Meteorologists need to keep an eye on atmospheric temperature fluctuations to predict the strength and direction of tropical storms and hurricanes, as well as everyday weather. In the same way, LLNL inertial confinement fusion (ICF) researchers want to know how heat is distributed and moves through a NIF hohlraum to better model and improve NIF implosions.
Hohlraum performance is a key factor in NIF’s ability to achieve ignition (see “How NIF Targets Work”). Understanding electron heat flow in a hohlraum’s plasma can provide researchers with a number of key insights into the factors that affect how well the hohlraum drives the target capsule, such as kinetic effects and self-generated magnetic fields.
Energy flow is important, said LLNL physicist John Moody, because “the temperature affects how the laser beams propagate; colder plasmas will tend to absorb laser light, and hotter plasmas absorb it less.
“Understanding the temperature distribution inside the hohlraum and how that evolves in time,” he said, “gives us a good understanding of how the laser beams actually move within the hohlraum as they go to the wall and deposit their energy—how efficient that whole process is.” This effects the efficiency with which x-rays are produced and therefore how efficiently and symmetrically the capsule is driven.
Moody and his colleagues utilized a novel technique to determine hohlraum temperature and plasma flow by placing spectroscopic tracer “dots” on a thin film inside the hohlraum and on the surface of the target capsule. The spectroscopic measurements are used to create a low-resolution temperature “map” of hohlraum plasma conditions, providing the first multi-location tests of particle and energy transport physics in a laser-driven x-ray cavity such as a hohlraum.
The results of the three-year effort to develop the dot-spectroscopy experimental platform were reported in a Physical Review Letters paper published online on August 31. The paper discusses the role of kinetic effects, self-generated magnetic fields, and instabilities in causing spatially dependent heat transport in the hohlraum.
“Predictive capability is a big effort in the ICF program,” Moody said. “One aspect of this involves trying to calculate the behavior of the hohlraum.
“Much about those calculations hasn’t yet been tested,” he said. “Finding a way to test these models is important because it gives us insight into some of the limitations in the designs and shows us where we need to make adjustments and improvements. These measurements are providing some of the most constraining tests of our modeling capability inside NIF ignition hohlraums.”
The researchers said the experimental data is consistent with classical heat flow near the target capsule, but reduced heat flow near the laser entrance hole.
Moody credited the success of the experiments to a collaboration involving experimentalists—led by former LLNL physicist Maria Barrios, lead author of the Physical Review Letters paper, along with Lab physicists Marilyn Schneider, Duane Liedahl, Larry Suter, Oggie Jones, and Mark Sherlock—and the target fabrication experts who created the specialized targets used in the experiments, especially Target Engineer Steve Johnson of LLNL and Target Designer Scott Vonhof of General Atomics.
“One of the key things that made it possible was (the target fabricators’) willingness and ability to help us develop these targets,” Moody said. “There was a lot of trying things and looking at the data and saying, ‘Let’s make this change,’ and Target Fab would help us with that.” He also credited LLNL physicists Nino Landen and Bob Kauffman with overseeing the experiments and “guiding us to keep us on track.”
Contributing to the paper along with Barrios, Moody, Schneider, Liedahl, Suter, Jones, Sherlock, Landen, and Kauffman were Hui Chen, William Farmer, Joe Kilkenny, Jeremy Kroll, Steve Maclaren, Nathan Meezan, Abbas Nikroo, Daniel Thorn, and Klaus Widmann of LLNL; Javier Jaquez of General Atomics; and Gabriel Pérez Callejo of the Clarendon Laboratory at the University of Oxford in the UK.