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A new type of MHD mode, provisionally termed the Wash Board (WB) mode, has been observed during H-mode plasmas in JET. It occurs in all types 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 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 frequncy 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 explain the power degradation of the empirically established H-mode scaling laws. ws.

Additional information

Authors: SMEULDERS P, JET Joint Undertaking, Abingdon (GB);CONWAY G D, JET Joint Undertaking, Abingdon (GB);ALPER B, JET Joint Undertaking, Abingdon (GB);BALET B, JET Joint Undertaking, Abingdon (GB);BARTLETT D V, JET Joint Undertaking, Abingdon (GB);BORBA D, JET Joint Undertaking, Abingdon (GB);DELIYANAKIS N, JET Joint Undertaking, Abingdon (GB);HENDER T C, UKAEA Fusion Association, Abingdon (GB);KWON O J, UKAEA Fusion Association, Abingdon (GB)
Bibliographic Reference: Article: JET-P(98)83
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