Charge exchange

Charge exchange (or charge exchange collision) is a process in which a neutral atom or molecule collides with an ion, resulting in the neutral atom acquiring the charge of the ion. The reaction is typically expressed as



This reaction has various diagnostic applications, such as in plasma physics and mass spectrometry.

Theory
When a neutral atom collides with an ion in a gas or a plasma, the ion can acquire an electron from the neutral atom as both electron shells overlap in the course of the collision. This can be used in various applications.

Charge-exchange spectroscopy
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.