Charge-exchange spectroscopy (abbreviated CES or CXS) is a technique commonly used in plasma diagnostics to analyze high-temperature controlled fusion plasmas. In fusion plasmas, the light elements tend to become fully ionized during operation, which makes it challenging to diagnose their properties using conventional optical diagnostics. To address this, a method was developed in the 1970s which involves the injection of a beam of neutral atoms, such as hydrogen or deuterium, into the plasma. This process results in the ionization of hydrogenic atoms the excitation of ions through charge exchange, as represented by the reaction:
where  represents the various possible charged states of the ions in the plasma.
Optical fibers are then strategically positioned to create "chords", lines of sight along which the measurements are taken. These chords pass through regions both with and without the neutral beam. By subtracting the signals from these two chords, emissions not generated by the neutral beam can be inferred. This allows for the determination of ion properties, such as its temperature and density. 
The technique can also be extended to include multiple chords to build spatial profiles of the plasma, such as its toroidal and poloidal rotation. This provides insights into how ions conduct heat and transport momentum within the plasma.
Charge-exchange spectroscopy is often referred to as charge-exchange recombination spectroscopy, which is acronymized as CXRS or CER.