Edge pedestal characteristics in JET and JT-60U tokamaks under variable toroidal field ripple

The effects of toroidal field (TF) ripple on the edge pedestal characteristics were examined in the TF ripple scan experiments at the plasma current I p of 1.1 MA in JET and JT-60U. The TF ripple amplitude δ R was defined as a value averaged over the existing ripple wells at the separatrix on the outer midplane. By the installation of ferritic inserts (FIs), δ R was reduced from 1% to 0.6% at 3.2T (0.5% at 2.2 T) in JT-60U. In JET, δ R was varied from 0.08% to 1% by feeding different currents to the odd and even set of coils out of 32 TF coils. The pedestal pressure p ped was similar for the cases before and after the installation of FIs in JT-60U. Similarly, no clear difference in p ped was also observed in the variation of δ R in JET. The core and edge toroidal rotation clearly shifted in the counter-direction by increased δ R . However, there were no changes in the spatial profiles of electron density, electron temperature and ion temperature. By the installation of FIs in JT-60U, the ELM frequency f ELM 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 δ R . In JET, f ELM increased only slightly with increased δ R while the edge toroidal rotation frequency decreased as δ R increased. From the inter-machine similarity experiment at 1.1 MA, TF ripple less than 1% does not strongly affect the pedestal pressure. However, in the single TF ripple scan at the higher I p of 2.6 MA in JET, it clearly decreases with the increased δ R , accompanying with a strong density pump out at large TF ripple. These results suggests that the effect of TF ripple on H-mode properties becomes stronger in the plasmas with higher I p or lower edge collisionality of ripple diffusion.