Avoidance of VDEs during plasma current quench in JT-60U

Abstract

Vertical displacement events (VDEs) during plasma current quench (Irho quench) are one of the serious problems encountered in designing tokamak fusion reactors, owing to the generation of enormously high electromagnetic forces on the vacuum vessel and in-vessel components, but they have been passively and actively avoided in JT-60U. In JT-60U `slow IP quench` is ended with very fast plasma current termination (final Irho termination), and the halo current is frequently measured at this final Irho termination. VDEs make the final Irho termination severe by increasing the halo current and the electromagnetic force. A strong dependence of VDE growth rate on the initial vertical position of the plasma current centre (ZJ) has been clarified experimentally, and a neutral point of ZJ for VDE has been found at ~15 cm above the midplane of the vacuum vessel. According to these measurements, VDE has been avoided by the selection of ZJ at the start of Irho quench (passive control) and by the control of ZJ during Irho quench (active control) eventually obtained owing to the small deviation of ZJ in real time calculations from its actual value. Furthermore, passive avoidance of VDEs by the injection of a neon ice pellet has been demonstrated