Thomson scattering is an important technique for plasma diagnostics in nuclear fusion facilities such as tokamaks and stellarators. It is used to measure both the electron temperature and density at many locations inside the plasma.
The measurement is conducted using a pulsed laser, typically a Ruby laser or Nd:YAG, that generate repeated pulses that are shone into the plasma. Some of the laser light (photons) is scattered of free electrons in the plasma resulting in spectral expansion due to the Doppler effect.
Avalanche photo diodes (APDs) detect the scattered photons and high-performance digitizers capture the resulting waveforms. The spectral expansion helps assess the plasma temperature whereas the number of scattered photons determine the plasma density.