EX/7-1Rb Effects of Lowering the Aspect Ratio on MHD Behaviour in a Reversed Field Pinch

Abstract. The reversed-field pinch (RFP) is a compact, high-beta magnetic confinement concept with a great advantage of weak external toroidal magnetic field. Lowering the aspect ratio (A) of RFP has the possibilities of enhanced neoclassical bootstrap current and simpler magnetic mode dynamics. Initial results from a low-A RFP machine RELAX is reported. The low-A equilibrium analyses with reconstruction technique have shown that the bootstrap current fraction is less than 5 % in the present RELAX plasmas, and, the fraction of ~25 % would be expected with improved plasma parameters having beta value of ~30%. The following indications of simple MHD mode dynamics have been observed. In round-topped discharges, transient QSH state dominated by m/n=1/4 mode appears A simple helical structure has been identified in a visible-light with fast camera diagnostic. An indication of Helical Ohmic State has been observed in round-topped discharges with very shallow reversal. These observations are consistent with MHD expectations in low-A RFP configuration. 1