Behaviour of discharges with deep reversal in HBTX1B

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Abstract
In HBTX1B Reversed Field Pinch, the normal discharges have theta =1.4 and F=-0.1. When the discharges are deeply reversed with F<-0.35, the plasma resistivity and density double and the edge magnetic field fluctuations increase from 1-2% to 5-10%. In these deeply reversed discharges, sawtooth-like activities are observed in the poloidal mode m=0 magnetic fluctuations. In normal discharges, the m=1 magnetic fluctuations are resonant inside the reversal surface and have a broad toroidal spectrum in the range of mod n mod =5-17. However, when F<-0.35, the m=1 magnetic fluctuations are dominated by a rotating helical magnetic structure resonant outside the reversal surface. A toroidal mode with n approximately=4 in these discharges was measured. These properties resemble the Taylor's helical minimum energy state (m=1 and n=4 for HBTX1B). According to Taylor (1974), the helical state emerges when theta reaches and saturates at 1.6. In experiment, the relationship between F and theta is not uniquely defined and theta >1.6 is observed, indicating that the plasma has not fully achieved the fully relaxed minimum energy state.