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

在反场箍缩(RFP)聚变反应堆的研制过程中，有三个关键的MHD问题需要解决。第一种是内部撕裂模式，它在RFP动力学中起着至关重要的作用。在未来RFP不可避免地配备阻性墙的情况下，对这种模式的控制或抑制仍然非常重要。另两种是理想扭结模式，它们随场穿透时间的时间尺度而增长。内扭结模式对于平坦的电流剖面是不稳定的，而外部扭结模式对于峰值电流剖面是不稳定的。在STE-2 RFP[R/a=0.4m/0.1m]中，我们试图用共振旋转螺旋场(RHF)来主动控制阻性壁模。该设备仅在场穿透时间(Tw)约为0.15 ms的SS真空室中运行，典型等离子体电流为60kA，放电持续时间为0.7ms。在θ=2的低收缩参数(8)区中，识别出随Tw的时间尺度增长的芯谐振模(m=1/n=8，9)…更多的是IED作为阻性壁撕裂模式。当在高位运行时？区域(θ&gt；2)，m=1/n=3，4个模随tw的时间尺度增长，它们被识别为阻性壁扭结模。我们安装了脉冲振荡器，以便在RFP放电期间以恒定的幅度施加RHF。频率可在10 kHz至30 kHz之间变化，最大电流为1 kA。当f=10 kHz时，边缘径向分量的微扰幅度[Bm]约为15G(当螺旋电流为1kA时)，而当f=30 kHz时，其微扰幅度减小到5G。目标或感兴趣的共振面位于r/a=0.4附近，微扰幅度在f=25 kHz时仍为4G，这将产生磁岛，其宽度为小半径的10%。在STE-2的标准RFP等离子体中，主模m=1/n=8和低模(n&lt；8)几乎被锁定在壁面上，而高模(n&gt；9)有时会沿与等离子体电流相反的方向(CTR方向)旋转。当我们施加RHF时，共振模倾向于沿着与所施加的RHF相同的方向旋转。模式的旋转速度低于外加的RHF。除了m=L/n=8模的共振外，相邻的m=L模和m=0模的转动也受到高频的影响。当微扰幅度大于0.6%(最高可达1%)时，在磁场起伏的原始数据中可以看到明显的模式旋转。此外，音调参数θ有时倾向于增加到高于2.5时。在这种高θ状态下，谐振模有时会以与外加RHF相同的速度旋转。主模的时间平均涨落能级降低了30-50%。核心谐振模式的较低水平表明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所致。结果表明，RHF有可能用于控制带阻壁的撕裂模式。较少

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