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The race is on to build fusion reactors that would provide limitless energy without nuclear waste or carbon emissions. Here's everything you need to know:

What is fusion?
Thermonuclear fusion is the nuclear reaction that powers the sun and all stars. It occurs when two nuclei of a lightweight element such as hydrogen collide at colossal speed, forcing them to fuse. Leftover mass is converted into enormous amounts of energy, according to Einstein's formula E = mc2. Unlike fission, in which atoms are split, fusion requires small amounts of ordinary fuel — the amount of hydrogen in a glass of water could provide enough energy for one person's lifetime — and does not create much radioactive waste, which is why it's been called "the holy grail for the future of nuclear power." Proponents believe fusion reactors could solve the climate-change crisis by providing inexhaustible energy with zero emissions and no chance of a meltdown. But the challenge of creating fusion reactions is enormous: Scientists and engineers essentially have to create a small star. Hydrogen must be heated to about 100 million degrees Celsius — six times hotter than the sun's core. At that temperature, hydrogen is no longer a gas but a plasma, a soupy mix of charged particles that is incredibly difficult to sustain. Scientists have been trying to contain the plasma using a tokamak, a doughnut-shaped structure with an extremely strong magnetic field, but thus far have been successful only for seconds.

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