Microinstabilities and Turbulent Transport in the Reversed Field Pinch

Microinstabilities and Turbulent Transport in the Reversed Field Pinch
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Total Pages: 0
Release: 2014
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The work presented in this thesis is concerned with addressing the nature of drift wave microturbulence in the reversed field pinch (RFP). Microturbulence is an important phenomenon and contributor to heat and particle transport in tokamaks, where it has been studied for several decades, but its role in the RFP is a rather new topic of study. As such, the nature of RFP drift waves and their relationship to their tokamak counterparts is still developing, and many of the results in this work are focused on addressing this challenge. Fundamental advances in microturbulence research have been made in recent decades through two parallel developments: the theoretical framework encompassed in the gyrokinetic model, and the computational power offered by massively-parallel, high-performance computing systems. Gyrokinetics is a formulation of kinetic theory in such a way that the fast timescale gyromotion of particles around magnetic field lines is averaged out. The implementation and use of RFP equilibrium models in gyrokinetic codes constitutes the bulk of this thesis. A simplified analytic equilibrium, the toroidal Bessel function model (TBFM), is used in the gyrokinetic code GYRO to explore the fundamental scaling properties of drift waves in the RFP geometry. Two drift wave instabilities, the ion temperature gradient (ITG) mode and the microtearing mode (MTM) are found to occur, and the relationship of their critical threshold in driving gradients and plasma beta is explored. The critical values in these parameters are found to be above those of similar tokamak cases by roughly a factor of the flux surface aspect ratio. The MTM is found to be stabilized by increasing the RFP pinch parameter theta, making it unlikely for it to unstable in the high-theta improved confinement pulsed poloidal current drive (PPCD) discharges. Efforts are also made to address microinstabilities in specific experimental discharges of the Madison Symmetric Torus (MST). A semi-analytic equilibrium, the adjusted circular model (ACM), is developed and implemented in the gyrokinetic code GENE to investigate representative PPCD discharges. The flexibility of the ACM enables it to be used for the high-theta PPCD discharges where the TBFM breaks down. The dominant linear instabilites for the discharges modeled here -- ITG and the trapped electron mode (TEM) -- are studied, as are their scaling properties in the PPCD regime. It is found that these instabilities are present outside of the reversal surface, where the driving gradients are strongest. Nonlinear simulations of the TEM turbulence are performed, the first such done for the RFP, and zonal flows are found to play an important role in the nonlinear saturation mechanism. These zonal flows lead to a large Dimits-like shift and suppressed transport. There is also evidence that residual global tearing mode fluctuations are a necessary part of modeling transport in the RFP, even in improved confinement PPCD discharges, and by modeling these residual fluctuations through the use of an externally imposed perpendicular magnetic field perturbation it is possible to bring simulated fluxes into agreement with experiment. Finally, the nature of the collisionless MTM, an instability seen to arise in some parameter regimes of the RFP, is investigated analytically using a fluid expansion in the drift-kinetic framework. Particular attention is paid to the role of magnetic drifts, and some evidence for their role in the collisionless instability is presented. Comparisons are made to gyrokinetic simulations and to earlier theory on the magnetic-curvature drift instability.

Energy Research Abstracts

Energy Research Abstracts
Author:
Publisher:
Total Pages: 782
Release: 1995
Genre: Power resources
ISBN:

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Characterizing the Role of Drift-wave Turbulence in a Reversed Field Pinch

Characterizing the Role of Drift-wave Turbulence in a Reversed Field Pinch
Author: Takashi Nishizawa
Publisher:
Total Pages: 0
Release: 2018
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ISBN:

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Trapped-electron-mode (TEM) microturbulence appears in the edge of MST RFP plas- mas that have reduced tearing instability and tokamak-level confinement using current profile control. High-frequency density fluctuations (k[modifier letter up tack][small rho]s = 0.2 − 0.4) emerge with a critical gradient threshold as the density profile steepens. These features are consistent with gyrokinetic simulations using GENE that include a small magnetic fluctuation mimicking residual tearing activity, which tends to disrupt zonal flow formation. Here we present direct measurements of impurity transport and zonal flows to investigate TEM turbulence saturation and transport. A new method of linearized spectrum cor- relation analysis for spectroscopic data resolves simultaneously the fluctuations in both the turbulent radial velocity (vr ∼ 1km/s) and impurity density. Their correlation reveals an inward flux of C2+ impurities, which is the first direct evidence for trans- port associated with TEM turbulence in the RFP. The C2+ ions are edge localized and evolve from graphite limiters. The profile of the plasma potential is measured in the edge using two multi-channel capacitive probes, each having 7 mm radial spatial reso- lution. An edge-localized flow is observed, and with the probes separated 180 degrees toroidally, the flow has a long-range correlation characteristic of zonal structure. The amplitude of the flow is modulated by the turbulence, as occurs in predator-prey-like dynamics. These measurements, together with the gyrokinetic modeling, suggest that transport in RFP plasmas will ultimately be regulated by microturbulence as occurs in tokamak and stellarator plasmas.

Science Abstracts

Science Abstracts
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Publisher:
Total Pages: 1228
Release: 1992
Genre: Physics
ISBN:

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Rotation And Momentum Transport In Magnetized Plasmas

Rotation And Momentum Transport In Magnetized Plasmas
Author: Philippe Ghendrih
Publisher: World Scientific
Total Pages: 336
Release: 2014-12-30
Genre: Science
ISBN: 9814644846

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This book compiles the contributions from various international experts on magnetized plasma physics, both in controlled fusion and in astrophysics, and on atmospheric science. Most recent results are presented along with new ideas. The various facets of rotation and momentum transport in complex systems are discussed, including atmospheric-ocean turbulence, the constraints, and the concept of potential vorticity. The close interplay between flows and magnetohydrodynamics dynamo action, instabilities, turbulence and structure dynamics are the main focus of the book, in the context of astrophysics and magnetic fusion devices like Tokamak, and Reversed Field Pinch. Both physicists and advanced students interested in the field will find the topics as interesting as researchers from other fields who are looking to broaden their perspectives.

Government Reports Annual Index

Government Reports Annual Index
Author:
Publisher:
Total Pages: 1088
Release: 1991
Genre: Government reports announcements & index
ISBN:

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