Dependence of Properties of Quasi-Single-Helicity States on Field Reversal Parameter in a Low-Aspect-Ratio Reversed Field Pinch

Dependence of several properties of Quasi-Single-Helicity (QSH) states on field reversal parameter F is investigated in low-aspect-ratio reversed-field-pinch (RFP) machine RELAX. QSH probability with dominant n = 5 or 6 (n is toroidal mode number) is higher in low F region than high F. QSH probability and duration with dominant n = 3 or 4 are higher in high F region. These suggest change of innermost resonant n with F as shown in an equilibrium reconstruction. QSH probability, duration and dominant mode amplitude with the dominant mode of possible innermost resonant n = 3, 4 or 5 are higher than other n, which is desirable for improved confinement. Averages of the spectral index N_S for flat-topped plasma current phases are similar to that predicted by a simulation which has shown N_S is lower as aspect ratio is lower. This agreement and the higher QSH probability of 2 - 3 % than another high aspect ratio RFP machine may show advantage of the low aspect ratio RFP in achieving QSH.