Dimensionless pedestal identity experiments in JT-60U and JET in ELMy H-mode plasmas

This paper reports the first results of collaborative experiments in JT-60U and JET to compare ELMy H-mode characteristics in the two machines. The JT-60U/JET identity experiments were aimed explicitly at matching edge pedestal parameters in plasmas of the same geometry. The similar size of the two devices allows plasma configurations and conditions to be established for which the match of the dimensionless parameters (ρ*, ν*, β and q95) and of the equilibrium geometry is carried out in plasmas that are also almost identical in dimensional terms.The results of the power and density scans at fixed (and matched) Ip − BT show that, in a common range of ρ*, the pedestal electron collisionality, , and normalized pressure, βp,ped do not match in the two machines, against expectations. In particular, the range of βp,ped of JT-60U ELMy H-modes with Type I ELMs is accessible to JET plasmas only with a Type III ELM edge, and in JT-60U is consistently higher by ≈2 than in JET, for the same ρ*. A comparison of ne and Te pedestal profiles indicates that ∇Te is comparable in the two devices, while JET ne profiles are steeper than JT-60U. The global energy confinement enhancement factor (H98y − 2) is higher in JET than in JT-60U, and part of this difference is attributed to the reduced pedestal pressure in JT-60U compared with JET.The two experiments are analysed in detail to try to identify 'hidden' physics that may be the cause of the failure to achieve the pedestal identity as predicted. Although a full explanation for the different behaviour of the edge pedestal in the two devices has not yet been found, the possible influence of aspect ratio effects on MHD stability, differences in the plasma current profile and ripple loss (and rotation) effects are discussed.