Pedestal and ELM response to impurity seeding in JET advanced scenario plasmas

Advanced scenario plasmas must often be run at low densities and high power, leading to hot edge temperatures and consequent power handling issues at plasma - surface interaction zones. Experiments at JET are addressing this issue by exploring the use of extrinsic impurity seeding and D(2) puffing to reduce heat fluxes. The experiments presented in this paper continue the line of advanced tokamak ( AT) scenario studies at high triangularity in JET by concentrating on the characterization of the edge pedestal and the ELM behaviour with deuterium and/or light impurity fuelling (neon, nitrogen). Both injection of extrinsic impurities and D2 puffing are shown to have a significant impact on the edge pedestal in typical JET AT conditions. The ELM energy loss, Delta W(ELM)/W(dia), can be reduced to below 3% and the maximum ELM penetration depth can be limited to r/a > 0.7, thus enhancing the possibility for sustainable internal transport barriers at large plasma radius. These conditions can be achieved in two separate domains, either at a radiated power fraction (F(rad)) of 30% or at a fraction of > 50%. At the lower Frad the ELMs are type I and a high pedestal pressure is maintained, but the occasional large ELM may still occur. At F(rad) > 50% the pedestal pressure is degraded by 30-50%, but the ELMs are degraded to type III. The intermediate regime at F(rad) similar to 40% is unattractive for ITB scenarios because large type I ELMs occur intermittently during the predominantly type III ELM phases (compound type I/III). F(rad) = 30% can be obtained with D(2) fuelling alone, whereas neon or nitrogen seeding is needed to achieve F(rad) > 50%. Only a limited number of tests have been carried out with nitrogen seeding, with the preliminary conclusion that the plasma edge behaviour is similar to that with neon seeding once the radiated fraction is matched.