High density operation at JET by pellet refuelling*

Abstract

Several approaches are used at JET to achieve operation at high densitywith good energy confinement. One of them is the injection of solid fuelpellets to realize efficient particle refuelling by deposition deepinside the plasma column. The new pellet launch system capable oflaunching from the torus magnetic high field side was investigated forits capability to fulfil this task in conventional ELMy H-modedischarges. Optimized pellet scenarios were developed for plasmaconfigurations with IP = 2.5 MA, Bt = 2.4 T, averagedtriangularity ⟨δ⟩≈0.34 and mainly neutral beam heatingat a level of approximately 17 MW. The accessible operational range wasextended by the pellet tool with respect to gas puff refuelling. Forexample, H-mode conditions could be maintained at densities beyond theGreenwald level. Plasma energy confinement was observed to becomedensity independent at high densities. When avoiding confinementdeterioration due to pellet triggered MHD activity or parasitic pelletborn gas in appropriate pulse schedules, enhanced particle inventorywith more peaked density profiles was achieved while maintaining theplasma pressure profile. First attempts indicate that pellet advantages canbe combined with the benefits achieved by higher triangularity. Veryhigh particle refuelling efficiencies were found for pellets injectedinto `ITER-like' discharges with an upper triangularity ⟨δu⟩ of0.53.