Sawteeth in the MST Reversed Field Pinch

Details Sawteeth in the MST Reversed Field Pinch Sawteeth in the MST Reversed Field Pinch

Jan 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990phdt.......183b&link_type=abstract

Thesis (PH.D.)--THE UNIVERSITY OF WISCONSIN - MADISON, 1990.Source: Dissertation Abstracts International, Volume: 51-12, Sectio

Astronomy and Astrophysics

Astrophysics

8

Plasma

Scientific paper

A dynamo mechanism has been used in astrophysics to explain the self-generation of the magnetic fields observed throughout the universe. This same type of phenomenon is believed to occur in the reversed field pinch (RFP) plasmas. The RFP dynamo has been a major theoretical and experimental investigation since the first observations of the self-reversal process in the early pinch research. A discrete dynamo event has been observed in the experimental RFP plasmas; this event is termed the RFP sawtooth. This phenomenon is similar to the sawtooth phenomenon observed in tokamak plasmas, but the two events differ in many respects. Both events are a result of the inward diffusion of the plasma current density. This radial diffusion peaks the current profiles causing the plasma to become unstable to the m = 1 tearing modes. In the tokamak, it is the (m,n) = (1,1) mode while in the RFP, several m = 1 modes can become unstable with the n spectra dominated by the n ~ 2R/a mode. It has been shown theoretically that the nonlinear interaction of these modes can generate the reversed toroidal field in the RFP. During the rise-phase of the sawtooth, the plasma is undergoing a purely diffusive process--no dynamo is occurring during this phase. The dynamo only occurs during the sawtooth crash. During the rise-phase, the m = 1 modes are observed to grow, and nonlinear interactions are observed prior to the sawtooth crash. At the time of the crash, many of the plasma profiles are flattened; these include the current density, the plasma temperature and the plasma density. The flattening of these profiles restabilizes the plasma to the m = 1 tearing modes. This thesis is a study of the RFP sawtooth phenomenon on the MST RFP. This includes experimental observations as well as 1-D numerical simulations of the sawtooth rise time. Comparisons will be made between the experimental observations observed on the MST and those observed on other RFPs throughout the world, as well as to the theoretical predictions.

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