H-mode access studies under conditions of very low plasma density provide
valuable information on the physics of the edge transport barrier formation
and are an important part of the operational scenario development for the
next step fusion device, ITER. JET operation with a very low edge electron
density, ¯ne = 0.8–1.4 × 1019 m−3, with the MkII gas box septum divertor was
found to have a significant effect on the power threshold for the L–H transition,
Pth. A sharp ‘turning point’ below which Pth increases with decreasing ¯ne
is found for Ip/Bt values of 2.5 MA/2.7 T, 2.5 MA/2.6 T, 2.5 MA/2.4 T and
2.2 MA/2.4 T. The pedestal electron temperature demonstrates a very similar
dependence on the edge ¯ne as Pth. The role of Zeff in the variation of the
H-mode power threshold with density is also considered. Shots with the divertor
septum present are compared with an identical ¯ne scan, run with the MkII
GB septum replacement plate divertor at 2.5 MA/2.7 T and 2.5 MA/2.6 T. The
results from this investigation indicate that removal of the septum may have
lowered the ¯ne ‘turning point’ for Pth. |