  This paper investigates the effect of the ITER-like wall (ILW) on runaway
electron (RE) generation through a comparative study of similar slow argon
injection JET disruptions, performed with different wall materials. In the
carbon wall case, a runaway electron plateau is observed, while in the
ITER-like wall case, the current quench is slower and the runaway current is
negligibly small. The aim of the paper is to shed light on the reason for these
differences by detailed numerical modelling to study which factors affected the
RE formation. The post-disruption current profile is calculated by a
one-dimensional model of electric field, temperature and runaway current taking
into account the impurity injection. Scans of various impurity contents are
performed and agreement with the experimental scenarios is obtained for
reasonable argon- and wall impurity contents. Our modelling shows that the
reason for the changed RE dynamics is a complex, combined effect of the
differences in plasma parameter profiles, the radiation characteristics of
beryllium and carbon, and the difference of the injected argon amount. These
together lead to a significantly higher Dreicer generation rate in the carbon
wall case, which is less prone to be suppressed by RE loss mechanisms. The
results indicate that the differences are greatly reduced above ~50% argon
content, suggesting that significant RE current is expected in future massive
gas injection experiments on both JET and ITER.
