Oscillating Field Current Drive in the Reversed Field Pinch
Scardovelli, Ruben Angelo Ugo
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https://hdl.handle.net/2142/70911
Description
Title
Oscillating Field Current Drive in the Reversed Field Pinch
Author(s)
Scardovelli, Ruben Angelo Ugo
Issue Date
1987
Department of Study
Nuclear Engineering
Discipline
Nuclear Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Fluid and Plasma
Abstract
Oscillating Field Current Drive (OFCD) was originally proposed as a means of maintaining a quasi steady-state plasma current in a RFP without building up the magnetizing flux threading the central hole of the torus. The MHD dynamics of OFCD is analyzed using a combination of a 1-D transport code, a linear resistive MHD stability model, and Kadomtsev nonlinear global reconnection model.
Using Faraday's law, Ohm's law and on-axis regularity conditions, it it shown that m = 1 modes are necessary for current drive, within the resistive MHD model. A class of m = 1 tearing modes that nonlinearly generate poloidal flux (a necessary condition for current drive) has been identified. These modes are destabilized by off-axis current peaks.
A scoping study of the F-$\Theta$ space has been undertaken to determine quantitatively the effectiveness of this class of tearing modes as a current drive mechanism. Results indicate that equilibria do exist, in regions of the F-$\Theta$ space that are easily accessible experimentally, that can yield significant amounts of poloidal flux generation without seriously deteriorating the confinement.
A scenario of the OFCD oscillations has been developed in terms of the evolution of the safety factor profile, by considering the stability of the RFP to current-driven tearing modes. The alternating compression and expansion of the plasma during the oscillations have been associated to two different classes of m = 1 tearing modes: the compression to modes driven by off-axis current peaks, that generate poloidal flux; the expansion to instabilities driven by on-axis current peaks, that generate toroidal flux.
Transport simulations of the single-circuit oscillations indicate that the RFP "dynamo" is definitely present during half of the oscillation period, while the other half cycle exhibits only small off-axis current peaks at best. Simulations of the two-circuit oscillations with a high current indicate, besides the dynamo, the presence of large off-axis current peaks. However, the peaks are too far in plasma radius to allow reconnection to the axis. Low current OFCD discharges excite off-axis current peaks that can reconnect to the axis and generate poloidal flux in amounts consistent with the experimentally measured increase. However, the continuous presence of the dynamo in the low current discharges makes it difficult to clearly single out the current drive effect.
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