A marriage of the world’s highest-energy lasers and a century-old technology is now producing new x-ray imaging data for NIF researchers. It’s called the NIF Survey Spectrometer, or NSS.
The NSS is an ultrasensitive x-ray spectroscopy device deployed in the facility in May following a five-year development campaign.
Mark May, an experimentalist with LLNL’s Weapons and Complex Integration Directorate, so eagerly awaited the new spectrometer that he came in on a Sunday night to observe its “first light.”
“There was a lot of anticipation for this,” he recalled as he watched technicians retrieve the shiny aluminum image plate module, about the size of a football, from the NSS perched high on the NIF Target Chamber. Next, they removed the image plate recording media—the modern equivalent of film negatives from Kodak box cameras used more than 100 years ago. Then they scanned the media for the results, a process similar to waiting for old Polaroid Instamatic negatives to be exposed to light.
And those first results were…?
…“Excellent,” May declares. “It’s got great sensitivity and resolution in a large spectral range. The beauty of the NSS is that it minimizes all the background (interference). Despite the bright target blowing up in the center of NIF, the data don’t have extraneous backgrounds.”
Physicist Marilyn Schneider of the Physics and Life Sciences Directorate also waited years for the NIF Survey Spectrometer, and she becomes animated when talking about its range of capabilities. A spectrometer can “see” specific areas of the color spectrum diffracted through a crystal, Schneider says. The NSS sees high-energy x rays from 6,000 electron volts (6 keV) to 150 keV using four quartz crystals in transmission geometry.
The spectral range of the NSS includes the gold L-band—x rays produced when the NIF laser deposits its energy on the inside surface of a gold hohlraum. “The exact spectral region and intensity of the L-band x rays tells us much about heating and cooling mechanisms in this hot gold plasma,” says Schneider.
Because it sits on a fixed port in the Target Chamber instead of being mounted and dismounted depending on the experiment, as was the original plan, the NSS can collect data on every NIF shot, which Schneider considers a great bonus.
The NSS is just one of dozens of precision diagnostics used in the 10-meter-diameter Target Chamber, but it arrives with a history and a persistent fan base.
LLNL engineer Perry Bell traces the longtime interest in developing the spectrometer back to 2002, when he was part of a group that built a survey spectrometer originally intended for NIF through the U.S. Naval Research Laboratory. The device went to the Omega Laser Facility at the University of Rochester for testing and calibration, and by the time NIF construction was completed it had found a permanent home in Rochester.
In 2012, the effort for a NIF-dedicated survey spectrometer restarted, with Bell working with some of the same engineers who are now part of a private company named Artep Inc., in Ellicott City, Maryland. Specifications for the instrument, however, changed course, design, and focus along the way.
The basic concept of the NSS is also basic photography, explains May. The media is a single-frame image, photographing the entire three-nanosecond NIF shot with basically a crystal, a filter, and a piece of modern photo media, but adapted to work in one of the world’s most advanced laser facilities to capture x-ray images with incredible precision.
While based on classic photography principles, the NSS is a curved-crystal transmission spectrometer, utilizing tungsten front aperture plates and an assembly that block x rays from sources other than the desired target and backlighters. One of the benefits of this diagnostic is that different crystals can be swapped in to tailor the energy range that is measured depending on the needs of the experiment.
As the light is transmitted from the crystals to the image plate, it passes through the tungsten cross-over slit. The slit plates further block the signal from background x-rays. A series of filters located in front of the slit allow specific energies to be filtered out. A light shield protects the image plate recording media from being erased by ambient light as the operators swap it out.
In this type of spectrometer, the incoming x rays hit the crystals and are spatially separated to focus on different parts of the image plate depending on their energy. The x rays that hit the top crystal are recorded on the bottom of the image place, and vice versa for the bottom crystals. This means that the signal has to cross over from the bottom to the top. The slit is located where the crossover occurs, allowing the desired signal to pass through to the image plate while background “noise” x rays that would otherwise pass straight through the diagnostic are rejected.
Bell says of the final product now on NIF, “One of the nice things about the spectrometer is it uses a cross-over aperture. You don’t get a lot of background from your source because it has this nice tungsten block that blocks out any radiation that might get into your image (and dilute the clarity).”
Since the NSS arrived at NIF last fall, engineer Nathaniel Thompson has been fielding the instrument, finding a way to squeeze the new diagnostic into the tight fit on its current perch and getting it operational. “The NSS is like a Swiss Army knife,” he says, pointing to the instrument high on the Target Chamber. “It offers a large field of view, high resolution, and a lot of options for configurability; it can do some great things up there.”
NIF experiments support the National Nuclear Security Administration (NNSA)’s Stockpile Stewardship Program to ensure the safety, security and reliability of the nation’s nuclear deterrent, while also providing scientists from around the world with unique conditions of temperature and pressure for fundamental science studies.