Impact and Mitigation of Disruptions with the ITER-Like Wall in JET

Disruptions are a critical issue for ITER because of the high thermal and magnetic energies that are released on short time scales, which results in extreme forces and heat loads. The choice of material of the plasma facing components (PFCs) can have significant impact on the loads that arise during a disruption. With the ITER-like wall (ILW) in JET made of beryllium in the main chamber and tungsten in the divertor, the main finding is a low fraction of radiation. This has dropped significantly with the ILW from 50­100% of the total energy being dissipated during disruptions in CFC wall plasmas, to less than 50% on average and down to just 10% for VDEs. All other changes in disruption properties and loads are consequences of this low radiation: long current quenches, high vessel forces caused by halo currents and toroidal current asymmetries as well as severe heat loads. Temperatures close to the melting limit have been locally observed on upper first wall structures during deliberate VDE and even at plasma currents as low as 1.5MA and thermal energy of about 1.5MJ only. A high radiation fraction can be regained by massive injection of a mixture of 10%Ar with 90%D2. This accelerates the current quench thus reducing the halo current and sideways impulse. The temperature of PFCs stays below 400oC. MGI is now a mandatory tool to mitigate disruptions in closed-loop operation for currents at and above 2.5MA in JET.
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