Thursday, June 21, 2018

The Twinkle in Mother Earth’s Eye: Laser Blasts Produce Promising Fusion Advances

What if you could have a miniature star powering your house, your computer, and your car? How cool would that be! Stars produce a lot of energy, and they get that energy through a process called fusion. Thanks to recent research at the National Ignition Facility (NIF), we’re now one step closer to using fusion as a power source—unlocking a virtually infinite supply of clean energy.

Technicians service the interior of the National Ignition Facility's spherical target chamber, where hundreds of extremely powerful lasers converge on a fuel pellet smaller than a pencil eraser.
Image credit: Philip Saltonstall/Lawrence Livermore National Laboratory
Fusion is when the nuclei of two small atoms collide and stick together into a single, larger atom. Sounds simple, but atoms really don’t like being forced together. Every atom's nucleus is positively charged. When these two positively charged nuclei are forced together, they repel each other, a lot like how two magnets repel one another when they’re facing the right way. But in stars, there is so much gravity pushing these atoms together that it overcomes that repulsion, and we get a new element!

If we combine several hydrogen atoms together, we get helium. But the mass of the new helium is less than the sum of the masses of the initial hydrogen atoms. So where did the missing mass go? According to Einstein’s famous equation E=mc2, the mass (m) becomes energy (E)! That means we can harness the extra energy, or missing mass, produced from this collision. This would provide another source of renewable energy to replace the fossil fuels that provide most of the electrical power in the modern world.

But harnessing energy like a star on Earth is really difficult. According to NASA, the sun fuses about 600 million tons of hydrogen every second. Fusion is why the sun is a huge fireball producing so much energy, and it's the power source for the most destructive weapons ever built. How could we possibly harness that safely here on Earth? Physicists have spent decades trying to answer that question, designing and building powerful fusion reactors to create and control the extreme pressure and temperature you find inside a star.

There are two main types of fusion that we have on Earth: Magnetic Confinement Fusion and Inertial Confinement Fusion. Our previous article on NIF goes into more detail, but Magnetic Confinement means that a magnetic field is used to hold the fusion fuel—a plasma of hot hydrogen isotopes—in place, because no container could withstand the heat. Ordinary hydrogen atoms are made of a single proton and a single electron, but the kind used in fusion have neutrons in their nuclei—adding one neutron makes deuterium, and two neutrons makes tritium. When heat is applied to a combination of tritium and deuterium, the fusion fuel as a gas gets so hot that the electrons are torn away from their atoms, leaving a pool of random electrons and nuclei: a plasma. Since temperature is a measure of how fast atoms are moving, getting the plasma hot enough causes the nuclei to slam into each other with enough force to produce helium, along with some highly energetic neutrons that will heat the walls of the chamber, eventually boiling water to produce electricity with a steam turbine.

The interior of a magnetic confinement fusion reactor, the Wendelstein 7-X.

Inertial confinement fusion, the method NIF uses, works a little differently. Instead of a fuel in the form of gas, there is a solid fuel pellet made of super cold deuterium and tritium, surrounded by a high-density capsule to hold it in one place. 192 lasers are aimed at the capsule from all sides, and fired all at once to transfer a huge amount of energy to it! The capsule absorbs this energy and expands, but the walls of the fusion reactor are strong, so they push back on the capsule and direct the energy inwards—causing the capsule and fuel pellet to implode. The pressure created by the implosion makes the gas become a plasma, which then produces the same helium, some neutrons, and energy. At a certain point, the system becomes self heating, meaning the heat and energy produced from the initial reaction continue the cycle to perpetuate more reactions.

NIF’s fusion experiments have been very a sense. Their recently published article, “Fusion Energy Output Greater Than the Kinetic Energy of an Imploding Shell at the National Ignition Facility”, has shown that the energy they are getting from their inertial confinement fusion is greater than the energy they are putting into the fuel pellet. Breaking even has always been a problem with regards to fusion experiments, because fusion requires so much energy to get started. The scientists at NIF are now getting about twice the amount of energy out (~54 kJ) as they put into the fuel pellet (~21 kJ). They have also worked on ways to encourage the system to self-heat. Self-heating has been difficult to achieve in an experimental setting, because energy can escape too quickly, but NIF says that once they increase how long the experiment is running—what’s called the confinement time—then self-heating can occur. That would make it easier to get more energy out from this process. The pressure inside of the reaction, or the hot spot, is now greater than the pressure at the center of the sun, meaning a star can effectively be made in their facility.

The device that holds the fusion pellet, pictured here, is called a hohlraum—German for "hollow room".
Image credit: Lawrence Livermore National Laboratory
If you're an avid Physics Central reader, you may remember our article about fusion published in 2014 called “In Depth: Fusion Strides at NIF”. In that article, fusion is explained in greater detail, and we talked about how the “High-foot” method, which alters the laser’s power throughout the reaction, allowed NIF to “break even” on energy in their experiments. Using High foot, NIF was able to refine the method that brought them to an equilibrium back in 2014 and get more energy out of the fusion reactor than in previous experiments.

Although NIF did get some positive net energy from their experiments, we've unfortunately got a long way to go before we're phasing out coal plants in favor of fusion ones. In 2014, NIF broke even, meaning that the amount of energy in their activated pellet was the same as the amount of energy that the process produced—but there was still a lot more energy going into the system. Every step in the process—turning fuel into electricity, electrical energy into laser light, then laser light into the heat and kinetic energy of the capsule—involves losses, so the total energy going into the system was still much greater than the energy produced by fusion. That part of the story hasn't changed much, but now NIF has been able to recover more energy: from their results, they got two times the amount they got back in 2014, but that’s still not enough to break even at a larger scale.

Despite the fact that fusion is still far from producing more energy than is requires to get it started, these latest results represent another step toward bridging the gap in energy that fusion is notorious for. Although there’s plenty left to learn about fusion, pursuing research into safer forms of power generation still promises an even brighter future for our planet—and the human race.

—Phoebe Sharp

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