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

Thun, C. P. Perez von; Maraschek, M.; Graca, S. da; Buttery, R. J.; Herrmann, A.; Stöber, J.; Eich, T.; Fuchs, Julien; Horton, L. D.; Igochine, V.; Kallenbach, A.; Loarte, A.; Müller, Hans; Nunes, I.; Saibene, G.; Sartori, R.; Sips, A. C. C.; Suttrop, W.; Wolfrum, E.

doi:10.1088/0741-3335/50/6/065018

Cited by 10 publications

(12 citation statements)

References 26 publications

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“…This is consistent with measurements presented in [17]. 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 [31,32]. ; and (g), (h) W ped /W th across a JET-C deuterium fuelling scan (green), a JET-ILW deuterium fuelling scan (magenta) and two JET-ILW nitrogen seeding scans (blue and red).…”

Section: Plasma Performancesupporting

confidence: 89%

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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Section: Plasma Performancesupporting

confidence: 89%

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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“…In both cases in the frequency range up to 180 kHz clear mode numbers from n= −2 to n= −8 (negative sign for counter-current or electron diamagnetic rotation in the lab frame) are observed. These modes were previously called washboard modes [39]. Additionally, the onset of these washboard (150 kHz) modes is earlier in the ELM cycle and more connected to the recovery of the pedestal density (c.f.…”

Section: Mode Detection On the Hfs Toroidal Mode Numbers And Estimatmentioning

confidence: 98%

High frequency magnetic fluctuations correlated with the inter-ELM pedestal evolution in ASDEX Upgrade

Laggner

Wolfrum

Cavedon

et al. 2016

Plasma Phys. Control. Fusion

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In order to understand the mechanisms that determine the structure of the high confinement mode (H-mode) pedestal, the evolution of the plasma edge electron density and temperature profiles between edge localised modes (ELMs) is investigated. The onset of radial magnetic fluctuations with frequencies above 200 kHz is found to correlate with the stagnation of the electron temperature pedestal gradient. During the presence of these magnetic fluctuations the gradients of the edge electron density and temperature are clamped and stable against the ELM onset. The detected magnetic fluctuation frequency is analysed for a variety of plasma discharges with different electron pressure pedestals. It is shown that the magnetic fluctuation frequency scales with the neoclassically estimated E × B velocity at the plasma edge. This points to a location of the underlying instability in the gradient region. Furthermore, the magnetic signature of these fluctuations indicates a global mode structure with toroidal mode numbers of approximately 10. The fluctuations are also observed on the high field side with significant amplitude, indicating a mode structure that is symmetric on the low field side and high field side. The associated fluctuations in the current on the high field side might be attributed to either a strong peeling part or the presence of non-adiabatic electron response.

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“…Furthermore, type-II ELMs are usually obtained in plasmas whose shapes have been highly tuned. In addition, this regime seems to be characterized by the presence of distinguishing broadband fluctuations near the plasma edge, as described by [14,15], and most recently by [16].…”

Section: Introductionmentioning

confidence: 95%

“…The requirements for obtaining type-II ELMs at AUG are already well-known, see e.g. [8,15,16] and references therein. In short, the regime of the type-II ELMs is reached using highly shaped plasmas in near double null (DN) configuration, with high triangularity δ, high elongation κ, and high q 95 .…”

Section: Type-ii Elm Experiments At Augmentioning

confidence: 99%

Characterization of broadband MHD fluctuations during type-II edge localized modes as measured in 2D with ECE-imaging at ASDEX Upgrade

et al. 2012

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The characterization of a broadband fluctuation that is typical for the type-II ELM regime at ASDEX Upgrade has been improved using the 2D capabilities of ECE-imaging. During the transition from the type-I to type-II ELMy phase, it has been found that electron temperature fluctuations form a broadband peak in the 19–65 kHz range. In the type-II phase, this broadband fluctuation reaches a maximum relative amplitude of almost 20% just inside the top of the pedestal. Simultaneously, the electron temperature profile is completely flattened at this location. The 2D distribution of the amplitude of this broadband fluctuation is such that, when averaged over time, a minimum occurs around the mid-plane. From the measurements of the nearby magnetic pickup coils, a similar broadband fluctuation seems visible in the same frequency range. However, this is peaked at a slightly lower frequency and does not show a similar minimum. From the analysis of the fluctuations on small timescales, the poloidal and toroidal mode numbers are estimated to be m ∼ 100 and n ∼ 21. Furthermore, activity reminiscent of beat waves has been observed, which might partially account for the fluctuation's broadband nature and the seeming velocity variation of single fluctuation passages. Overall, similarities between the characteristics of this broadband fluctuation and various precursors to type-I ELMs suggest that this fluctuation can play an important role in regulating the ELM cycle.

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

Cited by 10 publications

References 26 publications

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

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

High frequency magnetic fluctuations correlated with the inter-ELM pedestal evolution in ASDEX Upgrade

Characterization of broadband MHD fluctuations during type-II edge localized modes as measured in 2D with ECE-imaging at ASDEX Upgrade

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