In controlled nuclear fusion, heavy isotopes of hydrogen fuse into helium, releasing a huge amount of energy in the process. A large portion of the energy released by a laboratory fusion reaction goes into hot helium ash (an impurity in the plasma that bears no resemblance to ash from a fire). This ash is around 30 billion degrees Celsius, compared to 200 million degrees for the bulk plasma. For context, the temperature at the core of the sun is 15 million C. The ash energy may be captured by a plasma wave, via wave-particle interaction known as alpha channeling. The energy in the wave can then be absorbed by fuel ions, powering the fusion reaction.
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Source: Phys.org