Washboard modes as ELM-related events in JET

Perez, C. P.; Koslowski, H. R.; Hender, T. C.; Smeulders, P.; Loarte, A.; Lomas, P. J.; Saibene, G.; Sartori, R.; Bécoulet, M.; Eich, T.; Hastie, R. J.; Huysmans, G.; Jachmich, S.; Rogister, A.; Schüller, F. C.; Contributors, Jet

doi:10.1088/0741-3335/46/1/005

Cited by 63 publications

(83 citation statements)

References 37 publications

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“…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.…”

Section: Importance For Type-ii Elm Regimesupporting

confidence: 90%

“…Although JET experience suggests that this type of activity leads to enhanced (conductive) inter-ELM losses and a reduction of type-I ELM frequency [12], from their absence in certain type-II ELM scenarios and their behaviour during regime transitions it appears to be that this activity is actually not an essential ingredient of type-II ELM regimes on ASDEX Upgrade. Another type of activity is given by small repetitive bursts, and low frequency (∼ 5-25 kHz) magnetic precursor activity associated with these bursts.…”

Section: Summary and Discussionmentioning

confidence: 99%

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

Thun

Maraschek

Graca

et al. 2008

Plasma Phys. Control. Fusion

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Abstract. The properties of ASDEX Upgrade's type-II ELM regime, which combines good confinement with benign temporal variation of the power exhaust, make it an attractive candidate for ITER baseline scenario operation. As yet it is not understood what the physics origin of this regime is. In previous type-II ELM studies [Stober J et al 2001 Nucl. Fusion 41 1123, Stober J et al 2005 Nucl. Fusion 45 1213] a whole range of fluctuations have been reported. In this article the properties of these fluctuations are inspected in more detail, and their relevance for the regime investigated. For high frequency (∼ 150-220 kHz) magnetic fluctuation activity, from its absence in certain type-II ELM scenarios and behaviour during regime transitions it is concluded that it is not an essential ingredient of type-II ELM regimes on ASDEX Upgrade. A stronger contender to explain the type-II ELM regime appears to be given by small repetitive electromagnetic bursts, and low frequency (∼ 5-25 kHz) magnetic precursor activity associated with these bursts. It is demonstrated that these bursts give rise to small but frequent heat loads of the right order to influence ELM cycles. These are best detected in the vicinity of the inner strike point in the lower divertor through infra-red thermography and Langmuir probes. Their properties further suggest that the bursts are neither small type-I ELMs nor type-III ELMs. However, it remains to be shown that the losses associated with these bursts are high enough to replace the type-I ELM losses. Other enhanced density fluctuation activity at low frequency (10-30 kHz) is detected by reflectometry near the plasma boundary. The information available about this mode is more limited, but so far a causal implication of this mode in the type-II ELM regime cannot be excluded. Overall it can be concluded that, of the three potential dissipation mechanisms, the low frequency electromagnetic burst behaviour seems to be the most promising candidate to account for type II ELMs and their energy losses.Identifying the MHD signature associated with type-II ELMs 2

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Section: Importance For Type-ii Elm Regimesupporting

confidence: 90%

Section: Summary and Discussionmentioning

confidence: 99%

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

Thun

Maraschek

Graca

et al. 2008

Plasma Phys. Control. Fusion

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“…This is consistent with measurements presented in [Leyland 2013]. Furthermore, there is a change in magnetic fluctuation behaviour corresponding to washboard modes which are thought to regulate the build-up in pressure by enhanced inter-ELM transport [Perez 2004, Perez 2008]. …”

Section: Plasma Performancesupporting

confidence: 88%

The H-mode pedestal structure and its role on confinement in JET with a carbon and metal wall

Leyland

Beurskens

Frassinetti³

et al. 2014

Nucl. Fusion

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We present the pedestal structure, as determined from the high resolution Thomson scattering (HRTS) measurements, for a database of low and high triangularity (  0.22-0.39) 2.5MA, Type I ELMy H-mode JET plasmas after the installation of the new ITER-like Wall (JET-ILW). The database explores the effect of increasing deuterium fuelling and nitrogen seeding with a view to explain the observed changes in performance (edge and global). The low triangularity JET-ILW plasmas show no significant change in performance and pedestal structure with increasing gas dosing. These results are in good agreement with EPED1 predictions. At high triangularity, for pure deuterium fuelled JET-ILW plasmas, there is a 20-30% reduction in global performance and pressure pedestal height in comparison to JET-C plasmas. This reduction in performance is primarily due to a degradation of the temperature pedestal height. The global performance and pressure pedestal height of JET-ILW plasmas can be partially recovered to that of JET-C plasmas with additional nitrogen seeding [Giroud 2013]. This observed improvement in performance is predominately due to a significant increase in density pedestal height as well as a small increase in the temperature pedestal height. A key result with increasing deuterium fuelling for JET-ILW plasmas is there is no improvement in pressure pedestal height however the pedestal still widens which is inconsistent with the  = 0.076√ pol,ped scaling. Furthermore, a key result with increasing nitrogen seeding is the pressure pedestal widening is due to an increase in the temperature pedestal width whilst the density pedestal shows no clear trend. The comparison of EPED1 predictions with the measurements at high triangularity is complex as, for example, for pure deuterium fuelled plasmas there is very good agreement for the pedestal height but not the width. In addition, current EPED1 runs under-predict the pedestal height and width at high nitrogen seeding for JET-ILW plasmas however further work is required to determine the significance of these deviations. Understanding these deviations is essential as provides an insight to the physical mechanisms governing the pedestal structure and edge performance.

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“…In the same frequency range one finds enhanced density fluctuations close to the pedestal top with the farinfrared-interferometer [12]. In contrast to the type-II ELMs of ASDEX Upgrade, no high-frequent irregular, bursty events in the type-II phases in between type-I ELMs were found at JET [10,12,13]. Only recently, at the lowest plasma currents, i.e.…”

Section: Introductionmentioning

confidence: 77%

“…For a more detailed discussion of ELM sizes in mixed type-I/II ELMy phase see [12]. It was reported in [13] that the enhanced inter-ELM transport is related to an increase in the strength of broadband MHD modes moving in the direction of the electron diamagnetic drift with frequencies between 10 and 40 kHz, which are called at JET washboard-modes (WB) [14]. In the same frequency range one finds enhanced density fluctuations close to the pedestal top with the farinfrared-interferometer [12].…”

Section: Introductionmentioning

confidence: 97%

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

Stöber

Lomas²,

Saibene³

et al. 2005

Nucl. Fusion

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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.

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Washboard modes as ELM-related events in JET

Cited by 63 publications

References 37 publications

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

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

The H-mode pedestal structure and its role on confinement in JET with a carbon and metal wall

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

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