Charge exchange spectroscopy
A neutral beam heating system launches billions of billions of neutral atoms into the plasma at extremely high velocities. In collisions with the hot plasma they rapidly lose their electrons, quite often by passing them to plasma nuclei (hydrogen ions) or to heavier nuclei (‘impurity’ ions). Although the ions will soon lose the electrons in subsequent collisions, they can shine light in the meantime. By observing the characteristics of this very distinct light from impurity ions one can tell (thanks to the Doppler effect) what the temperature of the plasma ions is and what direction of flow the plasma has. Even more importantly, these data can be resolved to a precision of one centimeter as the light originates only in regions very close to the neutral beam.The JET system can observe six different impurity elements simultaneously, among them are Carbon, Helium, Neon, Beryllium, Nitrogen, Oxygen, Argon and/or beam emission.
JET also features an independent CXRS system to diagnose the colder edge region of plasma in order to observe the plasma-beam interaction from the top and bottom.