ELM pacing and trigger investigations at JET with the new ITER-like wall

Abstract

During the installation of the new ITER-like wall (ILW) at JET, the high frequency pellet injector has been further improved. The launching system is now capable of delivering reliable fuelling size pellets from the magnetic outboard side up to 15 Hz repetition rate. Pacing size pellets can be produced at rates up to 50 Hz but pellet trains suffer some losses during the transfer to the plasma. A significant fraction of the pellet train can arrive at the plasma when launched from the outboard, while only a few pellets make it to the vessel inboard launching site. Stable and reliable edge-localized mode (ELM) control was achieved when using outboard fuelling size pellets. This tool was successfully applied for scenario development purposes in the ITER baseline H-mode scenario at 2.5 MA. Employed for ELM sustainment and impurity control, pellets prevented the ELM frequency from becoming so low as to cause a radiative collapse of the discharge. Despite technical limitations, injecting outboard pacing size pellets resulted in a transient enhancement of the initial ELM frequency up to a factor 4.5. This could be achieved in cases where a continuous train of sufficiently large and fast pellets were arriving in the plasma at a frequency of up to 31 Hz. Pacing size pellets were also used to investigate the ELM trigger threshold. Three basic parameters could be identified for outboard pellet launch. The ELM triggering probability increased with (i) the time elapsed since the previous ELM occurred, (ii) pellet mass and (iii) pellet speed. An indication for dependence of the ELM trigger threshold on the poloidal pellet launch position has been found; inboard launched pellets seem to reveal a higher trigger capability than pellets launched from the outboard. Finally, we compared the pellet penetration depth required for ELM triggering in the actual JET configuration with plasma-facing components to similar previous experiments performed with a carbon wall. This comparison indicates that pellet ELM triggering requires deeper penetration in the ILW configuration.