The real breakthrough in fusion energy is still decades away
Last week inside a gold-plated drum in a northern California lab, a group of scientists briefly recreated the physics that powers the sun. Their late-night experiment involved firing 192 lasers into the capsule, which contained a peppercorn-sized pellet filled with hydrogen atoms. Some of these atoms, which usually repel each other, have been mixed and fused together, a process that produces energy. By the standards of Earth-bound fusion reactions, it was a plot of energy. For years, scientists have done this type of experiment only to find that it was less than the energy used to cook the fuel. This time, finally, they have passed it.
This feat, known as ignition, is a huge victory for those who study fusion. Scientists only had to look at the stars to know that such an energy source is possible – that the combination of two hydrogen atoms to produce one helium atom results in a loss of mass, and therefore, according to E = mc2, a release of energy. But it’s been a slow road since the 1970s, when scientists first defined the ignition target, sometimes also called the “break-even point”. Last year, researchers at the Lawrence Livermore Lab’s National Ignition Facility got close, generating about 70% of the laser energy they fired in the experiment. They continued the experiments. Then, on December 5, just after 1 a.m., they finally snapped the perfect shot. Two megajoules in it; 3 megajoules less. A 50% energy gain. “It shows it can be done,” Jennifer Granholm, US Secretary of Energy, said at a press conference earlier this morning.
For fusion scientists like Mark Cappelli, a Stanford University physicist who was not involved in the research, this is an exciting result. But he warns that those pinning their hopes on fusion as an abundant, carbon-free and waste-free energy source in the near future could be left hanging. The difference, he says, is how scientists define the break-even point. Today, NIF researchers said they had obtained as much energy as their laser fired at the experiment – a massive and long-awaited achievement. But the problem is that the energy of these lasers represents only a tiny part of the total power involved in triggering the lasers. By this measure, the NIF receives much less than it puts out. “It’s decades later. Maybe even half a century later.
The problem is ineffective lasers. Generating fusion energy using the NIF method involves shooting dozens of beams into a cylinder of gold called hohlraum, heating it to over 3 million degrees Celsius. Lasers do not target fuel directly. Instead, their goal is to generate “a soup of X-rays,” says University of Michigan fusion researcher Carolyn Kuranz. These bombard the tiny fuel pellet made of the hydrogen isotopes deuterium and tritium, and crush it.
It must be done with perfect symmetrical precision – a “stable implosion”. Otherwise, the pellet will wrinkle and the fuel will not heat up enough. To get to last week’s result, NIF researchers used improved computer models to improve the design of the capsule that holds the fuel and calibrate the laser beams to produce the correct X-ray scatter.
Currently, these lasers emit about 2 megajoules of energy per pulse. For fusion scientists, this is a huge and exciting amount of energy. This is roughly equivalent to the energy used in about 15 minutes of running a hair dryer, but delivered all at once, in a millionth of a second. Producing these beams at NIF involves a space almost the size of a football field, filled with flashing lights that excite the laser bars and propagate the beams. This alone takes 300 megajoules of energy, most of which is wasted. Add to that layers of cooling systems and computers, and you quickly have an energy input several orders of magnitude greater than the energy produced by fusion. So the first step in practical fusion, according to Cappelli, is to use much more efficient lasers.