X-ray conversion efficiency of high-Z hohlraum wall materials for indirect drive ignition

Abstract

We measure the conversion efficiency of 351 nm laser light to soft x-rays (0.1-5 keV) for Au, U and high Z mixtures "cocktails" used for hohlraum wall materials in indirect drive ICF. We use spherical targets in a direct drive geometry, flattop laser pulses and laser smoothing with phase plates to achieve constant and uniform laser intensities of 10 14 and 10 15 W/cm 2 over the target surface that are relevant for the future ignition experiments on NIF. The absolute time and spectrally-resolved radiation flux is measured with a multichannel soft x-ray power diagnostic. The conversion efficiency is then calculated by dividing the measured x-ray power by the incident laser power from which the measured laser backscattering losses is subtracted. After ~0.5 ns, the time resolved x-ray conversion efficiency reaches a slowly increasing plateau of 95% at 10 14 W/cm 2 laser intensity and of 80% at 10 15 W/cm 2 . The M-band flux (2-5 keV) is negligible at 10 14 W/cm 2 reaching ~1% of the total x-ray flux for all target materials. In contrast, the Mband flux is significant and depends on the target material at 10 15 W/cm 2 laser intensity, reaching values between 10% of the total flux for U and 27% for Au. Our LASNEX simulations show good agreement in conversion efficiency and radiated spectra with data when using XSN atomic physics model and a flux limiter of 0.15, but they underestimate the generated M-band flux. I IntroductionIn indirect drive inertial confinement fusion (ICF) experiments, intense laser or charged particle beams heat the interior of high-Z cylindrical cavities called hohlraums to efficiently generate soft x-rays. The role of the soft x-rays is to uniformly produce an ablation drive that compresses the DT filled capsule placed inside the hohlraum, driving it to ignition and burn [1]. Due to the relaxed requirements on laser-beam uniformity and reduced sensitivity to hydrodynamic instabilities the indirect drive scenario is the main approach for future ignition experiments at the National Ignition Facility (NIF) [2,3,4]. In such a hohlraum containing a capsule and heated by laser beams the power balance is given by [5,6]:η CE (P laser -P backscatter ) = P walls +P LEH + P capsule =σT R 4 [(1-α)A wall +A LEH +f capsule A capsule ] (1) where η CE is the laser conversion efficiency into soft x-rays (<2 keV), P laser is the incident laser power, P backscatter is the backscattered light by parametric laser-plasma instabilities, P capsule is the radiation absorbed by the capsule, P LEH is the radiation escaping through the hohlraum laser entrance holes (LEH) and P walls is the radiation loss in the hohlraum walls.Furthermore, σ is the Stefan-Boltzmann constant, T R is the hohlraum radiation temperature (≤300 eV), α is the x-ray albedo of the hohlraum wall [7], defined as the ratio between the re-emitted and incident soft x-ray flux at the hohlraum wall, A wall and A LEH are the areas of the hohlraum wall and LEH's, f capsule is the fraction of incident x-ray flux that is absorbed by the capsule and A ...

The role of a detailed configuration accounting (DCA) atomic physics package in explaining the energy balance in ignition-scale hohlraums

“…For example, for a 3 ns 10 14 W/cm 2 illumination of the Au sphere, the observed spectrum [25] at the time of peak emission (just before the end of the 3 ns pulse) had 2 approximately equal height peaks of emission: A w150 eV wide emission feature centered around a photon energy of 350 and a w300 eV wide emission feature centered around a photon energy of 750 eV. The XSN prediction was a single dominant peak at w750 eV, whereas the DCA model reproduced the data with 2 equal height peaks of the correct width and photon energy central positions.…”

“…As we shall see, despite the HFM's many past successes in correctly modeling high radiative fluxes seen in the Omega Laser Au sphere data [25,26] and in explaining the surprisingly high drive seen in NIC empty hohlraums [27,28], it met some resistance when applied to the NIC 2009 campaign. In part, this was because it was initially thought that the HFM was over-predicting the drive in this 2009 energetics campaign.…”

“…These experiments, and their analysis [25,26] are, in essence, the modern analog of the gold disk experiments of the 1970s [10]. Just as those old gold disks set the stage for the development of the standard model, the Au spheres set the stage for the new model, namely the HFM.…”

Cryogenic thermonuclear fuel implosions on the National Ignition Facility

“…With η CE being the x-ray conversion efficiency from laser power to soft x-rays [57,58]; α W and α CAP are the x-ray albedo of the hohlraum wall and the capsule, respectively. The albedo is defined as the ratio of re-emitted to incident x-rays.…”

Observation of High Soft X-Ray Drive in Large-Scale Hohlraums at the National Ignition Facility

“…Other recent experiments to measure the x-ray conversion efficiency using spherical gold targets have also required an increase in the flux limit from 0.05 to 0.15 in order to get agreement between the simulations and the experimental measurements [25].…”