Comparison of Pedestal Characteristics in JET & JT-60U Similarity Experiments Under Variable Toroidal Field Ripple
Conducting the TF ripple scan experiments at 1.1MA in JET and JT-60U, the effect of TF ripple on the edge pedestal characteristics are examined. The TF ripple amplitude is defined as a value averaged over the existing ripple wells at the separatrix on the outer midplane. By the installation of FIs, TF ripple was reduced from 1% to 0.6% at 3.2T (0.5% at 2.2T) in JT-60U. In JET, TF ripple was varied from 0.1% to 1% by feeding different currents to the odd and even set of coils out of 32 TF coils. The pedestal pressure was similar for the cases before and after the installation of FIs in JT-60U. In JET, no clear difference of the pedestal n-T diagram was also observed in the variation of the TF ripple. The core and edge toroidal rotation clearly decreases in counter direction by increased TF ripple. However, there are no changes in the spatial profiles of ne, Te and Ti. In JT-60U, by the installation of FIs, the ELM frequency decreased by ~20%, while the ELM energy loss increased by 50-150%. The increased ELM loss power by 30% suggests a reduction of inter-ELM transport with the reduced TF ripple. In JET, ELM frequency increases only slightly with increased TF ripple while the edge toroidal rotation frequency decreases as the TF ripple increased. From this inter-machine experiment at ~1MA, TF ripple less than 1% does not strongly affect the pedestal pressure. However, the ranges of other parameters relevant to plasma current where the TF ripple affects strongly on the performance might be important for higher Ip operation such as ITER.