Observation of Precursor Magnetic Oscillations to the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>H</mml:mi></mml:math>-Mode Transition of the ASDEX Tokamak

H-mode theory [K. C. Shaing and E. C. Crume. Jr., Phys. Rev. Lett. 63. 2369 (1989)j is extended to include realistic effects such as chargeexchange momentum loss associated with neutral particles, and the magnetic stresses induced by the magnetic islands. Local critical plasma parameters for the on-set of the bifurcation are obtained and can be compared quantatively with experimental measurements. It can be concluded from the extended theory that local critical parameters are more meaningful in quantifying H-mode transition than global power threshold scaling law.

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“…For example, in ASDEX, it was observed that before the H-mode transition, rn = 4, n = 1 magnetic islands are stabilized (TOI, et al 1989). Here.…”

Section: Effects Of Magnetic Stressesmentioning

confidence: 99%

L-H transition in tokamaks and stellarators

Shaing

Hsu

Christenson

1994

Plasma Phys. Control. Fusion

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“…The propagation of the modes is discussed in Section 3. In ASDEX, beam heated plasmas, in particular those with H-type confinement behaviour, were the favoured subject of MHD investigations [4][5][6][7][8][9][10]. Modes arising from major resonant magnetic surfaces (in the majority of cases differing in m by only one, while n = l for each of them) may propagate at different angular velocities and even in different directions.…”

Section: F Wagnermentioning

confidence: 99%

MHD mode structure and propagation in the ASDEX tokamak

et al. 1991

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The Mirnov oscillations observed in the ASDEX tokamak are analysed with regard to the generally accepted interpretation scheme according to which (1) Mirnov oscillations are caused by currents flowing parallel to the magnetic field on rational magnetic surfaces and (2) the field perturbation is frozen within the plasma. If the second statement holds, the frequency is obtained from the profiles of the electron density, the ion temperature and, if applicable, the toroidal or poloidal rotation velocity. It is shown that there are modes which are consistent with the above interpretation. On the other hand, mode coupling is observed. Mode coupling is also invoked to reconcile the experimental findings with the predictions of a theory based on statement (1); according to this theory, there is a poloidal variation of both the phase velocity and the amplitude of the Mirnov oscillations. While the observed phase velocity fits well into this picture, the poloidal variation of the amplitude cannot be ascribed to only one mode in the majority of cases. In addition, the possibility of coherent MHD activity due to currents in the scrape-off layer is discussed.

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“…MHD activity is often modified sharply at the L-H transition, and usually low frequency MHD modes are suppressed at the transition (e.g., ASDEX [22], W7-AS [23]), although they are also sometimes triggered by it (LHD [24][25][26]). Interestingly, MHD fishbone activity has recently been identified as a trigger of the L-H transition at HL-2A [27].…”

Section: Introductionmentioning

confidence: 99%

Causal impact of magnetic fluctuations in slow and fast L–H transitions at TJ-II

Estrada

Carreras²,

Ascasíbar

et al. 2016

Physics of Plasmas

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This work focuses on the relationship between L-H (or L-I) transitions and MHD activity in the low magnetic shear TJ-II stellarator. It is shown that the presence of a low order rational surface in the plasma edge (gradient) region lowers the threshold density for H-mode access. MHD activity is systematically suppressed near the confinement transition.We apply a causality detection technique (based on the Transfer Entropy) to study the relation between magnetic oscillations and locally measured plasma rotation velocity (related to Zonal Flows). For this purpose, we study a large number of discharges in two magnetic configurations, corresponding to 'fast' and 'slow' transitions. With the 'slow' transitions, the developing Zonal Flow prior to the transition is associated with the gradual reduction of magnetic oscillations. The transition itself is marked by a strong spike of 'information transfer' from magnetic to velocity oscillations, suggesting that the magnetic drive may play a role in setting up the final sheared flow responsible for the H-mode transport barrier. Similar observations were made for the 'fast' transitions. Thus, it is shown that magnetic oscillations associated with rational surfaces play an important and active role in confinement transitions, so that electromagnetic effects should be included in any complete transition model.

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Observation of Precursor Magnetic Oscillations to theH-Mode Transition of the ASDEX Tokamak

Cited by 41 publications

References 12 publications

L-H transition in tokamaks and stellarators

L-H transition in tokamaks and stellarators

MHD mode structure and propagation in the ASDEX tokamak

Causal impact of magnetic fluctuations in slow and fast L–H transitions at TJ-II

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