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, induced by 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. The sheared flow is generated by the turbulence itself via the Reynolds Stress mechanism. Thus, the H-mode is the consequence of a self-organizing process in the plasma. The details of this mechanism are the subject of ongoing studies.

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.