Indications of flow near maximum compression in layered deuterium-tritium implosions at the National Ignition Facility

An accurate understanding of burn dynamics in implosions of cryogenically layered deuterium (D) and tritium (T) filled capsules, obtained partly through precision diagnosis of these experiments, is essential for assessing the impediments to achieving ignition at the National Ignition Facility. We present measurements of neutrons from such implosions. The apparent ion temperatures $T_{\mathrm{ion}}$ are inferred from the variance of the primary neutron spectrum. Consistently higher DT than DD $T_{\mathrm{ion}}$ are observed and the difference is seen to increase with increasing apparent DT $T_{\mathrm{ion}}$. The line-of-sight rms variations of both DD and DT $T_{\mathrm{ion}}$ are small, $\sim 150\mathrm{eV}$, indicating an isotropic source. The DD neutron yields are consistently high relative to the DT neutron yields given the observed $T_{\mathrm{ion}}$. Spatial and temporal variations of the DT temperature and density, DD-DT differential attenuation in the surrounding DT fuel, and fluid motion variations contribute to a DT $T_{\mathrm{ion}}$ greater than the DD $T_{\mathrm{ion}}$, but are in a one-dimensional model insufficient to explain the data. We hypothesize that in a three-dimensional interpretation, these effects combined could explain the results.

Figure 1

Ratio of apparent DD to DT $T_{\mathrm{ion}}$ as a function of apparent DT $T_{\mathrm{ion}}$. The data points come from cryogenically layered NIF DT implosions, with HiFoot-driven 195-μm CH ablator shots in green, 175-μm CH ablator shots in blue, 165-μm CH ablator shots in pink, HDC shots in purple, and shots driven with an adiabat-shaped laser pulse in orange. A square point indicates that the implosion utilized a depleted uranium hohlraum; circular points represent Au hohlraum implosions. Red points indicate shots with a known asymmetry in the laser drive and/or fuel. Gray error bars represent the systematic error and black error bars represent the statistical error. Also shown are simulated ratios from 1D HYDRA simulations (black crosses) and predicted ratios in apparent $T_{\mathrm{ion}}$ due to a nonthermal contribution from flows according to Ref. [23], assuming a thermal $T_{\mathrm{ion}}=0\mathrm{keV}$ (solid purple line), thermal $T_{\mathrm{ion}}=1.5\mathrm{keV}$ (dot-dashed line), and thermal $T_{\mathrm{ion}}=2\mathrm{keV}$ (double-dot–dashed line).

Figure 2

Ratios of DT to DD yield as a function of DT $T_{\mathrm{ion}}$. Points without error bars represent the measured DT and DD yields uncorrected for neutron scattering, framed data points represent estimated birth-yield ratios determined from the measured data using a DSR-dependent correction (plotted vs measured apparent DT $T_{\mathrm{ion}}$, with the same color coding as in Fig. 1). Error bars are statistical only; the systematic error is represented by the dotted curves compared to the dashed curve average for the framed data points. The solid black curve represents the expected birth-yield ratio (plotted vs $T_{\mathrm{thermal}})$ calculated using Bosch-Hale reactivities [38] assuming 50%:50% D:T; the dot-dashed black curve is the same for 60%:40% D:T.

Figure 3

$T_{\mathrm{ion}}$ isotropy data as a function of apparent DT $T_{\mathrm{ion}}$. The color coding is the same as in Fig. 1. (a) Observed variation in the DT apparent $T_{\mathrm{ion}}$ between the five available viewing directions, calculated as the rms between the individual data points and the average of all values for each shot. The vertical error bars represent the 68% confidence interval calculated by taking the square root of the ratio of the variance of the LOS $T_{\mathrm{ion}}$ measurements to the reduced ${\chi }^2$ for the number of degrees of freedom corresponding to the upper and lower probabilities defining the confidence interval; the gray region represents an estimate of the systematic uncertainty. (b) Same as (a) for the three available viewing directions for apparent DD $T_{\mathrm{ion}}$.