Taming the ‘Bubble’ for Symmetric Implosions with Full NIF Energy

Researchers from LLNL and Los Alamos National Laboratory (LANL) have developed a new model to guide the design of NIF experiments aimed at minimizing the effects of the hohlraum "bubble" that contributes to target capsule asymmetries and interferes with NIF implosion performance.

The bubble results when NIF’s intense outer-cone laser beams hit the high-Z (gold or uranium) hohlraum wall, causing the wall to heat up and expand inward toward the hohlraum axis. The bubble robs energy from the inner-cone beams, causing an oblate, or pancake-shaped, implosion (see "Improving the Energy Balance in NIF Hohlraums").

In a Physics of Plasmas paper published online on March 20, the researchers noted that implosion symmetry depends on controlling the balance between laser energy deposited at the midplane or "waist" of the hohlraum and the laser energy deposited near the ends of the hohlraum, close to the laser entrance holes. Symmetry control is lost when the inner beams can no longer reach the hohlraum waist.

While it’s possible to compensate for the energy loss caused by the bubble by increasing the power of the inner beams relative to the outer beams, they said, "this comes at a cost because the NIF laser is designed for ‘33% cone fraction’—1/3 of the beams are inner beams and 2/3 are outer beams. Anytime we operate at a cone fraction that is not 33%, we reduce the total amount of power and energy that can be delivered to the hohlraum.

"In order to maximize the neutron yield," they said, "we want to symmetrically drive the largest capsule that we can on the NIF to high velocity by using all of NIF’s power and energy (up to 500 terawatts of power and 1.8 megajoules of energy)."

The new "data-based" implosion model is based on the hypothesis that implosion symmetry becomes more oblate as the high-Z bubble size becomes large compared to the hohlraum radius, or the capsule size becomes large compared to the hohlraum radius. Using the model, researchers can design targets for NIF inertial confinement fusion experiments by taking into consideration the constraints from both the capsule and the hohlraum together from the outset.

"Previous designs were generally done by optimizing the capsule by using a radiation drive source and then designing a hohlraum to produce that radiation drive source," the researchers said. "The capsule and the hohlraum were then iterated to find a design. Since we believe that hohlraum drive asymmetry has been one of the main sources of yield degradation, optimizing the design including the hohlraum from the beginning seems a promising path.

"We can use this data-based model to form the basis for new target designs for the NIF with the goal of symmetrically driving the largest capsule that we can with the NIF laser at near 33% cone fraction," they said. "Since the capsule yield and the stagnation pressure are strong functions of the implosion velocity and the capsule radius, we want designs that are symmetric near NIF’s optimal 33% cone fraction, so that we can get the maximum amount of laser power and energy out of the laser. We plan to start testing designs using this methodology this year."

Lead author Debbie Callahan was joined on the paper by LLNL colleagues Omar Hurricane, Joseph Ralph, Cliff Thomas, Kevin Baker, Robin Benedetti, Laura Berzak Hopkins, Dan Casey, Tom Chapman, Chris Czajka, Eddie Dewald, Laurent Divol, Tilo Döppner, Denise Hinkel, Matthias Hohenberger, Charlie Jarrott, Shahab Khan, Andrea Kritcher, Nino Landen, Sebastian LePape, Steve MacLaren, Laurent Masse, Nathan Meezan, Art Pak, Jay Salmonson, Tod Woods, Nobuhiko Izumi, Tammy Ma, Derek Mariscal, and Sabrina Nagel, along with John Kline, George Kyrala, Eric Loomis, Austin Yi, Alex Zylstra, and Steve Batha from LANL.