Toroidal simulations of forced Quasi-Single Helicity in RFPs with the NIMROD code

The Quasi-Single Helical (QSH) state of Reversed Field Pinches (RFPs) is dominated by one of the m=1 helical modes with non-dominant modes also existing at finite amplitudes. Toroidal 3D NIMROD computations of MST-like RFPs with high current, no reversal and restricted five-fold toroidal periodicity were performed to promote an equilibrium QSH state with a dominant m=1, n=5 mode at saturation. Our results show characteristic Single Helical Axis (SHAx) behavior with preservation of closed helical flux surfaces near the core, surrounded by field line stochasticity due to low amplitude secondary modes. Poloidal flow profiles have a more complicated mode structure than the global m=1 flow pattern expected from the dominant mode alone. Recent core-localized 3D ion flow measurements in the SHAx state of MST also show complex flow patterns, indicating the role of toroidal and non-linear processes in sustaining the SHAx state. Inclusion of two-fluid effects and complete toroidal resolution in NIMROD simulations is the subject of ongoing work that can give us further insights into mechanisms at play in the QSH regime.