Jiangxi Province Key Laboratory of Fusion and Information Control, Department of Physics, Nanchang University, Nanchang 330031, China.
Rev Sci Instrum. 2023 Jan 1;94(1):013509. doi: 10.1063/5.0125948.
The field null configuration of a poloidal magnetic field is one of the critical conditions for achieving Ohmic breakdown during the initial discharge of a new tokamak. The issue of the Ohmic breakdown on the NanChang Spherical Tokamak (NCST) is still not solved satisfactorily although plasma currents of about 2 kA were found. Hence, a removable three-dimensional magnetic probe (RTMP) system consisting of 25 magnetic probes was designed, calibrated, and constructed on the NCST to evaluate the field null inside a vacuum vessel. After repeated tests, the RTMP system exhibited outstanding performance in terms of accuracy and stability with errors of about 1%. Meanwhile, the RTMP system successfully measured the toroidal field (TF) coil ripples at the magnetic axis. During experiments, the stray field arising from the TF coil implied a strong link between the flexible connection of the TF coil and the Ohmic breakdown on the NCST. After the field null was effectively modified by using a new flexible connection of the TF coil and controlling the induced current in the poloidal field coil, the NCST tokamak reproducibly obtained 20 kA plasma current with the limiter configuration during the plasma current flat-top phase.
环向磁场的场零点是新托卡马克装置初始放电时实现欧姆击穿的关键条件之一。尽管在南昌球形托卡马克（NCST）上发现了约 2kA 的等离子体电流，但欧姆击穿问题仍未得到满意解决。因此，设计、校准并在 NCST 上构建了由 25 个磁场探针组成的可移动三维磁场探针（RTMP）系统，以评估真空室内的场零点。经过反复测试，RTMP 系统在精度和稳定性方面表现出色，误差约为 1%。同时，RTMP 系统成功测量了磁轴处的环形磁场（TF）线圈纹波。在实验过程中，来自 TF 线圈的杂散场表明 TF 线圈的柔性连接与 NCST 上的欧姆击穿之间存在很强的联系。通过使用 TF 线圈的新柔性连接和控制极向场线圈中的感应电流来有效修正场零点后，NCST 托卡马克在等离子体电流平顶阶段，在限流器配置下，可重复获得 20kA 的等离子体电流。
