CONTROL OF RFP DYNAMICS WITH HELICAL FIELDS

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Control of the dynamics of reversed field pinches (RFPs) is one of the urgent issues for the improvements of RFP experiments. In RFPs with a resistive wall, both resistive and ideal modes can become unstable in the time scale of field penetration time of the wall. The growth of core resonant tearing modes would result in highly stochastic magnetic field, leading to significant degradation. Theories have predicted that the growth of tearing modes or their saturation amplitudes can be reduced by toroidal rotation of the plasma.In this research project, first attempts have been made to drive magnetic fluctuation mode and/or plasma rotation using rotating M/N=1/8 resonant helical field, where M (N) is the poloidal (toroidal) mode number of the applied field.In standard STE-2 (R/α=0.4m/0.1m) discharges with only the vacuum vessel whose time constant TィイD2wィエD2?0.15 ms, the plasma current IィイD2pィエD2 is around 60 kA with discharge duration TィイD2dィエD2 around 0.7 ms. The discharge resistance is … More about 50% higher than with a close-fitting copper shell of 2 ms time constant, and TィイD2dィエD2 shorter by about 30-40%. After attaining the RFP configuration, the magnetic fluctuations grow whose dominant toroidal mode number n appears to be 8, maintaining the structure for the rest of discharge. The m/n=1/8 mode is the tearing mode whose resonant surface is located at r/α〜0.4. The toroidally rotating helical field (RHF) was applied with two pairs of helical coils together with alternating current sources with a phase difference of π/2. The frequency was 10-20kHz, and the toroidal phase velocity of the rotating field was 4-7 km/s. Since we have used LC damping oscillation to obtain the alternating current in the present research project, effective duration of RHF is restricted to not longer than 0.3-0.4ms. The amplitude of the RHF |BィイD2rαィエD2| is defined as an average peak value from the second to the fifth peak values measured inside the vessel. The RHF was applied at t=0.3 ms, with f〜15 kHz and relative amplitude |BィイD2rαィエD2|/BィイD2θαィエD2 of 0.5%, where BィイD2θαィエD2 is the edge poloidal field. The amplitude of magnetic fluctuations is reduced, and, furthermore, the fluctuations rotate toroidally in the direction of the applied RHF for the amplitude level higher than 0.4%. The phase velocity of magnetic fluctuations is slightly lower than that of the RHF. The time evolution of the n=7, 8, 9 mode amplitudes with the RHF of relative amplitude of 0.5% shows that the growth of these mode amplitudes is suppressed transiently for 0.2-0.3 ms, and, moreover, the amplitudes are reduced for the rest of the discharge.It is the first demonstration in the RFP of the direct interaction between the rotating M=1 helical field with inherent m=1 core resonant tearing modes. Less