In tokamak disruptions the Ohmic current is often replaced by a current of runaway electrons, which is likely to be more peaked in the centre of the discharge than the pre-disruption current. This raises the question of the resistive stability of the post-disruption plasma, where the equilibrium current is entirely carried by the runaway electrons while the cold (∼ 10 eV) background plasma is relatively resistive. It is found that the linear properties of the classical tearing mode are essentially determined by the cold bulk plasma, and the growth rate is approximately the same as in a plasma without runaways but with the same current profile. The nonlinear saturation amplitude is different however. In a symmetric plasma slab, the saturated island size is larger when the current is carried by runaway electrons than in the Ohmic case, and undergoes a nonlinear bifurcation when the stability index ∆ ′ exceeds a critical value.

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Since the installation of an ITER-like wall, the JET programme has focused on the consolidation of ITER design choices and the preparation for ITER operation, with a specific emphasis given to the bulk tungsten melt experiment, which has been crucial for the final decision on the material choice for the day-one tungsten divertor in ITER. Integrated scenarios have been progressed with the re-establishment of long-pulse, high-confinement H-modes by optimizing the magnetic configuration and the use of ICRH to avoid tungsten impurity accumulation. Stationary discharges with detached divertor conditions and small edge localized modes have been demonstrated by nitrogen seeding. The differences in confinement and pedestal behaviour before and after the ITER-like wall installation have been better characterized towards the development of high fusion yield scenarios in DT. Post-mortem analyses of the plasma-facing components have confirmed the previously reported low fuel retention obtained by gas balance and shown that the pattern of deposition within the divertor has changed significantly with respect to the JET carbon wall campaigns due to the absence of thermally activated chemical erosion of beryllium in contrast to carbon. Transport to remote areas is almost absent and two orders of magnitude less material is found in the divertor.