High performance capsule implosions on the OMEGA Laser facility with rugby hohlraums

Abstract

Rugby-shaped hohlraums have been proposed as a method for x-ray drive enhancement for indirectly driven capsule implosions. This concept has recently been tested in a series of shots on the OMEGA laser facility ͓T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 ͑1997͔͒. In this paper, experimental results are presented comparing the performance of D 2 -filled capsules between standard cylindrical Au hohlraums and rugby-shaped hohlraums. The rugby hohlraums demonstrated 18% more x-ray drive energy as compared with the cylinders, and the high-performance design of these implosions ͑both cylinder and rugby͒ also provided Ϸ20ϫ more deuterium ͑DD͒ neutrons than any previous indirectly driven campaign on OMEGA and Ϸ3ϫ more than ever achieved on NOVA ͓E. M. Campbell, Laser Part. Beams 9, 209 ͑1991͔͒ implosions driven with nearly twice the laser energy. This increase in performance enables, for the first time, a measurement of the neutron burn history and imaging of the neutron core shapes in an indirectly driven implosion. Previous DD neutron yields had been too low to register this key measurement of capsule performance and the effects of dynamic mix. A wealth of additional data on the fuel areal density from the suite of charged particle diagnostics was obtained on a subset of the shots that used D 3 He rather than D 2 fuel. Comparisons of the experimental results with numerical simulations are shown to be in very good agreement. The design techniques employed in this campaign, e.g., smaller laser entrance holes and hohlraum case-to-capsule ratios, provide added confidence in the pursuit of ignition on the National Ignition Facility ͓J. D. Lindl, P. Amendt, R. L. Berger et al., Phys. Plasmas 11, 339 ͑2004͔͒.

Progress of LMJ-relevant implosions experiments on OMEGA

“…These experiments demonstrated nevertheless a pancake (oblate) symmetry with normalized Legendre polynomial coefficient P 2 /P 0 = −20% to −28% [8]. This was found to be consistent with a 10-20% backscatter on the inner cones [8].…”

“…Moreover this increased x-ray drive leads to capsule implosion performances with record neutron yield in indirect-drive on OMEGA reaching Y n = 1.5 10 10 neutrons (D 2 fuel) for a laser energy of only 19.5 kJ. The increased drive on capsule is also confirmed by a smaller energy downshift of secondary proton spectra in rugby due to greater mass ablation [7,8,12].…”

“…This was found to be consistent with a 10-20% backscatter on the inner cones [8]. We performed therefore a symmetry tuning with a predicted cone power balance between outer and inner cones: a 0.8x factor was applied on outer cone power (59 • beams).…”

“…It has been shown that rugby-shaped hohlraums [5] present significant advantages in terms of coupling efficiency and symmetry control with a 1/2-1/2 energy balance [6] and several tuning campaigns have been performed for capsule implosions in empty and gas-filled hohlraums. The energetics benefit of rugby over cylinder has been demonstrated [7] in both cases, leading to the highest D 2 yields to date on OMEGA in indirect-drive [8]. These highperformance implosions enable the use of a complete suite of nuclear diagnostics, as for example the neutron temporal diagnostic for determination of the burn history [9].…”

Diagnosing indirect-drive inertial-confinement-fusion implosions with charged particles

“…Measurements of the charged-particle properties provide important information about fusion yields, fuel and shell temperatures, areal densities, implosion symmetry and dynamics, and fields in specific experiments, as well as information of general interest on such topics as the slowing down of energetic ions in ICF relevant plasmas [14]. Figure 1 schematically shows a typical vacuum, gold (Au) hohlraum and a warm plastic capsule filled with D 3 He gas that were used in the indirect-drive experiments on OMEGA [33,34] described in this article. This OMEGA "scale-1" hohlraum had a 1.6-mm diameter, a 2.45-mm length, 50% laser-entrance holes (LEH), and a 30-µm-thick Au wall.…”