Detached scrape-off layer tokamak plasmas

Kesner, J.

doi:10.1063/1.871284

Cited by 10 publications

(5 citation statements)

References 9 publications

(4 reference statements)

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“…Firstly, with two measurements of the plasma temperature on a given field line and an estimate of the heat flux to the divertor plate, the two constants of integration and the volumetric heat source required for a solution of the conduction equation can be evaluated. The interpolated field line calculation has been carried out with Alcator C-Mod data, including radiation losses, by LaBombard et al [20], and including both radiation loss and momentum loss due to collisions with neutrals by Kesner [38]. In both cases, it has been found that the temperature drops only at a very short distance from the target (poloidally and along a field line), giving a detachment Lfront' parallel to the plate.…”

Section: Neutral Particle Flux Modelmentioning

confidence: 99%

Neutral particle dynamics in the Alcator C-Mod tokamak

et al. 1997

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This thesis presents an experimental study of neutral particle dynamics in Alcator C-Mod -a high power-and particle-density tokamak with an advanced poloidal divertor. The primary diagnostic used in the study is a set of six neutral pressure gauges, including special-purpose gauges built for in situ tokamak operation.While a low main chamber neutral pressure coincides with high plasma confinement regimes, a high divertor pressure is required for heat and particle flux dispersion in future devices such as ITER. Thus we examine conditions that optimize divertor compression, defined here as a divertor-to-midplane pressure ratio. We find that both pressures depend primarily on the edge plasma regimes defined by the scrape-off-layer heat transport. While the maximum divertor pressure (pdiv =30-60 mTorr) is achieved at high core plasma densities ( Ke =2-4.1 020 m-3), corresponding to the detached divertor state, the maximum compression (-70) is achieved in the high-recycling regime. Variations in the divertor geometry have a weaker effect on the neutral pressures. For otherwise similar plasmas the divertor pressure and compression are maximized when the strike point is located at the bottom of the vertical target plate.We introduce a simple flux balance model, which allows us to explain the divertor neutral pressure across a range of plasma densities. In particular, a high pressure sustained in the detached divertor (despite a considerable drop in the recycling -3-source) can be explained by scattering of neutrals off the cold plasma plugging the divertor throat. Because neutrals are confined in the divertor through scattering and ionization processes (provided the mean-free-paths are much shorter than a typical escape distance) tight mechanical baffling is unnecessary. The analysis suggests that two simple structural modifications may increase the divertor compression in Alcator C-Mod by a factor of -5. Widening the divertor throat would increase the divertor recycling source, while closing leaks in the divertor structure would eliminate a significant neutral loss mechanism.

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Section: Neutral Particle Flux Modelmentioning

confidence: 99%

Neutral particle dynamics in the Alcator C-Mod tokamak

et al. 1997

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“…One operational regime that maximizes the influence of neutral particle collisions is that of the detached plasma regime. [3][4][5][6][7] A detached plasma is characterized in part by formation of a cool dense neutral layer in front of a material surface. Conditions are such that T e ϳ1→5 eV and the plasma density is ϳ10 20 →10 21 m Ϫ3 ͑Ref.…”

Section: Introductionmentioning

confidence: 99%

Improved elastic collision modeling in DEGAS 2 for low-temperature plasmas

et al. 2000

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Recent emphasis on low-temperature divertor operations has focused attention on proper treatment of neutral-elastic collisions in low-temperature environments. For like species collisions, as in D++D, quantum mechanical indistinguishability precludes differentiation of small-angle elastic scattering from resonant charge exchange for collision energies <2 eV. The current work improves the low-temperature simulation capabilities of the DEGAS 2 Monte Carlo neutral transport code [D. P. Stotler and C. F. F. Karney, Contrib. Plasma Phys. 34, 392–397 (1994)] by employing a set of quantum mechanical collision cross sections produced by Oak Ridge National Laboratory to provide a comprehensive treatment of ion-neutral elastic collisions. Ion-molecular collisions in the form of D++D2 are included for the first time. An integration technique is utilized that reduces the total collision cross section while keeping the other transport cross sections invariant. The inclusion of ion-molecular elastic collisions results in significant increases in energy exchange between background ions and neutral test species.

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“…All dimensions are in metres. [8,12]. This model is based on the two-point model [13,14], but has terms to account for momentum transfer from the ions to neutral atoms in the divertor and a term to account for the power lost by radiation.…”

Section: Neutral Density Measurementsmentioning

confidence: 99%

Measurements of neutral density based on emissivity in the Alcator C-Mod divertor

Kurz

LaBombard

Lipschultz

et al. 1997

Plasma Phys. Control. Fusion

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Neutral density profiles have been derived along the outboard divertor target of Alcator C-Mod and inside the last closed flux surface (LCFS). Local H, emission rates are obtained by a tomographic inversion of measured Ha brightness profiles. Electron temperatures and densities provided by a divertor Langmuir probe array are then used to calculate the local neutral density, nn, from the Ha emissivity profile. For points inside the LCFS, ECE and interferometry data were used for electron density and temperature, respectively. Along the divertor target, steep gradients in neutral density persist during regular ohmic attached discharges, with nn a 2 x 10" m-3. Plasma detachment leads to values of n, that can reach up to several times 1020 m-3 in the bottom of the divertor with much shallower pressure gradients to the midplane. Modelling of the parallel plasma momentum decrease due to charge exchange (CX) indicates that neutral densities between one and two orders of magnitude below the values observed are sufficient to explain the measured drop in parallel plasma pressure. This suggests that significant momentum is carried by the neutrals in the divertor resulting in decreased efficiency of momentum transfer between ions and neutrals.

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Detached scrape-off layer tokamak plasmas

Cited by 10 publications

References 9 publications

Neutral particle dynamics in the Alcator C-Mod tokamak

Neutral particle dynamics in the Alcator C-Mod tokamak

Improved elastic collision modeling in DEGAS 2 for low-temperature plasmas

Measurements of neutral density based on emissivity in the Alcator C-Mod divertor

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