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 (W-Q) 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 W-Q of targets with a CH ablator for the megajoule laser with wavelength lambda = 0.53 mu m. 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, W-Q remains substantially below unity for the laser with wavelength lambda = 0.53 mu m. The ignition margin increases by a factor of similar to 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 lambda = 0.35 mu m. In this case, if SBS is reduced by a factor of 3-4 via shifting the laser emission lines in the neighboring channels by Delta mu approximate to 10-20 A, the ignition margin W-Q similar to 2 and a fusion energy yield of similar to 50 MJ are obtained in the 1D calculation for a laser energy of similar to 2 MJ and the third harmonic of a Nd laser.