EFD-C(13)03/32 ITER Like Wall Impact on MHD Instabilities in JET Discharges

An important effect observed in ILW discharges is the change of q = 1 MHD activity in hybrid discharges. The q = 1 activity with the JET C-wall had usually shown no effect on plasma confinement, except for NTM triggering, its main experimental signature being a rotating kink activity interleaving sawteeth crashes. In the C-Wall hybrid scenario sawteeth had low amplitude and frequency, and the rotating precursor had often fishbone characteristic. The radial profile of the mode fluctuation has been calculated using a coherence technique between one pick up coil and 96 ECE measurements along the red L.o.S in Figure 1. As explained in ref.[1], the coherence phase radial profile shows a -jump whenever an island is crossed moving along the plasma radius, this effect is due to the radial displacement of magnetic field lines produced by the rotating island, which has opposite directions on the outer side of the island x-point than on the inner side of the x-point. In the C-Wall hybrid pulses the ECE fluctuation at the q = 1 magnetic frequency did not show any radial phase jump, as can be seen in Figure 2, and therefore had kink characteristic.