Microtearing modes in the tokamak pedestal




Larakers, Joseph Leland

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An economical thermonuclear reactor has the potential to serve as an on-demand, clean, and abundant energy source. The main difficulty is confining the fusion fuel to the large pressures required for the fuel to react. The energy produced from fusion reactions must be collected and confined such that it activates further reactions. Magnetic confinement is a promising strategy. Magnetic confinement devices, such as tokamaks, have steadily improved by identifying and suppressing different mechanisms of heat transport and instability.

This dissertation focuses on a single mechanism known as the microtearing mode (MTM). The microtearing mode is an electromagnetic excitation that is localized about rational magnetic surfaces and is driven unstable by electron temperature gradients. The mode tears magnetic surfaces and modifies their structure. The resulting topology relaxes the radial temperature gradient via fast parallel motion. The MTM has recently gained attention as a potentially important instability in the pedestal region of H-mode tokamaks. It is theorized to be responsible for the anomalous electron heat transport and discrete bands of magnetic fluctuations observed experimentally.

Here, we revisit the conventional microtearing theory and extend it to study features pertinent to the pedestal region. In doing so, we identify a new crucial parameter for MTM linear stability. This extended theory matches with the experimental observations of magnetic fluctuations and provides an explanation of their discrete nature. With an understanding of the linear dispersion characteristics, we proceed to study the nonlinear evolution of the mode. The dispersion suggests a strong mode-mode resonance between MTM harmonics. A weak turbulence model has been developed to study the nonlinear consequences of these resonances.



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