Three techniques are adopted for plasma diagnostic (electron temperature, electron density): Optical Emission Spectrum (OES), Thompson scattering, and Langmuir probe.
Optical emission Spectroscopy (OES) . When atoms are excited, they relax to lower energy state through emitting photons. The emission spectrum is the characteristic of an atom. Based on this principle, OES becomes the most popular technique to investigate plasma, due to simplicity and no perturbation in the plasma. Commonly, the ratio of two characteristic emission line intensity is measured and based on that electron temperature and density can be calculated.
Langmuir Probe. It is another simple but valuable techniques employed to characterize the plasma parameters. The obtained I-V characteristics can be used to determined the electron temperature and density.
Thompson scattering. Scattering of monochromatic electromagnetic radiation from hot electrons in a plasma leads to a spectral broadening of the scattering spectrum due to the Doppler effect. Thomson scattering method adopts this principle to determine a spectral width of the Thomson scattered signal, and then calculate electron density from scattered signal intensity.
Reference: http://dx.doi.org/10.1016/j.reffit.2017.04.002