Nonlinear three-dimensional magnetohydrodynamic (MHD) simulations were applied to a reversed-field pinch (RFP) plasma to reveal the physical mechanism of the formation of helical structures such as the so-called quasi-single helicity and single helical axis states. The simulations were executed using the MHD Infrastructure for Plasma Simulation (MIPS) code in a realistic experimental geometry of the REversed field pinch of Low-Aspect ratio eXperiment (RELAX) device with reconstructed initial equilibria calculated by the RELAXFit code. Long-term evolutions showed remarkable formation of n = 4 structures as a result of the dominant growth of resistive modes. The resultant relaxed helical state consists of a bean-shaped, hollow pressure profile in the poloidal cross section for both the cases of resonant and non-resonant triggering instabilities.
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