Nonlinear evolution of double tearing modes

The linear and nonlinear behaviors of the tearing mode is systematically st udied for nonmonotonic q-profiles on the basis of the reduced magnetohydrod ynamics (MHD) equations in cylindrical geometry, and some new features abou t the double tearing mode are revealed. The linear eigenmode scales as a re sistive internal mode for a small distance between two rational surfaces wi th the same q-value, Deltar(s), and as the tearing one for large Deltar(s). New nonlinear phenomena appear in the tearing mode regime and for shorter Deltar(s). The linear eigenfunction shows sharply localized fluid motion at both resonant surfaces and small but global convective motion between the resonant surfaces; consequently the mode goes through a Rutherford-type reg ime. When the islands have grown enough, the mode shows explosive growth. T his results from the nonlinear coupling among the higher harmonics, so that the inner and outer magnetic islands interact with each other leading to a n internal disruption. (C) 2000 American Institute of Physics. [S1070-664X( 00)04211-7].