Washboard modes as ELM-related events in JET
Washboard (WB) modes 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. 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 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.
Bibliographic Reference: An article published in: Plasma Physics and Controlled Fusion 46 (January 2004) 61�87
Availability: This article can be accessed online by subscribers, and can be ordered online by non-subscribers, at: http://stacks.iop.org/0741-3335/46/61
Record Number: 200417323 / Last updated on: 2004-01-07
Original language: en
Available languages: en