Processed NIF implosion radiographs taken when the imploding capsule shell is at about 200 microns. (a) target photograph displaying the capsule held in place by a supporting “tent”; (b) x-ray radiograph from an October 2013 shot showing the prolate (football-shaped) inflight shape of a 2D convergent ablator target driven by the low-foot (LF) laser pulse. The two horizontal scars are produced by the hydrodynamic interaction of the tent with the capsule material. (c) x-ray radiograph from an October 2014 shot driven by an adiabat-shaped (AS) pulse; both experiments were performed using the same nominal tent thickness of 45 nanometers. Note that the scarring is barely visible in the 2014 shot, strongly supporting the conclusion that the AS pulse mitigates the ablation-front instability.
(Download Image)‘Adiabat-shaped’ Pulse Design Improves NIF Implosions
In a Physics of Plasmas paper published online on Dec. 10, LLNL researchers reported on integrated hohlraum simulations and experiments showing that minimal changes in the picket drive of the NIF laser pulse can substantially improve the hydrodynamic stability of high-compression implosions.
This “adiabat-shaped” pulse is tailored to achieve resistance to ablation-front hydrodynamic instability growth, similar to the recent “high-foot” pulses which produced record neutron yields on NIF, but with a low fuel adiabat (internal capsule energy) to achieve high compression.
The adiabat-shaped pulse designs were evaluated in a dedicated NIF campaign that included all aspects of inertial confinement fusion implosions, from laser propagation and hohlraum x-ray conversion to capsule implosion and burn. “Keyhole” targets were used in preparatory tuning experiments to infer the implosion adiabat by performing shock-timing measurements. The hydro-growth radiography (HGR) platform was used to measure and verify the reduction in ablation-front instability growth comparable to that achieved in the high-foot experiments. Finally, low-mode shape distortions were measured through in-flight 2D radiography experiments.
This series was followed by a layered DT implosion experiment that demonstrated a significant improvement in yield over all low-foot shots. The new pulse designs produced a factor of 3-10x improvement in the neutron yield (>40% of predicted 2D simulated yield) over similar implosions conducted during the National Ignition Campaign, the researchers said, while maintaining a relatively high fuel compression of >1g/cm2. There are a number of other pulse-shaping technique ideas that can now be explored to further improve the stability of high convergence implosions.
Lead author Jose Milovich was joined on the paper by LLNL colleagues Harry Robey, Dan Clark, Kevin Baker, Dan Casey, Charlie Cerjan, John Field, Andy MacPhee, Arthur Pak, Prav Patel, Luc Peterson, Vladimir Smalyuk, and Chris Weber.
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