Edge-Localized-Mode Suppression through Density-Profile Modification with Lithium-Wall Coatings in the National Spherical Torus Experiment

Maingi, R.; Osborne, T.H.; LeBlanc, B.P.; Bell, R. E.; Manickam, J.; Snyder, P. B.; Ménard, J.; Mansfield, D.K.; Kugel, H.; Kaita, R.; Gerhardt, S. P.; Sabbagh, S.A.; Kelly, F. A.

doi:10.1103/physrevlett.103.075001

Cited by 163 publications

(161 citation statements)

References 28 publications

(23 reference statements)

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“…Subsequent analysis of the MHD stability of these profiles has revealed that the edge plasma is close to the stability boundary for peeling-ballooning modes for the discharges without lithium which exhibit ELMs. However, in the discharges with lithium, the changes in both the edge profile and the resulting equilibrium move the stability boundary away from the experimental values [20]. The source of the increased radiation in the discharges with lithium appears to be metallic impurities, notably iron, while the increase in the Z eff is more attributable to an increase in the carbon impurity content [21].…”

Section: Discussionmentioning

confidence: 87%

Plasma Response to Lithium-Coated Plasma-Facing Components in the National Spherical Torus Experiment

2009

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Experiments in the National Spherical Torus Experiment (NSTX) have shown beneficial effects on the performance of divertor plasmas as a result of applying lithium coatings on the graphite and carbonfiber-composite plasma-facing components. These coatings have mostly been applied by a pair of lithium evaporators mounted at the top of the vacuum vessel which inject collimated streams of lithium vapor towards the lower divertor. In NBI-heated, deuterium H-mode plasmas run immediately after the application of lithium, performance modifications included decreases in the plasma density, particularly in the edge, and inductive flux consumption, and increases in the electron and ion temperatures and the energy confinement time. Reductions in the number and amplitude of ELMs were observed, including complete ELM suppression for periods up to 1.2 s, apparently as a result of altering the stability of the edge. However, in the plasmas where ELMs were suppressed, there was a significant secular increase in the effective ion charge Z eff and the radiated power as a result of increases in the carbon and medium-Z metallic impurities, although not of lithium itself which remained at a very low level in the plasma core, <0.1%. The impurity buildup could be inhibited by repetitively triggering ELMs with the application of brief pulses of an n = 3 radial field perturbation. The reduction in the edge density by lithium also inhibited parasitic losses through the scrape-off layer of ICRF power coupled to the plasma, enabling the waves to heat electrons in the core of H-mode plasmas produced by NBI. Lithium has also been introduced by injecting a stream of chemically stabilized, fine lithium powder directly into the scrape-off layer of NBI-heated plasmas. The lithium was ionized in the SOL and appeared to flow along the magnetic field to the divertor plates. This method of coating produced similar effects to the evaporated lithium but at lower amounts.

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Section: Discussionmentioning

confidence: 87%

Plasma Response to Lithium-Coated Plasma-Facing Components in the National Spherical Torus Experiment

2009

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“…ELM suppression with lithium coatings was shown to coincide with a shift of the density pedestal away from the separatrix, as well as significant increase in its width. By shifting the peak density gradient away from the separatrix, lithium reduced the bootstrap current at the edge (where it had the largest effect on stability), suppressing ELMs [14,17]. With inclusion of the additional data in Figure 2(a-b), it is now apparent that the density pedestal shifted and widened continuously with increasing lithium deposition, even after ELMs were completely suppressed.…”

Section: Evolution With Increasing Lithiummentioning

confidence: 94%

“…Elements of the first experiment have been described previously [8,9,[14][15][16][17][18][19]; it was the first experiment to use the upgraded dual evaporator LITER lithium deposition system [8,10] and began with an entirely lithium-free baseline. In that experiment, medium triangularity (δ~0.5) discharges with lithium coatings >400 mg and neutral beam power P NBI = 4 MW reached global stability limits and disrupted at ~0.3s.…”

