stage since SRS can be found to saturation at a similar level in the cases with or without a finite bandwidth.In the nonlinear stage,wave-particle interactions are the dominant mechanisms for saturation,the bandwidth effects need further studies in this kinetic process.To enhance the suppression effects,one may choose a proper frequency modulation parameter to decrease the growth rate.Finally,we have proposed a strategy to totally suppress parametric instabilities.A theo-retical model has been proposed to describe the coupling of different frequency beamlets.By use of the dispersion relations,we found that strong coupling and enhanced SRS take place only when the unstable regions for each beamlet are overlapped in the wave-number space.Hence a threshold of the beam frequency difference for their decoupling is found as a function of their intensity and plasma density.Based upon the threshold,a strategy to suppress SRS completely are proposed by use of so called decoupled broadband lasers?DBLs?.A DBL is composed of many monochromatic laser beamlets each with slightly different frequency,where the frequency of neighboring is large enough that there is no coupling between them.Comparing with the sinusoidal-frequency-modulation laser,DBLs have better suppression effect.When the band-width of DBLs is larger than the Langmuir wave frequency,forward SRS can be seeded between different components of the DBLs.PIC simulations indicate that DBLs can suppress the linear convective SRS in inhomogeneous plasmas.In practical application,it is difficult to produce an intense laser with a large bandwidth.A possible way to produce such DBLs with plasma optical modulators are proposed.