The reversed field pinch (RFP) is characterized by low-q (safety factor), high-shear magnetic configuration, dominated by magnetic fluctuations excited by resistive MHD instabilities. Recent success in controlling the resistive instabilities by current profile modification has resulted in confinement improvement, leading to the tokamak quality confinement. As in tokamaks, the resistive wall modes (RWMs) are thought to be serious problems in RFPs with life time much longer than the field penetration time of the vessel. In this paper, we will discuss the RWM dynamics in the STE-2 RFP with rotating helical field (RHF).
The STE-2 is a small RFP machine (R/a=0.4m/0.1m, I_p<80 kA, τ_d <1 ms) operated only with a SS vacuum chamber.[1,2] In typical RFP configurations, the safety factor on axis is about 0.15, decreasing towards the edge to about -0.05. It has been observed that, when the pinch parameter Θ is lower than about 2, internally resonant m=1/n=8-9 modes grow, while the nonresonant m=1/|n|=3,4 modes grow instead, when Θ is higher than 2. Here Θ is the ratio of the edge poloidal field B_θa to the average toroidal field <B_φ>. The dependence on Θ of the dominant toroidal mode number, and their growth time as well, are in good agreement with the linear theory prediction of RWM in an RFP.
The efforts have been made to control these RWMs with resonant RHF. We have applied resonant M/N=1/8 RHF with frequency from 10 kHz to 30 kHz, where M (N) stands for the poloidal (toroidal) mode number of the RHF. In the present experiments, the perturbation level has been increased up to 1 % (radial field amplitude normalized by the poloidal field at the edge). The maximum perturbation would produce an island with the width of 10 % of minor radius at the resonant surface. At the frequency of 10 kHz, the resonant m=1/n=8 mode could be driven to rotate at the same velocity as that of RHF, while no significant effect was observed on the neighboring m=1/n=7 or m=1/n=9 modes. The intrinsic m=1/n=8 mode amplitude decreased by 50 % with RHF; the neighboring mode amplitudes were not influenced very much by RHF. The rather limited effect of the RHF may be an indication of the weak viscous coupling of the dominant magnetic island with neighboring ones.

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