TOROIDAL COUPLING OF DISPARATE HELICAL TEARING MODES

An analytical model has been developed for toroidal coupling of tearin g modes with different helicities in the low-beta and large aspects ra tio tokamaks. A standard characteristic value problem has been natural ly composed according to the structure of magnetohydrodynamic (MHD) so lutions. The explicit general dispersion relation has been obtained by the asymptotic matching. The growth rates (characteristic values) and corresponding flux perturbations (characteristic functions) of the to roidal tearing mode have been derived. The toroidal coupling plays a r ole mainly through the correction for the ideal MHD solutions. Without loss of generality, variation tendency of growth rates has been analy zed for a toroidal tearing mode with cylindrical components m/n and (m + 1)/n, the results indicate that (1) The toroidal coupling has a des tabilizing effect on the tearing modes; (2) the ''beating'' of the gro wth rates of two components leads to strong coupling, even if the coup ling parameter C is quite small, and the coupling effect does not expl icitly rely on magnitude of DELTA(m)'(0) of cylindrical component, so that DELTA(m)'(0) is similar to epsilon is neither a necessary nor suf ficient condition for strong coupling.