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Induction of Microstreaming by Nonspherical Bubble Oscillations in an Acoustic Levitation System
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Energetic Geodesic Acoustic Modes Associated with Two-Stream-like Instabilities in Tokamak Plasmas.
Z S Qu1, M J Hole1, M Fitzgerald2
1Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia.
Physical Review Letters
|March 19, 2016
Summary
A new unstable energetic geodesic acoustic mode (EGAM) was discovered, acting like a two-stream instability. This low-frequency mode has a low fast ion density threshold, matching experimental observations.
Area of Science:
- Plasma physics
- Fusion energy research
Background:
- Energetic geodesic acoustic modes (EGAMs) are crucial for understanding plasma behavior in fusion devices.
- Previous studies have explored EGAMs, but a specific unstable branch remained unidentified.
Purpose of the Study:
- To identify and characterize a new unstable branch of the energetic geodesic acoustic mode (EGAM).
- To explain the low-frequency behavior and low density threshold of this newly found EGAM.
Main Methods:
- Utilizing fluid theory to model plasma behavior.
- Analyzing fast ions with narrow energy distributions and collective transit characteristics.
Main Results:
- Discovery of an unstable EGAM branch with a frequency significantly lower than the thermal GAM frequency.
- Identification of this mode as a reactive instability, analogous to the two-stream instability.
- Determination of a low fast ion density threshold for the mode, dependent on the fast ion transit frequency relative to the thermal GAM frequency.
Conclusions:
- The newly identified reactive EGAM explains experimental observations of mode onset in DIII-D experiments.
- The study provides insights into the transition dynamics between reactive EGAMs and velocity gradient-driven EGAMs.
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