For low-beta plasma in heliotron devices, electromagnetic Kelvin-Helmholtz instability can be excited by poloidal velocity shear due to radial electric field. According to nonlinear calculation of single helicity 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 and saturation is obtained by the quasi-linear effect on the poloidal shear 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) and (m, n)=(4, 4). In the saturation state magnetic flux fluctuation psi is nearly equal to the electrostatic potential fluctuation phi , which is different from the usual interchange modes with phase difference of pi /2 between psi and phi . Magnetic island structure is also seen in resistive plasmas, although linear growth rate is not affected by the resistivity.
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