A new type of MHD mode, provisionally termed the wash Board mode[1], 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 #beta#. The WB mode appears to be linked with saturation in the plasma ocnfinement and central plasma temperatures. These modes have high m and n numbers and are localised in the outer part of the plasma, typically from the q=2 surface to the plasma edge. They rotate with the electron diamagnetric frequency and have a strong ballooning character. They are regarded as a possible candidate to play a role in the confinement degradation of H-mode plasmas.
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