Characteristics of energy transport of Li-conditioned and non-Li-conditioned plasmas in the National Spherical Torus Experiment (NSTX)

Ding, Siye; Kaye, S. M.; Bell, R. E.; Kaita, R.; Kugel, H.; LeBlanc, B.P.; Paul, S. F.; Wan, Baonian

doi:10.1088/0741-3335/52/1/015001

Cited by 14 publications

(11 citation statements)

References 28 publications

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“…On the other hand, the core χ e , χ i and χ φ at r/a = 0.35 were insensitive to or weakly increasing with the pre-discharge lithium deposition, as shown in panel (d). These results agree with analysis [41] of a broader dataset, which included a couple of the discharges from this scan. For reference, the random uncertainties for the quantities shown in figure 8 are 20-30%.…”

Section: Description Of Experiments With Increasing Lithium Wall Coat...supporting

confidence: 87%

The effect of progressively increasing lithium coatings on plasma discharge characteristics, transport, edge profiles and ELM stability in the National Spherical Torus Experiment

Maingi¹,

Boyle²,

Canik³

et al. 2012

Nucl. Fusion

115

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Lithium wall coatings have been shown to reduce recycling, suppress edge-localized modes (ELMs), and improve energy confinement in the National Spherical Torus Experiment (NSTX). Here we document the effect of gradually increasing lithium wall coatings on the discharge characteristics, with the reference ELMy discharges obtained in boronized, i.e. non-lithiated conditions. We observed a continuous but not quite monotonic reduction in recycling and improvement in energy confinement, a gradual alteration of edge plasma profiles, and slowly increasing periods of ELM quiescence. The measured edge plasma profiles during the lithium-coating scan were simulated with the SOLPS code, which quantified the reduction in divertor recycling coefficient from ∼98% to ∼90%. The reduction in recycling and fuelling, coupled with a drop in the edge particle transport rate, reduced the average edge density profile gradient, and shifted it radially inwards from the separatrix location. In contrast, the edge electron temperature (T e) profile was unaffected in the H-mode pedestal steep gradient region within the last 5% of normalized poloidal flux, ψ N ; however, the T e gradient became steeper at the top of the H-mode pedestal for 0.8 < ψ N < 0.94 with lithium coatings. The peak pressure gradients were comparable during ELMy and ELM-free phases, but were shifted away from the separatrix in the ELM-free discharges, which is stabilizing to the current-driven instabilities thought to be responsible for ELMs in NSTX.

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Section: Description Of Experiments With Increasing Lithium Wall Coat...supporting

confidence: 87%

The effect of progressively increasing lithium coatings on plasma discharge characteristics, transport, edge profiles and ELM stability in the National Spherical Torus Experiment

Maingi¹,

Boyle²,

Canik³

et al. 2012

Nucl. Fusion

115

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“…6a, it is seen that B T τ E,th ∼ ν * e −0.79±0. 10 . The scalings as a function of ν * e are even stronger when the variation of ρ * is taken into account through the Bohm (Fig.…”

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confidence: 99%

The dependence of H-mode energy confinement and transport on collisionality in NSTX

et al. 2013

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Understanding the dependence of confinement on collisionality in tokamaks is important for the design of next-step devices, which will operate at collisionalities at least one order of magnitude lower than in present generation. A wide range of collisionality has been obtained in the National Spherical Torus Experiment (NSTX) by employing two different wall conditioning techniques, one with boronization and between-shot helium glow discharge conditioning (HeGDC+B), and one using lithium evaporation (Li EVAP). Previous studies of HeGDC+B plasmas indicated a strong and favorable dependence of normalized confinement on collisionality. Discharges with lithium conditioning discussed in the present study generally achieved lower collisionality, extending the accessible range of collisionality by almost an order of unity. While the confinement dependences on dimensional, engineering variables of the HeGDC+B and Li EVAP datasets differed, collisionality was found to unify the trends, with the lower collisionality lithium conditioned discharges extending the trend of increasing normalized confinement time with decreasing collisionality when other dimensionless variables were held as fixed as possible. This increase of confinement with decreasing collisionality was driven by a large reduction in electron transport in the outer region of the plasma. This result is consistent with gyrokinetic calculations that show microtearing and Electron Temperature Gradient modes to be more stable for the lower collisionality discharges. Ion transport, near neoclassical at high collisionality, became more anomalous at lower collisionality, possibly due to the growth of hybrid TEM/KBM modes in the outer regions of the plasma.

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“…Unfortunately, the E = E H98y2 typically obtained in spherical tokamaks with boron wall coatings is more typically 0.8-1.0, albeit with different dependences than the ITER H98y2 scaling [3]. The recent deployment of lithium wall coatings has helped [4][5][6] by increasing E = E H98y2 up to 1:1-1:2 times the value for the H98y2 scaling, but still short of the desired value for future STs. We note that this problem is not specific to spherical tokamaks, i.e., higher aspect ratio tokamaks rarely achieve E = E H98y > 1:2, and in those cases, the high confinement phase is often transient.…”

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confidence: 99%

Triggered Confinement Enhancement and Pedestal Expansion in High-Confinement-Mode Discharges in the National Spherical Torus Experiment

et al. 2010

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We report observation of a new high performance regime in discharges in the National Spherical Torus Experiment, where the H mode edge "pedestal" temperature doubles and the energy confinement increases by 50%. The spontaneous transition is triggered by a large edge-localized mode, either natural or externally triggered by 3D fields. The transport barrier grows inward from the edge, with a doubling of both the pedestal pressure width and the spatial extent of steep radial electric field shear. The dynamics suggest that 3D fields could be applied to reduce edge transport in fusion devices.

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Characteristics of energy transport of Li-conditioned and non-Li-conditioned plasmas in the National Spherical Torus Experiment (NSTX)

Cited by 14 publications

References 28 publications

The effect of progressively increasing lithium coatings on plasma discharge characteristics, transport, edge profiles and ELM stability in the National Spherical Torus Experiment

The effect of progressively increasing lithium coatings on plasma discharge characteristics, transport, edge profiles and ELM stability in the National Spherical Torus Experiment

The dependence of H-mode energy confinement and transport on collisionality in NSTX

Triggered Confinement Enhancement and Pedestal Expansion in High-Confinement-Mode Discharges in the National Spherical Torus Experiment

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