Abstract
A dynamical model describing oscillations between multiple and single helicity configurations in the quasi-single helicity (QSH) state of the reversed field pinch [P. W. Terry and G. G. Whelan, Plasma Phys. Controlled Fusion 56, 094003 (2014)] is extended to include electron temperature profile dynamics. It is shown that QSH dynamics is linked to the electron temperature profile because the suppression of mode coupling between tearing modes proposed to underlie QSH also suppresses magnetic-fluctuation-induced thermal transport. Above the threshold of dominant-mode shear that marks the transition to QSH, the model produces temperature-gradient steepening in the strong shear region. Oscillations of the dominant and secondary mode amplitudes give rise to oscillations of the temperature gradient. The phasing and amplitude of temperature gradient oscillations relative to those of the dominant mode are in agreement with experiment. This provides further evidence that the model, while heuristic, captures key physical aspects of the QSH state.
Highlights
A revealing feature of the self-organized dynamics is the quasi-single helicity (QSH) state, in which the normally broad-band magnetic fluctuation spectrum undergoes a transition to a spectrum dominated by a single helical mode
The steep electron temperature gradient at the outer edge of the helical core in QSH plasmas has been cited as evidence of a transport barrier
Given the connection between transport barriers and turbulence suppression in the H-mode, it is natural to extend the dynamical QSH model based on suppression of nonlinear mode coupling13 to include thermal transport
Summary
The nonlinear evolution and interaction of globally unstable tearing modes in the reversed field pinch (RFP) are aspects of a self-organized state. A revealing feature of the self-organized dynamics is the quasi-single helicity (QSH) state, in which the normally broad-band magnetic fluctuation spectrum undergoes a transition to a spectrum dominated by a single helical mode. Theoretically, QSH transitions can either be induced or occur spontaneously. The limit-cycle behavior above a critical plasma current and the increase in QSH persistence with rising current have been captured successfully by a dynamical model based on an MHD description of the nonlinear coupling of an unstable dominant tearing mode and stable secondary modes under the influence of the shears in the magnetic field and flow of the dominant mode.. The relative magnitude of the temperature gradient excursion as a response to excursions of the dominant-mode amplitude is in rough agreement These results and their agreement with experiment represent an important test of the dynamical model and buttress its fundamental hypothesis that suppression of mode coupling by either the magnetic or flow shears of the dominant mode underlies the QSH state.
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