Edge Pedestal Characteristics in JET and JT-60U Tokamaks Under Variable Toroidal Field Ripple
The effect of Toroidal Field (TF) ripple on the edge pedestal characteristics were examined in the TF ripple scan experiments at the plasma current Ip of 1.1MA in JET and JT-60U. The TF ripple amplitude dR 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), dR was reduced from 1% to 0.6% at 3.2T (0.5% at 2.2T) in JT-60U. In JET, dR 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 pped was similar for the cases before and after the installation of FIs in JT-60U. Similarly, no clear difference in pped was also observed in the variation of dR in JET. The core and edge toroidal rotation clearly decreased in counter direction by increased dR. 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 fELM 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 dR. In JET, fELM increased only slightly with increased dR while the edge toroidal rotation frequency decreased as dR increased. From the inter-machine similarity experiment at 1.1MA, TF ripple less than 1% does not strongly affect the pedestal pressure. However, in the single TF ripple scan at the higher Ip of 2.6MA in JET, it clearly decreases with the increased dR, accompanying with a strong density pump out at large TF ripple. These results suggets that the effect of TF ripple on H-mode properties becomes stronger in the plasmas with higher Ip or lower edge collisionality.