Single helicity: a new paradigm for the reversed field pinch

Abstract

The reversed field pinch (RFP) is a configuration for magnetic confinementakin to the tokamak, but its toroidal field has an amplitude comparable to thepoloidal one, and reverses in the outer region. Recent progress in experimentsand theory open a path beyond the standard paradigm that a bath of magneticturbulence is intrinsic to the RFP. In RFX, the largest present RFPexperiment, the existence of plasma states with a hot helical core has beenproven by soft x-ray tomography. Such states have been seen transiently inseveral RFPs, but also permanently in discharges in RFX. This last fact makesthese states germane to the stationary single-helicity (SH) states displayedby three-dimensional visco-resistive MHD simulations. These simulations alsodisplay multiple-helicity (MH) states whose features, in particular magneticchaos, are analogous to the traditional turbulent state of RFP plasmas. Thenumerical MH states bifurcate to SH ones when the product(viscosity×resistivity) is increased. The SH states correspond to anintegrable magnetic field with good flux surfaces, a feature favourable togood confinement. SH states are not Taylor states, but they may be viewed asthe nonlinear state of a resistive kink mode stabilized by the toroidal fieldreversal.