Diagnostic signatures of performance degrading perturbations in inertial confinement fusion implosions

Abstract

We present 3D radiation-hydrodynamics simulations of indirect-drive inertial confinement fusion experiments performed at the National Ignition Facility (NIF). The simulations are carried out on two shots from different NIF experimental campaigns: N130927 from the high foot series and N161023 from the ongoing high density carbon series. Applying representative perturbation sources from each implosion, synthetic nuclear diagnostics are used to post-process the simulations to infer the stagnation parameters. The underlying physical mechanisms that produce the observed signatures are then explored. We find that the radiation asymmetry and tent scar perturbations extend the nuclear burn width; this is due to an asymmetric stagnation of the shell that causes the delivery of mechanical PdV work to be extended compared to an idealised implosion. Radiation asymmetries seed directed flow patterns that can result in a difference in the inferred ion temperature ranging from 80 eV to 230 eV depending on the magnitude and orientation of the asymmetry considered in the simulation; the tent scar shows no such temperature difference. For N130927, radiation asymmetries dominate the yield and inferred ion temperature and the tent scar has the largest influence on the neutron burnwidth. For N161023, the fill tube decreases the burn width by injecting mix into the hot spot, leading to a smaller hot spot and increased energy losses. Both the radiation asymmetry and the fill tube generate directed flows that lead to an anisotropic inferred temperature distribution. Through existing and novel synthetic neutron imaging techniques, we can observe the hot spot and shell shape to a degree that accurately captures the perturbations present.

Perturbation modifications by pre-magnetisation of inertial confinement fusion implosions

“…1 The simulations in this paper build upon the work by Perkins et al by using a perturbation source (time-dependent radiation asymmetry) that was expected to be the dominant yield degradation mechanism for the high-foot shot simulated (N130927). 14,15 While the yield in this case is found to double by the application of a 50 T magnetic field, the amplification factor is expected to be highly dependent on the initial unmagnetised implosion performance. As Perkins et al demonstrated, the amplification factor for current experiments is greater if the unmagnetised implosion yield is higher.…”

“…Capsule-only simulations are performed using the 3-D radiation extended-magnetohydrodynamics code Gorgon, 11,17 which uses the same framework as in the Chimera code. 14 The radiation transport is non-diffusive, using P 1/3 automatic flux limiting. Full radiation transport is only used during the 2-D drive-phase simulations, while an optically thin radiative loss model is used here for the 3-D capsule stagnation phase to lower computation time.…”

“…The grid resolution at this stage is 1 lm radially and 1 in the polar direction. The prescribed 2-D time-dependent radiation drive asymmetry 14 is incorporated from the beginning of the drivephase. Magnetic field effects on the hohlraum (which can affect the radiation asymmetry 27,28 ) are not considered here.…”

Burn regimes in the hydrodynamic scaling of perturbed inertial confinement fusion hotspots

“…mass redistribution from the perturbation [24] causing larger momentum differences around the shell than for MM. Higher P dV power coincides with better heating of the hotspot.…”

“…The radiation transport algorithm uses 54 non-uniform radiation groups, chosen to provide sufficient resolution around features such as the carbon K-edge [24]. These 1D simulations use a frequency-dependent x-ray drive spectrum, including the m-band component of the radiation flux [24]. Figure 1 shows the time-evolution of a self-heating capsule, a scale 0.9 version of N130927.…”