The halo current detection system in JET was refurbished in 2001. The collected data confirm previous results, obtained with a smaller number of sensors, and are consistent with the ITER assumptions. An analysis of recent disruptions shows that the local halo current density in JET is linked to the average plasma current quench rate (the slower the disruption, the larger the local halo load), rather than the plasma current. The largest local halo current densities are more likely to occur when the equilibrium boundary safety factor is low and when it decreases to a critical value, close to 1, during the disruption. The largest halo current fractions are found for the most vertically unstable configurations (i.e. low ratio of resistive vessel time to vertical displacement growth time). The information from the new pick-up coils and Rogowski coils and the only surviving old sensor (a pick-up coil located outboard of the new system) provides an estimate of the size, and an improved picture of the movement, of the halo/wall interaction region during disruptions.
展开▼
