H-mode: Difference between revisions

340 bytes added ,  29 August 2009
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== Physical mechanism ==
== 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.  
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.
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>
<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>
The sheared flow can be generated by the turbulence itself via the Reynolds Stress mechanism.
 
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.
<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>
<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>
The details of this feedback mechanism are the subject of ongoing studies.
The details of the feedback mechanism between turbulence and sheared flow are the subject of ongoing studies.
<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>
<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>
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.
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.
<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>
<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>
Thus, the H-mode is the consequence of a self-organizing process in the plasma.
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]].
The mechanism is probably closely related to the mechanism for forming an [[Internal Transport Barrier]].