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Field-Line Localized Destabilization of Ballooning Modes in Three-Dimensional Tokamaks.
M Willensdorfer1, T B Cote2, C C Hegna2
1Max Planck Institute for Plasma Physics, 85748 Garching, Germany.
Physical Review Letters
|September 28, 2017
Summary
Localized ballooning modes were observed in high confinement mode plasmas due to 3D perturbations. These modes agree with stability calculations, indicating reduced plasma stability caused by 3D magnetic shear distortion.
Area of Science:
- Plasma physics
- Fusion energy research
- Magnetohydrodynamics
Background:
- High confinement mode (H-mode) plasmas are crucial for fusion energy.
- Edge localized modes (ELMs) and external magnetic fields can influence plasma stability.
- Understanding plasma behavior at the edge is key to controlling fusion reactions.
Purpose of the Study:
- To investigate the occurrence and localization of field-line ballooning modes.
- To analyze the impact of rotating 3D perturbations on plasma stability.
- To compare experimental observations with theoretical stability calculations.
Main Methods:
- Observation of ballooning modes in ASDEX Upgrade H-mode plasmas.
- Induction of rotating 3D perturbations using an external n=2 error field.
- Application of edge localized mode mitigation techniques.
- Comparison with infinite-n ideal ballooning stability calculations in 3D magnetohydrodynamic equilibria.
Main Results:
- Field-line localized ballooning modes were observed at the plasma edge.
- Modes localized to specific field lines experiencing radial flux surface displacement zero crossings.
- Excellent agreement between observed mode localization and calculated stability growth rates.
- Analysis predicted reduced plasma stability compared to axisymmetric cases.
Conclusions:
- Rotating 3D perturbations significantly influence plasma edge stability.
- 3D distortion of local magnetic shear is the primary cause of reduced stability.
- Experimental findings support theoretical predictions of ballooning mode behavior in 3D equilibria.
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