The primary diagnostic for measuring ion temperature, toroidal rotation, and impurity density profiles inside fusion plasmas
Charge exchange recombination spectroscopy relies on the capture of an electron from an injected neutral beam atom by a fully stripped impurity ion in the plasma. In the most common implementation, a neutral deuterium beam intersects the plasma and donates electrons to carbon C6+ or neon Ne10+ ions, producing excited C5+ or Ne9+ ions that promptly emit characteristic spectral lines as they radiatively decay. The line shape encodes the ion velocity distribution at the intersection volume.1
From a single spectral line—most commonly C VI at 529.1 nm—three quantities are extracted simultaneously: the Doppler width gives the ion temperature Ti, the Doppler shift gives the bulk toroidal (or poloidal) rotation velocity, and the line intensity gives the local impurity density. This trio of measurements from a single diagnostic makes CXRS uniquely valuable for transport and stability studies.2
A CXRS system consists of a neutral beam injector, collection optics viewing the beam–plasma intersection at multiple radial points, fiber-optic bundles routing light to high-throughput spectrometers, and fast CCD or CMOS detectors. Spatial resolution is set by the beam width and viewing geometry, typically 1–3 cm radially, while temporal resolution reaches 5–10 ms on modern systems.3
On ITER, CXRS will use a dedicated diagnostic neutral beam to measure Ti, rotation, and helium ash density—the latter being critical for monitoring fuel dilution. Rotation measurements are equally important because toroidal rotation stabilizes resistive wall modes and influences turbulent transport, both of which affect confinement quality and achievable fusion gain.4
The diagnostic faces challenges in burning plasmas where neutron-induced background emission and radiation damage to optical components must be mitigated through careful shielding and remote maintenance of front-end mirrors.