Characteristics of a new class of transport-related MHD modes in JET H-mode plasmas

A new type of magnetohydrodynamic (MHD) mode, provisionally termed the wash board (WB) mode, has been observed during H-mode plasmas in JET. It occurs in all types of H-mode discharges, but is not seen during L-mode even at high values of . The WB mode appears to be linked with saturation in the plasma confinement and central plasma temperatures. These modes have high m and n numbers and are localized in the outer part of the plasma, typically from the q = 2 surface to the plasma edge. They rotate with the electron diamagnetic frequency and have a strong ballooning character.