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. [1] In the H-mode, the energy confinement time is
significantly enhanced, i.e., typically by a factor of 2 or more. [2] [3] H-mode profiles have a
characteristic 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. [4]
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. [5] [6]
This radial current can also actively be produced by electrode biasing. [7]
Sheared flow may be favoured by reduced viscous damping, which might explain the
dependence on rational surfaces observed in the stellarator W7-AS. [8]
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. [9] [10]
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.
References
1. ↑ F. Wagner et al, Development of an Edge Transport Barrier at the H-Mode Transition
of ASDEX, Phys. Rev. Lett. 53 (1984) 1453 - 1456
2. ↑ M. Keilhacker, H-mode confinement in tokamaks, Plasma Phys. Control.
Fusion 29 (1987) 1401-1413
3. ↑ The International Global H-mode Confinement Database
4. ↑ F. Wagner, A quarter-century of H-mode studies, Plasma Phys. Control.
Fusion 49 (2007) B1-B33
5. ↑ P.H. Diamond and Y.-B. Kim, Theory of mean poloidal flow generation by turbulence,
Phys. Fluids B 3 (1991) 1626
6. ↑ S.B. Korsholm et al, Reynolds stress and shear flow generation, Plasma Phys.
Control. Fusion 43 (2001) 1377-1395
7. ↑ R.J. Taylor et al, H-mode behavior induced by cross-field currents in a tokamak,
Phys. Rev. Lett. 63 (1989) 2365-2368
8. ↑ H. Wobig and J. Kisslinger, Viscous damping of rotation in Wendelstein 7-AS,
Plasma Phys. Control. Fusion 42 (2000) 823-841
9. ↑ 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
10. ↑ M.A. Malkov and P.H. Diamond, Weak hysteresis in a simplified model of the L-H
transition, Phys. Plasmas 16 (2009) 012504