Comparison of deuterium pellet injection from different locations on the DIII-D tokamak

Baylor, L. R.; Jernigan, T. C.; Parks, P.B.; Antar, G.; Brooks, N.H.; Combs, S. K.; Fehling, D. T.; Foust, C. R.; Houlberg, W. A.; Schmidt, G. L.

doi:10.1088/0029-5515/47/11/023

Cited by 59 publications

(88 citation statements)

References 27 publications

(48 reference statements)

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“…Furthermore, the impact of the pacing on the more crucial peak heat flux on the divertor has to be investigated. While DIII-D reports a reduction in proportion of at least the ELM frequency enhancement [6], recent investigations at AUG [33] and JET [22] found virtually no reduction. In particular, recent ELM pacing experiments at JET increasing the ELM frequency by a factor of 4 reduced the ELM-induced thermal load to the divertor by a factor of only about 1.5; for the peak heat flux virtually no reduction was found [22].…”

Section: Figure 8: Most Extreme Cases Observed For Triggering Lag Timmentioning

confidence: 87%

ELM control at the L → H transition by means of pellet pacing in the ASDEX Upgrade and JET all-metal-wall tokamaks

Lang¹,

Meyer²,

Birkenmeier³

et al. 2015

Plasma Phys. Control. Fusion

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In ITER pellets are envisaged for ELM control and fuelling. More important, ELM control, particularly control of the first ELM needs to be demonstrated already in the non-nuclear phase of ITER during operation in H or He. Whilst D pellets have been established as ELM control technique in the stationary phase with D target plasmas in devices with C as plasma-facing component, the question of other isotopes and non-stationary phases are not so well known. Here, we report on new pellet triggering experiments in ASDEX Upgrade and JET mimicking specific ITER operating scenarios. Both machines are equipped with an all-metal wall, where recent investigations have shown that pellet triggering and pacing become more intricate. In both machines ELM triggering by D pellets injected into D plasmas during extended ELM-free phases, often following the L → H transition, has been demonstrated. In both devices the pellets are found to induce ELMs under conditions far from the stability boundary for type-I ELMs. Near the L → H transition, induced ELMs in some cases might more likely have type-III rather than type-I characteristics. Furthermore, in ASDEX Upgrade this study was conducted during L → H transitions in the current ramp-up phase as envisaged for ITER. In addition, the pellet ELM trigger potential was proven in ASDEX Upgrade with a correct isotopic compilation for the non-nuclear phase in ITER, viz. H pellets into either He or H plasmas. Results from this study are encouraging since they have demonstrated the pellets' potential to provoke ELMs even under conditions quite far from the stability boundaries attributed to the occurrence of spontaneous ELMs. However, with the recent change from carbon to an allmetal plasma-facing components examples have been found in both machines where pellets failed to establish ELM control under conditions where this would be expected and needed. Consequently, a major task of future investigations in this field will be to shed more light on the underlying physics of the pellet ELM triggering process to allow sound predictions for ITER. Keywords: ELM control, tokamak, pelletExtended 41th EPS contributed paper O3.115 Accepted Version V4.0, 2.3.2015 2 INTRODUCTIONOperating ITER in the reference inductive scenario at the design values I P = 15 MA and Q DT = 10 relies on good H-mode confinement facilitated by the presence of a strong edge transport barrier and a sufficiently high plasma pressure pedestal. The steep gradients evolving at the edge can drive MHD instabilities resulting in an Edge-Localized Mode (ELM) producing a rapid energy burst from the pedestal region. Without dedicated ELM control, the resulting transient heat loads on plasma-facing materials in ITER become critical for operation at a plasma current I P ≈ 9.5 MA [1]; progressing to a higher I P would result in an intolerably short lifetime of the divertor plates [2]. Currently, several options are being considered for this inevitable ELM actuation, but all of them need further validation for the ITER tasks. Obviously...

