Disruption heat loads on the JET MkIIGB divertor

Combined analysis of divertor thermocouple and IR camera measurements during JET disruptions can provide valuable information on the distribution of the energy loads, even if the stored energy of the JET plasmas is small compared to that foreseen for the next-generation tokamaks. Typically the energy collected at the divertor represents a small fraction of the pre-disruption plasma energy; this is consistent with the high level of radiation observed and with part of the magnetic energy being transferred to the plasma-coupled conductors. The data for this paper are taken from the whole set of disruptive plasmas of JET operation in the years 2000 and 2001. In most of the MkIIGB disruptions, the plasma displaces upwards (away from the divertor); therefore, only a small number of downward events are available for analysis. However, divertor heat loads seem to be more strongly correlated to the delay of the loss of the X-point with respect to the thermal quench than the direction of the plasma displacement. When the plasma thermal energy is lost with the plasma still in X-point configuration, the septum and the tiles wetted by the strike-points, often more than one tile per strike-point, experience a sharp increase in temperature, equivalent to up to 1 MJ m(-2). When the thermal quench occurs at the same time as, or after, the loss of plasma vertical control, no significant divertor tile temperature increase can be observed for both upwards and downwards events. Most of the disruptions purposely made to produce runaway electrons went towards the divertor and, although not systematically, lead to local (mostly at the septum) temperature increase equivalent to a load up to 2 MJ m(-2), often toroidally asymmetric.