RFP plasma response to external helical perturbations

Plasma response to four kinds of static helical perturbations with different helicities has been studied in the STE-2 reversed field pinch (RFP). With application of externally non-resonant helical fields which have no resonance between the field reversal surface and shell, RFP discharges have been slightly improved with characteristics of higher pinch parameter and suppressed coupling of m = 0 modes for perturbation level of up to . On the other hand, degradation of RFP discharges have been caused by low level of helical fields resonant inside the reversal surface with indications of enhanced m = 1 mode coupling. The degradation is shown to be caused by sufficient overlapping of the static and inherent magnetic islands, the latter being associated with m = 1 tearing modes.