Quantitative simulation of non-thermal charge-exchange spectra during helium neutral beam injection

Abstract

Non-thermal He II spectra for discharges with helium beam fuelling are analysed. Simulated spectra are used to study the effects of plasma temperature, plasma density and Z eff on observed charge-exchange (CX) spectra. Differences in modelling the non-thermal velocity distribution function with a numerical Fokker-Planck code or alternatively using analytical expressions are investigated. The intensities and spectral shapes of both active, localized CX spectra and competing, non-localized, passive electron-impact excitation components are simulated and compared with observations. The 'plume' contributions of electron-impact excited He +, * particles are found to be quite appreciable and uncertainties in the plume calculation lead to non-negligible errors in the extraction of the active signal from the total spectrum. However, for experimental conditions with magnetic field configurations minimizing the plume effect good agreement can be found between fast-particle densities derived from the numerical calculations and the experimental observations. Significant problems in deriving absolute He 2+ densities are encountered when a helium beam also acts as a CX diagnostic beam. For the case of dominant passive emission components, simulated fast spectral intensities for the core lines of sight agree within a factor of two with experimental data.

Modelling of passive charge exchange emission and neutral background density deduction in JET

“…The horizontal spectrometers have in addition to their 12 ACX viewing lines a few lines of sight at the side of the NBI from which horizontal passive emission can be detected. Due to halo effects from beam produced neutral atoms and plume contribution [8], these passive charge exchange measurements tend to be difficult to interpret. Optical fibres guide line radiation from the vessel to the spectrometers behind a concrete wall, which shields from neutron radiation.…”

Review of atomic data needs for active charge-exchange spectroscopy on ITER

“…The atomic data incorporated in the ADAS database 2 has been extensively used for charge-exchange and XUV diagnostics on fusion devices such as JET, 3 ASDEX, 4 TEXTOR, 5 or TORE SUPRA. 6 The assessment of the quality of these data can be done either on the base of experimental results or by means of independent calculations including multistep processes.…”

Validation of fast-ion D-alpha spectrum measurements during EAST neutral-beam heated plasmas

“…The substantial differences between the two codes are that, compared with FIDASIM applying a multiparticle Monte Carlo code, the fast ion slowing-down function in SOS is the anisotropic function from the analytical solution of the Fokker-Planck neutral injection equation. 12 Thus, the most possible reason for the difference of "Fast-ion CX" part between two codes is the value for the on-axis source rates from A-and F-injector combined, that is, the neutral beam stopping calculation and the slowing-down velocity distribution functions.…”