Investigations on the ELM cycle by local 3D perturbation experiments

Lang, P. T.; Gál, K.; Hobirk, J.; Kálvin, S.; Kocsis, G.; Mertens, V.; Neuhäuser, J.; Maraschek, M.; Suttrop, W.; Veres, G. I.

doi:10.1088/0741-3335/48/5a/s13

Cited by 9 publications

(16 citation statements)

References 10 publications

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“…Notably, still as in AUG-C prompt ELMs are triggered very soon (typically in less than 0.1 ms) after the onset of pellet ablation. Thus, as shown in a dedicated earlier study [38], the pellet-imposed perturbation finally causing (or growing into) the ELM is located within the pedestal. Normalized energy losses for pellet-triggered ELMs were derived as indicated in the lower box of figure 6; for cases with no pronounced pellet-induced energy drop (can be identified by dW MHD /dt > 0 at the end of pellet ablation) as in case (A) the difference in W MHD between the onset and end of pellet ablation is taken.…”

Section: Figure 5: To Distinguish Between Pellets Triggering An Elm Asupporting

confidence: 53%

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ELM pacing and high-density operation using pellet injection in the ASDEX Upgrade all-metal-wall tokamak

et al. 2014

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Abstract. ELM triggering and pacing in an all-metal wall environment shows significant differences to a firstwall configuration containing carbon. Here we report on experiments performed at ASDEX Upgrade revisiting the issue with all plasma-facing surfaces now fully replaced by tungsten. This investigation was motivated by experimental findings indicating that ELM triggering becomes more intricate when the carbon is replaced by a metal wall. ELM pacing could no longer be achieved by magnetic triggering in ASDEX Upgrade under conditions that previously showed a positive response. Also, recent investigations at JET indicate that a lag time occurs in pellet ELM triggering when operating with the new ITER-like wall. The ASDEX Upgrade centrifugebased launching system was revitalized and upgraded for this study, now allowing detailed analysis of the ELM trigger response. The appearance of a lag time for pellet ELM triggering in an all-metal wall environment was confirmed. While different lag time durations were found for several type-I ELMy H-mode scenarios, the magnitude of the pellet perturbation was found to cause no difference. Reducing the auxiliary heating power for ELM triggering clearly makes the pellet tool less efficient for ELM control purposes; however, this affords a major benefit when applied for fuelling. Plasma operation with benign ELM behaviour at core densities far beyond the Greenwald limit was demonstrated, this being fully reversible and not affecting the energy confinement.

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Section: Figure 5: To Distinguish Between Pellets Triggering An Elm Asupporting

confidence: 53%

“…The plot displays data obtained from reference phases for spontaneous ELMs, for a few paced ELMs and for triggering probe pellets injected at 10 Hz. The data set shows consistent behaviour for the evolution of achievable ELM size along an ELM cycle; the trigger lag time is about 8 -10 ms. [38].…”

Section: Trigger Lag Times In Different Scenariosmentioning

confidence: 99%

ELM pacing and high-density operation using pellet injection in the ASDEX Upgrade all-metal-wall tokamak

et al. 2014

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“…Again this change in the balance of power efflux demonstrates the capacity of extrinsic seeding fundamentally to influence the character of ILW H-modes. More generally, the persistence of a dominant ELM component in time-average exhaust re-emphasizes the probable need ultimately to combine extrinsic seeding with some form of active mitigation [149][150][151][152][153][154][155][156][157][158][159][160][161][162] of Type I fluctuations.…”

