Charge exchange recombination spectroscopy (CXRS) is a widely extended technique to evaluate the ion temperature, density and rotation in tokamaks. The technique is based on the observation of spectral lines, typically fully-ionized light impurities which emit light after a charge exchange reaction with diagnostic neutrals.

The CXRS suite at ASDEX Upgrade consists of 5 core diagnostics and 6 edge systems viewing either beam source #3 of the neutral beam injection (NBI) box 1 injecting 60 keV deuterium particles, beam source #8 of NBI box 2 (92 keV) or a thermal neutral gas puff. The diagnostics provide high-resolution measurements of impurity ion temperature, density and rotation profiles both in the toroidal and poloidal direction and at the low-field and high-field side of the tokamak.

The edge CXRS diagnostic suite

ASDEX Upgrade is equipped with two edge CXRS diagnostics at the LFS and two systems at the HFS. The LFS systems are focused on the center of beam #3, while the HFS diagnostics are based on CXRS on a thermal gas puff. On both sides, one diagnostic is viewing the plasma in the toroidaland one in the poloidal direction, thus allowing CXRS measurements at two different positions along the flux surfaces. Combining the measurements of a poloidal and toroidal diagnostic also enables the evaluation of the edgeradial electric field using the radial force balance equation:


More details on the edge CXRS diagnostics at AUG can be found in:
[1] E. Viezzer et al., High-resolution charge exchange measurements at ASDEX upgrade, Rev. Sci. Instrum. 83, 103501 (2012).
[2] M. Cavedon et al., A fast edge charge exchange recombination spectroscopy system at the ASDEX Upgrade tokamak, Rev. Sci. Instrum. 88, 043103 (2017)

Fig. 1.

The upgraded HFS edge CXRS diagnostics

The HFS CXRS system is located at the inner central column of ASDEX Upgrade, behind the first wall. This recently upgraded system is based on a piezoelectric actuator that allows high accurate control of the neutral flowrate. Dedicated calibrations allow the injection of multiple gas species as: D2, N2, He and Ar. The light produced by the CX reactions is collected by two optical heads, each one composed of 16 optical fibres.

More information on the HFS edge CXRS system can be found in:
[1] D.J. Cruz-Zabala et al., Upgrade of the edge Charge Exchange Recombination Spectroscopy system at the High Field Side of ASDEX Upgrade, JINST 14, C11006 (2019).

Fig. 2.

The edge main ion CXRS diagnostics

Although the measurements of the bulk plasma properties are important for understanding fusion plasmas, they have rarely been done for many years. CXRS measurements on impurity ions are typically preferred, and main ion properties are inferred from them. This is because the photoemission processes become more complex when the measured ions represent the majority bulk plasma ions (deuterium), making the interpretation of the main ion spectra challenging.

Recent progress has been made in direct measurements of the main ion properties [1,2]. At the ASDEX Upgrade tokamak, a feasibility study for a new edge main ion charge exchange recombination spectroscopy has been done [2]. This diagnostic uses the new edge fast-ion Dα (FIDA) spectrometers and the optical heads of the existing edge CXRS systems. This system enables the first edge main ion profiles at AUG, which will be compared to impurity properties and neoclassical predictions.

Fig. 3. (a) The active edge main ion Dα spectra at AUG. The gray dashed line illustrates the area of influence of the wire which is used to block the cold edge emission. The fit (red) and fitted components (thermal Dα in blue, Fast Ion Dα emission in pink and Beam Emission in green) are shown. (b) Weighted residuals characterize the goodness of the fit.

Fig. 4. Typical edge neutral density of the direct charge exchange (DCX) and halo populations at the ASDEX Upgrade tokamak calculated by the FIDASIM code. The separatrix is shown in black, the neutral beam injector is shown in a blue dashed line and two lines of sight are shown in red for reference. It can be seen that the halo density is of the same order of magnitude as the DCX population, and spreads further away from the beam.

More details on main ion CXRS diagnostics at DIII-D and AUG can be found in:
[1] B.A. Grierson et al., Active spectroscopic measurements of the bulk deuterium properties in the DIII-D tokamak, Rev. Sci. Instrum. 83, 10D529 (2012).
[2] S.R. Haskey et al.,
Main ion and impurity edge profile evolution across the L- to H-mode transition on DIII-D, Plasma Phys. Control. Fusion 60, 105001 (2018).
[3] P. Cano-Megias et al., Feasibility study for an edge main ion charge exchange recombination spectroscopy system at ASDEX Upgrade, JINST 14, C10040 (2019).