What enabled the big boost in fusion energy announced this week?
Two megajoules of laser yielded three megajoules of fusion energy.
JOHN TIMMER , 12/14/22
On Tuesday, the US Department of Energy (DOE) confirmed information that had leaked out earlier this week: its National Ignition Facility had reached a new milestone, releasing significantly more fusion energy than was supplied by the lasers that triggered the fusion. "Monday, December 5, 2022 was an important day in science," said Jill Hruby, head of the National Nuclear Security Administration. "Reaching ignition in a controlled fusion experiment is an achievement that has come after more than 60 years of global research, development, engineering, and experimentation."
In terms of specifics, the lasers of the National Ignition Facility deposited 2.05 megajoules into their target in that experiment. Measurements of the energy released afterward indicate that the resulting fusion reactions set loose 3.15 megajoules, a factor of roughly 1.5. That's the highest output-to-input ratio yet achieved in a fusion experiment.
Before we get to visions of fusion power plants dotting the landscape, however, there's the uncomfortable fact that producing the 2 megajoules of laser power that started the fusion reaction took about 300 megajoules of grid power, so the overall process is nowhere near the break-even point. So, while this was a real sign of progress in getting this form of fusion to work, we're still left with major questions about whether laser-driven fusion can be optimized enough to be useful. At least one DOE employee suggested that separating it from its nuclear-testing-focused roots may be needed to do so.
During today's announcement, the DOE's Marv Adams described the National Ignition Facility's process for using lasers to trigger fusion (使用雷射來啟動「核融合反應」的步驟與過程). It involves placing a small target sphere containing hydrogen isotopes inside a metal cylinder, and then zapping the cylinder with lasers. "192 laser beams entered from the two ends of the cylinder and struck the inner wall, Adams said. "They didn't strike the capsule (即上述的target sphere – 卜凱), they struck the inner wall of this cylinder and deposited energy. And that happened in less time than it takes light to move 10 feet. So it's kind of fast."
The cylinder released much of the energy it received in the form of X-rays, which compress the small hydrogen target—another speaker compared the compression to smashing a basketball down to the size of a pea. The intense heat and compression (熱量與壓縮) set off fusion among the hydrogen isotopes, releasing energetic photons and neutrons. These carry much of the excess energy produced during the reactions.
The Department of Energy built the National Ignition Facility partly because hydrogen fusion is at the heart of many of its nuclear weapons and because fusion is a potential power source that produces far less—and far less dangerous—nuclear waste than nuclear fission (核分裂反應).
The press conference makes clear that, over the past several years, the team operating the National Ignition Facility has gradually improved yields (產出量) through an iterative process. They have several knobs they can turn—different ways to distribute the lasers' power across individual beams, different ways of managing small defects on the target, etc. These could alter not only the amount of fusion that starts in the target, but how its energy spreads into the surrounding hydrogen isotopes.
"That plasma wants to immediately lose its energy—it wants to blow apart, it wants to radiate, it's looking for ways to cool down," said Livermore Labs' Mark Herrmann. "But the fusion reactions are depositing heat in that plasma, causing it to heat up, so there's a race between heating and cooling. And if the plasma gets a little bit hotter, the fusion reaction rate goes up, creating even more fusion, which gets even more heating. And, so the question is, can we win the race. And for many, many decades, we lost the race. We got more cooling out than we got the heating up."
But all those lost races led to better models of the reaction conditions, including recent ones refined by machine-learning studies of past tests, as well as improved manufacturing of targets. And that led to a tweak of the distribution of laser energy that led to a more symmetric target compression in the recent experiment. And that, apparently, made enough of a difference to produce this unusually high yield, even though Annie Kritcher, who leads the experimental design team, indicated that the pre-tweak experiments only produced about 1.2 megajoules of output energy.
The DOE indicated that the conditions haven't been replicated yet, but it brought in a panel of outside experts to review its measurements before making this announcement. (Or at least not replicated in the real world. "I had vivid dreams of all possible outcomes from the shot," Kritcher said. "This always happens before a shot from, like, complete success to utter failure.")
About that energy yield
As we noted above, the 3 MJ released in this experiment is a big step up from the amount of energy deposited in the target by the National Ignition Facility's lasers. But it's an enormous step down from the 300 MJ or so of grid power that was needed to get the lasers to fire in the first place.
But many speakers emphasized that the facility was built with once-state-of-the-art technology that's now over 30 years old. And, given its purpose of testing conditions for nuclear weapons, keeping power use low wasn't one of the design goals. "The laser wasn't designed to be efficient," said Herrmann, "the laser was designed to give us as much juice as possible to make these incredible conditions happen in the laboratory."
Tammy Ma leads the DOE's Inertial Fusion Energy Institutional Initiative, which is designed to explore its possible use for electricity generation. She estimated that simply switching to current laser technology would immediately knock 20 percent off the energy use. She also mentioned that these lasers could fire far more regularly than the existing hardware at the National Ignition Facility.
Which gets into all the other problems that laser-driven fusion faces. Kim Budil, director of Lawrence Livermore National Lab, mentioned the other barriers. "This is one igniting capsule one time," Budil said. "To realize commercial fusion energy, you have to do many things; you have to be able to produce many, many fusion ignition events per minute. And you have to have a robust system of drivers to enable that." Drivers like consistent manufacturing of the targets, hardware that can survive repeated neutron exposures, and so on.
So, while laser-driven fusion may have reached major energy milestones, there's a huge list of unsolved problems that stand between it and commercialization. By contrast, magnetic confinement in tokamaks, an alternative approach, is thought to mostly face issues of scale and magnetic field strength and to be much closer to commercialization, accordingly.
