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In a tokamak, a toroid with toroidal and poloidal magnetic fields, Deuterium and Tritium (D-T) fuse at ~10 keV to produce an alpha particle (3.5 MeV) and neutron (14.1 MeV).

The magnetic fields confine the charged particles to move in gyrations (Larmor orbits) around the toroidal magnetic field lines. The alpha particles are supposed to interact with the plasma of D-T particles, heating them further, leading to self-sustaining fusion.

In this fusion reaction, I suspect that half of the alpha particles will move in the negative direction of the plasma current, effectively cooling down the plasma just as much as it heats it. All of the literature claims that the alpha particles contribute 3.5 MeV to heat the plasma, so could someone clarify what I am missing?

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  • $\begingroup$ Since the alpha particles represent fusion "ash", I'm wondering how they are removed from the plasma. For any responders, I would appreciate you including that detail in your answer. $\endgroup$
    – David White
    Commented Aug 3, 2022 at 22:44
  • $\begingroup$ iopscience.iop.org/article/10.1088/0029-5515/40/1/307 $\endgroup$
    – anna v
    Commented Aug 4, 2022 at 4:01
  • $\begingroup$ this earlier iopscience.iop.org/article/10.1088/0029-5515/40/1/307/pdf $\endgroup$
    – anna v
    Commented Aug 4, 2022 at 4:07
  • $\begingroup$ @WillWillCUB "I suspect that half of the alpha particles will move in the negative direction of the plasma current," this is wrong, the charge of the alpha does not change! $\endgroup$
    – anna v
    Commented Aug 4, 2022 at 4:48
  • $\begingroup$ @annav I guess I meant that in the fusion reaction, the resulting particles will fly off in all directions. Some of the alphas will move with velocity parallel to the directions that the Deuterium and Tritium are moving. Some of the alphas will move in the opposite direction. The ones moving in the opposite direction will presumably slow down any ions they collide with. This is what I'm confused about still. I will do some calculations, but maybe there is a solution in considering that heating occurs via electron-ion collision, and that the alpha particles move slower than the electrons. $\endgroup$
    – WillWillCUB
    Commented Aug 4, 2022 at 18:42

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Alphas heat the plasma by slowing down through kinetic energy transfer. It is a Coulomb collision problem of 2 moving particles. The slowing down equation is established from the coefficients of friction and diffusion of a fast ion of velocity V interacting with an ionic and electronic population of velocity v.

The "ashes" which are a mixture of helium, fuel and impurities coming from the walls, are recovered in the edge zone of the plasma in traps which neutralize them, then they are pumped, sorted and reinjected for some: this is active pumping, and by the way, it helps to control the fuel content of the plasma.

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