Modal analysis of power imbalance and pointing errors for direct drive and tetrahedral hohlraums

Abstract

This article extends the work of Skupsky and Lee (Skupsky, S., Lee, K., Appl. Phys. 54 (1983) 3662) and Murakami et al. (Murakami, M., et al., J. Appl. Phys. 74 (1993) 802) in presenting analytic formulas that predict the laser flux asymmetry, in various inertial confinement fusion (ICF) targets from low order modes, due to beam geometry, variations in beam power and beam pointing errors. The variance per l mode σ2l is given for both direct drive and tetrahedral hohlraums, as a function of mode number, the beam geometry, and the pointing and power balance errors. For the National Ignition Facility (NIF), a proposed 1.8 MJ laser, the flux asymmetry on a direct drive capsule is dominated by power balance errors in modes 1 and 2. For the Omega, a 30 kJ laser at the Laboratory for Laser Energetics at the University of Rochester, the flux asymmetry on a direct drive capsule is dominated by pointing errors in modes 1 to 5. The flux asymmetry on a tetrahedral indirect drive target is dominated by the Y3,2 mode resulting from the laser entrance holes.

Three-dimensional modeling of capsule implosions in OMEGA tetrahedral hohlraums

“…This property provides some inherent symmetry advantages for the tetrahedral hohlraum, causing all lϭ1, 2, and 5 spherical-harmonic components of the radiation drive to be identically zero. 8 For the specific hohlraum designs used in this paper, the x-ray drive nonuniformity on the capsule ( rms ) is almost entirely dominated by the Y 32 spherical harmonic mode 9,13 and, in the optimal designs, is less than 1% during most of the laser pulse. This highly uniform drive has been confirmed by x-ray images of imploded cores that are essentially round to within the resolution of the instrument.…”