ion temperature is the temperature of a 📝plasma's ion population — the temperature that directly sets the 📝fusion rate, because it is the ions that fuse.
Its near-neighbor is 📝electron temperature, and the two are routinely different in the same plasma: each species equilibrates with itself far faster than the light electrons and heavy ions exchange energy with each other. The distinction is not academic. Fusion is a collision between nuclei, so the reaction rate rises steeply with ion temperature while caring nothing directly for the electrons — a plasma with hot electrons and cool ions produces essentially no fusion power. The 📝D-T reaction reaches practical reactivity in the range of 10 to 20 📝keV, which is why that band defines the operating point of reactor designs.
Raising and reading the ion temperature each take dedicated machinery. Neutral beam injection and ion cyclotron resonance heating deposit power preferentially into ions, supplementing ohmic heating that favors electrons. Measurement is spectroscopic: the Doppler broadening of light emitted by trace impurity ions reveals how fast the ions are moving, and charge-exchange recombination spectroscopy turns a heating beam itself into a diagnostic, mapping ion temperature and rotation across the plasma.