ENERGY SECURITYWhy Nuclear Fusion Is So Exciting

By Clea Simon

Published 15 December 2022

The Lawrence Livermore National Lab in California last week achieved fusion with a net energy gain. Harvard scientist Adam Cohen breaks down breakthrough that might prove major turning point in clean energy efforts — but not any time soon.

The Lawrence Livermore National Lab in California last week achieved fusion with a net energy gain,the U.S. Department of Energy reported on Thursday. That is, by focusing 192 giant lasers on a bit of frozen deuterium and tritium, the lab’s National Ignition Facility created a reaction that produced more energy than it used, a threshold called “ignition.” The long-sought result is a major breakthrough in nuclear fusion, with exciting, if still very far off, implications for renewable energy. Clea Simon of the Harvard Gazette asked Adam E. Cohen, a professor of chemistry, chemical biology, and physics, to explain what happened and why it matters. The interview has been edited for clarity and length.

Clea Simon: What is fusion?
Adam Cohen: Fusion is the process of colliding light nuclei with each other to form heavier nuclei. This process can release huge amounts of energy as the nuclei combine.

Simon: It sounds like what the scientists did was smash two hydrogen isotopes together to make helium, which has slightly less mass. But how does that create energy?
Cohen: Einstein  taught us more than a century ago, in his famous formula e = mc2, that you can convert mass into energy. So a little bit of the mass of the hydrogen isotopes that are getting fused together goes into energy, which comes out of this reaction.

Simon: Why doesn’t it just stay as mass? Why aren’t there just extra bits of mass flying around?
: Mass comes in discrete chunks, and if you add up the mass of a helium and the neutron that comes flying out too in this process, there’s a little bit of a difference. Another way of thinking about it is that helium has two protons and two neutrons, and those protons and neutrons are bound to each other. They stick to each other very hard, very strongly. And when the hydrogen isotopes fuse to make that helium nucleus in the process of them sticking to each other, that releases a lot of energy. They attract each other, just the way the north and south pole of a magnet might attract each other. And as they smash into each other, they release a lot of energy.