Diffusion-dominated mixing in moderate convergence implosions

High-$Z$ material mixed into the fuel degrades inertial fusion implosions and can prevent ignition. Mix is often assumed to be dominated by hydrodynamic instabilities, but we report Omega data, using shells with $\sim 150\mathrm{nm}$ deuterated layers to gain unprecedented resolution, which give strong evidence that the dominant mix mechanism is diffusion for these moderate temperature ($\lesssim 6$ keV) and convergence ($\sim 12$) implosions. Small-scale instability-driven or turbulent mix is negligible.

Figure 1

(a) Radial cross section of the capsules used: $15\text{−}\mu \mathrm{m}$-thick plastic shells, $860\mu \mathrm{m}$ diameter, with a $0.15\mu \mathrm{m}$ deuterated layer. The capsules were filled with 9 atm of equimolar ${\mathrm{H}}_2$ and ${\mathrm{T}}_2$ gas. (b) Example framing camera image of the stagnated plasma on shot 85184, where the grayscale represents x-ray surface brightness.

Figure 2

Nuclear data versus CD recession: DT neutron yield (a), and DT/TT yield ratio (b). Model curves are shown by the green curves. The blue horizontal dashed and dotted lines represent values from 2% D in the gas or contamination D (0.05%). Red circles are individual shots simulated with the diffusion-only model.

Figure 3

Ion species number fraction (for H, C, T, D, O) versus normalized radius for the turbulence + diffusion no. 1 model (top row) and diffusion-only model (bottom row) for CD recession depths of 0 $\mu \mathrm{m}$ (left) and 0.3 $\mu \mathrm{m}$ (right). The cyan dashed curve shows the DT reaction rate ($n_{\text{D}}{n}_{\text{T}}\langle \sigma v\rangle r^2$) versus radius.