Volume recombination and opacity in Alcator C-Mod divertor plasmas

Abstract

Volume recombination within the Alcator C-Mod [I.H. Hutchinson et al., Phys. Plasmas 1 1511 (1994)] divertor plasma is measured and found to be a significant fraction of the total ion sink under detached divertor conditions. The recombination occurs in regions where Te ~1 eV and ne ~ 101 m-3 . Measurements of the spatial distribution of the recombination are presented. The determinations of the recombination rates are made by measuring the DO Balmer spectrum and by using a collisional radiative model describing the level populations, ionization and recombination of Do. The concept of 'recombinations per Balmer series photon' is developed to simplify the determinations. Measurements of the opacity of Lyo emission are presented. It is observed that up to 50% of the Lyo emission is trapped, indicating that Ly, is strongly trapped in some cases. The effects of Ly,, trapping on the 'recombinations per photon' curves are calculated and considered in the recombination rate determinations. Observations indicating the presence of Molecular Activated Recombination are discussed.+ Also at I.V. Kurchatov Institute of Atomic Energy, Moscow, Russian Federation

Physics of ultimate detachment of a tokamak divertor plasma

“…Trapping of the radiation (in particular, of the hydrogen Ly α and Ly β lines) in the edge plasmas, which was predicted long ago (Krasheninnikov & Pigarov 1987) and observed in experiments (see e.g. Terry et al 1998), also causes violation of the binary collision approximation.…”

“…the Lyman series). Such radiation trapping, seen in experiments Terry et al (1998), strongly modifies the rate constants of the hydrogen-related atomic processes and, due to the radiation transport, makes them non-locally dependent on the plasma parameters. Therefore, the most sophisticated collisional-radiative models for the hydrogen-related processes include the effects of the radiation transport (e.g.…”

The role of particle sinks and sources in Alcator C-Mod detached divertor discharges

“…Modeling has also shown that volume recombination should be significant in such low temperature detached divertor plasmas and potentially provide the mechanism for the observed divertor plate current reduction [10][11][12][13][14] . Recombination has been experimentally shown to exist in the divertor [15][16][17][18][19][20][21][22][23][24] and its magnitude to be significant 15,17,19,21,22,24 in comparison to the ion current reduction to the divertor plates. Measurements made on linear devices lead to similar conclusions 25,26 .…”

Electron density measurements in the National Spherical Torus Experiment detached divertor region using Stark broadening of deuterium infrared Paschen emission lines

“…We use the astrophysical notation for the series lines: for example, H10 is used for the Balmer 10 − 2 transition, and P10 for the Paschen 10 − 3 transition. The corresponding ultraviolet (UV) H6-H14 and near-infrared (NIR) P8-P14 lines have been observed in many divertor MCF experiments [3][4][5][6]. While the Stark-broadened Balmer lines, as well as the Balmer and Paschen continua emissions have been extensively used for diagnosing tokamak detached divertor plasma conditions, no application of the NIR Paschen lines has been reported.…”