H-mode access on JET and implications for ITER

One of the critical issues for ITER is access to an H-mode regime with good confinement, H98 = 1. The most basic scaling laws for power threshold for the L–H transition, Pth, take the variation with plasma density, magnetic field and plasma size into account. However, the large variations in the Pth data from the values estimated with such simple scaling laws indicate other underlying dependencies. Another important consideration for ITER is that H-modes with higher values of energy confinement factors are often obtained with input power values much greater than Pth. This paper presents results from recent studies on JET to assess possible hidden variables for H-mode access over a wide range of plasma conditions. Experimental results demonstrate that sensitivity to the magnetic shaping and divertor geometry could account for some of the scatter in the international power threshold database. Hysteresis in the L–H transition Pth has been studied in detail for the first time on JET by comparing values of Pth at the forward and back H-mode transitions over a range of densities. The impact of the edge plasma rotation on H-mode access has also been considered on JET with a toroidal field ripple scan across the L–H and H–L transitions. Finally, the total input power required relative to the measured value of Pth for access to a steady-state H-mode with H98 = 1 has been examined for a highly shaped magnetic configuration. The implications of these results for the attainment of H-mode with good confinement on ITER are discussed.