Absorption of infra-red radiation as a general technique for the determination of plasma temperature

It is shown that the continuum absorption coefficient K( lambda ) of a plasma at temperature T and electron density N - for radiation of wavelength lambda has the dependence K( lambda ) varies as z lambda 2 N - 2 T -3 2/. The temperature here is the free electron temperature and z is the average ionic charge in the plasma. Recent techniques of laser interferometry enable N - to be measured to 2 percent and therefore it is proposed that an infra-red absorption measurement could enable T to be assessed to better than 7 percent for plasmas of known N - and z. Full calculations of K( lambda ) are given for radiations of 3.39 mu and 10.6 mu wavelength, corresponding to the prominent transitions of the helium-neon and carbon dioxide lasers respectively. The outstanding features of this proposed technique of temperature measurement are that plasma local thermodynamic equilibrium is not required, large magnetic fields in the plasma are permitted and no information is required on the chemical composition of the plasma.