The role of edge plasma parameters in H-mode density limit on the JET-ILW

A study of a dataset of JET H-mode plasma with the Be/W ITER-like wall (JET-ILW) shows that reaching the edge MHD ballooning limit leads to confinement degradation. However, unlike JET plasmas with a carbon wall (JET-C), the JET-ILW plasmas stay in a marginal dithering phase for a relatively long period, associated with a higher (approximate to 20%) H-mode density limit (HDL) than JET-C equivalents. This suggests that ITER could be operated in H-mode with higher density than the scaling based on carbon wall devices, but likely with a dithering phase plasma with lower confinement. A new, reliable estimator for JET E (r, min) has been derived by combining HRTS measurements of pedestal gradient and edge-SOL decay lengths. JET radial E (r) ETB wells are observed in the range of -15 to -60 kV m(-1) in high performance H-modes, consistent with previous CXRS results in ASDEX Upgrade. The results imply that a higher positive E x B shear in the near SOL plays a role in sustaining a marginal phase in JET-ILW which leads to a higher HDL than that in JET-C. The results of the JET-ILW dataset show agreement with the Goldston finite collisionality HD model for SOL broadening at high collisionality. A hypothesis for the dithering H-mode phase is proposed: as n (e,SOL) increases, nu (*,SOL) increases, SOL broadens, E (r) shear decreases, triggers L-mode; n (e) drops, nu (*,SOL) decreases, SOL becomes narrower, and E (r) shear increases, triggering H-mode, resulting in a cycle of H-L-H- oscillations. For burning plasma devices, such as ITER, operating just below the MHD limit for the dithering phase could be a promising regime for maximising core density, and fusion performance while minimising plasma-material interaction. The oscillatory signal during the dithering phase could be used as a precursor of undesirable plasma performance for control purposes.