Impact and mitigation of disruptions with the ITER-like wall in JET

TLDR

Disruptions in ITER release large thermal and magnetic energies on short timescales, producing extreme forces and heat loads, and the material of plasma‑facing components significantly influences the resulting loads. Massive gas injection of a 10 % Ar/90 % D₂ mixture restores high radiation, accelerates the current quench, reduces halo currents, and is now mandatory for disruption mitigation in JET at currents ≥2.5 MA. With the ITER‑like wall, radiation during disruptions falls below 50 % on average and to 10 % for vertical displacement events, leading to longer current quenches, higher vessel forces, and severe heat loads, with temperatures approaching the melting limit on upper first‑wall structures even at modest plasma currents.

Abstract

Disruptions are a critical issue for ITER because of the high thermal and magnetic energies that are released on short timescales, 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 50–100% of the total energy being dissipated during disruptions in CFC wall plasmas, to less than 50% on average and down to just 10% for vertical displacement events (VDEs). All other changes in disruption properties and loads are consequences of this low radiation: long current quenches (CQs), 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 1.5 MA and thermal energy of about 1.5 MJ only. A high radiation fraction can be regained by massive injection of a mixture of 10% Ar with 90% D2. This accelerates the CQ thus reducing the halo current and sideways impulse. The temperature of PFCs stays below 400 °C. MGI is now a mandatory tool to mitigate disruptions in closed-loop operation for currents at and above 2.5 MA in JET.