Section: Introductionmentioning

confidence: 99%

“…Previous work has reported on many aspects of the original lithium scan: the gradual suppression of ELMs [9], the stabilization of kink/peeling modes that cause ELMs [14], the continuous reduction in transport at the pedestal top [15,16], the systematic modification of the pedestal profiles [17], and the continuous evolution of recycling and confinement with increasing lithium deposition [18,19]. However, the effect of additional lithium beyond the point of ELM suppression on plasma profiles and performance was not determined due to the gap in the original data.…”

Section: Introductionmentioning

confidence: 99%

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Varying the Pre-discharge Lithium Wall Coatings to Alter the Characteristics of the ELM-free H-mode Pedestal in NSTX

2012

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A previous experiment in the National Spherical Torus Experiment (NSTX) showed pre-discharge lithium deposition gradually suppresed edge-localized modes (ELMs) and had nearly continuous relationships with reduced recycling and transport. In this paper, additional data filled gaps in the earlier experiment, and demonstrates that recycling, confinement, and pedestal structure continued to improve with additional lithium, even after ELMs were completely suppressed. New analysis shows that toroidal rotation and ion temperature also increased continuously with additional lithium. Besides its evolution with additional lithium, we also characterize the time evolution of the ELM-free H-mode pedestal as average density rose and impurities accumulated. We find that the pedestal structure, divertor heat flux and Dalpha profiles, and inferred recycling coefficient did not change significantly, at least until radiative losses become dominant. This suggests that the low-recycling properties of lithium were not significantly degraded over the duration of the discharge.

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“…In previous work [26], we compared the profiles and stability of the endpoints of the experiment, i.e., ELMy pre-lithium discharges versus completely ELM-free discharges with thick lithium coatings. The conclusions were that lithium widened the density pedestal and shifted it away from the separatrix, causing similar changes in the edge pressure and current profiles, and that this caused the plasma to be farther from its peeling-ballooning stability boundary, i.e., more stable.…”

Section: Lithium Wall Coatings and Elmsmentioning

confidence: 99%

The Relationships Between ELM Suppression, Pedestal Profiles, and Lithium Wall Coatings in NSTX

Boyle¹,

Maingi²,

Snyder³

et al. 2012

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Abstract.Recently in the National Spherical Torus Experiment (NSTX), increasing lithium wall coatings suppressed edge localized modes (ELMs), gradually but not quite monotonically. This work details profile and stability analysis as ELMs disappeared throughout the lithium scan. While the quantity of lithium deposited between discharges did not uniquely determine the presence of ELMs, profile analysis demonstrated that lithium was correlated to wider density and pressure pedestals with peak gradients farther from the separatrix. Moreover, the ELMy and ELM-free discharges were cleanly separated by their density and pedestal widths and peak gradient locations. Ultimately, ELMs were only suppressed when lithium caused the density pedestal to widen and shift inward. These changes in the density gradient were directly reflected in the pressure gradient and calculated bootstrap current. This supports the theory that ELMs in NSTX are caused by peeling and/or ballooning modes, as kink/peeling modes are stabilized when the edge current and pressure gradient shift away from the separatrix. Edge stability analysis using ELITE corroborated this picture, as reconstructed equilibria from ELM-free discharges were generally farther from their kink/peeling stability boundaries than ELMy discharges. We conclude that density profile control provided by lithium is the key first step to ELM suppression in NSTX.Boyle, PPCF paper 2

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Edge-Localized-Mode Suppression through Density-Profile Modification with Lithium-Wall Coatings in the National Spherical Torus Experiment

Cited by 163 publications

References 28 publications

Plasma Response to Lithium-Coated Plasma-Facing Components in the National Spherical Torus Experiment

Plasma Response to Lithium-Coated Plasma-Facing Components in the National Spherical Torus Experiment

Varying the Pre-discharge Lithium Wall Coatings to Alter the Characteristics of the ELM-free H-mode Pedestal in NSTX

The Relationships Between ELM Suppression, Pedestal Profiles, and Lithium Wall Coatings in NSTX

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