Measurements of the temperature and velocity of the dense fuel layer in inertial confinement fusion experiments

The apparent ion temperature and mean velocity of the dense deuterium tritium fuel layer of an inertial confinement fusion target near peak compression have been measured using backscatter neutron spectroscopy. The average isotropic residual kinetic energy of the dense deuterium tritium fuel is estimated using the mean velocity measurement to be $\sim 103$ J across an ensemble of experiments. The apparent ion-temperature measurements from high-implosion velocity experiments are larger than expected from radiation-hydrodynamic simulations and are consistent with enhanced levels of shell decompression. These results suggest that high-mode instabilities may saturate the scaling of implosion performance with the implosion velocity for laser-direct-drive implosions.

Figure 1

(a) A schematic of neutron scattering in an ICF target near peak compression that consists of a central low-density, high-temperature hot spot (HS, red), surrounded by a low-temperature, high-density shocked shell (SS, blue), which is surrounded by a low-density and temperature free-falling shell (FS, light blue) which is still imploding radially inward. A rebounding shock (dashed line) separates the shocked shell and free-falling material and propagates radially outward. (b) An example of the neutron tritium backscatter kinematics assuming the triton has 1 keV of kinetic energy and moves toward (blue solid arrow) or away (red dashed arrow) from the incident neutron. (c) Two nT edge neutron time-of-flight signals (shaded curves) from OMEGA implosions along with the forward fit (solid and dashed lines). (d) The unfolded neutron energy spectra used to forward fit the data in (c), along with the best fit values for the mean $\overline{v}$ and standard deviation ${\mathrm{\Delta }}_v$ of the triton velocity distribution used in the forward fit. The signals in (c) and (d) have been normalized to the primary DT yield to highlight the shape differences.

Figure 2

(a) The measured (points) and lilac calculated (lines) values for ${\mathrm{\Delta }}_v$ (blue points and solid line) and $\overline{v}$ (orange squares and dashed line) as functions of the calculated peak implosion velocity. (b) The lilac calculated scattering fraction occurring in the hot spot (red), shocked shell (dark blue), and free-falling shell (green) as functions of the calculated peak implosion velocity.

Figure 3

(a) The hot-spot (upper blue points and lines) and shell (lower orange squares and lines) ion temperature as a function of the peak implosion velocity. The calculated thermal ion temperature (dashed lines) and apparent ion temperature (solid lines) are calculated using lilac while the measured apparent ion temperature (points and squares) are inferred from the nTOF spectrum. (b) The relationship between the implosion stability metric $\eta$ and the peak implosion velocity.

Figure 4

The ratio between the measured and simulated values for various physics parameters a function of the implosion stability metric $\eta$. The dashed lines are linear fit for each parameter.