Runaway Beam Studies During Disruptions at JET-ILW

Runaway electrons during disruptions are a concern for future all-metal tokamaks including ITER. First experimental campaigns with the tungtsen/beryllium JET ITER-Like-Wall (JET-ILW) showed that runaways were very rare in spontaneous disruptions, unlike with the carbon-wall configuration (JET-C). However, runaway beams up to 380kA could be obtained using massive injection of argon in divertor pulses. This shows that the runaway generation is not completely avoided with a metallic environment. Runaway energies up to 20MeV have been measured, and the transition from no-runaway domain to runaway domain occur at approximately the same location. However higher runaway currents have been obtained inside the runaway domain with JET-ILW when compared to JET-C. This trend might be due to the influence of the metallic wall on the current quench plasma impurity content or to different runaway loss mechanisms. Impacts of runaway beams on the beryllium plasma facing components have been observed, with up to 900C from IR measurements. Simple 1D heat diffusion simulations of the tile cooling time show that 1 to 2mm heat deposition depth must be assumed to match the temperature evolution after the impact. More refined 3D simulations of the runaway energy deposition and heat diffusion inside the tile have also been carried out using the ENDEP/MEMOS suite of codes. They confirm the deposition depth and show that is likely that no melting has occurred up to 100kA of runaway beam impact.
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