Abstract
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. There is a good correlation between increasing plasma pressure and the growth of both the spectral extent and amplitude of the WB modes. Changes in the electron temperature profile also correlate well with changes in the amplitude of these modes. They are, therefore, regarded as a possible candidate to play a role in the confinement degradation of H-mode plasmas. Although these modes appear in all types of H-modes in JET, most of the analysis has been carried out during the hot-ion H-mode period of the 100% T discharges, since these modes are practically the only MHD activity left and the small-scale density fluctuations can, therefore, be analysed.
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