A forward model for the helium plume effect and the interpretation of helium charge exchange measurements at ASDEX Upgrade

Abstract

The analysis of the charge exchange measurements of helium is hindered by an additional emission contributing to the spectra, the helium 'plume' emission (Fonck et al 1984 Phys. Rev. A 29 3288), which complicates the interpretation of the measurements. The plume emission is indistinguishable from the active charge exchange signal when standard analysis of the spectra is applied and its intensity is of comparable magnitude for ASDEX Upgrade conditions, leading to a significant overestimation of the He 2+ densities if not properly treated. Furthermore, the spectral line shape of the plume emission is non-Gaussian and leads to wrong ion temperature and flow measurements when not taken into account. A kinetic model for the helium plume emission has been developed for ASDEX Upgrade. The model is benchmarked against experimental measurements and is shown to capture the underlying physics mechanisms of the plume effect, as it can reproduce the experimental spectra and provides consistent values for the ion temperature, plasma rotation, and He 2+ density.

Simulation of Spectra Code (SOS) for ITER Active Beam Spectroscopy

“…The modeling of the plume effect is illustrated on the example of the He + spectrum. In order to derive the number of plume ions reaching the observation line of sight we need to solve the continuity and transport equation following the description given by Kappatou et al [25].…”

“…One should emphasize that the plume effect has been a challenging goal for many years of CXRS modeling efforts (cf. [22][23][24][25]). One of the reasons are the difficulties in modeling the dynamics of H-like ions exactly.…”

“…For example, neither gyro-motion nor the diffusion perpendicular to magnetic lines of H-like ions have been included so far. Therefore, the modeling of plume in spite of the recent progress in modeling and experimental evidence and validation [25] remains one of the open challenges in the CXRS spectroscopy. Ultimately, a reliable assessment of experimental and theoretical uncertainties, for example, in any CXRS based helium ash measurement in a fusion plasma, depends on error estimates for each of the composite HeII spectral components.…”

“…The 'plume' effect [22][23][24][25] is evident mostly in the case of the CXRS HeII spectrum: the excitation by electrons of resulting H-like He can be comparable with the CXRS signal thus extending the emission zone due to the thermal motion of H-like ions. The rapid decay of excitation rate coefficient as (~1/n 3 ) where n is the principal quantum number prevents the appearance of plume in all other impurities except of He (see also PEC values for HI, HeII, and CVI in Figure 2h).…”

“…The halo effect [19][20][21] refers to the cloud of neutrals accompanying the beam penetration path interacting with the plasma environment via collisions with electrons and ions and creating a supplementary spectrum representing the same atomic transition as the CX line emission. The 'plume' effect [22][23][24][25] is evident mostly in the case of the CXRS HeII spectrum: the excitation by electrons of resulting H-like He can be comparable with the CXRS signal thus extending the emission zone due to the thermal motion of H-like ions. The rapid decay of excitation rate coefficient as (~1/n 3 ) where n is the principal quantum number prevents the appearance of plume in all other impurities except of He (see also PEC values for HI, HeII, and CVI in Figure 2h).…”