Pedestal width and ELM size identity studies in JET and DIII-D; implications for ITER

The dependence of the H-mode edge transport barrier width on normalized ion gyroradius (ρ* = ρ/a) in discharges with type I ELMs was examined in experiments combining data for the JET and DIII-D tokamaks. The plasma configuration as well as the local normalized pressure (β), collisionality (ν*), Mach number and the ratio of ion and electron temperature at the pedestal top were kept constant, while ρ* was varied by a factor of four. The width of the steep gradient region of the electron temperature (Te) and density (ne) pedestals normalized to machine size showed no or only a weak trend with ρ*. A ρ1/2 or ρ1 dependence of the pedestal width, given by some theoretical predictions, is not supported by the current experiments. This is encouraging for the pedestal scaling towards ITER as it operates at lower ρ* than existing devices. Some differences in pedestal structure and ELM behaviour were, however, found between the devices; in the DIII-D discharges, the ne and Te pedestal were aligned at high ρ* but the ne pedestal shifted outwards in radius relative to Te as ρ* decreases, while on JET the profiles remained aligned while ρ* was scanned by a factor of two. The energy loss at an ELM normalized to the pedestal energy increased from 10% to 40% as ρ* increased by a factor of two in the DIII-D discharges but no such variation was observed in the case of JET. The measured pedestal pressures and widths were found to be consistent with the predictions from modelling based on peeling-ballooning stability theory, and are used to make projections towards ITER