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Absolute stimulated Raman side scatter in direct-drive laser-produced plasmas
摘要： Production of suprathermal electrons by stimulated Raman scattering (SRS) is a principal concern for contemporary direct-drive inertial confinement fusion experiments at the National Ignition Facility and similar systems since such electrons penetrate and preheat the target core, preventing efficient implosion. The higher temperatures and longer scale lengths in these experiments favor SRS over two-plasmon decay, which predominated in earlier experiments. In particular, current experiments are expected to exceed the threshold for absolute Raman side scatter, which would then dominate the interaction since it grows temporally until saturated by nonlinear mechanisms such as hot-electron production. Until recently, analyses of SRS side scatter have treated the case of a single laser beam incident on a plasma, but the direct-drive approach to laser fusion employs a multitude of beams to drive the implosion. Here, we present an analysis that can be applied to an arbitrary number of beams with varied angles of incidence and polarizations. In the case of a single beam, it allows a physically motivated derivation and verification of an analytic threshold formula. In the general case of multiple beams and arbitrary orientation and polarizations, the threshold is found by numerical integration of a set of first-order linear partial differential equations.
关键词： direct-drive,stimulated Raman scattering,inertial confinement fusion,laser-produced plasmas,suprathermal electrons
更新于2025-09-23 15:21:01
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Effect of fast electrons on the gain of a direct-drive laser fusion target
摘要： The results of numerical and theoretical studies of the gain of a direct-drive inertial con?nement fusion target, which includes a kinetic description of energy transfer by laser-accelerated fast electrons, are presented. The range of the initial temperature of the fast electrons and the fraction of laser energy contained in these particles were chosen based on the results of recent experiments at the National Ignition Facility. The effect of ‘wandering’ of fast electrons is taken into account which is due to the remoteness of the region of fast electron generation from the ablation surface of the imploded target. As a result a signi?cant fraction of particles do not fall into the compressed part of target. The ‘wandering’ effect leads to a decrease in the negative effect of fast electron generation on the gain of the target.
关键词： implosion,energy transfer,fast electrons,direct-drive targets,inertial con?nement fusion
更新于2025-09-23 15:19:57
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Calculation of Direct Drive Targets for Megajoule Laser Facilities with Radiation in the Second and Third Harmonics of Nd Laser
摘要： The numerical results for cryogenic direct drive targets of megajoule facilities with radiation in the second and third harmonics of a Nd laser are presented. The calculations were performed with the 1D radiation hydrodynamics code ERA with the laser light absorption model that takes into account stimulated Brillouin scattering (SBS), generation of fast electrons in the processes of two-plasmon decay (TPD), and stimulated Raman scattering (SRS). The verification of the developed models was carried out on the basis of the comparison with experiments performed at the OMEGA and NIF facilities. The ignition margin (WQ) of nonuniform fusion targets with an allowance for energy losses due to radiation transfer and heat conduction from the hot spot was the objective of the target optimization. The calculations showed that SBS and target heating by fast electrons generated in TPD and SRS fatally reduce WQ of targets with a CH ablator for the megajoule laser with wavelength λ = 0.53 μm. The possibilities of decreasing these effects by replacing a CH ablator with a glass ablator and reducing the laser intensity upon increasing the target aspect ratio are considered. However, in both cases, WQ remains substantially below unity for the laser with wavelength λ = 0.53 μm. The ignition margin increases by a factor of ~2 upon transition from the second to the third harmonic of a Nd laser. A glass ablator almost eliminates fast electrons in calculation with the laser wavelength λ = 0.35 μm. In this case, if SBS is reduced by a factor of 3–4 via shifting the laser emission lines in the neighboring channels by Δλ ≈ 10–20 ?, the ignition margin WQ ~ 2 and a fusion energy yield of ~50 MJ are obtained in the 1D calculation for a laser energy of ~2 MJ and the third harmonic of a Nd laser.
关键词： megajoule laser facilities,two-plasmon decay,fast electron generation,direct drive targets,stimulated Raman scattering,thermonuclear ignition margin of target,stimulated Brillouin scattering
更新于2025-09-23 15:19:57