The MIT spectroscopy tech behind the NIF fusion triumph
In December 2022, researchers in the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory achieved the greatest breakthrough in the long, frustrating history of nuclear fusion research. For the first time, an inertial confinement fusion (ICF) experiment produced more energy than was delivered (3.15 megajoules from 2.05 MJ). Since then, output has increased to almost 4MJ.
Thousands of researchers and engineers contributed to the success, which extended the hope that by duplicating the energy process that powers the sun we may someday create a powerful energy source that is safe, continuous, and carbon free. Yet, the landmark may not have happened without the contributions of the High Energy Density Physics (HEDP) group at MIT’s Plasma Science and Fusion Center (PSFC). The HEDP group develops, oversees, and analyzes output from fusion reactions using a large magnetic recoil neutron spectrometer (MRS), which together with other neutron spectrometers provides key information about the performance of ICF implosions. By measuring the emitted particles, the HEDP group has helped to improve laser targeting to increase gain.
“Over the 13 years we have been working to achieve ignition at NIF, we needed to understand every detail of what was going on in these tiny implosions before we could improve them,” says Maria Gatu Johnson, a principal research scientist in the HEDP group and the leader of several diagnostics projects at the NIF. “Our diagnostics have been essential to making that progress. We are now developing new techniques to improve the measurements and help advance fusion toward commercial viability.
Over the 13 years we have been working to achieve ignition at NIF, we needed to understand every detail of what was going on in these tiny implosions before we could improve them.
Gatu Johnson runs neutron spectrometers at both the NIF and the OMEGA Laser Facility for ICF at the Laboratory for Laser Energetics at the University of Rochester. “We're continuously developing new diagnostic ideas, installing them at the facilities, and processing and analyzing the data from the experiments,” says Gatu Johnson, who was recently elected an American Physical Society Division of Plasma Physics Fellow. “In addition, we conduct basic science experiments with the ICF platform and our developed diagnostics.”
Gatu Johnson also manages the PSFC-HEDP linear electrostatic ion accelerator lab for diagnostic development while finding time to pursue her own nuclear astrophysics research. As excited as Gatu Johnson is about fusion research, she is even more passionate about the potential for ICF and neutron spectrometry to help answer some of the great questions of astrophysics (see farther below).
The principal spectrometry system used by the MIT HEDP group in the NIF fusion experiments uses Magnetic Recoil Spectrometry (MRS) technology. The MRS concept was developed in the 1990s at Sweden’s Uppsala University for the Joint European Torus (JET) tokamak project in the UK by researchers led by MIT PSFC’s current HEDP group leader, Johan Frenje. Gatu Johnson followed Frenje from Uppsala to the UK and then in 2010 to MIT where she leads MRS applications and development.
The MRS concept required some modifications to transition from monitoring the magnetic confinement fusion (MCF) at JET to ICF. MCF uses magnets to generate a donut-shaped plasma for running continuous fusion reactions within a plasma over a period of seconds, whereas ICF uses lasers to generate higher power from intermittent implosions on the micron-scale.
At the NIF, 192 lasers target a BB-sized fuel pellet of deuterium and tritium housed in a cannister. The lasers generate enough heat to convert the solid fuel to plasma and cause an implosion that enables the fusion of atomic nuclei.
The MRS indirectly records the neutrons emitted from fusion implosions using a conversion foil placed 26 centimeters away. “It's really hard to detect neutrons directly, but some of the neutrons emitted in the implosion knock out from the foil more easily detectable deuterons -- heavy isotopes of hydrogen with one neutron and one proton,” says Gatu Johnson. “Forward-scattered deuterons are selected by an aperture in front of a magnet that bends and directs the deuterons to different locations on a detector array, depending on their energy. The deuterons leave large tracks in the detector’s CR-39 plastic that are easy to identify with a microscope after etching.”
The number of deuteron tracks enable the researchers to calculate the number of neutrons emitted. Because one neutron is generated in each fusion reaction, they can then determine the number of fusion reactions, which in turn reveals the energy gain. By combining this data with analysis of the energy spectra, they can calculate the heat of the plasma and determine how well it is assembled.
To achieve net gain, the researchers must optimize compression to uniformly squeeze a 2mm fuel pellet into 100 microns of plasma. “It’s like compressing a basketball to the size of a green pea,” says Gatu Johnson. “To make it more challenging we need to retain its shape. Imagine trying to compress a balloon uniformly from all sides.”
By comparing the number of neutrons that lost energy due to tighter compression (down-scattered neutrons) and neutrons that were unaffected (primary neutrons), the researchers can infer the compression. Doing this in several directions enables them to see how uniformly it compresses and the direction of the non-uniformities.
The MRS also proved helpful in recording the energy shift of the primary neutrons, which helps identify uniformity. “This was a key contribution because it helped us to improve other spectrometers that recorded the implosion, thereby giving us a full vector of the non-uniformities to help improve the shape,” says Gatu Johnson.
