Washboard modes as ELM-related events in JET

Washboard (WB) modes (Smeulders P et al 1999 Plasma Phys. Control. Fusion 41 1303) are a very common edge instability regularly observed in the H-mode regime in JET. They are detected as (normally several) bands of continuously fluctuating magnetic activity rotating in the direction of the electron diamagnetic drift with typical frequencies in the range of 10-90 kHz. The time evolution of the WB mode frequency is found to follow qualitatively the evolution of the electron temperature measured near the pedestal top, probably due to the strong diamagnetic drift associated with the large pedestal gradients. Evidence for their involvement in the pedestal and ELM dynamics will be presented. Increasing WB mode amplitude is correlated with an increase in the time between consecutive type-I ELMs. In situations in which a sudden increase (decrease) of WB mode activity is observed, the build-up of the pedestal temperature (and, linked to this, also of the pedestal pressure) of the electrons is seen to become slower (faster). This is a strong indication that the WB mode activity has a regulating effect on the pedestal and that it is responsible for an enhanced transport of energy across the separatrix. The occurrence of a class of type-I ELM precursor modes commonly observed in JET in discharges with low to moderate collisionality (v(e)(*) < 2, roughly) (Perez C P et al EFDA-JET Preprint EFD-P(02)11) is found to be associated with a weakening of the WB modes. The underlying mechanism for this interaction has not been yet identified. In contrast to low triangularity discharges, WB activity is found to increase with gas puffing at high triangularity. This can provide an explanation for the regime recently identified on JET that has been called the mixed type-I/type-II ELM regime (Saibene G et al 2002 Plasma Phys. Control. Fusion 44 1769). A modified version of the peeling-ballooning cycle for type-I ELMs on JET that takes into account the WB mode phenomenon and is consistent with the experimental observations is proposed.