Section: Inter-elm Heat Load and Elm Lossesmentioning

confidence: 99%

Contrasting H-mode behaviour with deuterium fuelling and nitrogen seeding in the all-carbon and metallic versions of JET

et al. 2014

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The former all-carbon wall on JET has been replaced with beryllium in the main torus and tungsten in the divertor to mimic the surface materials envisaged for ITER. Comparisons are presented between Type I H-mode characteristics in each design by examining respective scans over deuterium fuelling and impurity seeding, required to ameliorate exhaust loads both in JET at full capability and in ITER. Attention is focused upon a common high-triangularity, single-null divertor configuration at 2.5 MA, q 95 ≈ 3.5 yielding the most robust all-C performance. Contrasting results between the alternative linings are found firstly in unseeded plasmas, for which purity is improved and intrinsic radiation reduced in the ITER-like wall (ILW) but normalised energy confinement is ≈ 30% lower than in all-C counterparts, owing to a commensurately lower (electron) pedestal temperature. Divertor recycling is also radically altered, with slower, inboard-outboard asymmetric transients at ELMs and spontaneous oscillations in between them. Secondly, nitrogen seeding elicits opposite responses in the ILW to all-C experience, tending to raise plasma density, reduce ELM frequency, and above all to recover (electron) pedestal pressure, hence global confinement, almost back to previous levels. A hitherto unrecognised role of light impurities in pedestal stability and dynamics is consequently suggested. Thirdly, while heat loads on the divertor outboard target between ELMs are successfully reduced in proportion to the radiative cooling and ELM recurrence effects of N in both wall environments, more surprisingly, average power ejected by ELMs and estimated scrape-off-layer pressure ratio during the approach to outboard detachment also both decline in the same proportion for the ILW. Finally, inter-ELM W sources in the ILW divertor tend to fall with N input, although core accumulation possibly due to net ELM erosion still leads to significantly less steady conditions than in all-C plasmas. This limitation of ILW H-modes so far will be readdressed in future campaigns to continue progress towards a fully-integrated scenario suitable for D-T experiments on JET and for "baseline" operation on ITER. The diverse changes in behaviour between all-C and ILW contexts demonstrate essentially the strong impact which boundary conditions and intrinsic impurities can have on tokamak-plasma states. ‡ That is, contours are interpolated between the torus-pressure values for each all-C and ILW shot at its position in the normalised confinement versus density plane.

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“…Accordingly, the question is if and how triggered ELMs differ from spontaneous ones. Applied in a perturbative way (pellet rate much smaller than ELM frequency), small and shallow pellet injection may also be seen as a tool to investigate pedestal stability, time scales for communication from the seed perturbation around the torus, non-linear mode growth and filament ejection [21,22]. For the present analysis, the hfs pellet injection system is used with velocities between 240 and 1000 m/s.…”

Section: Pellet Induced Versus Spontaneous Elmsmentioning

confidence: 99%

Structure and dynamics of spontaneous and induced ELMs on ASDEX Upgrade

et al. 2008

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In order to assess the contribution of edge localized modes (ELMs) to plasma-wall interaction in future fusion experiments like ITER, a sound experimental data basis for model validation and extrapolation, and, to be prepared for the unfavourable case, the development of tools for ELM mitigation are required. On ASDEX Upgrade, exploiting the fast edge/divertor diagnostics as well as active ELM control tools, especially ELM pacing by cryogenic pellet injection, a large amount of experimental information has been accumulated on the structure and dynamics of natural and induced ELMs, as well as on related wall effects. In this paper, a survey of ELM structure related diagnostics and results is given first. Recent progress in spontaneous ELM analysis is given then with emphasis on magnetic probe analysis, 2d Thomson scattering snapshots of pedestal density and temperature perturbations in comparison with electron cyclotron emision, and combined Langmuir, magnetic and thermographic investigation of filament dynamics in the scrape-off layer wing. Pellet induced ELMs are characterized in comparison to spontaneous ones. The combined experimental evidence is finally discussed in terms of simple considerations based on presently available, still rudimentary models.

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Investigations on the ELM cycle by local 3D perturbation experiments

Cited by 9 publications

References 10 publications

ELM pacing and high-density operation using pellet injection in the ASDEX Upgrade all-metal-wall tokamak

ELM pacing and high-density operation using pellet injection in the ASDEX Upgrade all-metal-wall tokamak

Contrasting H-mode behaviour with deuterium fuelling and nitrogen seeding in the all-carbon and metallic versions of JET

Structure and dynamics of spontaneous and induced ELMs on ASDEX Upgrade

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