Drive characteristics of a fast movable limiter in the JT-60 tokamak

Abstract

A pair of fast-acting movable limiters are to be installed in the vacuum chamber of the JT-60 tokamak being designed at JAERI. Their purpose is to suppress skin current in the plasma column. They should travel across the vacuum chamber over a stroke of about 1 m in 0.1 s during the build-up phase of the plasma current. Each movable limiter system consists of the drive system, a vacuum seal, a bearing usuable at high temperature in a vacuum, and a molybdenum rail limiter head. For the drive a hydraulic mechanism is used with servovalves to control the oil flow. To develop satisfactory movable limiters for JT-60, the full-scale model was constructed and tested. Overall tests with the model showed high reliability, reproducibility, controllability, and safety of the movable limiter. The drive pattern measured is in good agreement with that simulated. The movable limiters should prevent the skin effect of plasma current in the current rising phase and also provide information on particle diffusion during the flat-top phase of plasma current.