Computational modeling of quasi-single helicity states in an RFP*

Experiments have shown that RFP plasmas can self-organize to a quasi-single helicity (QSH) equilibrium with a helical axis [D.F. Escande, et al., Phys. Rev. Lett. 85, 1662 (2000)]. These states have improved confinement and lower magnetic turbulence levels compared to a standard RFP plasma. These experiments all have circular, or nearly-circular cross-sections. This work explores the impact of boundary shaping on access to quasi-single helicity states in reverse-field-pinch (RFP) plasmas. The VMEC code can obtain computational ideal MHD equilibria with a helical axis and a symmetric boundary [J.D. Hanson, et al., Nucl. Fusion 53, 083016 (2013)]. In this work, we analyze the VMEC input parameters that control access to QSH states and test the impact of 2D-shaping of the boundary on RFP equilibria. The effect of increasing elongation and triangularity are tested systematically. Increased elongation results in lower plasma current for the same safety factor profile and a larger radial excursion of the helical axis in a QHS state. Optimization of the boundary coefficients targeting an increased radial excursion of the helical axis is undertaken. Results will be presented.