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Section: Figure 8: Most Extreme Cases Observed For Triggering Lag Timmentioning

confidence: 87%

ELM control at the L → H transition by means of pellet pacing in the ASDEX Upgrade and JET all-metal-wall tokamaks

Lang¹,

Meyer²,

Birkenmeier³

et al. 2015

Plasma Phys. Control. Fusion

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“…by adjusting the pellet perturbation magnitude or by changing the poloidal launching position. There are strong indications reported from DIII-D [46] and JET-ILW [18] that the location of the injection path has a significant influence. However, since the AUG system is restricted to the injection geometry shown in figure 1, it is currently not possible to vary this parameter.…”

Section: Trigger Threshold and Edge Stability Considerationsmentioning

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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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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“…However, pellet triggered ELMs introduce enhanced particle losses during and after the deposition process. For fuelling size pellets injected into a type-I ELM phase an initial ELM is already triggered while ablation is still ongoing [15,16]. Moreover, in the phase following the ablation, enhanced ELM activity is observed, causing increased particle losses from the pellet particle deposition zone.…”

Section: Introductionmentioning

confidence: 99%

High-density H-mode operation by pellet injection and ELM mitigation with the new active in-vessel saddle coils in ASDEX Upgrade

et al. 2012

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Recent experiments at ASDEX Upgrade demonstrate the compatibility of ELM mitigation by magnetic perturbations with efficient particle fuelling by inboard pellet injection. ELM mitigation persists in a high density, high collisionality regime even with the strongest applied pellet perturbations. Pellets injected into mitigation phases trigger no type-I ELM like events unlike when launched into unmitigated type-I ELMy plasmas. Furthermore, the absence of ELMs results in improved fuelling efficiency and persistent density build up. Pellet injection is helpful to access the ELM-mitigation regime by raising the edge density beyond the required threshold level, mostly eliminating the need for strong gas puff. Finally, strong pellet fuelling can be applied to access high densities beyond the density limit encountered with pure gas puffing. Core densities of up to 1.6 times the Greenwald density have been reached while maintaining ELM mitigation. No upper density limit for the ELM-mitigated regime has been encountered so far; limitations were set solely by technical restrictions of the pellet launcher. Reliable and reproducible operation at line averaged densities from 0.75 up to 1.5 times the Greenwald density is demonstrated using pellets. However, in this density range there is no indication of the positive confinement dependence on density implied by the ITERH98P(y,2) scaling.Final version, 6.1.2012 2 IntroductionThe power load deposited by type-I ELMs onto the first wall and divertor presents a severe danger for ITER. Currently, there are several options for ELM mitigation: Operating with plasma scenarios which develop no or only benign ELMs, ELM pace-making to enhance the ELM frequency by at least a factor of 15 -30 in order to reduce individual ELM losses by this same factor, or ELM mitigation or ELM suppression with non-axisymmetric magnetic perturbations. Pace-making tries to reduce the type-I ELM size by triggering the instability through an external perturbation, e.g. pellet injection with a rate higher than the natural ELM frequency. Avoidance of type-I ELMs is preferred over ELM pace-making if it can be achieved without significant deleterious impact on the plasma performance. Nonaxisymmetric magnetic perturbations have been investigated in DIII-D where full suppression or at least significant mitigation of ELMs has been achieved over a wide operational range [1]. This successful first demonstration prompted further experiments at JET [2,3], NSTX [4] and MAST [5] with different perturbation field configurations. In theses experiments, quite different results with respect to ELM mitigation were obtained, and so far full ELM suppression has not been reproduced in any of them. Recently, the ASDEX Upgrade tokamak has been enhanced with a set of in-vessel coils [6] in order to study ELM amelioration, shed light on the physics involved, and to improve the extrapolation base for ITER. In a first stage of this enhancement, n=2 magnetic perturbations are found to allow suppressing type-I ELMs and replace th...

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Comparison of deuterium pellet injection from different locations on the DIII-D tokamak

Cited by 59 publications

References 27 publications

ELM control at the L → H transition by means of pellet pacing in the ASDEX Upgrade and JET all-metal-wall tokamaks

ELM control at the L → H transition by means of pellet pacing in the ASDEX Upgrade and JET all-metal-wall tokamaks

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

High-density H-mode operation by pellet injection and ELM mitigation with the new active in-vessel saddle coils in ASDEX Upgrade

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