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

Abstract

Recent experiments on the National Ignition Facility [M. J. Edwards et al., Phys. Plasmas 20, 070501 (2013)] demonstrate that utilizing a near-vacuum hohlraum (low pressure gas-filled) is a viable option for high convergence cryogenic deuterium-tritium (DT) layered capsule implosions. This is made possible by using a dense ablator (high-density carbon), which shortens the drive duration needed to achieve high convergence: a measured 40% higher hohlraum efficiency than typical gas-filled hohlraums, which requires less laser energy going into the hohlraum, and an observed better symmetry control than anticipated by standard hydrodynamics simulations. The first series of near-vacuum hohlraum experiments culminated in a 6.8 ns, 1.2 MJ laser pulse driving a 2-shock, high adiabat (α ∼ 3.5) cryogenic DT layered high density carbon capsule. This resulted in one of the best performances so far on the NIF relative to laser energy, with a measured primary neutron yield of 1.8 × 10 15 neutrons, with 20% calculated alpha heating at convergence ∼27×. Inertial confinement fusion (ICF) experiments implode millimeter-scale deuterium-tritium (DT) filled spherical capsules, compressing and heating the DT fuel to fusion conditions and releasing energy [1]. Indirect-drive ICF places the fuel-filled capsule at the center of a high-Z radiation enclosure (hohlraum) and strikes the inside walls of the hohlraum with laser power. This produces an internal bath of x rays-a radiation drive which ablates and implodes the fuel-filled capsule. 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]. The intended effect is to maintain an open path for laser propagation to the wall for the full pulse duration.In this Letter, we report on the first experimental campaign on the NIF using near-vacuum hohlraums (NVH) to drive a high convergence cryogenic DT layered capsule implosion. A NVH has a hohlraum fill density of 0.03 mg=cm 3 helium, more than an order of magnitude lower density than conventional gas-filled hohlraums (0.96-1.6 mg=cm 3 ). Symmetry of implosions driven with the NVH is controlled through direct adjustments to the inner and outer beam power balance rather than relying on beam wavelength separations [10] and the resultant crossbeam energy transfer [11,12]. Unlike earlier research on true "vacuum" hohlraums [13][14][15], the NVH...

Progress towards a more predictive model for hohlraum radiation drive and symmetry

“…More recent NIF experiments using HDC ablators and hohlraum He fill densities from 0.03 to 0.6 mg/cc are in better agreement with HFM calculations than the earlier CH ablator experiments, but a discrepancy of ∼300 ps or 10% in the peak drive persists 16 . Experiments in which the hohlraum gas fill density was systematically varied up to 1.6 mg/cc showed that for 5.75 mm diameter hohlraums and 2 mm diameter HDC capsules, the bang time discrepancy increases for helium fills above 0.6 mg/cc, 17,18 rising to 800 ps at a fill density of 1.6 mg/cc.…”

Near-vacuum hohlraums for driving fusion implosions with high density carbon ablatorsa)

“…The timeresolved equatorial x-ray emission data for N131212 showed less than 5% swing in symmetry, confirming that the hot spot did not undergo symmetry swings during burn despite the purposefully driven swing in flux symmetry, i.e., these designed symmetry swings in drive did not produce symmetry swings in hot spot emission. 18 Increased case-to-capsule ratio Another option to mitigate hindered inner beam propagation is to design a hohlraum with a larger diameter to increase the distance between the capsule and wall; such a design is currently being experimentally explored. For this series of experiments, the diameter of the hohlraum was increased to 6.72 mm (from the nominal size of 5.75 mm), and the length was increased to 11.26 mm (from 10.13 mm) (Fig.…”

Cryogenic tritium-hydrogen-deuterium and deuterium-tritium layer implosions with high density carbon ablators in near-vacuum hohlraums

“…4,6,18 This makes sense: during its development, the high-flux model was bench-marked against vacuum hohlraum experiments performed on the NOVA and OMEGA laser facilities. The first demonstration of the high-flux model's utility for designing NIF experiments was its ability to reproduce x-ray intensity data from vacuum hohlraums, as reported by Olson.…”

“…19 The high efficiency of near-vacuum hohlraums for driving implosions is described in more detail in a recent paper by Hopkins. 18 The x-ray conversion efficiency g CE % 90% for near-vacuum hohlraums on the NIF. For a given amount of laser energy, a near-vacuum hohlraum can drive a capsule with 30%-40% more x-ray intensity than one filled with helium gas of q տ 1 mg/cm 3 .…”

“…The 10% ad-hoc reduction in laser power likely compensates for a slight over-estimation of the emissivity of gold plasma by the high-flux model in HYDRA; however, it could also be due to uncertainty in the x-ray opacity of carbon. For the experiments described here, laser backscatter and hot electron generation were too small to be significant 18 and were not included in post-shot simulations.…”