Improved ELM scaling with impurity seeding in JET

Argon-seeded H-modes in low triangularity pumped-limiter-like, and medium to high triangularity divertor, configurations on JET are described, focusing upon ELM effects. Appropriate fuelling and seeding strategies in each geometry have combined good confinement with density around the Greenwald level, accompanied by ELMs shown to have explicitly Type I character. For lower triangularity, argon injection leads to a reduction in normalized ELM energy losses averaged over several fluctuations, relative to unseeded scaling at comparable frequency and confinement but lower density. This generalizes a similar result in earlier studies [14,15]. Optimized seeding tends always to decrease ELM frequencies too, so that the average energy efflux in ELMs is significantly diminished, down to only ≈10% of input power in the lowest example included. At least for lower triangularity again, electron pedestal temperature is cooled by argon, which tends also to lower peak pedestal pressure before each consequently smaller fluctuation. However, the pedestal density is raised, and at higher radiation fractions this can restore unseeded peak pedestal pressure and roughly ELM size, but still at lower frequency. The underlying physics of Type I instabilities defined by pedestal collisionality appears to prevail in all cases both with and without seeding. High performance H-modes with both reduced core transport and moderated steady and transient power loads through added impurities therefore seem to come closer to a fully integrated scenario for Q = 10 operation in ITER.