Single helicity: a new paradigm for the reversed field pinch

The reversed held pinch (RFP) is a configuration for magnetic confinement akin to the tokamak, but its toroidal field has an amplitude comparable to the poloidal one, and reverses in the outer region. Recent progress in experiments and theory open a path beyond the standard paradigm that a bath of magnetic turbulence is intrinsic to the RFP. In RFX, the largest present RFP experiment, the existence of plasma states with a hot helical core has been proven by soft x-ray tomography. Such states have been seen transiently in several RFPs, but also permanently in discharges in RFX. This last fact makes these states germane to the stationary single-helicity (SH) states displayed by three-dimensional visco-resistive MHD simulations. These simulations also display multiple-helicity (MW) states whose features, in particular magnetic chaos, are analogous to the traditional turbulent state of RFP plasmas. The numerical MH states bifurcate to SH ones when the product (viscosity x resistivity) is increased. The SH states correspond to an integrable magnetic field with good flux surfaces, a feature favourable to good confinement. SH states are not Taylor states, but they may be viewed as the nonlinear state of a resistive kink mode stabilized by the toroidal field reversal.