Localized mix-induced radiative cooling in a capsule implosion at the National Ignition Facility

We present direct measurements of electron temperature variations within an inertially confined deuterium-tritium plasma caused by localized mix of higher-Z materials into the central hot spot. The data are derived from newly developed differentially filtered penumbral imaging of the bremsstrahlung continuum emission. Our analysis reveals distinct localized emitting features in the stagnated hot-spot plasma, and we infer spatial variations in the electron temperature: the mixed region is $660\pm 130\mathrm{eV}$ colder than the surrounding hot-spot plasma at $3.26\pm 0.11\mathrm{keV}$. Our analysis of the energy flow shows that we measure approximately steady-state conditions where the radiative losses in the mix region are balanced by heat conduction from the surrounding hot deuterium-tritium plasma.

Figure 1

(a) Cross section of the beryllium capsule with Cu dopant fractions [25]. (b) Diagram of the indirect drive hohlraum configuration, showing the x-ray penumbral imaging configuration and raw data for implosion N180121-2, imaged through an array of approximately 100 penumbral apertures with $100\mu \mathrm{m}$ diameter each and filtered with a series of x-ray differential filters. (c) Early-time, soft x-ray image captured from the north pole of the target chamber, approximately 50 ps before peak x-ray emission, tracking the bright feature coming from the direction of the fill tube. (d) Reconstructed penumbral image of the equatorial hot-spot emission (axis 90–100), orthogonal to the fill-tube axis. Increased x-ray emission from the fill-tube jet is clearly visible in the center.

Figure 2

(a)–(c) Differentially filtered, reconstructed penumbral images of the hot spot from (90 to 100) line-of-sight. (d) X-ray image of the hot spot without the local mix feature from the $103.5\mu \mathrm{m}$ titanium channel. (e) X-ray image of the extracted mix region with normalized color scale. Images in (a)–(d) are on the same color scale.

Figure 3

(a) Calculated x-ray emission spectra for the self-emission of the pseudohomogeneous hot spot including shell absorption and additional filtration. The gray shaded area is bound by predictions from a clean 1D simulation and one that includes a mix model to match the observed nuclear yield and down-scattered neutron ratio. (b) Histograms representing the electron temperature probability distributions in the mix region and in the hot D-T plasma inferred by Monte Carlo sampling of the experimental data using uncertainties of plasma composition, shell optical depth, filter thicknesses, detected photon yield, and image plate detector response.

Figure 4

(a) Simplified picture of the temperature evolution in the mix with different possible initial conditions for the mix center (solid yellow) and mix edge (dotted red) assuming a temperature equilibration time of $\tau =10\mathrm{ps}$ and a constant D-T temperature (solid blue). Early times (shaded) may not be present in the measured, time-integrated x-ray images as the x-ray emission is below the detection threshold at those temperatures. (b) Sketch of the regions that have been spatially resolved and the inferred temperatures.