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

Abstract

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 of H-mode discharges, but is not seen during L-mode even at high values of β. 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 localised 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 explain the power degradation of the empirically established H-mode scaling laws.

Identifying the MHD signature and power deposition characteristics associated with type-II ELMs in ASDEX Upgrade

“…This agrees with the range of mode numbers reported earlier for the high frequency turbulence [2]. Most interestingly, a shortlived series of bands is discernable at 4.015s (early ELM period) with n = −3, −4, −5 and −6, resembling the standard appearance of washboard (WB) modes on JET [12,13]. At this time the bands with −7 and −8 represent the natural continuation towards higher mode numbers and frequency.…”

Spectra of magnetic perturbations triggered by pellets in JET plasmas

“…activity with negative toroidal mode number is strongly reduced during the pellet-fuelled phase, although coherent modes with short duration can still be detected during the inter-ELM phases. These modes are likely to be Washboard modes (WB) [14][15]. They are always observed on JET during H-mode plasmas, propagate in the electron drift direction, are localized at the edge and their evolution in frequency follows qualitatively that of the edge electron temperature [14].…”

“…These modes are likely to be Washboard modes (WB) [14][15]. They are always observed on JET during H-mode plasmas, propagate in the electron drift direction, are localized at the edge and their evolution in frequency follows qualitatively that of the edge electron temperature [14]. In the experiments shown herein, the typical saturation in the WB mode frequency is not observed, most probably because the inter-ELM phase is too short and because the time evolution of temperature is not regular as in the cases studied in Refs.…”

“…In the experiments shown herein, the typical saturation in the WB mode frequency is not observed, most probably because the inter-ELM phase is too short and because the time evolution of temperature is not regular as in the cases studied in Refs. [14][15].…”

“…The power spectral density associated with negative toroidal mode numbers gradually increases, reaching a maximum during the crash, then it rapidly ). (f) Toroidal mode number spectrum, P (n), obtained integrating P (n, s) over scales log 2 (s j ) = [11,14], during the time windows indicated in the plot.…”