Chirped Laser Inhibits The Instability Of The Plasma Parameters

In indirect drive Inertial Confinement Fusion(ICF), the interaction between the laser and the plasma produced by the irradiated hohlraum is the first step of the energy coupling process. It directly determines the overall hohlraum coupling efficiency(capsule absorbed energy/laser energy). During the laser-plasma interaction(LPI), many parametric instabilities would be excited such as SBS, SRS, filamentation, etc. It would largely reduce the x-ray conversion efficiency and destroy the symmetry of the radiation field. In addition, when the strength of parametric instability reaches a certain level, there would be interconnections and couplings between different instabilities, which makes laser-plasma interaction complicated and unpredictable. Therefore this uncertainty has become the largest risk in achieving the ignition goal. In order to suppress parametric instabilities, laser-plasma interaction relevant to laser fusion schemes is an important topic in need of further research.With the eventual goal of suppressing parametric instabilities using chirped laser in mind, the interactions between the plasma and the laser of monochromatic, single chirp and multiple chirps are studied based on PIC simulation and experiment. The preliminary result shows the uncompressed broadband chirped pulse has the ability of suppressing parametric instabilities, and it is worth being studied in depth as a new potential ICF laser driver.The main results we have obtained are as follows.1. The fluid-Maxwell equations, Nishikawa equations and mode coupling equations for parametric process description are systematically reviewed. The dispersion relationship, growth rate and threshold for SBS and SRS in homogeneous and inhomogeneous plasma are derived from these equations. The importance of the mode coupling equations is emphasized.2. For large-scale plasma in the future ignition, the mechanisms of suppressing parametric instabilities relevant to LPI physics are reviewed, which include increasing linear Landau damping, particle trapping, secondary parametric process and pump depletion.3. In laser driven indirect ICF, the goal of suppressing parametric instabilities also decides the design of beam smoothing. Therefore, theories and techniques relevant to the laser beam for suppressing parametric instabilities are reviewed, which include spatial smoothing such as RPP, CPP, PS, small f number, etc., and temporal smoothing such as chirped laser, SSD, ISI, etc.4. The interaction of laser and homogeneous plasma under conditions of Te=817eV, Ti=255eV, mi=50000me, Z=1, L=50λ0,1×1014-1×1016Wcm-2, tpulse=16000τ0, ne=0.1nc is studied using 1-dimensional PIC simulation. We focus on the saturation of EPW, which caused by the nonlinear frequency shift and the LDI, hence results in bursts of the SRS scattered light. Although the peak reflectivity of SRS scattered light reaches 30%, the overall fraction is below 2%. For the SBS, the overall fraction of scattered light reaches 10%-30%, and exhibits no saturation in the absence of the similar saturation mechanism such as ion trapping and 2-dimensional TID.5. The simulation shows that the single chirp laser of different bandwidth(δω/ω0=0.2%,δω/ω0=0.6%,δω/ω0=2%,δω/ω0=6%,δω/ω0=20%) has no effect on SRS even if the bandwidth of the laser is much larger than the common SRS growth rate(δω/γ0,R>10). However, when the bandwidth is much larger than the common SBS growth rate(δω/γ0,B>10), the chirp's bandwidth could directly detune the SBS, hence weaken the scattered light. For the multiple chirp laser case, we choose three different bandwidth(δω/ω0=0.2%,δω/ω0=0.6%,δω/ω0=2%), and four different chirp number(10,20,50,100) during an interaction time of 16000τ0. The result shows the chirp number does not affect SRS either. When the bandwidth is small (δω/ω0=0.2%), the number of chirp affects SBS little. However, when the bandwidth is much larger than the growth rate(δω/ω0=0.6%,δω/ω0=2%), it shows an anti-correlation between chirp number and overall fraction of SBS scattered light.6. A preliminary experiment is carried out on the XG-Ⅱlaser facility. The laser is incident of 1053 nm wavelength,8 nm bandwidth,1ns pulse length and 100 J energy, the average intensity on target is about 5×1013-7×1014W/cm2. For comparison with the past experiments, the planar target of Ta, Au, AuCH, Au foam and hohlraum target of Au are adopted. By changing the target material, the incident intensity and angle, the backscattered light and harmonics are studied. The results show that (a)the second-harmonic is weak, its maximal fraction is below 4×10-4, (b)the maximal SRS backscattered fraction does not exceed 0.56%, which is much smaller than in the similar experiments in the past(>10%), (c)the maximal backscattered fraction of SBS is 14.3% for Au target, which is slightly smaller than in the hohlraum experiments on SG-Ⅰ(20-30%).