How Is It Possible To Measure Extreme Temperatures? (>2 Billion Kelvin)

Fusion is plasma physics, and in plasma physics the temperature is defined by the average kinetic energy of the ions and electrons in the plasma.

Therefore the kinetic energy distributions have to be measured and they have developed ingenious methods of doing so. Fitting with the black body radiations curves allows an estimate of the temperature.

There are more ingenious methods developed, using Thomson scattering, which allow by scattering light on different ions accurate measurements of temperature.

Very high temperatures can be measured for the stars from their black body radiation and the spectral distribution of the light reaching us also.