138 research outputs found
Global Linear and Nonlinear Gyrokinetic Simulations of Tearing Modes
To better understand the interaction of global tearing modes and
microturbulence in the Madison Symmetric Torus (MST) reversed-field pinch
(RFP), the global gyrokinetic code \textsc{Gene} is modified to describe global
tearing mode instability via a shifted Maxwellian distribution consistent with
experimental equilibria. The implementation of the shifted Maxwellian is tested
and benchmarked by comparisons with different codes and models. Good agreement
is obtained in code-code and code-theory comparisons. Linear stability of
tearing modes of a non-reversed MST discharge is studied. A collisionality scan
is performed to the lowest order unstable modes (, ) and shown to
behave consistently with theoretical scaling. The nonlinear evolution is
simulated, and saturation is found to arise from mode coupling and transfer of
energy from the most unstable tearing mode to small-scale stable modes mediated
by the tearing mode. The work described herein lays the foundation for
nonlinear simulation and analysis of the interaction of tearing modes and
gyroradius-scale instabilities in RFP plasmas
Quasi-single helicity spectra in the Madison Symmetric Torus
Evidence of a self-organized collapse towards a narrow spectrum of magnetic instabilities in the Madison Symmetric Torus [R. N. Dexter, D. W. Kerst, T. W. Lovell, S. C. Prager, and J. C. Sprott, Fusion Technol. 19, 131 (1991)] reversed field pinch device is presented. In this collapsed state, dubbed quasi-single helicity (QSH), the spectrum of magnetic modes condenses spontaneously to one dominant mode more completely than ever before observed. The amplitudes of all but the largest of the m=1 modes decrease in QSH states. New results about thermal features of QSH spectra and the identification of global control parameters for their onset are also discussed
Locked modes and magnetic field errors in the Madison Symmetric Torus
In the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 13 1 ( 1991)] magnetic oscillations become stationary (locked) in the lab frame as a result of a process involving interactions between the modes, sawteeth, and field errors. Several helical modes become phase locked to each other to form a rotating localized disturbance, the disturbance locks to an impulsive field error generated at a sawtooth crash, the error fields grow monotonically after locking (perhaps due to an unstable interaction between the modes and field error), and over tens of milliseconds of growth confinement degrades and the discharge event.ually terminates. Field error control has been partially su&essful in eliminating locking
Nonambipolar Magnetic-Fluctuation-Induced Particle Transport and Plasma Flow in the MST Reversed-Field Pinch
First direct measurements of nonambipolar magnetic fluctuation-induced charge transport in the interior of a high-temperature plasma are reported. Global resistive tearing modes drive the charge transport which is measured in the vicinity of the resonant surface for the dominant core resonant mode. Finite charge transport has two important consequences. First, it generates a potential well along with locally strong electric field and electric field shear at the resonant surface. Second, this electric field induces a spontaneous E B driven zonal flow
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