Performance and Mix Measurements of Indirect Drive Cu-Doped Be Implosions

Abstract

The ablator couples energy between the driver and fusion fuel in inertial confinement fusion (ICF). Because of its low opacity, high solid density, and material properties, beryllium has long been considered an ideal ablator for ICF ignition experiments at the National Ignition Facility. We report here the first indirect drive Be implosions driven with shaped laser pulses and diagnosed with fusion yield at the OMEGA laser. The results show good performance with an average DD neutron yield of ∼2 × 10 9 at a convergence ratio of R 0 =R ∼ 10 and little impact due to the growth of hydrodynamic instabilities and mix. In addition, the effect of adding an inner liner of W between the Be and DD is demonstrated. DOI: 10.1103/PhysRevLett.114.205002 PACS numbers: 52.57. Fg, 52.70.La In inertial confinement fusion (ICF) experiments, like those performed at the National Ignition Facility (NIF) [1] and the OMEGA laser facility [2], capsules of deuterium and tritium fuel are imploded to high densities and temperatures to initiate fusion burn. The indirect drive ICF concept uses a laser to irradiate a hohlraum, which produces a nearly uniform thermal x-ray drive. The x-ray drive then ablates the outer capsule material imploding the remaining cryogenically frozen DT shell-mass inward. The conditions necessary for ignition are related to a minimum requirement of the energy density delivered to the DT hot spot and the confinement time of that energy, or equivalently Pτ, the product of the hot-spot pressure (P), a measure of the hotspot energy density, and the energy confinement time (τ) [3]. Betti et al. [4] showed that P is related to the implosion velocity (v) by balancing the hot-spot internal energy to the shell kinetic energy via 2πPR 3 ∼ θ 1 2 Mv 2 , where R is the radius of the hot spot, θ is the fraction of the shell kinetic energy converted to hot-spot energy, and M is the mass of the shell. Note that assuming a thin shell results in M ∼ 4πR 2 × ρR; where ρR is the areal density of the imploded shell. From Newton's law, the hot-spot confinement time is related to the shell inertia like τ ∼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi M=4πPR p . Combining these expressions shows that Pτ ∝ v × ρR [4]. This means that a successful ignition experiment must achieve high v and high ρR. In the attempt to achieve these conditions, several ablator materials have been considered, i.e., plastic (CH), high density carbon (HDC), and beryllium. The choice of ablator material has important consequences for the implosion hydrodynamics. For example, the mass ablation rate ( _ m) can be quite different for different materials. Olson et al.[5] measured _ m (in mg=cm 2 =ns) experimentally and found it to be 0.75T 3 r for Be (with and without 1% Cu dopant), 0.50T 3 r for HDC, and 0.35T 3 r for CH (0.6% Ge dopant), where T r is the drive radiation temperature in units of hundreds of eV (or heV). Because the ablation velocity is V a ¼ _ m=ρ, where ρ is the ablator density, Be has the highest V a for the same in-flight ρ. Als...

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