The excitation temperature in (helium) plasmas

Measuring excitation temperatures is a widely used way to characterise plasmas. In this paper we show that in most (atmospheric) helium plasmas this temperature is hardly related to any plasma quantity. It will appear that due to the strong non-equilibrium character of these plasmas the excitation temperature is lower than the electron temperature. The reason for the fact that, in particular, helium plasmas are far from equilibrium is the easy ambipolar diffusion of helium ions. Together with the relatively high electron temperature (necessary to sustain a helium plasma) this results in an excitation temperature, which strongly depends on the ionisation energies of the levels used to determine this temperature, rather than on the electron temperature. Since due to the specific atomic structure of helium only a limited amount of excited levels can be observed, always more or less the same excitation temperature is found.