NONLINEAR BEHAVIOR OF KELVIN-HELMHOLTZ INSTABILITY IN A HELIOTRON PLASMA AND POLOIDAL VELOCITY SHEAR

For low-beta plasma in heliotron devices, electromagnetic Kelvin-Helmh oltz instability can be excited by poloidal velocity shear due to radi al electric field. According to nonlinear calculation of single helici ty instability at iota = 1 resonant surface in a straight Heliotron E model, growth phase is governed by (m, n) = (3, 3) and (4, 4) modes an d saturation is obtained by the quasi-linear effect on the poloidal sh ear flow, where iota is a rotational transform and m(n) is a poloidal( toroidal) mode number. The linearly unstable mode with (m, n) = (1, 1) is strongly energized by the mode coupling between (m, n) = (3, 3) an d (m, n) = (4, 4). In the saturation state magnetic flux fluctuation < (phi)over tilde> is nearly equal to the electrostatic potential fluctu ation <(phi)over tilde>, which is different from the usual interchange modes with phase difference of pi/2 between <(psi)over tilde> and <(p hi)over tilde>. Magnetic island structure is also seen in resistive pl asmas, although linear growth rate is not affected by the resistivity.