Atomic collision processes with ions at the edge of magnetically confined fusion plasmas

Abstract

Spectra of the hydrogen isotopes, the major atomic constituents of magnetically confined fusion plasmas, are of particular importance for understanding the physical processes occurring in the plasma edge. A detailed analysis of the Zeeman-split Balmer lines reveals a number of subtle effects related to the formation of the radiating atoms by molecular dissociation, as well as charge-exchange recombination, and their subsequent heating by atom–ion collisions. We discuss and compare two of the possible physical processes whereby the spectra are broadened, through collisions transferring momentum between fast ions from the interior of the plasma and slow (‘cold’) atoms at the plasma edge. The evaluation of momentum transfer cross-sections for these processes is considered, and rate coefficients are compared with typical plasma conditions of interest. The picture of the heating process, as presented in an earlier paper, is modified in several respects, both as regards the calculation of the rate coefficients and the identification of the major channels (pathways between states) of interest.

Development of multiwavelength-range fine-resolution spectrometer for hydrogen emissions and its application to large helical device periphery plasmas

“…10 The velocity distribution of hydrogen atoms of each location is assumed to be expressed by a sum of three Maxwellian distributions. 6 Details of the analysis method are described in Ref. 3.…”

“…1,2 Spectroscopic diagnostics of the dynamics of the neutral hydrogen ͑or deuterium͒ has been proposed by many authors. [3][4][5][6][7] From the observation of the line shape of the hydrogen Balmer-␣ emission from large helical device 3 ͑LHD͒ and Torus of Research Institute for Applied Mechanics ͑TRIAM-1M͒, 4 the emission locations of the atomic hydrogen in the peripheral region were determined from the Zeeman effect, together with the flow velocity and the temperature estimated from the Doppler effect. Regarding the molecular hydrogen, the Fulcher-␣ band spectra were measured for TRIAM-1M, and then the emission locations and the rotational and vibrational temperatures of hydrogen molecules were determined 5 with a similar analysis to that of the hydrogen Balmer-␣ spectra.…”

Plasma particle sources due to interactions with neutrals in a turbulent scrape-off layer of a toroidally confined plasma

“…The energy exchange rate between neutrals is low compared to their average life time, and they are thus assumed to maintain their initial energy until ionization. The assumption is supported experimentally, where distinct highly populated energies in the neutral velocity spectrum has also been observed, both from spectroscopic measurements of the Zeeman-split Balmer lines 25 and from laser-induced fluorescence 26 on the TEXTOR tokamak. The experimental findings report slightly lower typical energies for the dissociated neutrals than what is assumed here, most likely due to the dissociation into atoms in the n = 3 state, instead of the ground state.…”

Spectral Line Shape Modelling and Ion Temperature Fluctuations in Tokamak Edge Plasmas

“…Here we only consider the contribution from the class of neutrals created locally by charge exchange, I(∆λ, z, t) → I cx (∆λ, X). In fact, neutrals created via other channels, i.e molecular dissociation and charge exchange in the inner plasma are not strongly coupled to the ions at the edge [8], i.e. to edge turbulence.…”