Fusion Breakthroughs in Context: Professors Holdren and Bunn Reflect on Fusion Ignition Announcement

| December 28, 2022 | Leave a Comment

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Date of Publication: December 14

Year of Publication: 2022

Publication City: Cambridge, MA

Publisher: Harvard Kennedy School - Belfer Center

Author(s): John Holdren

Tuesday’s announcement of a breakthrough in fusion energy research at the Department of Energy’s Livermore Lab was just the latest in a recent flurry of fusion-breakthrough announcements at labs around the world. What is going on?

After seven decades of increasingly costly research—including an unprecedented degree of cooperation across nations — are safe, clean, affordable fusion reactors now just around the corner?

Fusion reactions power the stars, and they have allowed the building of thermonuclear bombs of unlimited explosive power. But, aside from capturing energy from the sun, harnessing these reactions on Earth’s surface to help meet society’s energy needs has proven to be incredibly challenging. No technology devised for this purpose so far has been able to produce as much energy as needed to operate it.

This energy gap has gotten more or less steadily smaller over the years, however, for both of the main approaches to meeting the fusion challenge; “Magnetic fusion”, using a cage of magnetic fields to confine the immensely hot fusion fuel; and “Inertial- confinement fusion”, using powerful, pulsed lasers or particle beams to compress and heat tiny fuel pellets essentially instantaneously, producing a series of fusion micro-explosions.

The Livermore breakthrough used the world’s most powerful laser, bigger than three football fields, to create a single micro-explosion that, for the first time anywhere, yielded a bit more fusion energy than the laser energy arriving at the target. That fusion yield, however, was about 250 times smaller than the amount of electrical energy supplied to the laser for the “shot”. A practical fusion reactor based on this approach would need 10-20 times more fusion yield per shot than the electricity supplied to the laser, hence 2500-5000 times the yield of the Livermore experiment, and it would need to operate at rate of about 10 shots per second. The current Livermore device can manage 2 shots per day.

Thus, while the Livermore result represents real progress, it is miles short of the performance needed for a practical fusion reactor. The daunting energy gap just mentioned, moreover, takes no account of other, as yet unsolved problems relating to structural damage by fusion neutrons, efficient and secure breeding and recycling of the radioactive tritium needed for an adequate reaction rate, and more. The current Livermore laser-fusion system, however, is quite useful for the less-advertised, national-defense function that has paid most of the facility’s bills: studying the physics of thermonuclear explosions without blowing up real bombs.

The magnetic approach to controlled fusion, around which a number of physics and technology breakthroughs have also been announced in recent months, is actually considerably closer than laser fusion to achieving reactor-relevant yield. (Beneficially for arms-control purposes, it also has little relevance to thermonuclear-weapon physics.) Even so, the additional hurdles that need to be surmounted in order to arrive at a practical magnetic-fusion reactor are formidable. It seems very unlikely that fusion reactors of any type will be contributing significant electricity to the power grid before the second half of this century; and, even on that timescale, it is not obvious that they can be made both reliable and inexpensive enough to compete with other options.

Is it worth the continuing effort and expense needed to find out? I believe the answer is yes. Uncertainties relating to the adequacy of other carbon-free energy technologies for meeting all of society’s energy needs for the long term are large, and the potential attractions of fusion are considerable: practically inexhaustible fuel from sea water, less polluting than fossil fuels, safer and less proliferation-prone than fission energy systems, free of the intermittency and land requirements of wind and solar. It is also the only known energy source potentially powerful enough to carry humans beyond the solar system, should society want or need to go there.

The full article about the laser-fusion experiment at the Livermore Lab appeared on the Belfer Center website alongside an even shorter piece by  Matthew Bunn.

A more detailed PowerPoint presentation on Understanding the Recent Results from the National Ignition Facility can be downloaded from the link above.

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