Science Fusion energy with lasers

SCIENCE EDITORS: Daniel Pain and Hayley Power

Oliver Lamb, Deputy Editor, explains how the next source of green energy could have been created

IN December, scientists at the National Ignition Facility in California took another small step towards nuclear fusion power by achieving an energy gain for the first time.

Fusion works by fusing smaller isotopes — usually deuterium and tritium, which are isotopes of hydrogen — to form larger ones — usually helium. (Isotopes are variations of chemical elements: The number of neutrons in an atom’s nucleus may vary without substantially changing the element’s chemical properties). Nuclear fission, where an isotope is split, is the basis of existing forms of nuclear power.

Whereas most fusion research focuses on magnetic confinement fusion, the NIF breakthrough involved inertial confinement fusion. A laser pulse consisting of 192 beams was fired at a gold cylinder containing a capsule of hydrogen isotopes. The cylinder converted the laser light into x–rays, which caused the fuel to implode, and thence react. 3.15 megajoules of energy were produced from a 2.05MJ laser shot. Never before had a fusion reaction generated more energy than was put in.

A laser pulse was fired at a gold cylinder containing a capsule of hydrogen

Fusion’s potential rewards are colossal. Like fission, it produces no greenhouse gas; unlike fission, it produces only low-level, short–lived nuclear waste. Furthermore, the conditions required for a fusion reaction are so extreme, so hard to maintain, that a disastrous chain reaction cannot occur. Yet fusion produces four times more energy per kilogram of fuel than fission, and nearly four million times more than burning oil or coal. What’s more, deuterium can be extracted from seawater at low cost, and tritium, though rare in nature, can be produced by reacting the lithium-6 isotope with neutrons from fission reactors. Theoretically, a few grams of these isotopes could gen erate a terajoule

Yet fusion produces four times more energy per kilogram of fuel than fission for 60 years. In short, nuclear fusion promises clean, safe, practically limitless power.

Stars run on fusion, and it is for the stars that scientists are metaphorically reaching as they pursue this near–utopian vision. Fusion reactions in the heart of the Sun take place at ten million degrees Celsius — that, combined with the immense pressure, is required to persuade the isotopes to fuse. On Earth, because of the lower pressure, the threshold is 100 million degrees.

Longview Fusion hopes within five years to begin building a power plant that meets the ten–per–second target. They are just one start–up in a field that is buzzing with excitement and working to identify the best design and components. Focused Energy are pioneering a two–laser variant of ICF known as fast ignition. The Omega Laser Facility at the University of Rochester advocates getting rid of the cylinder. First Light Fusion is experimenting with projectiles instead of lasers.

Further challenges relate specifically to ICF. Generating the laser pulse itself requires hundreds of megajoules, so a viable ICF system would have to produce much more energy per shot. Moreover, the NIF can currently produce one shot a day; an ICF power plant may need to fire ten every seconds.

Magnetic confinement fusion, which uses magnetic fields to confine hot plasma, is still the primary subject of fusion research and the likeliest future basis for fusion power, at least commercially, given its superior ability to sustain reactions. But its little sibling is a dark horse to end the decades–long pursuit of energy production’s holy grail.