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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.
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<ref>[http://link.aps.org/doi/10.1103/PhysRevLett.53.1453 F. Wagner et al, ''Development of an Edge Transport Barrier at the H-Mode Transition of ASDEX'', 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>[http://dx.doi.org/10.1088/0741-3335/29/10A/320 M. Keilhacker, ''H-mode confinement in tokamaks'', Plasma Phys. Control. Fusion '''29''' (1987) 1401-1413]</ref>
<ref>[http://efdasql.ipp.mpg.de/HmodePublic/ The International Global H-mode Confinement Database]</ref>
 
== 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>[http://dx.doi.org/10.1088/0741-3335/49/12B/S01 F. Wagner, ''A quarter-century of H-mode studies'', 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>[http://dx.doi.org/10.1063/1.859681 P.H. Diamond and Y.-B. Kim, ''Theory of mean poloidal flow generation by turbulence'', Phys. Fluids B '''3''' (1991) 1626]</ref> <ref>[http://dx.doi.org/10.1088/0741-3335/43/10/308 S.B. Korsholm et al, ''Reynolds stress and shear flow generation'', Plasma Phys. Control. Fusion '''43''' (2001) 1377-1395]</ref>
* This radial current can also actively be produced by electrode biasing. <ref>[http://link.aps.org/doi/10.1103/PhysRevLett.63.2365 R.J. Taylor et al, ''H-mode behavior induced by cross-field currents in a tokamak'', 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>[http://dx.doi.org/10.1088/0741-3335/42/7/306 H. Wobig and J. Kisslinger, ''Viscous damping of rotation in Wendelstein 7-AS'', 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>[http://link.aps.org/doi/10.1103/PhysRevLett.72.2565 P.H. Diamond et al, ''Self-Regulating Shear Flow Turbulence: A Paradigm for the L to H Transition'', Phys. Rev. Lett. '''72''' (1994) 2565 - 2568]</ref>
<ref>[http://link.aip.org/link/?PHPAEN/16/012504/1 M.A. Malkov and P.H. Diamond, ''Weak hysteresis in a simplified model of the L-H transition'', 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 ==
<references />
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