JET Disruption Studies in Support of ITER

Full text: Disruptions are a key issue for all future large tokamaks, due to the large mechanical and thermal loads they can place on the tokamak structure. Given that size (and field and current) are a key determinant of disruptions loads, JET is best placed to understand ITER disruption issues. This paper summarises recent key advances in understanding on JET, in the disruption area. Results will be presented on halo currents and asymmetric disruptions, where large sideways forces may occur (up to 4 MN) - which are very significant if simply extrapolated to ITER. Considerable effort is being devoted to understanding this asymmetry, its rotation and how it should be extrapolated to ITER. A second significant consequence of disruptions is the resulting thermal loads. IR data show in general that the outer limiter and the divertor each handle comparable energies. A third disruption consequence is runaway electrons (REs). IR imaging shows distinct localised impacts on the upper dump plate by the RE beam leading to an increase of the surface temperature of up to ∼ 1500oC. The effects of applied non-axisymmetric n =1 or n = 2 fields on the runaway beam have been studied in JET but at the applied values no appreciable effect on the runaway confinement is observed. Intrinsic to mitigation of disruptions is a means to predict that they are going to occur and also an understanding of the sequence events that ultimately leads to disruptive termination. By focussing not only on the disruptions root causes, but also on these chains, an improved probability of disruption mitigation and avoidance can be achieved. A fast valve (Disruption Mitigation Valve-DMV) has been recently installed on JET to study disruption mitigation by MGI. The results show MGI leads to significant halo force reductions and to > 50% of the disruptive energy loss being radiative. An aspect of these studies is that the MGI induced Ip-quenches tend to be faster than those occurring in natural disruptions/VDEs. Overall substantial progress has been made on JET in terms of quantifying disruption consequences and disruption control methods, leading to an improved physics basis for ITER. Work partly funded by UK EPSRC and EURATOM, and conducted under EFDA. (author)