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

Vertical displacement events (VDEs) during plasma current quench (I-p 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 I-p quench' is ended with very fast plasma current termination (final I-p termination), and the halo current is frequently measured at this final Ip termination. VDEs make the final I-p 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 (Z(j)) has been clarified experimentally and a neutral point of Z(j) for VDE has been found at similar to 15 cm above the midplane of the vacuum vessel. According to these measurements, VDE has been avoided by the selection of Z(j) at the start of I, quench (passive control) and by the control of Z(j) during I-p quench (active control) eventually obtained owing to the small deviation of Z(j) in real time calculations from its actual value. Furthermore, passive avoidance of VDEs by the injection of a neon ice pellet has been demonstrated.