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Plasma temperatures in a tokamak are around 100 million C.

Presumably for a power plant that will run for a long period of time, fuel will have to be injected and "burnt" products will have to be removed.

How will this be done without destroying the fuel injector from the superhot plasma? Is that even possible, or will we be forced to always shut down the operation when replacing fuel?

EDIT: Any device to inject or remove something must be nearby the plasma (not necessarily contacting it). I imagine the black body radiation of a 100 Million C substance causing some extreme radiation damage, not necessarily from alpha/beta particles, but simply from intense EM radiation.

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    $\begingroup$ I rarely cite patents, but this one is a fun piece of "fusion is the energy source of the future (and always will be)": google.com/patents/US3713967. Neutral particle injection was an idea that was conceived and demonstrated long before the machines had reached the point where one could even dream about a serious fusion reactor. These neutral beams can penetrate the magnetic field and can heat, fuel and modify the composition and spatial density distribution of the plasma. $\endgroup$ – CuriousOne Jul 6 '16 at 4:47
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    $\begingroup$ Why does the fuel injector have to be in the plasma? $\endgroup$ – Jon Custer Jul 6 '16 at 11:12
  • $\begingroup$ @JonCuster Doesn't hafta be directly contacting the plasma, but it does hafta be nearby. I imagine the black body radiation of a 100 Million C substance causing some extreme radiation damage, not necessarily from alpha/beta particles, but simply from intense EM radiation. $\endgroup$ – DrZ214 Jul 7 '16 at 1:39
  • $\begingroup$ @CuriousOne Right, but in that case, how is the injector not damaged/destroyed from the intense radiative heat of the nearby plasma? $\endgroup$ – DrZ214 Jul 7 '16 at 1:40
  • $\begingroup$ The injectors are completely outside of the reactor. There can't be anything much in there, anyway, as all the neutrons are needed desperately for the breeding blanket, which at the same time is the radiation shield. The major problems with fusion, at this point, will be the blankets for the fuel cycle... tritium doesn't grow on trees and it can't be mined, the only place to make it is in the reactors, themselves, unless we want to have half a dozen fission breeders for every fusion plant. $\endgroup$ – CuriousOne Jul 7 '16 at 7:07
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The ash is removed by the divertor. Since everything in the ring is ionised you can selectively remove ions with a magnetic field using their charge to mas ratio. This is done by the divertor, which pulls out heavier ions while leaving the tritium and deuterium in the ring.

As for fuelling, that's basically easy as you don't need to inject much fuel since fusion is so efficient. Just inject hydrogen ions using a simple accelerator or even fire in tiny nuggets of frozen hydrogen and let them vaporise.

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  • $\begingroup$ Indeed. I once interviewed at JET (in the late 80's), and was was offered a job to work on the fuel injection system that shot (neutral) frozen pellets into the plasma. You need sufficient speed, and aim, so the pellet ionizes when it's in the hottest / densest part of the plasma. I didn't take it in the end... $\endgroup$ – Floris Jul 6 '16 at 15:08

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