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

在反场捏缩（RFP）聚变反应堆的开发过程中，需要解决三个关键的MHD问题。第一种是内部撕裂模式，在RFP动力学中起着至关重要的作用。在未来的RFP中，不可避免地会配备阻力墙，控制或抑制这种模式仍然非常重要。另外两种是理想的扭结模式，它们随电场穿透壁时间的时间尺度而增长。内部扭结模式在平坦电流曲线下不稳定，而外部扭结模式在峰值电流曲线下不稳定。在STE-2 RFP [R/a=0.4m/0.1m]中，利用共振旋转螺旋场（RHF）主动控制了阻壁模态。该机器仅与SS真空容器一起操作，其场穿透时间（tw）约为0.15 ms。典型的等离子体电流为60 kA，放电持续时间为0.7 ms。在θ=2的低夹紧参数(8)区，芯层共振模式（m=1/n=8,9）随时间尺度为2而增长，主要为阻壁撕裂模式。在高位操作时？Regime （θ>2）、m=1/n=3、4种模态随时间尺度tw的增大而增大，这些模态被确定为电阻壁结模态。我们安装了脉冲振荡器，以便在RFP放电期间以恒定幅度施加RHF。频率可以在10khz到30khz之间变化，最大电流为1ka。在f= 10khz时，边缘径向分量的扰动幅值[Bm]约为15G（螺旋电流为1ka时），而在f= 30khz时则降至5g。感兴趣的谐振面位于r/a=0.4附近，即使在f=25 kHz时，那里的扰动幅值仍为4 G，这将产生宽度为小半径10%的磁岛。在STE-2的标准RFP等离子体中，优势的m=1/n=8和较低的（n<8）模式几乎锁定在壁上，而较高的模式（n>9）有时与等离子体电流（CTR方向）相反方向旋转。当我们施加RHF时，谐振模式倾向于与施加的RHF方向相同的旋转。模态旋转速度低于应用的RHF。除了共振的m=l/n=8模式外，邻近的m=l模式和m=0模式的旋转也受到RHF的影响。当我们将扰动幅值增加到高于0.6%（高达1%）时，可以在磁波动的原始数据中看到明显的模式旋转。此外，螺距参数θ有时有增大到2.5以上的趋势。在这个高θ区，谐振模式有时以与外加RHF相同的速度旋转。优势模态的时间平均波动水平降低了30-50%。磁芯谐振模式的低电平表明RFP发电机的活跃程度较低，这与较高的θ趋势一致。虽然等离子体电流增大，放电电阻减小，但RFP特性没有明显改善，这可能是由于等离子体电流增大时缺乏足够的平衡控制。目前的研究结果表明，RHF可以用于控制具有电阻壁的撕裂模式。少

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