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High Confinement Mode and Edge Localized Mode Characteristics in a Near-Unity Aspect Ratio Tokamak.
K E Thome1, M W Bongard1, J L Barr1
1Department of Engineering Physics, University of Wisconsin-Madison, 1500 Engineering Drive, Madison, Wisconsin 53706, USA.
Physical Review Letters
|May 14, 2016
Summary
Experiments with near-unity aspect ratio tokamaks reveal unique insights into H-mode plasma confinement. These ultralow-aspect-ratio devices show a significantly higher power threshold for the H-mode transition, offering new data on plasma behavior.
Area of Science:
- Plasma Physics
- Fusion Energy Research
- Magnetic Confinement Fusion
Background:
- The H-mode regime is crucial for achieving sustained fusion reactions.
- Understanding plasma confinement at different aspect ratios is key to optimizing tokamak design.
- Previous studies focused on conventional aspect ratios (A~3), leaving ultralow-aspect-ratio physics less explored.
Purpose of the Study:
- To investigate the H-mode plasma confinement regime in tokamaks with near-unity aspect ratio (A≲1.2).
- To analyze the impact of ultralow aspect ratio on the L-H power threshold and Edge Localized Modes (ELMs).
- To enable novel measurements of edge current density (J_edge) during ELMs in these unique plasma conditions.
Main Methods:
- Conducting Tokamak experiments at aspect ratios close to unity (A≲1.2).
- Measuring the power required for the L-mode to H-mode (L-H) transition (P_LH).
- Observing Edge Localized Mode (ELM) instabilities and their characteristics, including toroidal mode numbers.
- Utilizing ELMs to perform in-situ measurements of edge current density (J_edge(R,t)).
Main Results:
- The L-H power threshold (P_LH) in ultralow-aspect-ratio tokamaks is approximately 15 times higher than predicted by standard scaling laws.
- P_LH was found to be insensitive to magnetic topology variations.
- ELM instabilities exhibited a shift towards lower toroidal mode numbers as the aspect ratio (A) decreased.
- Complex, multimodal ELM collapses and the ejection of current-carrying filaments were observed, providing unprecedented J_edge(R,t) data.
Conclusions:
- Ultralow-aspect-ratio tokamak operation significantly alters the physics of the H-mode transition and ELM behavior.
- The observed high and topology-insensitive P_LH suggests unique confinement properties at A≲1.2.
- These findings challenge existing models and highlight the need for revised scaling laws for ultralow-aspect-ratio devices.
- The ability to measure J_edge(R,t) during ELMs in these regimes opens new avenues for understanding plasma-wall interactions and stability.
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