Schematic of a NIF ignition hohlraum. The fuel capsule is placed inside a gold or other high-Z (high atomic number) cylinder called the hohlraum. The laser beams enter the target at the ends of the cylinder through laser entrance holes and strike the inside of the hohlraum to generate x rays. The laser beams are arranged in two cones—an inner cone that is pointed toward the waist of the hohlraum and an outer cone that is pointed near the ends. The two cones are independently controlled to help achieve a symmetric implosion.
(Download Image)Ignition: Improving Efficiency with Low-Fill Hohlraums
Third in a series of articles exploring the progress and challenges of LLNL’s Inertial Confinement Fusion (ICF) Program.
Article 2—"Taming Symmetry With the 2Shock"
During a National Ignition Facility experiment, or "shot," NIF’s 192 laser beams inject millions of joules of laser energy into a small eraser-sized cylindrical gold-lined container called a hohlraum which contains a fusion target capsule at its center.
Controlling what happens inside this hohlraum is an important step on the path to generating fusion ignition and gain in the laboratory, and an LLNL team is focused on just that.
When the laser beams enter the hohlraum and strike its walls, the laser energy rapidly turns the hohlraum into an x-ray oven. These x rays bathe the target, causing the capsule shell to vaporize and drive the fuel inward at nearly 400 kilometers a second, with the goal of getting the center of the implosion to reach temperatures of more than 100 million degrees Celsius and pressures that exceed 100 billion times Earth’s atmosphere.
In order to drive the implosion symmetrically, the placement of and relative energies in the inner and outer cone laser beams have to be carefully balanced. This is complicated, however, by the fact that as the lasers hit the inside wall of the hohlraum, the walls turn into an expanding plasma, impeding the laser beam path and adversely affecting the x-ray drive symmetry—a necessary condition for ignition.
To combat this plasma, experiments during the 2006-2012 National Ignition Campaign (NIC) and the current "high-foot" campaign used hohlraums with a high helium gas fill to hold back this plasma expansion. While the high-foot experiments were the first to achieve fuel gain and alpha heating, they also showed that the high-gas-fill approach was compromised by high levels of laser plasma instabilities (LPI) that steal usable laser energy and cause drive asymmetries.
This led the program to pursue an alternate approach using hohlraums with a low gas fill to lower the LPI. The results have been promising—significantly reduced LPI, 20 to 25 percent efficiency improvement in the conversion from laser energy to x rays, and a much closer match between models and experimental results.
Low-fill hohlraums come with their own set of challenges—primarily, expansion of the hohlraum walls is less restricted by the gas fill. To combat this, the program decided to shorten the laser pulse, shaving it to 6 to 7 nanoseconds, less than half the length of pulses used in the high-foot campaign.
A change in capsule material is one way to move to shorter pulses. This approach uses targets made from high density carbon (HDC), or diamond, which has a density three times that of the plastic used in previous campaigns. This results in a much thinner capsule shell, important because pulse length is primarily determined by the time it takes for the first shock wave to travel through the capsule.
"It’s a race against time," said Sebastian Le Pape, experimental lead for the HDC campaign. "At some point, your hohlraum is going to fill with gold, but if you can drive your implosion with short enough laser pulses before it closes, you’ll be fine. That’s what HDC allows you to do, and the advantage of low gas fill means that you don’t have LPI and aren’t using energy to heat up the gas. This combination is a more efficient platform."
A perennial concern for ignition is implosion symmetry—absolutely critical for ignition. The combination of low-fill hohlraums and HDC is paying dividends in this area. "Based on the measurement techniques that we have, we believe that these implosions are driven pretty symmetrically through the entire pulse," said Debbie Callahan, associate division leader for ICF in LLNL’s Design Physics division, and deputy program lead for Integrated Experiments.
"A long-term concern, even prior to the NIC, was that it would be impossible to control the symmetry in the hohlraum," Callahan said. "Symmetry is a big challenge, and it’s a major accomplishment that we’ve been able to do that."
In addition, a more symmetric implosion is more efficient, and that is showing in the yield. "While our total yield hasn’t gone up, our yield per unit laser energy has," Callahan noted. "We’re getting the same yield, but with less laser energy. What that says is that we’ve improved the quality of the implosion, converting the energy into yield in a more efficient way."
Now that the implosions are driven more symmetrically, the team has been able to get a clearer look at engineering features that impact the implosion, such as the fill tube used to fill the capsule with deuterium-tritium fuel and the capsule support that holds the capsule in place in the hohlraum. The program is now working on reducing the impact of those components, and this will be explored in a future article in this series.
Looking ahead, the program now plans to focus on applying what has been learned to further improve target designs.
"We’ve progressed an amazing amount, building on the original NIC efforts," said Laura Berzak Hopkins, design lead for the HDC Campaign. "We’ve been able to nearly remove backscatter as a source of energy loss and potential laser damage. We’ve been able to work with platforms where the code is more predictive of drive and where the hohlraum reaches higher temperatures with the same amount of laser energy.
"And in the past year," she said, "we’ve been able to predictably design and measure implosions in which we can control symmetry much better. What we hope to do now is to use what we’ve learned to try to make further improvements to target performance."
