This colorized image of a NIF “Bigfoot” deuterium-tritium (DT) implosion was taken on Feb. 7, 2016. This shot was the Inertial Confinement Fusion program’s first layered DT fusion implosion using the Bigfoot strategy in a sub-scale high density carbon (diamond) ablator. The Bigfoot platform uses a shortened three-shock pulse and a thinner DT ice layer that puts the fuel and the diamond ablator on a higher adiabat (resistance to compression) than previous designs. The 5.75-millimeter-diameter hohlraum used a low gas fill (0.3 mg/cc) to limit laser-plasma instability and cross-beam energy transfer. Credit: Don Jedlovec.
(Download Image)‘Bigfoot’ Prowls the NIF Target Chamber
First there was Lowfoot. Then came Highfoot. And now LLNL researchers are conducting NIF experiments using Bigfoot—not the fabled Sasquatch of the Pacific Northwest, but an experimental platform designed to control implosion symmetry and hydrodynamic instabilities, improve predictability, and enhance the delivery of laser energy to NIF’s targets.
The campaign combines a short, six-to-seven-nanosecond (billionths of a second), high-energy laser pulse with a strong initial shock, or foot, (hence the name "Bigfoot") to drive a high-velocity target capsule implosion. The revised pulse shape increases the fusion fuel’s adiabat, or resistance to compression, but the researchers believe the lack of compression can be offset by higher velocity.
The campaign also provides a better understanding of the trade-offs that stand in the way of improved implosion performance.
The Bigfoot approach "is similar in strategy to the Highfoot, but a more extreme version," said Omar Hurricane, chief scientist for LLNL’s Inertial Confinement Fusion (ICF) Program (see "Ignition Experiments"). "You back off of compression to try to get a more controlled implosion, and it gives you a chance to be closer to your model expectation" than in previous experiments.
"We can make this thing just scream—it can go very, very, very fast."–Brian Spears
"All the other (implosion) parameters matter, but the velocity matters a lot more than anything else," he said. "A little bit more velocity buys you a lot in performance." Higher velocity means higher temperatures, and the Bigfoot campaign "has accessed some of the highest temperatures and the highest velocities of any ICF implosions to date, on the order of 420 to 430 kilometers per second. Velocity also buys you compression—your ability to densify the implosion.
"What the Lowfoot (campaign) was trying to do with lower velocity and low adiabat, the Bigfoot team is trying to do with higher velocity and higher adiabat," Hurricane said, "but the ratio of the two in the Bigfoot is better. It’s been a great success."
To reach ignition conditions, implosions need to achieve high peak implosion velocity, good energy coupling between the imploding capsule shell and central hot spot, and high assembled fuel areal density, or compression, at the point when the implosion stagnates. "Achieving these simultaneously is extremely challenging," said Dan Casey, the Bigfoot experimental campaign’s lead researcher, "partly because of inherent tradeoffs between these three coupled requirements."
To address this challenge, while also further refining the computer models that simulate NIF experiments, Bigfoot shots try to take advantage of the benefits of predictability, control, and energy coupling. "In this approach we attempt to minimize uncertainty," said Cliff Thomas, the Bigfoot design campaign lead, "and instead, seek a target that is predictable."
Casey said a key to the success of the campaign to date has been the platform’s ability to take full advantage of NIF’s laser energy. "The power of operating in a place that’s predictable," he said, "is that you can design a system that can optimally use NIF. That’s how we’ve been able to get all the way to 500 terawatts (of peak power)—by having a model of the hohlraum that has worked, and engineering the system such that inner and outer (laser) cones can be at the same power, maxed out at 500 terawatts. There’s a trade-off in engineering a system that doesn’t get to as high a compression, but you gain back this ability to design an experiment that tests just what you want."
Another feature of the Bigfoot campaign is the continued progress toward near-record energy yields demonstrated over the last few months.
"With a strong predictive hohlraum model and accumulated knowledge from our previous campaigns, and a new (design) concept that is a little bit more conservative," said Brian Spears, a member of the experimental design team, "we were able to increase the yield by a factor of 100," in a chronological sequence of eight shots using deuterium-tritium fuel.
"Our theories and simulation codes tell us how to trade off competing effects" such as velocity and compressibility, Spears said. "But our best predictions in the past have said that we should have as low an adiabat or as compressible an implosion as we can, and we should give up velocity to get there.
"So now we’re going to allow this target to be less compressible, and the tradeoff that we get back for it is that we can make this thing just scream—it can go very, very, very fast. And it looks like we win more in that trade than our simulation codes predict.
"If in that progression from Lowfoot to Highfoot to Bigfoot, there’s something going on that’s different in reality than in our codes, we can learn that way. We can go back and modify our codes to represent what’s going on in the experiment, or adjust our experiment to put ourselves in a regime that looks more like our codes. Finding things we don’t expect is how we’re going to actually get better."
"Does the system behave the way we want, or is reality a little different?" Thomas added. "If we’re successful in this approach, we should end up with the information that will guide us to doing even better."
