Tokamak turbulence, L-H transition and density limit

5 problems

• Physical mechanism of the tokamak density limit

  Open

  Tokamaks usually disrupt or lose good confinement when the fuel density passes an empirical ceiling set by the plasma current, the Greenwald limit. Fusion power grows with density squared, so knowing what really sets this ceiling matters for reactors.

• Intrinsic toroidal rotation of ITER plasmas without external torque

  Open

  Tokamak plasmas spin around the doughnut by themselves even when nothing pushes them, because turbulence moves momentum. ITER will have little external push, and its spin matters for suppressing instabilities, but the self-generated spin cannot be predicted.

• Why heavier hydrogen isotopes confine better than theory predicts

  Partially resolved

  Simple turbulence theory says heavier fuel ions should leak heat faster, yet in experiments deuterium and tritium plasmas confine heat better than hydrogen. The reason has to be known to extrapolate to deuterium-tritium reactors.

• Origin of the density minimum and isotope dependence of L-H threshold

  Open

  The heating power needed to enter H-mode is lowest at one particular plasma density and is about twice as large in hydrogen as in deuterium. Predicting it for ITER's hydrogen, helium and deuterium-tritium phases needs an explanation of both facts.

• Physical trigger of the L-H transition

  Open

  When enough heating power is applied, a tokamak's edge turbulence switches off within a fraction of a millisecond and a steep insulating edge forms. More than forty years after its discovery, what causes the switch is not agreed.