Effect of toroidal field ripple and toroidal rotation on H-mode performance and ELM characteristics in JET/JT-60U similarity experiments

Abstract

The effect of the toroidal field (TF) ripple and edge toroidal rotation (VT) on H-mode and pedestal performance as well as on ELM characteristics are investigated both in JET and JT-60U using matched plasma shape. In JT-60U, the amplitude of TF ripple ( δr) was reduced from ∼1.2% to ∼0.5% (averaged value at BT = 2.2 T) after the installation of ferritic steel tiles (FSTs). In addition with FSTs, δr can also be varied by changing the toroidal magnetic field for a given separatrix position. In JET, the TF system can be configured to feed different currents to the odd and even coil sets out of 32 TF coils. In this operation mode, δr can actively be varied by selecting the appropriate differential current between each coil set. In both devices, it is observed that the edge VT, which was in the same direction as the plasma current (co-VT), reduced with increasing δr. Even at the same δr of 0.5%, the achievable edge VT in JT-60U was still lower than that in JET due to higher neutral beam fast ion losses in JT-60U. A series of power and density scans performed at several δr indicated that plasmas with smaller ripple amplitude and/or larger co-VT are favourable to achieve higher pped and HH factor in both devices. As for ELM characteristics, larger co-VT seems to increase the ELM energy loss together with the reduction of the ELM frequency.