Probing the Mysteries of Hohlraum Plasmas

Information that will literally shed new light on the dynamics of NIF fusion implosions will soon be available to researchers, thanks to an important diagnostic now under development.

When NIF’s laser beams enter a hohlraum, they generate an electrically charged plasma—a turbulent mix of ions and free electrons (see "Targets"). The plasma interacts with the laser beams and causes backscatter that can drain laser energy needed to symmetrically implode the fuel capsule. Gaining a better understanding of how laser-plasma interactions affect the hohlraum’s ability to successfully implode a target capsule could be a key to improving NIF’s implosion performance during the coming months.

Scientists have known for decades that one of the best ways to measure the characteristics of a plasma, such as its temperature and density, is with a diagnostic technique called optical Thomson scattering (OTS). Shining a strong laser beam called a "probe" into a plasma stimulates the electrons to oscillate and produce radiation in the form of visible or ultraviolet light that can be measured by the OTS diagnostic.

The resulting data will be critical to understanding many aspects of hohlraum dynamics, including the transfer of energy when laser beams cross, the motion of the hohlraum walls, and the mixing of the hohlraum wall material and fill gas—all of which can significantly affect symmetry control and implosion performance. The diagnostic will also be used for direct-drive experiments, where Thomson scattering can probe the plasma conditions in the corona to validate laser coupling and thermal transport modeling.

"Thomson scattering is one of the very few diagnostics that gives us a very detailed, localized measurement," said physicist Steven Ross. "We have a lot of other diagnostics that can look at very integrated parameters, but Thomson scattering is really a measurement at one place. That’s great for comparing experimental results to the models, because it’s very easy to tell how well they’re doing at modeling the target—or how not well they’re doing."

Ross said the biggest uncertainties in modeling hohlraum dynamics center on hydrodynamic instabilities and plasma conditions. "We think we have a reasonably good understanding of the laser-plasma interaction processes, but modeling the exact plasma temperature and density tends to be what the codes have the most trouble with."

The OTS diagnostic can peer into the laser entrance hole regions, where the lasers enter the target, and "tell us if we’re getting the temperature right, if we’re getting the plasma flow velocity right, if we’re getting the density right," Ross said. "We have very detailed models on cross-beam energy transfer, but those are all predicated on understanding the plasma conditions. Verifying that we know the plasma conditions will help us better plan experiments going forward."

And if improved understanding of hohlraum characteristics leads to improved models, "that will in turn allow us to improve the targets," Ross said. "It’s this kind of continuous loop of modeling and experiment, to understand what’s going on and how can we make it better, that we need to carry out with our experiments and our targets."

The OTS diagnostic has had to overcome a number of tough scientific and engineering challenges to reach its current stage of development. "We had to find the right approach that would have the highest probability of success without having to build a complete new NIF beamline-scope project," Ross said.

The result is the combination of a sophisticated multi-part optical detector in a diagnostic instrument manipulator (DIM), and a small-scale laser mounted directly on the NIF Target Chamber. The detector, which has already been deployed on NIF experiments, can be used stand-alone at the ultraviolet (351-nanometer) wavelength of the NIF beamlines known as three-omega (3ω). When the OTS laser is completed, it can be tuned to the deep-ultraviolet (5ω) portion of the electromagnetic spectrum, with wavelengths ranging from 185 to 215 nanometers, for integrated Thomson scattering tests.

"The five-omega laser was chosen for a particular reason," said Phil Datte, the project’s lead engineer. "The NIF system has three colors in the Target Chamber which come from the main laser system—one omega (infrared), two omega (green), and three omega (ultraviolet)—and the unconverted light has a large background signal. The noise band, or the background band, is very strong in the two-omega and the three-omega range, but it falls off down to the 200-nanometer range."

Because the five-omega wavelength is not in the same band as "all the other wavelengths that are flying around in the chamber," he said, "it will allow us to have a better signal-to-noise ratio." Datte noted that overcoming the background radiation at higher wavelengths would require a much more powerful laser, which could heat the plasma and distort the OTS measurements.

In a recent series of experiments, the detector obtained good measurements of hohlraum background radiation, which will be used to determine how energetic the probe laser needs to be for the OTS diagnostic to obtain usable data from the low-signal-level Thompson scattering light.

One significant challenge facing the development team was dealing with the intense flux of x rays produced when the laser beams strike the inside surface of the hohlraum. These x rays heat the surface of the target capsule and drive the capsule’s implosion.

"When the target is emitting radiation in the x-ray band, and you have a window in your system, it makes that window become opaque" to OTS light, Datte said. "We’ve come up with a way to shield the x rays and allow the visible light, the light we’re interested in measuring, to pass through."

Under the guidance of experimental physicist George Swadling, the team successfully tested their concept for mitigating x-ray "blanking" on the OMEGA Laser at the University of Rochester. They placed a diagnostic snout filled with pressurized xenon gas between the hohlraum and the OTS diagnostic. "The x rays are absorbed by the xenon and never make it to the window," Datte said, "but the xenon is transparent to the visible light and the UV light that we measure.

"This is one of the first diagnostics where we’re actually inside the Target Chamber, close to the target, where we’re collecting data in the visible band and the UV band. Historically that was almost impossible to do at these x-ray fluxes. With this anti-blanking concept we’ve broken that barrier, and we’re able to collect measurements in that wavelength band without the interference of the x rays blanking out your system. That’s a really powerful mechanism that will open up the range of uses for these types of diagnostics."

Ross said the diagnostic has the potential to characterize plasmas for other NIF platforms and Discovery Science experimental campaigns. The team plans to demonstrate the mitigation of x-ray blanking on NIF when experiments resume in April for additional calibration of the OTS detector.

Among the other challenges faced by the diagnostic's development team are designing the architecture for precisely aligning the detector and the laser beam inside the Target Chamber, and obtaining large-scale CLBO (cesium-lithium-boron-oxygen) nonlinear crystals, which convert the laser light to 5ω, that can withstand the intense ultraviolet energy of the laser beams.

Both Ross and Datte expressed confidence that the project is proceeding according to plan. "There’s definitely work to be done," Ross said. "It’s incredibly challenging; unfortunately, we don’t spend a lot of time working on easy problems. But so far, the measurements have been very, very nice and there’s a very optimistic outlook for the project as a whole."

"We’re really excited about the performance of this device," added Datte. "I’m sure we’ll have some growing pains, but this thing is really behaving very nicely."

Joining Ross and Swadling on the OTS physics team are John Moody, Laurent Divol, Nino Landen and Pierre Michel of LLNL; Dustin Froula, Joe Katz, Chuck Sorce and David Turnbull of the Laboratory for Laser Energetics at the University of Rochester; David Montgomery and John Kline of Los Alamos National Laboratory; Jim Weaver of the Naval Research Laboratory; Siegfried Glenzer of the SLAC National Accelerator Laboratory; and Wojciech Rozmus of the Unversity of Alberta.

Along with Datte, members of the LLNL engineering/operations team are Jason Beagle, Ron Bettencourt, Mike Borden, Kelly Burns, Mike Fedorov, Gene Frieders, Justin Galbraith, Mike Hardy, Ben Hatch, Sukhdeep Heerey, Ray Iaea, Glen James, Steve Kramer, Brandi Lechleiter, Tony Lee, Stacie Manuel, Warren Massey, Tom Mccarville, Bill Molander, Kevin Person, Brad Petre, Mike Rayce, Mai Thao, Gene Vergel de Dios, Mike Vitalich, Scott Wilcox, Reg Wood, Tony Golod, and Valier Pacheu.