Plasma recombination and molecular effects in tokamak divertors and divertor simulators

Abstract

Analysis of the experimental data from tokamaks and linear divertor simulators leads to the conclusion that plasma recombination is a crucial element of plasma detachment. Different mechanisms of plasma recombination relevant to the experimental conditions of the tokamak scrape-off layer (SOL) and divertor simulators are considered. The physics of Molecular Activated Recombination (MAR) involving vibrationally excited molecular hydrogen are discussed. Although conventional Electron–Ion Recombination (EIR) alone can strongly alter the plasma parameters, MAR impact can be substantial for both tokamak SOL plasma and divertor simulators. Investigation of the effects of EIR on the plasma flow in divertor simulators shows that due to the balances of (a) energy transport and electron cooling, and (b) the plasma flow and recombination, that EIR extinguishes the simulator plasma at an electron temperature about 0.15 eV.

Physics of ultimate detachment of a tokamak divertor plasma

“…For the detached divertor conditions, T ion ∼ 5 eV and does not vary strongly, since for a lower temperature, the ionization rate constant drops sharply, whereas the neutrals cannot penetrate to the higher temperature regions due to the high plasma density and fast ionization. Then, for the case where T w is low and the heat flux to the target associated with the kinetic energy of the electrons and ions can be ignored, from (4.1) we find (Krasheninnikov 1996;Krasheninnikov et al 1997)…”

“…To sort out the issues with the plasma source and sink in the detached tokamak plasma, we will follow Krasheninnikov (1996), Krasheninnikov et al (1997) and consider the global balance of the plasma particles and energy in the tokamak SOL, which can be written as…”

Volume recombination and opacity in Alcator C-Mod divertor plasmas

“…Experimental investigations seeking to identify and measure MAR in tokamak divertors are of interest because MAR has been predicted to be an important pathway for volume recombination [8]. We have modelled measured Lyman series spectra emitted from a relatively low density discharge for which the inner divertor was de- In an initial attempt to model these intensity ratios, it was assumed that the emission was from a single isothermal region.…”

“…Modelling has shown that recombination should be significant in these divertor plasmas, and that it is potentially responsible for removing a large fraction of the ion flux [4][5][6][7][8][9]. The first experimental measurements of significant volume recombination in a tokamak divertor were made on Alcator C-Mod [10,11] where its quantitative signature was found in the emission spectrum of D 0 , and where the ion sink due to recombination was found to be comparable to the ion loss to the plates.…”

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

“…During detachment, T e can reach very low values (~ 1 eV) [8][9] . 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.…”