Astro Bob: All about fusion — how the sun keeps the lights on
With nuclear fusion in the news, we look to the sun. An old hand at the process, it squeezed its first hydrogen atoms together 4.6 billion years ago.
I've spent the past two days removing nearly two feet of snow from two back-to-back snowstorms. In the process I used chemical energy derived from food I ate to shovel the sidewalk and deck, then switched over to gasoline to power my snowblower for the heavy lifting. Chemical energy is stored in the bonds between atoms and molecules. Break and rearrange those bonds the right way, and you'll get the energy you need to get the work done.
Someday I hope a scene from the movie Back to the Future Part II becomes a reality. Dr. Emmett "Doc" Brown returns from the future in his modified DeLorean time machine. In need of fuel, he flips the lid off a nearby garbage can, grabs a handful of waste and drains a partially-full can of Miller beer into the car's Mr. Fusion engine. For good measure he tosses in the can, too. If only it could be so easy.
Feeling sore in the back at the moment, I'm dreaming of AI snowblowers equipped on Mr. Fusion engines. But I hear it'll be at least 30 years before fusion leaves the laboratory and becomes commercially available and probably twice that before I can pour beer into one. That doesn't take away from this week's great news that scientists successfully bombarded a peppercorn of hydrogen with 192 lasers, producing about 50 percent more energy than the amount expended by the beaming lasers.
That team and others around the globe have been working for decades to create an energy-plus fusion reaction, making the recent success reason to celebrate this nugget of progress toward finding a safe, carbon-free form of energy to power humanity's many needs.
What is fusion anyway? Look up at the sun. All the heat, light and energy it radiates begins with fusion deep within its roasty, toasty core, where the temperature simmers around 27 million degrees Fahrenheit (15 million C). In the extreme heat, under the crushing pressure of overlying matter, hydrogen meets hydrogen in an atomic relay race that forges a new element — helium — and the energy critical to our survival. That energy radiates outward from the core and takes up to a million years to reach the surface as light and heat. Let that journey sink in the next time you feel the sun's warmth on your face.
Our star has been in the fusion business since its nuclear furnace fired up nearly five billion years ago. Let's take a look at exactly how it works. Hydrogen, which makes up 73 percent of the sun's mass, normally consists of a proton, or nucleus, orbited by a single electron. But the fierce heat inside the core has stripped away hydrogen's electrons, creating a soup of protons and electrons some 160 times denser than water.
These circumstances, however hellish, provide the perfect environment for two protons to collide and stick together. One of them emits two tiny particles in the process — a positively charged speck called a positron and a neutrino — transforming itself into a neutron (neutral particle). Together, the neutron and proton form deuterium, a variant of hydrogen called an isotope .
Next, the deuterium particle collides with another proton, making a new atom with two protons and one neutron and emitting a burst of energy in the form of a gamma ray. Different elements are defined by the number of protons in their nucleus. Hydrogen has just one, carbon six and oxygen eight. Since the new atom has two protons, it's no longer hydrogen but an unstable form of helium called helium-3.
In the final step two helium-3 atoms collide. Two of the neutrons and two of the protons combine to make a stable helium-4 atom — the same gas found in floating party balloons — while the remaining two protons fly off to start a new round of nuclear reactions.
When said and done, four hydrogen nuclei (protons) join together to create a brand new atom, helium. In the process, 0.7 percent of the total matter involved is lost as energy in the form of gamma rays and speeding positrons and neutrinos. The gamma radiation migrates from the core to the surface. Slamming into atomic nuclei along the way, it gradually loses energy and transforms into visible light. Arriving at the surface it streams off into space as sunshine.
Incredibly, any particular hydrogen nucleus has to wait an average of one billion years to start the reaction. But because there are so many zillions of them (not an official number!) the process is ongoing. In fact, every second the sun converts 600 million tons of hydrogen into 596 million tons of helium.
What happens to the missing four million tons? Fusion has converted that small fraction into the energy that powers the sun. It takes so little stuff to create so much firepower that matter is really just an incredibly concentrated form of energy. Like one of those protein bars from a health food store. Isn't fusion grand?
At Lawrence Livermore National Laboratory scientists mimicked the fusion process, essentially creating a tiny, artificial sun, by packing deuterium and tritium into a fuel capsule and heating them with 192 lasers to over 100 million degrees. When the isotopes fused, a tiny fraction of their mass was converted to pure energy. Just like what happens inside the sun. Also, energetic neutrons released in the reaction struck the container walls, producing heat. Heat can be captured and converted to electricity.
True, the sun got a head start in fusion. Over time we came to understand the deep power of the atom in part informed by the sun. Now, 4.6 billion years later, we're on the cusp of building our own stars, though on a smaller and more manageable scale.