Burning plasmas and alpha-particle physics

4 problems

• Alpha-particle losses driven by Alfven eigenmodes in ITER

  Open

  Fusion-born alpha particles move about as fast as magnetic waves in the plasma, so they can feed those waves, and the waves can push alphas outward before they deliver their heat. How many alphas ITER would lose this way is not reliably predicted.

• Do $\alpha$ particles reduce or increase turbulent heat loss

  Contested

  Besides heating, $\alpha$ particles change the plasma's turbulence. Simulations disagree on whether this makes confinement better or worse.

• Thermal stability and control of a self-heated tokamak plasma

  Open

  When most heating comes from fusion itself, a small temperature rise can raise fusion power and heat the plasma further. Whether a reactor plasma stays steady and can be steered with available controls has not been tested.

• Can helium ash be exhausted fast enough in a reactor

  Open

  Each fusion reaction leaves a helium nucleus behind, and if helium accumulates it dilutes the fuel and stops the burn. It must be removed faster than a certain rate, and it is not certain reactors can do so.