Impurity-seeded ELMy H-modes in JET, with high density and reduced heat load

Abstract

Experiments performed at JET during the past two years show that, in high triangularity H-mode plasmas with Ip = 2.5 MA, ne/nGr ≈ 1.0, it is possible to radiate separately up to ≈40% of the total injected power on closed flux surfaces in the pedestal region (argon seeding) and up to ≈50% of the injected power in the divertor region (nitrogen seeding), while maintaining the confinement improvement factor at the value required for ITER, H98(y, 2) ≈ 1.0. The total radiated power fraction achieved in both cases (65–70%) is close to that required for ITER. However, Type I ELMs observed with impurity seeding have the same characteristics as that observed in reference pulses without seeding: decreasing plasma energy loss per ELM with increasing pedestal collisionality. One has to reach the Type III ELM regime to decrease the transient heat load to the divertor to acceptable values for ITER, although at the expense of confinement. The feasibility of an integrated scenario with Type-III ELMs, and q95 = 2.6 to compensate for the low H factor, has been demonstrated on JET. This scenario would meet ITER requirements at 17 MA provided that the IPB98 scaling for energy content is accurate enough, and provided that a lower dilution is obtained when operating at higher absolute electron density.