Observations of Centrally Peaked Impurity Profiles Following Pellet Injection in the Alcator-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>C</mml:mi></mml:math>Tokamak

Petrasso, R. D.; Sigmar, D. J.; Wenzel, K. W.; Hopf, James E.; Greenwald, M.; Terry, J.; Parker, J.

doi:10.1103/physrevlett.57.707

Cited by 34 publications

(36 citation statements)

References 12 publications

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“…(9). For carbon this relation gives excellent agreement with the experimental observations [2]. For highly collisional molybdenum, in the trace impurity limit and in the Pfirsch-Schhiter regime, an exact form for the profiles is given by Eq.…”

Section: Discussionsupporting

confidence: 74%

“…The Mo impurity in Alcator C cannot be simply treated with the expressions (2)- (4) valid for one impurity frictioning on the main ion, because the Mo transport is dominated by effects from C [2], another impurity. In their review of neoclassical impurity transport,…”

Section: Molybdenummentioning

confidence: 99%

“…However, those discharges in the Alcator C tokamak with frozen fuel pellets injected into the plasma showed enhanced energy confinement [1] and strongly peaked, nearly neo-classical impurity density profiles [2]. More recent experiments on ASDEX [3][4][5], TEXT [6,7], JET [8] and TFTR [9] have also demonstrated impurity peaking on axis following the injection of frozen hydrogen pellets or the transition to other improved confinement regimes.…”

Section: Introductionmentioning

confidence: 99%

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Neoclassical analysis of impurity transport following transition to improved particle confinement

Wenzel

Sigmar

1990

Nucl. Fusion

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Strongly peaked impurity density profiles have been observed in Alcator C after frozen hydrogen pellet injection. More recent experiments in ASDEX, PBX, TEXT, JET, and TFTR have exhibited similar impurity accumulation during regimes of improved confinement. In this context, we present calculations of the neoclassically predicted equilibrium profiles of the intrinsic impurities in Alcator C. These theoretical calculations were performed for comparison with the experimentally determined peaked profiles observed after pellet fueling. Profiles of the main impurities in Alcator, carbon (C) and molybdenum (Mo), were measured using soft x-ray diagnostics. C exists in the plateau collisionality regime, and its transport is dominated by collisions with the hydrogen background ions and temperature gradient effects. Mo is in the Pfirsch-Schliiter regime, and it is driven mostly by collisions with C inside r/a ~ 0.25 and temperature gradients outside this radius. The rigorous multi-ion, mixed regime calculation necessary for the Mo transport is shown here. The predicted C profile is in excellent agreement with observation, and the profile predicted for Mo (which is not as close to equilibrium as C) is in fair agreement with observation.

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Section: Discussionsupporting

confidence: 74%

Section: Molybdenummentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Neoclassical analysis of impurity transport following transition to improved particle confinement

Wenzel

Sigmar

1990

Nucl. Fusion

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“…This suggests that strong accumulation may be avoided operationally by reduction of impurity influxes and lowering of the overall impurity levels, as has been observed. 14 These results, concurrent work on the Alcator-C experiment, 15 earlier work on smaller experiments, 2,16 and the possibility of large impurity-impurity-driven transport all emphasize the need for a full characterization of all ion constituents in the study of transport in any impure plasma, and especially when attempting to identify causes of strong central impurity accumulation.…”

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confidence: 94%

Observation ofz-dependent impurity accumulation in the PBX tokamak

et al. 1987

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Charge-dependent convective impurity transport is observed in the central core of PBX //-mode discharges by measurement of radial profiles for both low-and high-Z intrinsic impurities in the presence of strong sawtooth activity. The time-evolving Z e ff profile and central metallic densities are modeled to yield a diffusion coefficient and a Z-dependent inward convective pinch which are in general agreement with estimates of neoclassical transport in the central plasma region.

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“…The ratio of the diffusion coefficients, V/D, increases although individually V and D decrease after injection. Studies on TEXT have shown the impurity transport to be consistent with neoclassical theory [44]. Further support of the ITG model is seen in the reduction or suppression of density fluctuations in TEXT following pellet injection [45,46].…”

Section: Pellet Modementioning

confidence: 81%

Measurement of particle transport coefficients on Alcator C-Mod

Luke¹

1994

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The study of particle transport is increasingly important for fusion research. During the 1993 and 1994 campaigns of Alcator C-Mod, one observed phenomenon was the divertor detachment. This phenomenon was characterized by reduced particle and energy flows to the divertor plates and is of interest as a solution to the heat loading problem of the first wall. A sharp rise in the central electron density was observed after detachment. The goal of this thesis was to study the behavior of the plasma transport during the divertor detachment in order to explain this density rise.The measurement of particle transport coefficients requires sophisticated diagnostic tools. A two color interferometer system was developed and installed on Alcator C-Mod to measure the electron density with high spatial (~ 2 cm ) and high temporal (5 1.0 ms) resolution. The system consists of 10 CO 2 (10.6 pm) and 4 HeNe ( .6328 4m) chords that are used to measure the line integrated density to within 0.08 CO 2 degrees or 2.3x 1016 m-2 theoretically.Using the two color interferometer, a series of gas puffing experiments were conducted. The density was varied above and below the threshold density for detachment at a constant magnetic field and plasma current. Using a gas modulation technique, the particle diffusion, D, and the convective velocity, V, were determined. Profiles were inverted using a SVD inversion and the transport coefficients were extracted with a time regression analysis and a transport simulation analysis. Results from each analysis were in good agreement.Measured profiles of the coefficients incriased with the radius and the values were consistent with measurements from other experiments. The values exceeded neoclassical predictions by a factor of 10. The profiles also exhibited an inverse dependence with plasma density. The scaling of both attached and detached plasmas agreed well with this inverse scaling. This result and the lack of change in the energy and impurity transport indicate that there was no change in the underlying transport processes after detachment. On the contrary, measurements of the Ha radiation and argon transport were consistent with a change in the electron source at the edge. Thus, the rise in central density was due to an increase in the overall electron source or a improvement in fueling efficiency and not due to an improvement in the core particle transport.

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Observations of Centrally Peaked Impurity Profiles Following Pellet Injection in the Alcator-CTokamak

Cited by 34 publications

References 12 publications

Neoclassical analysis of impurity transport following transition to improved particle confinement

Neoclassical analysis of impurity transport following transition to improved particle confinement

Observation ofz-dependent impurity accumulation in the PBX tokamak

Measurement of particle transport coefficients on Alcator C-Mod

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