In other words, why is it so hard to get the Q value above 1 (ration between energy output and input)
In: 179
ELI5: The main issues with nuclear fusion generally goes back to confinement. A common analogy is trying to squeeze a balloon to smaller size with a bunch of rubber bands while blowing a torch inside the balloon. The rubber bands take energy. The torch takes energy. Not only does the [torch burn the balloon walls (sparks)](https://m.youtube.com/watch?v=IhHsOwLdCu4), the material that comes off the walls poisons the fusion reaction and when running the torch in a confined space it’s hard to get rid of the byproducts (exhaust) which also kills the reaction. So you need holes (an inlet and outlet) on this crazy balloon contraption. And fusion is actually much much more complicated (obviously) – I haven’t even started on how to get the heat out to do work – the process where power from the reactor can be put back into powering the rubber bands and torches.
Further:
Nuclear fusion as done by the sun, uses a giant sphere of gas and plasma to confine the reactants – the components that get squeezed together. The sun’s confinement process is efficient in that the materials fusing are also the walls of the reaction vessel and the reaction products are a small proportion of the reaction mass. There’s an interesting reactor design that uses molten lithium as the confinement vessel walls to help with plasma eating the chamber.
Now switch to Earth. We can’t use gravity like the sun so we’re trying to use magnetic field and heat to drive up *plasma density* (and temperature) – which is pretty much proportional to fusion efficiency. So we need powerful magnets (electricity) and plasma generators (more electricity) to confine and bang the atoms together. Then we need to add hydrogen (or deuterium and tritium) and we need to exhaust helium. And hope we can get the heat to drive a turbine to make enough energy to power the magnets and plasma generators. *Plus a little extra.* It’s pretty hard to do.
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