Charge-exchange spectroscopy in fusion plasmas entails the use of optical transitions that follow electron transfer from a neutral atom into an excited state of an impurity ion. In most applications, the sources of neutral particles are high-energy beams employed either for heating or for the specific purpose of active plasma diagnosis. The transitions following charge exchange are particularly useful for determining the densities of fully stripped low-Z ions and for measuring ion temperatures and plasma rotation, although they have also been exploited for other purposes. In this review, the atomic physics considerations for interpreting the data, including the influence of the plasma environment, are reviewed, and examples of recent applications to fusion studies are presented.

https://iopscience.iop.org/article/10.1088/0741-3335/36/2/001/pdf

An overview of charge-exchange spectroscopy as a plasma diagnostic

https://hal.archives-ouvertes.fr/jpa-00241125/document

The Astrophysical Journal; T. XXI ; 1905

Impurity toroidal rotation has been observed in the centre of Alcator C-Mod ion cyclotron range of frequencies (ICRF) heated plasmas, from the Doppler shifts of argon X ray lines. Rotation velocities greater than 1.2 × 107 cm/s (ω = 200 krad/s) in the co-current direction have been observed in H mode discharges that had no direct momentum input. There is a correlation between the increase in the central impurity rotation velocity and the increase in the plasma stored energy (confinement enhancement), induced by ICRF heating, although other factors may be at play. The toroidal rotation velocity is highest near the magnetic axis, and decreases with increasing minor radius. A radial electric field of 300 V/cm at r/a = 0.3 has been inferred from the force balance equation. The direction of the rotation changes when the plasma current direction is reversed, remaining co-current. Impurity toroidal rotation in ICRF heated plasmas is in the direction opposite to the rotation in ohmic L mode plasmas; co-current rotation has also been observed during purely ohmic H modes. When the ICRF heating is turned off, the toroidal rotation decays with a characteristic time of order 50 ms, similar to the energy confinement time, and much shorter than the calculated neoclassical momentum damping time.

https://iopscience.iop.org/article/10.1088/0029-5515/38/1/306/pdf

Observations of central toroidal rotation in ICRF heated Alcator C-Mod plasmas

Co-current central impurity toroidal rotation has been observed in Alcator C-Mod plasmas with on-axis ICRF heating. The rotation velocity increases with plasma stored energy and decreases with plasma current. Very similar behaviour has been seen during ohmic H modes, which suggests that the rotation, generated in the absence of an external momentum source, is not mainly an ICRF effect. A scan of ICRF resonance location across the plasma has been performed in order to investigate possible influences on the toroidal rotation. With a slight reduction of toroidal magnetic field from 4.7 to 4.5 T and a corresponding shift of the ICRF resonance from r/a = -0.36 to -0.48, the central toroidal rotation significantly decreased together with the formation of an internal transport barrier (ITB). During the ITB phase, electrons and impurities peaked continuously for |r/a| ≤ 0.5. Comparison of the observed rotation and neoclassical predictions indicates that the core radial electric field changes from positive to negative during the ITB phase. Similar rotation suppression and ITB formation have been observed during some ohmic H mode discharges.

Observations of impurity toroidal rotation suppression with ITB formation in ICRF and ohmic H mode Alcator C-Mod plasmas

Modern trends in the spectroscopy of multicharged ions

Characteristic x-ray spectra of plasma impurity ions have been measured in the region $\ensuremath{\lambda}=4.3\ensuremath{-}5.1$ \AA{}. The observation of strong principal $n=2$ to $n=1$ x-ray lines for H-like sulfur and for He-like sulfur and chlorine in the density range (1-6) \ifmmode\times\else\texttimes\fi{} ${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ is reported. Dielectric recombination and inner-shell excitation satellite lines are weak. The results, which are of interest for both atomic physics and plasma diagnostic applications, are compared with atomic calculations of line spectra and transition rates.

/pdf/observation-of-h-and-he-like-x-ray-line-emission-in-high-5prkgnat1z.pdf

Observation of H- and He- like x-ray line emission in high density tokamak plasmas

Measurements of H- and He-like X-ray spectra of elements between Z = 16 and 26 (hv = 2 to 8 keV) are discussed and illustrated with recent results obtained for argon. Examples of instrumental development are presented together with the outlook for future experiments.

Plasma atomic X-ray spectroscopy of tokamaks

Electron temperature measurements from line ratios of He‐ and H‐like argon in the Alcator C tokamak

/pdf/electron-temperature-measurements-from-line-ratios-of-he-and-3hxc4mpcn6.pdf

Resultats des premieres mesures radiales des transitions Rx, n=2 a n=1, des elements Z eleves dans des plasmas de deuterium de tokamaks. Les rapports des intensites des raies observees ne peuvent s'expliquer par des processus d'excitation par impact electronique et recombinaison dielectronique. La recombinaison radiative et de transfert de charge est proposee comme mecanisme de population additionnelle. Les raies emises dans la cascade suivant la capture constituent un diagnostic unique de la composante neutre des plasmas

Observations of recombination population of n=2 excited states of Ar16+ in tokamak plasmas

X-ray spectra of He-like argon (Ar/sup +16/) have been obtained for the transitions 1snp..-->..1s/sup 2/, with 3< or =n..Ar/sup +16asterisk/(1snp /sup 1/P)+H/sup +/ are presented and their cross sections are estimated.

Observation of charge-transfer population of high-n levels in Ar+16 from neutral hydrogen in the ground and excited states in a tokamak plasma.

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