High density operation at JET by pellet refuelling

Several approaches are used at JET to achieve operation at high density with good energy confinement. One of them is the injection of solid fuel pellets to realize efficient particle refuelling by deposition deep inside the plasma column. The new pellet launch system capable of launching from the torus magnetic high field side was investigated for its capability to fulfil this task in conventional ELMy H-mode discharges. Optimized pellet scenarios were developed for plasma configurations with I_p = 2.5 MA, B_t = 2.4 T, averaged triangularity <δ> ≈ 0.34 and mainly neutral beam heating at a level of approximately 17 MW. The accessible operational range was extended by the pellet tool with respect to gas puff refuelling. For example, H-mode conditions could be maintained at densities beyond the Greenwald level. Plasma energy confinement was observed to become density independent at high densities. When avoiding confinement deterioration due to pellet triggered MHD activity or parasitic pellet born gas in appropriate pulse schedules, enhanced particle inventory with more peaked density profiles was achieved while maintaining the plasma pressure profile. First attempts indicate that pellet advantages can be combined with the benefits achieved by higher triangularity. Very high particle refuelling efficiencies were found for pellets injected into 'ITER-like' discharges with an upper triangularity <δ^u> of 0.53.