Experimental investigation of quasi-steady current driving for spheromak plasmas by means of magnetic helicity injection.

为了证明用磁螺旋度注入法维持球马克等离子体磁位形的可行性，对球马克等离子体的等离子体电流进行了外部驱动。在东京大学TS-3球形马克机上对螺旋度注入的传导型和感应型进行了检验。在适当的当前驾驶条件下。Spheromak配置保持的时间比非驱动条件长三到五倍。本文介绍了一种用于观测等离子体不稳定运动的可见光计算机层析成像系统。本实验采用一种新研制的三向十通道计算机层析成像系统，观测了等离子体中平面上可见光二维发射轮廓的时间演化。该系统可以通过基于Fourier-Bessel-Neumann展开的Cormack方法重建发射轮廓。从重建的图像，松弛被发现是由逆级联现象，其中具有较高的环形模式数的不稳定性被激发，其次是能量传输到那些具有较低的模式数。还进行了磁测量，并观察到与逆级联现象相关的磁通量从极向到环向的转换。还进行了三维磁流体动力学计算机模拟，研究了球马克位形在螺旋度注入期间的动力学行为。结果表明，由电流峰化阶段和随后的MHD弛豫阶段组成的磁弛豫周期在磁通量从极向向环向的转换中起着非常重要的作用。在磁通变换过程中，模数n=2和n=3的电流驱动MHD不稳定性发生非线性耦合。这些发现与断层观测的MHD活动定性一致。

External drive of the plasma current of spheromak plasmas has been performed to demonstrate the feasibility of sustaining their magnetic configurations by the magnetic helicity injection method. The both of the conductive type and the inductive type of the helicity injection have been examined in the Tokyo University spheromak machine TS-3. Under appropriate current driving conditions. the spheromak configuration is maintained three to five times longer than non-driving conditions. A visible light computer tomography system has been developed to observe unstable plasma motions in the pasmas. In this measurement, the time evolutions of two dimensional emission profile of visible light on the mid-plane of plasmas have been observed by a newly developed three-direction ten-channel computer tomography system. This system can reconstruct the emission profile by the Cormack method based on the Fourier-Bessel-Neumann expansions. From reconstructed images, the relaxation is found to be caused by the inverse-cascade phenomena, in which instabilities with higher toroidal mode numbers are excited first, followed by the energy transmission to those with lower mode numbers. Magnetic measurements have also been performed and the flux conversion from poloidal to toroidal associated with the inverse-cascade phenomena is observed. Three dimensional magnetohydrodynamic computer simulations have also been carried out to examine the dynamic behaviors of the spheromak configuration in helicity injection periods. It is found that a magnetic relaxation cycle, consisting of the current peaking phase and the subsequent MHD relaxation phase, plays very important roles in the magnetic flux conversion from poloidal to toroidal. The nonlinear coupling of the current driven MHD instabilities with mode numbers of n=2 and n=3 takes place in the flux conversion. These findings are qualitatively consistent with the tomographic observations of the MHD activities.