Ikeyama Taeko, Hiroi Masanori, Nemoto Yuuichi, Nogi Yasuyuki
College of Science and Technology, Nihon University, Tokyo, Japan.
Rev Sci Instrum. 2008 Jun;79(6):063501. doi: 10.1063/1.2937205.
A confinement field is disturbed by magnetohydrodynamic (MHD) motions of a field-reversed configuration (FRC) plasma in a cylindrical conductor. The effect of the conductor should be included to obtain a spatial structure of the disturbed field with a good precision. For this purpose, a toroidal current in the plasma and an eddy current on a conducting wall are replaced by magnetic dipole and image magnetic dipole moments, respectively. Typical spatial structures of the disturbed field are calculated by using the dipole moments for such MHD motions as radial shift, internal tilt, external tilt, and n=2 mode deformation. Then, analytic formulas for estimating the shift distance, tilt angle, and deformation rate of the MHD motions from magnetic probe signals are derived. It is estimated from the calculations by using the dipole moments that the analytic formulas include an approximately 40% error. Two kinds of experiment are carried out to investigate the reliability of the calculations. First, a magnetic field produced by a circular current is measured in an aluminum pipe to confirm the replacement of the eddy current with the image magnetic dipole moments. The measured fields coincide well with the calculated values including the image magnetic dipole moments. Second, magnetic probe signals measured from the FRC plasma are substituted into the analytic formulas to obtain shift distance and deformation rate. The experimental results are compared to the MHD motions measured by using a radiation from the plasma. If the error included in the analytic formulas and the difference between the magnetic and optical structures in the plasma are considered, the results of the radiation measurement support well those of the magnetic analysis.
在圆柱形导体中，约束场会受到场反转构型（FRC）等离子体的磁流体动力学（MHD）运动的干扰。为了高精度地获得干扰场的空间结构，应考虑导体的影响。为此，等离子体中的环形电流和导电壁上的涡电流分别用磁偶极矩和镜像磁偶极矩代替。利用这些偶极矩，针对径向位移、内部倾斜、外部倾斜和n = 2模变形等MHD运动，计算了干扰场的典型空间结构。然后，推导了根据磁探针信号估算MHD运动的位移距离、倾斜角度和变形率的解析公式。利用偶极矩进行计算估计，解析公式包含约40%的误差。进行了两种实验来研究计算的可靠性。首先，在铝管中测量圆形电流产生的磁场，以确认用镜像磁偶极矩代替涡电流。测量的场与包括镜像磁偶极矩的计算值吻合良好。其次，将从FRC等离子体测量的磁探针信号代入解析公式，以获得位移距离和变形率。将实验结果与利用等离子体辐射测量的MHD运动进行比较。如果考虑解析公式中的误差以及等离子体中磁结构和光学结构之间的差异，辐射测量结果很好地支持了磁分析结果。
