Design of an ignition target for the laser megajoule, mitigating parametric instabilities

Abstract

Laser plasma interaction (LPI) is a critical issue in ignition target design. Based on both scaling laws and two-dimensional calculations, this article describes how we can constrain a laser megajoule (LMJ) [J. Ebrardt and J. M. Chaput, J. Phys.: Conf. Ser. 112, 032005 (2008)] target design by mitigating LPI. An ignition indirect drive target has been designed for the 2/3 LMJ step. It requires 0.9 MJ and 260 TW of laser energy and power, to achieve a temperature of 300 eV in a rugby-shaped Hohlraum and give a yield of about 20 MJ. The study focuses on the analysis of linear gain for stimulated Raman and Brillouin scatterings. Enlarging the focal spot is an obvious way to reduce linear gains. We show that this reduction is nonlinear with the focal spot size. For relatively small focal spot area, linear gains are significantly reduced by enlarging the focal spot. However, there is no benefit in too large focal spots because of necessary larger laser entrance holes, which require more laser energy. Furthermore, this leads to the existence, for a given design, of a minimum value for linear gains for which we cannot go below.

Progress of LMJ-relevant implosions experiments on OMEGA

“…At OMEGA scale backscattering losses are not very different in cylinders and rugby hohlraum. At MJ scale LPI in rugby hohlraum is an open question but various mitigation options are available including hohlraum wall shape [2,4]. The energetics benefit of rugby-shaped hohlraum could be useful either to increase the x-ray drive (and so favor higher capsule implosion velocity) for the same laser power, to achieve the same drive at reduced laser power or for high-gain target designs [13].…”

“…The first implosion experiments and ignition attempts on LMJ [1] will be performed in a two-cone irradiation scheme resulting in a 1/2-1/2 ring balance configuration in the hohlraum [2]. CEA is pursuing an experimental program at the OMEGA Laser Facility [3] to validate the conditions relevant to LMJ point design [4].…”

Low initial aspect-ratio direct-drive target designs for shock- or self-ignition in the context of the laser Megajoule

“…NIF and LMJ are ID-designed facilities [7,10,11] and planed to provide moderate thermonuclear gains [12,13].…”

First High-Convergence Cryogenic Implosion in a Near-Vacuum Hohlraum

“…The National Ignition Facility (NIF) [2,3] drives this process using 192 frequency-tripled laser beams (351 nm at 3ω), which enter a cylindrical hohlraum through laser entrance holes (LEHs) at each end. The laser beams are pointed through the LEHs to provide various angles of drive to the capsule such that the superposition of drives can be spherically symmetric throughout the implosion time.Typically, ICF experiments on the NIF have utilized plastic (CH) capsules inside gold hohlraums filled with helium at densities ranging from 0.96 [4] to 1.6 mg=cm Although vacuum hohlraums were initially considered, the long pulse duration for CH capsules led to choosing higher densities of hohlraum fill [7,8] to minimize expansion of the interior gold wall [ Fig. 1(a)] [9].…”

“…Typically, ICF experiments on the NIF have utilized plastic (CH) capsules inside gold hohlraums filled with helium at densities ranging from 0.96 [4] to 1.6 mg=cm Although vacuum hohlraums were initially considered, the long pulse duration for CH capsules led to choosing higher densities of hohlraum fill [7,8] to minimize expansion of the interior gold wall [ Fig. 1(a)] [9].…”