X-ray observations of up-down impurity density asymmetries in Alcator C-Mod plasmas

Rice, J. E.; Terry, J. L.; Marmar, E.S.; Bombarda, F.

doi:10.1088/0029-5515/37/2/i09

Cited by 35 publications

(48 citation statements)

References 24 publications

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“…While an up/down asymmetry in argon X-ray emission has previously been reported on C-Mod [13] using an array of single-chord spectrometers, this research expands those measurements over a substantial portion of C-Mod's Ohmic operating space by exploiting an X-ray imaging crystal spectrometer [14]. Measurements of the poloidal variation are larger than predicted by neoclassical theory and display a thresholdtype behavior, rather than scaling linearly withn e B t /I 2 p .…”

Section: Introductionmentioning

confidence: 84%

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Non-neoclassical up/down asymmetry of impurity emission on Alcator C-Mod

Reinke¹,

Rice²,

Hutchinson³

et al. 2013

Nucl. Fusion

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We demonstrate that existing theories are insufficient to explain up/down asymmetries of argon X-ray emission in Alcator C-Mod Ohmic plasmas. Instead of the poloidal variation, n z / n z , being of order the inverse aspect ratio, ǫ, and scaling linearly with B tne /I 2 p , it is observed over 0.8 < r/a < 1.0 to be of order unity and exhibits a threshold behavior between 3.5 < B tne /I p < 4.0 [T10 20 m −3 /MA]. The transition from symmetric to asymmetric impurity distribution is shown to occur at densities just below those that trigger a reversal of the core toroidal rotation direction, thought to be linked to the transition between the linear and saturated Ohmic confinement regimes. A possible drive is discussed by which anomalous radial transport might sustain the impurity density asymmetry as the ratio of the perpendicular to parallel equilibration times, τ ⊥,z /τ ,z , approaches unity. This explanation requires a strong up/down asymmetry in radial flux which, while not observable on C-Mod, has been measured in TEXT and Tore Supra Ohmic plasmas.

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Section: Introductionmentioning

confidence: 84%

“…The up/down asymmetry of soft x-ray emission from highly-ionized Ar has previously been observed on Alcator C-Mod [13] using a 5-channel von Hamos spectrometer [30], where chords could be scanned poloidally between shots. By repeating discharges, the full poloidal cross-section could be scanned at fixed field, current and density .…”

Section: Introductionmentioning

confidence: 99%

Non-neoclassical up/down asymmetry of impurity emission on Alcator C-Mod

Reinke¹,

Rice²,

Hutchinson³

et al. 2013

Nucl. Fusion

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“…There is an enhancement of the emission from the top of the plasma between 0.8≤ r/a ≤ 1.0. This implies that there is an up/down impurity density asymmetry, with an excess at the top of the plasma, in the direction opposite to the ion B×∇B drift direction [3,4]. This enhancement does not depend on the X-point location (upper versus lower null).…”

Section: Methodsmentioning

confidence: 99%

“…This brightness ratio is indicative of the edge up/down impurity density asymmetry [3,4]. For low density LOC plasmas, the impurity densities are up/down symmetric, and become asymmetric at higher electron densities, with an impurity density surplus in the direction opposite to the ion B×∇B drift direction.…”

Section: Methodsmentioning

confidence: 99%

Non-local heat transport, rotation reversals and up/down impurity density asymmetries in Alcator C-Mod ohmic L-mode plasmas

Rice¹,

Gao²,

Reinke³

et al. 2013

Nucl. Fusion

155

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Several seemingly unrelated effects in Alcator C-Mod Ohmic L-mode plasmas are shown to be closely connected: non-local heat transport, core toroidal rotation reversals, energy confinement saturation and up/down impurity density asymmetries. These phenomena all abruptly transform at a critical value of the collisionality. At low densities in the linear Ohmic confinement regime, with collisionality ν * ≤ 0.35 (evaluated inside of the q=3/2 surface), heat transport exhibits non-local behavior, core toroidal rotation is directed co-current, edge impurity density profiles are up/down symmetric and a turbulent feature in core density fluctuations with k θ up to 15 cm −1 (k θ ρ s ∼ 1) is present. At high density/collisionality with saturated Ohmic confinement, electron thermal transport is diffusive, core rotation is in the counter-current direction, edge impurity density profiles are up/down asymmetric and the high k θ turbulent feature is absent. The rotation reversal stagnation point (just inside of the q=3/2 surface) coincides with the non-local electron temperature profile inversion radius. All of these observations can be unified in a model with trapped electron mode prevalence at low collisionality and ion temperature gradient mode domination at high collisionality.

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“…Ar towards the plasma edge [28,29] so that the diagnostic can be potentially used to derive the neutral density profile at the plasma edge. For instance, the increased ratio between n Ar 17+ to n Ar 16+ (figure 2 d) towards the edge shows the role of the charge-exchange recombination in and n e profiles and a defined Ar gas puff outside the last closed flux surface, the temporal …”

Section: Temperature and Density Profile Inferencementioning

confidence: 99%

Argon impurity transport studies at Wendelstein 7-X using x-ray imaging spectrometer measurements

et al. 2017

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Abstract. In the first operational phase of the stellarator Wendelstein 7-X (W7-X), the X-ray Imaging Crystal Spectrometer (XICS) system has been commissioned for measuring radial profiles of ion and electron temperature, T i and T e , plasma rotation velocities v P , and selected impurity densities n Z . This paper shows the first measurements of the spectrometer and gives an initial calculation of impurity transport parameters derived from an Ar impurity transport study. Using Bayesian analysis, the temporal evolution of Ar impurity density profiles after an Ar gas puff could be observed with a time resolution of up to 5 ms, yielding a maximum value for the diffusion coefficient of D = 1.5 m 2 /s at ~0.5 and small pinch velocities in the inner plasma region.

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X-ray observations of up-down impurity density asymmetries in Alcator C-Mod plasmas

Cited by 35 publications

References 24 publications

Non-neoclassical up/down asymmetry of impurity emission on Alcator C-Mod

Non-neoclassical up/down asymmetry of impurity emission on Alcator C-Mod

Non-local heat transport, rotation reversals and up/down impurity density asymmetries in Alcator C-Mod ohmic L-mode plasmas

Argon impurity transport studies at Wendelstein 7-X using x-ray imaging spectrometer measurements

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