Impact of the ITER-like wall on divertor detachment and on the density limit in the JET tokamak

Abstract L-mode and H-mode density limits with the ITER-like wall (ILW) have been investigated in the recent experimental campaign and compared with experiments in the JET carbon material configuration. The density limit is up to 40% higher in the JET-ILW than in the JET-CFC machine. This is linked to the formerly higher radiation fraction and, correspondingly, to earlier divertor detachment in the JET-CFC. In the ILW configuration, the discharge demonstrates a stable operation with a completely detached outer divertor in L- and H-mode. In contrary to the well-known “heating power independent” Greenwald limit, the L-mode densities limit increases moderately with rising heating power ( ∼ P heat 0.4 ) independently of the wall material. The H–L transition constitutes an effective undisruptive density limit for an H-mode plasma. Detachment itself does not trigger the H–L back transition and does not present a limit on plasma density. In the range of neutral beam heating 8–10.5 MW, no dependence of the H-mode density limit on the heating power was observed.