Diamond Capsules Show Near-Term Promise
Could diamonds be a NIF researcher’s best friend?
In a study of the relative advantages and disadvantages of the three ablator (target capsule) materials used in NIF inertial confinement fusion (ICF) experiments—high-density carbon (HDC), or diamond; glow discharge polymer plastic (CH); and beryllium—diamond emerged as the most promising ablator material to improve NIF’s performance, at least in the near term (see "How NIF Targets Work").
The study, which compared the three options in capsule-only simulations of current NIF experiments and potential future designs, was reported in a Physics of Plasmas paper published online on March 7. It looked at how the ablators responded to implosion perturbations caused by the fill tube used to inject deuterium-tritium fuel into the capsule, the support "tent" that suspends the capsule in the hohlraum, and the capsule’s surface roughness—both one at a time and in combination. The perturbations interfere with NIF implosion performance and degrade energy yields.
"According to the simulations," the authors said, "each ablator is impacted by the various perturbation sources differently, and each material poses unique challenges in the pursuit of ignition on NIF."
The simulations used the radiation-hydrodynamics code HYDRA and were run in both a post-shot mode to model past experiments and a pre-shot or predictive mode to assess the prospects of future designs intended to exploit the full power and energy of NIF. They predicted that the relative impacts of the different perturbation sources vary from post-shot models to extrapolated designs.
"For example," the researchers said, "the tent impacts CH most in the post-shot simulations but impacts beryllium most in the extrapolated designs. This variation underscores the important point that the ablator choice is not the single determinant of perturbation growth but that pulse shape and other design choices strongly influence the results. Indeed, selecting the optimal pulse shape is an essential choice in developing better performing designs."
The researchers also noted that "there are many ablator-specific features such as opacity inhomogeneities from contaminants, material strength effects, and crystalline micro-structure which are not addressed here but deserve attention in future work.
"Based on current understanding and the results described (in the paper)," they said, "the HDC ablator design looks best positioned to reach higher (energy) yields on NIF in the near term. In fact, implosion designs similar to the scaled-up HDC design…are the subject of active work aiming for higher yield in the very near future. The CH design looks similarly promising, although it absorbs less energy at a given scale than HDC and cannot be driven as efficiently.
"The future performance of the beryllium design looks most uncertain due principally to the higher surface roughness of current beryllium shells and inadequacies in current modeling," the researchers said. "However, the dataset of beryllium implosions on NIF is extremely limited and less effort has been devoted to improving capsule quality in recent years, so more experience will be needed before reliable predictions can be made."
Joining lead author Dan Clark on the paper were LLNL colleagues Annie Kritcher, Steve Haan, and Chris Weber along with Austin Yi and Alex Zylstra of Los Alamos National Laboratory.
