H-mode

When a magnetically confined plasma is heated strongly, it may spontaneously transition from a low confinement (or L-mode) state to a high confinement (or H-mode) state. ^[1] In the H-mode, the energy confinement time is significantly enhanced, i.e., typically by a factor of 2 or more. ^[2]

This transport bifurcation is the consequence of the suppression of turbulence in the edge plasma, induced by a sheared flow layer and an associated edge radial electric field. ^[3] The precise mechanism governing this phenomenon is the subject of ongoing studies.

The transition occurs spontaneously when a certain threshold value of the heating is exceeded.

The steep edge gradients (of density and temperature) lead to quasi-periodic violent relaxation phenomena, known as Edge Localized Modes (ELMs), which have a strong impact on the surrounding vessel.

References

[1] F. Wagner et al, Development of an Edge Transport Barrier at the H-Mode Transition of ASDEX, Phys. Rev. Lett. 53 (1984) 1453 - 1456

[2] M. Keilhacker, H-mode confinement in tokamaks, Plasma Phys. Control. Fusion 29 (1987) 1401-1413

[3] F. Wagner, A quarter-century of H-mode studies, Plasma Phys. Control. Fusion 49 (2007) B1-B33

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