"There's a lot of commonalities between the two where we can learn from each other," Ma said optimistically. "There's burning plasma physics, material science, reactor engineering, and we're very supportive of each other in this community. A win for either inertial or magnetic confinement is a win for all of us." But another speaker noted that magnetic confinement works at much lower densities than laser-driven fusion, so not all of the physics would apply.
But Ma also suggested that, for laser-driven fusion to thrive, it may need to break away from its past in weapons testing. "Where we are right now is at a divergent point," she said. "We've been very lucky to be able to leverage the work that the National Nuclear Security Administration has done for inertial confinement fusion. But if we want to get serious about [using it for energy production], we need to figure out what an integrated system looks like... and what we need for a power plant. It has to be simple, it has to be high volume, it needs to be robust." None of those things had been required for the weapons work.
Budil had the most optimistic take about the different approaches and uses, saying, "Many technologies will grow out of both fields, in addition to the path to a fusion power plant."
There is no “breakthrough”: NIF fusion power still consumes 130 times more energy than it creates
If you gave me $400 and I gave you $3.15, would you consider yourself wealthier? That's a financial analogy for the supposed fusion power "breakthrough."
Tom Hartsfield, 12/18/22
* In 2021, NIF’s laser fusion energy output jumped by 2,500%, a legitimate breakthrough.
* This year, NIF reports that it has achieved "ignition" — that is, it has achieved slightly more fusion energy output than laser energy input.
* However, to produce commercial fusion power, NIF would need to increase the fusion output of each experiment by at least 100,000%. The technological hurdles are absolutely enormous.
Here we go again. In 2021, the National Ignition Facility (NIF) announced a scientific breakthrough in its pursuit of fusion power technology. One year later, they’re making another announcement, heralded as “game-changing,” “transformative,” and “a moment of history.” But this is not a meaningful breakthrough for practical, commercial fusion power: NIF still drains at least 130 times more energy from the power grid than it produces.
A legitimate breakthrough in 2021
Last year’s big news was that NIF dramatically increased the fusion output of its experiments. At the time, I wrote about NIF and the scientific background of its accomplishment. They earned most of their hype. Here’s a quick recap:
“[NIF] was built for two missions. Performing research in support of the Stockpile Stewardship Program is the foremost duty, but the sign over the door doesn’t say “National Stockpile Research Facility.” NIF is named after its other task: to further our quest to understand and harness energy from nuclear fusion. A recent breakthrough in this fusion mission has made headlines across the scientific community.”
“One of two critical parts of NIF’s fusion mission is “ignition“: release of a quantity of fusion energy greater than the laser energy required to drive the implosion. After the failure of the National Ignition Campaign, many scientists believed that ignition at NIF was impossible. That goal remains just beyond our grasp, but it is now far closer than before. The bigger news is that we may have seen the first sign of the other important fusion goal: thermonuclear burn.”
A hyped breakthrough in 2022
In that work, NIF’s laser fusion energy output — measured in megajoules, MJ — jumped by 2,500%, a sign of a significant physics breakthrough on the crucial problem of thermonuclear burn. This week’s announcement is an increase in fusion energy output, relative to laser energy input, from 70% in 2021 to 154% in 2022. This incremental, possibly incidental, progress toward thermonuclear burn is not a breakthrough.
The facility has, at last, achieved slightly more fusion output than laser input: ignition. On paper that is a major symbolic victory. In practice, it’s of little consequence. Here’s why.
The laser energy delivered to the target was 2.05 MJ, and the fusion output was likely about 3.15 MJ. According to multiple sources on NIF’s website, the input energy to the laser system is somewhere between 384 and 400 MJ. Consuming 400 MJ and producing 3.15 MJ is a net energy loss greater than 99%. For every single unit of fusion energy it produces, NIF burns at minimum 130 units of energy.
In terms of electrical power, 3.15 MJ would not quite power one 40-watt refrigerator light bulb for a day. Charging NIF steadily over the same day would draw 4,600 watts from the power grid. (NIF is actually charged much more quickly, but at the cost of a much higher draw in watts — more energy per unit time, over less time — but the total energy is the same.)
Getting to viable fusion power
To produce useful power, NIF would need to increase the fusion output of each experiment by at least 100,000%. That’s an enormous scientific challenge to resolve before commercial operation can even be considered.
The scientific challenge is equaled and possibly exceeded by others. A power plant needs to produce steady power. NIF currently executes, at best, one experimental blast per day. A commercial plant would need to blast fusion-producing capsules at a rate of tens of thousands per day.
Each blast requires strict conditions: temperatures a few degrees (Kelvin) above absolute zero; a spherical capsule, mechanically perfect in shape with an error of less than 1% the width of a hair; and a vacuum chamber environment. Most blasts suffer from slightly imperfect conditions and produce less fusion.
Either way, the machine takes hours to recover from each experiment. The fact that NIF is able to do this once per day is a technical achievement that took years to perfect. Making it happen 10,000 times faster is absurdly difficult. If it could be done, still more engineering then would be required to extract the energy in the form of heat for practical electricity generation.
Finally, there is a supply problem. The pellets contain deuterium and tritium. Deuterium is plentiful, but the world’s entire supply of tritium is something like 50 pounds. In 2020, the market cost of tritium was nearly $1 million per ounce. Livermore scientists estimate that a commercial operation modeled on NIF would require two pounds per day. Producing more tritium itself will be a challenge.
As in 2021, we should laud the scientific accomplishments of NIF. Many years (and careers) of hard work are producing progress on one of the most difficult applied science problems ever tackled. Scientifically, it’s symbolic progress. But it’s not a breakthrough, a game-changer, or the herald of imminent clean fusion power. NIF is still decades away from economically viable fusion.
Editor's note: This article was updated on December 13, 2022 to provide more accurate, up-to-date figures.
本部落格編者註：NIF, National Ignition Facility 或 美國融合式核能研究所
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