Study of grounding system of large tokamak device JT-60

In the critical plasma testing facility JT-60 constructed by the Japan Atomic Energy Research Institute, high voltage, large current is required in an instant. Accordingly, for the protection of human bodies and the equipment, and for realizing the stable operation of the complex, precise control and measurement system, a large scale facility of grounding system is required. In case of the JT-60 experimental facility, the equipments with different functions in separate buildings are connected, therefore, it is an important point to avoid high potential difference between buildings. In the grounding system for the JT-60, a reticulate grounding electrode is laid for each building, and these electrodes are connected with a low impedance metallic duct called grounding trunk line. The power supply cables for various magnetic field coils, control lines and measurement lines are laid in the duct. It is a large problem to grasp quantitatively the effect of a grounding trunk line by analysis. The authors analyzed the phenomenon that large current flows into a grounding system by lightning strike or grounding. The fundamental construction of the grounding system for the JT-60, the condition for the analysis and the result of simulation are reported. (Kako, I.)