Study on Stabilization of Resistive Wall Modes in RFP's with the use of Rotating Helical Field

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Three critical MHD issues are to be addressed in the course of development of a reversed field pinch (RFP) fusion reactor. The first one is the internal tearing mode that plays essential roles in the RFP dynamics. In future RFP's inevitably equipped with resistive wall, control or suppression of this mode remains very important. The other two are ideal kink modes that grow with the time scale of field penetration time of the wall. Internal kink modes are unstable for flat current profile, while external kinks become unstable for peaked current profile. In STE-2 RFP [R/a=0.4m/0.1m], efforts have been made to actively control the resistive wall modes with resonant rotating helical field (RHF). The machine has been operated only with a SS vacuum vessel whose field penetration time(tw) is about 0.15 ms.Typical plasma current is 60 kA with discharge duration of 0.7 ms. In low pinch parameter (8) regime where θ=2, core resonant modes (with m=1/n=8,9) growing with time scale of tw are identif … More ied as the resistive wall tearing modes. When operated in high ? Regime (θ>2), m=1/n=3,4 modes grow with time scale of tw, and these are identified as resistive wall kink modes. We have installed pulsed oscillators so that RHF can be applied with constant amplitude during RFP discharge duration. The frequency can be changed from 10 kHz to 30 kHz, with maximum current of 1 kA. The perturbation amplitude of radial component at the edge [Bm] is about 15G (for helical current of 1 kA) at f=1O kHz, while it decreases to 5 G at f=30 kHz. The resonant surface of or interest lies near r/a=0.4, and the perturbation amplitude there is 4 G even at f=25 kHz, which would produce the magnetic island with the width of 10 % of the minor radius. In standard RFP plasmas in STE-2, the dominant m=1/n=8 and lower (n<8) modes are almost locked to the wall, while the higher modes (n>9) sometimes rotate in the opposite direction to the plasma current (CTR direction). When we apply the RHF, the resonant mode tends to rotate in the same direction as the applied RHF. The mode rotation velocity is lower than the applied RHF. In addition to the resonant m=l/n=8 mode, rotation of the neighboring m=l modes and m=0 modes also, are influenced by the RHF. When we increase the parturbation amplitude higher than 0.6% (to as high as 1%), a clear mode rotation can be seen in raw data of magnetic fluctuations. In addition, the pitch parameter θ sometimes tends to increase to higher than 2.5. In this high-θ regime, the resonant mode sometimes rotates at the same velocity as the applied RHF. The time-averaged fluctuation level of the dominant mode reduces by 30-50%. The lower level of core resonant modes indicates less active RFP dynamo, consistent with the higher θ trend. Although the plasma current increases and the discharge resistance decreases, no appreciable improvement is evident in RFP characteristics, which may be due to the lack of sufficient equilibraium control with increased plasma current. The present results have shown the possible use of RHF for the control of tearing modes with resistive wall. Less