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.    H-mode profiles have a characteristic edge pedestal.
This transport bifurcation is due to 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, or favour its growth:
- The main process for sheared flow generation is generation by the turbulence itself via the Reynolds stress mechanism. Simply put, transport generated by the fluctuations produces a radial current jr that spins up the plasma via the j × B Lorentz force.  
- This radial current can also actively be produced by electrode biasing. 
- Sheared flow may be favoured by reduced viscous damping, which might explain the dependence on rational surfaces observed in the stellarator W7-AS. 
- Sheared flow can also be generated 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.
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