View source for 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. 
<ref>F. Wagner et al, ''Development of an Edge Transport Barrier at the H-Mode Transition of ASDEX'', [[doi:10.1103/PhysRevLett.53.1453|Phys. Rev. Lett. '''53''' (1984) 1453 - 1456]]</ref>
In the H-mode, the [[Energy confinement time|energy confinement time]] is significantly enhanced, i.e., typically by a factor of 2 or more.
<ref>M. Keilhacker, ''H-mode confinement in tokamaks'', [[doi:10.1088/0741-3335/29/10A/320|Plasma Phys. Control. Fusion '''29''' (1987) 1401-1413]]</ref>
<ref>F. Wagner et al., ''H-mode of W7-AS stellarator'', [[doi:10.1088/0741-3335/36/7A/006|Plasma Phys. Control. Fusion '''36''' (1994) A61]]</ref>
<ref>[http://efdasql.ipp.mpg.de/HmodePublic/ The International Global H-mode Confinement Database]</ref>
H-mode profiles have a characteristic ''[[Pedestal|edge pedestal]]''.

== Physical mechanism ==

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.
<ref>F. Wagner, ''A quarter-century of H-mode studies'', [[doi:10.1088/0741-3335/49/12B/S01|Plasma Phys. Control. Fusion '''49''' (2007) B1-B33]]</ref>

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 ''j<sub>r</sub>'' that spins up the plasma via the ''j'' &times; ''B'' [[:Wikipedia:Lorentz force|Lorentz force]]. <ref>P.H. Diamond and Y.-B. Kim, ''Theory of mean poloidal flow generation by turbulence'', [[doi:10.1063/1.859681|Phys. Fluids B '''3''' (1991) 1626]]</ref> <ref>S.B. Korsholm et al, ''Reynolds stress and shear flow generation'', [[doi:10.1088/0741-3335/43/10/308|Plasma Phys. Control. Fusion '''43''' (2001) 1377-1395]]</ref>
* This radial current can also actively be produced by electrode biasing. <ref>R.J. Taylor et al, ''H-mode behavior induced by cross-field currents in a tokamak'', [[doi:10.1103/PhysRevLett.63.2365|Phys. Rev. Lett. '''63''' (1989) 2365-2368]]</ref>
* Sheared flow may be favoured by reduced viscous damping, which might explain the dependence on rational surfaces observed in the stellarator W7-AS. <ref>H. Wobig and J. Kisslinger, ''Viscous damping of rotation in Wendelstein 7-AS'', [[doi:10.1088/0741-3335/42/7/306|Plasma Phys. Control. Fusion '''42''' (2000) 823-841]]</ref>
* Sheared flow can also be generated by external momentum input.

The details of the feedback mechanism between turbulence and sheared flow are the subject of ongoing studies.
<ref>P.H. Diamond et al, ''Self-Regulating Shear Flow Turbulence: A Paradigm for the L to H Transition'', [[doi:10.1103/PhysRevLett.72.2565|Phys. Rev. Lett. '''72''' (1994) 2565 - 2568]]</ref>
<ref>M.A. Malkov and P.H. Diamond, ''Weak hysteresis in a simplified model of the L-H transition'', [[doi:10.1063/1.3062834|Phys. Plasmas '''16''' (2009) 012504]]</ref>

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]].

== See also ==

* [[Edge Localized Modes]]

== References ==
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