Small ELM regimes with good confinement on JET and comparison to those on ASDEX Upgrade, Alcator C-mod and JT-60U

Abstract

Since it is uncertain if ITER operation is compatible with type-I edge localized modes (ELMs), the study of alternative ELM regimes is an urgent issue. This paper reports on experiments on JET aiming to find scenarios with small ELMs and good confinement, such as the type-II ELMs in ASDEX Upgrade, the enhanced D-alpha H-mode in Alcator C-mod or the grassy ELMs in JT-60U. The study includes shape variations, especially the closeness to a double-null configuration, variations of q 95 , density and beta poloidal. H-mode pedestals without type-I ELMs have been observed only at the lowest currents ( 1.2 MA), showing similarities to the observations in the devices mentioned above. These are discussed in detail on the basis of edge fluctuation analysis. For higher currents, only the mixed type-I/II scenario is observed. Although the increased inter-ELM transport reduces the type-I ELM frequency, a single type-I ELM is not significantly reduced in size. Obviously, these results do question the accessibility of such small ELM scenarios on ITER, except perhaps the high beta-poloidal scenario at higher q 95 , which could not be tested at higher currents at JET due to limitations in heating power.

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

“…The higher band (red coloured) suffers from aliasing (the natural phase difference between the two closest coils approaches π) and has in reality n = −8 (in this shot one of the magnetic pick-up coils was not functioning; when this coil works the aliasing is no longer observed). 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].…”

“…Most experiments were done in deuterium plasmas, although transient type-II ELM phases have also been observed in hydrogen plasmas. Type-II ELM regimes have also been reported on DIII-D [3,4] and JET [2,[5][6][7]. On DIII-D the suppression of type-I ELMs was achieved in single null (with no mention of DN proximity) by a simultaneous increase of the plasma elongation (κ 1.9) and triangularity (δ 0.5), at high edge safety factor (q 95 ∼ 7).…”

“…It would be desirable to have a characteristic MHD signature as a third criterium for the regime. Indeed, in type-II ELM plasmas a number of MHD observations have been made [1,2] identifying particular signatures present with type-II ELMs. On the other hand, cases of type-II ELM discharges have been reported where this MHD activity was missing [2].…”

“…The above evidence is not supportive of the high frequency magnetic fluctuation activity being an essential ingredient of ASDEX Upgrade's type-II ELM regime. It is also worth mentioning that Stober et al [2] has speculated whether the small ELM regime observed in the hybrid scenario and so far classified as type-II could be in reality a grassy ELM regime [10] (which on ASDEX Upgrade have been observed at similar high beta values and similar plasma shape, but significantly higher q 95 ∼ 6.5 [11]). This offers the possibility that the small ELM regime at high beta is different to the type-II ELM regime at moderate beta.…”

“…A whole range of fluctuations has been observed (also varying with the particular type of scenario run), whose relevance is not known. It is not even clear whether the "type-II ELMs" themselves are inherently bursting or continuous phenomena (cf results in [1] and [2]). To understand its physics and the extrapolability to ITER, it is important to identify which modes are necessitated to sustain the regime (e.g.…”