H-mode access in the low density regime on JET

Abstract

1 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, , 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 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 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 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 ‘turning point’ for Pth.