NIFS-814

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Author(s):

T. Watari, Y. Hamada, T. Notake, N. Takeuchi and K. Itoh

Title:

Geodesic Acoustic Mode Oscillation in the Low Frequency Range

Date of publication:

Sep. 2005

Key words:

geodesic acoustic mode, low frequency GAM, neo-classical polarization current

Abstract:

In this paper, the Geodesic Acoustic Mode theory is reconsidered with regard to the linear dispersion relation obtained previously by use of the drift kinetic equation. Specifically, kinetics effects in the low frequency range are handled more precisely, suggesting an answer to the question if GAM has only a single frequency of oscillation. It is shown, for a high-aspect-ratio tokamak with circular cross-section, there are two GAM frequencies. The existence of a new solution with a lower frequency is attributed to the unique features of the geodesic response functions defined in this paper. It is also shown that inclusion of kinetic effects renders the geodesic current smaller and narrows the range of GAM existences in the configuration- and the plasma parameter-spaces.
It is also shown that a system of two mixed helicity (consisting of helical and toroidal ripples) have two modes in a wide range of plasma parameters: The one with the lower frequency mode is a small modification from the tokamak type GAM and the one with higher frequency transits between tokamak GAM to helical GAM depending on the relative size of the curvature of magnetic lines of force. With the transitional plasma parameter, there could be four mode frequencies.

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