Plasma current asymmetries during disruptions in JET

A key feature of disruptions during vertical displacement events, discovered in JET in 1996, is the toroidal variation in the measured plasma current I-p, i.e. the plasma current asymmetries, lasting for almost the entire current quench. The unique magnetic diagnostics at JET (full set of poloidal coils and saddle loops recorded either from two toroidally opposite or from four toroidally orthogonal locations) allow for a comprehensive analysis of asymmetrical disruptions with a large scale database. This paper presents an analysis of 4854 disruptions over an 18 year period that includes both the JET carbon (C) wall and the ITER-like (IL) wall (a mixed beryllium/tungsten first wall). In spite of the I-p quench time significantly increasing for the IL-wall compared to C-wall disruptions, the observed toroidal asymmetry time integral (similar to sideways force impulse), did not increase for IL-wall disruptions. The I-p asymmetry has a dominantly n = 1 structure. Its motion in the toroidal direction has a sporadic behaviour, in general. The distributions of the number of rotation periods are found to be very similar for both C-and IL-wall disruptions, and multi-turn rotation was sometimes observed. The I-p asymmetry amplitude has no degradation with rotation frequency for either the C-or IL-wall disruption. Therefore dynamic amplification remains a potentially serious issue for ITER due to possible mechanical resonance of the machine components with the rotating asymmetry.