To recap: the MRS records the number of fusion reactions, the heat produced in implosion, the compression of the fuel, and the energy shift, which helps to measure implosion symmetry. All this data is “critical for improving the implosions to reach ignition and then prove that we achieved it,” says Gatu Johnson.
The next challenge is to dramatically improve the gain, which requires understanding what is happening during implosion at the sub-nanosecond level. Gatu Johnson leads the development team for a next-generation neutron spectrometer based on the MRS that will provide time-resolved measurements to accomplish this. “It’s a big undertaking,” she says. “We will still use the conversion foil and magnet, but instead of CR-39 detectors, we might use a pulse dilation drift tube that de-skews the signal and stretches it out in time. This might allow us to take snapshots within an incredibly short burn time of 0.1 billionths of a second.”
One major challenge will be dividing the burn time into several time windows that measure how the implosion compression and temperature evolve during the burn. An equal challenge will be raising enough funds to deliver proof-of-concept. “Typically, with the NIF, you want to prove a technology elsewhere before you install it on the NIF because the engineering is so expensive there,” says Gatu Johnson.
If fusion has any hope of achieving commercial viability, the repetition rate for implosions needs to accelerate dramatically. Here, Gatu Johnson’s team can help by reducing turnaround time for analytics. “Right now, the gap between the experiments is as long as once a month, but to achieve continuous energy production, we need to get that to 1-10 times per second,” says Gatu Johnson. “Using a CR-39 detector, which requires etching for six hours in sodium hydroxide and then scanning for four hours, is not going to cut it.”
The HEDP group is also working on improving the MRS design to handle higher yields. “Now that we are producing almost four megajoules, the MRS is reaching its saturation level,” says Gatu Johnson. She is leading the work on developing a new type of detector for the MRS that is placed further from the target to handle higher yields. Another plus: The detector is electronic so it can immediately report the data.
One major factor contributing to the NIF net-gain success was the use of predictive modeling backed by machine learning and cognitive simulations. The HEDP group has recently begun to apply these technologies to diagnostics analysis. “I expect AI and cognitive simulations to contribute a lot to fusion projects over the next few years,” says Gatu Johnson.
ICF and spectrometry for astrophysics research In addition to supporting nuclear fusion research, Gatu Johnson and her colleagues are using the MRS and other diagnostics equipment at the NIF and OMEGA Lasers to conduct basic nuclear physics research. This includes answering astrophysics questions about nuclear reactions in stars, magnetic reconnection, astrophysical jets, shocks in space, and more.
“Until recently, nuclear physics researchers were limited to using particle accelerators to test their theories,” says Gatu Johnson. “Accelerators shoot a mono-energetic beam of ions onto a solid or gaseous target. But in stars and in much of space everything exists in a plasma state. The ability to study nuclear reactions in plasma using ICF allows for studies that better mimic the conditions in stars. We can mockup scenarios that are directly relevant to astrophysics, which is extremely cool.”
One major project is to develop a technique to study plasma screening. “The probability of reactions happening in plasma is enhanced by the presence of negatively charged electrons, which reduce the repelling force between positively charged ions,” explains Gatu Johnson. “It is kind of like trying to push two magnets together. No one has ever measured this effect, but if we can develop the right tools, we may eventually be able to accomplish it.”
Many nuclear reactions in space happen in an excited state due to high temperature or particle bombardment. Together with collaborators, Gatu Johnson is now developing an experiment to determine how excited states impact the probability for certain reactions.
Another question Gatu Johnson is pursuing is the nature of fusion reactions between helium-3 ions, which powers nearly half the sun's energy. At the NIF and OMEGA, she is leading the effort of shooting lasers at capsules of helium-3 gas to mimic the conditions in the sun, create fusion reactions, and measure them using a particle spectrometer.
“It is difficult to study this reaction at solar-relevant energies, because the reactions happen at about a ten times lower temperature in the sun compared to what is easily accessible at the fusion facilities,” says Gatu Johnson. (Strange but true: ICF conditions must be ten times hotter than the sun, which can achieve fusion at cooler temperatures due to higher density and a greater volume of fuel.)
In another project, the HEDP group is exploring spectrometry for charged particles -- the ions produced in fusion reactions such as protons and alpha particles. The researchers have developed a compact “wedge range filter” spectrometer that uses a thin aluminum filter to measure the charged particle spectrum. The spectrometers are placed around an implosion to study directional variation in the proton emission due to non-uniformities in the compression.
Gatu Johnson recently launched an experiment at OMEGA to determine the ion distributions in ICF implosions. “Do they relax to thermal distribution or are there some faster ions that contribute more to the nuclear reactions?” she wonders. “In all these plasma nuclear experiments that are executed to study the probability for reactions in stars, it's critical to understand how the ions distribute themselves.”
To further their research, “We are working on new X-ray diagnostics, as well as a gamma spectrometer that would look at the gammas born in ICF implosions,” says Gatu Johnson. “We have a lot of ideas for projects that should improve ICF implosions for fusion while advancing our plasma nuclear experiments and learning more about astrophysical events.”