Halo currents and VDEs in COMPASS-D

A series of experiments were performed on COMPASS-D in a single null ITER-like configuration (R = 0.557 m, a = 0.232 m, b = 0.385 m) to investigate the properties of poloidal and toroidal halo currents flowing in the vacuum vessel during vertical displacement events, or VDEs. Both poloidal and toroidal halo currents are induced during a disruption and these currents can produce large forces on the vacuum vessel; these forces are a design consideration for ITER and also have serious consequences for in-vessel components and the vacuum vessel on devices such as JET. The toroidally symmetric (n = 0) portion of the poloidal halo currents produces vertical forces on the vacuum vessel, while the asymmetric portion produces torques, lateral forces, and locally high pressures. Scans of elongation k, predisruption plasma current Ip(0), and toroidal field Bφ were performed. Different disruption types such as high βp, H-mode, n-bare limit, ELM-induced and control failures were investigated. The results of experiments performed on COMPASS-D indicate that the symmetric portion of the halo current is bounded by ∼ 1.2Ip(0)/q(95), and that it flows in a paramagnetic sense. The toroidally asymmetric portion of the halo current is fixed spatially in COMPASS-D, and although the asymmetry can be quite large, no reversal of the halo current direction is observed. Attempts to change the halo currents by means of external helical fields are described. (author). 7 refs, 5 figs