H-mode

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

Physical mechanism

This transport bifurcation is the consequence of the suppression of turbulence in the edge plasma. There is substantial evidence that the suppression of turbulence is the consequence of the formation of a sheared flow layer and an associated edge radial electric field. The local suppression of turbulence leads to a reduction of transport and a steepening of the edge profiles.

A variety of mechanisms can give rise to sheared flow. The main process for sheared flow generation is generation by the turbulence itself via the Reynolds Stress mechanism. The details of the feedback mechanism between turbulence and sheared flow are the subject of ongoing studies.

However, other factors can also contribute, such as the viscous damping, which might explain the dependence on rational surfaces observed in the stellarator W7-AS. Sheared flow can also be generated by imposing an external radial electric field (biasing), or by external momentum input.

In summary, the H-mode is the consequence of a self-organizing process in the plasma. The mechanism is probably closely related to the mechanism for forming an Internal Transport Barrier.

ELMs

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. Although Quiescent H-modes exist (without ELMs), they are generally considered not convenient due to the accumulation of impurities. To achieve steady state, an ELMy H-mode is preferred and this mode of operation is proposed as the standard operating scenario for ITER, thus converting ELM mitigation into a priority.