Ideal saturated MHD helical structures in axisymmetric hybrid plasmas

Kinked saturated m = 1 helical structures are frequently observed in tokamak hybrid plasmas and in reversed field pinches (RFP). These modes occur when an extremum in the safety factor is close to, but necessarily resonant with, a low order rational (typically qmin ≈ 1/1 in tokamaks, and qmax ≈ 1/7 in RFPs). If the exact resonance can be avoided, the essential character of these modes can be modelled assuming ideal nested magnetic flux surfaces. The methods used to characterize these structures include linear and nonlinear ideal magnetohydrodynamic stability calculations, which evaluate the departure from an axisymmetric plasma state, or equilibrium calculations using a 3D equilibrium code. The extent to which these approaches agree in tokamaks and reverse field pinches is investigated, and compared favourably for the first time with an analytic nonlinear treatment that is valid for arbitrary toroidal mode number.