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Nonlinear Stability and Saturation of Ballooning Modes in Tokamaks
C J Ham1, S C Cowley1, G Brochard1
1CCFE, Culham Science Centre, Abingdon, Oxfordshire OX14 3DB, United Kingdom.
Physical Review Letters
|June 25, 2016
Summary
Tokamak plasma stability is analyzed for nonlinear ballooning displacements. Above critical pressure, finite displacements lower stored energy (metastability), while higher pressures cause instability, potentially explaining rapid confinement loss.
Area of Science:
- Plasma Physics
- Fusion Energy Research
- Magnetohydrodynamics
Background:
- Tokamak devices are crucial for fusion energy research.
- Understanding plasma stability is essential for achieving controlled fusion.
- Nonlinear effects in plasma confinement are not fully understood.
Purpose of the Study:
- To present a theory for tokamak stability against nonlinear ballooning displacements.
- To investigate the conditions leading to plasma metastability and instability.
- To relate theoretical predictions to experimental observations of confinement loss.
Main Methods:
- Theoretical analysis of elliptical magnetic flux tube displacements.
- Examination of plasma stability at varying pressure profiles.
- Modeling of finite (non-infinitesimal) displacements.
Main Results:
- A critical pressure profile is identified, above which finite displacements lead to metastability.
- A higher pressure profile marks the boundary for linear and nonlinear instability.
- Predicted saturated flux tube displacements are comparable to the pressure gradient scale length.
Conclusions:
- Nonlinear ballooning displacements can lead to plasma metastability and instability in tokamaks.
- These displacements and associated plasma transport may explain experimental rapid confinement loss.
- The theory provides a framework for understanding tokamak operational limits.
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