How Doppler effect is used to measure the temperature of plasma

Doppler effect is used for measuring many physical parameters, one such is to measure the

temperature of plasma.

In thermonuclear fusion experiments we usually come across extremely hot gases or plasma. The

temperature is of very high order of degree Celsius.

At such high order of temperature the molecules

of glowing gas which are now moving randomly, also moves away and towards the observer with high speeds.

Due to this motion their wavelength of a particular spectral line is changed apparently due to Doppler effect. The molecules that are moving directly towards the observer show an apparent increase in wavelength and on the other hand the molecules moving directly away from the observer show an apparent decrease in wavelength(obeying E= hc/λ).

The spectral line is thus seen to be broadened. The breadth of the line can be measured by using a diffraction grating since λ & c are known, velocity v can be calculated. This velocity can be further used to calculate the temperature of the plasma.