Quasi-single-helicity reversed-field-pinch plasmas

The reversed field pinch (RFP) is a configuration for plasma magnetic confi nement. It has been traditionally viewed as dominated by a bath of MHD inst abilities producing magnetic chaos and high energy transport. We report exp erimental results which go beyond this view. They show a decrease of magnet ic chaos and the formation of a coherent helical structure in the plasma, w hose imaging and temperature profile are provided for the first time. These quasi-single-helicity states are observed both transiently and in stationa ry conditions. The last case is consistent with a theoretically predicted b ifurcation. Our results set a new frame for improving confinement in high c urrent nonchaotic RFP's.