Dimensionless Identity Experiments in JT-60U and JET
This paper summarises results of dimensionless identity experiments in JT-60U and JET, aimed at the comparison of the H-mode pedestal and ELM behaviour in the two devices. Given their similar size, dimensionless matched plasmas are also similar in their dimensional parameters (in particular a is the same in JET and JT-60U). Power and density scans were carried out at two values of Ip, providing a q scan (q95 = 3.1 and 5.1) with fixed (and matched) field. Contrary to expectations, a dimensionless match between the two devices has been quite difficult to achieve. pped in JT-60U is lower than in JET and, at low q, the pedestal pressure of JT-60U with a Type I ELM edge is matched in JET only in the Type III ELM regime. At q95 = 5.1, a dimensionless match in r*, n* and bp,ped is obtained with Type I ELMs, but only with low-power JET H-modes. These results motivated a closer investigation of experimental conditions in the two devices, to identify possible "hidden" physics that prevents obtaining a good match of pedestal values over a large range of plasmas parameters. Ripple-induced fast ion losses of the large bore plasma used in JT-60U for the similarity experiments are identified as the main difference with JET. The magnitude of the JT-60U ripple losses is sufficient to induce counter-toroidal rotation in co-injected plasma. The influence of ripple losses was demonstrated at q95 = 5.1: reducing ripple losses by ~2 (from 4.3 to 1.9MW) by replacing Positive with Negative Neutral Beam injection at ~ constant Pin, increased pped in JT-60U, providing a good match to full power JET H-modes. At the same time, the counter-toroidal rotation decreased. Physics mechanisms relating ripple losses to pedestal performance are not yet identified, and the possible role of velocity shear in the pedestal MHD stability, as well as the possible influence of ripple on thermal ion transport are briefly discussed. Ripple losses in the ITER Q = 10 reference scenario are negligible, and should not affect the plasma rotation. Although ITER plasmas will rotate at ~1/10 of the frequency of typical JET H-modes, results so far do not indicate that this should have a large effect on the pedestal MHD stability. On the other hand, the possible effect of ripple on thermal transport may deserve more attention, since the ripple magnitude of ITER is intermediate between that of JET and JT-60U.