Measured electron energy distribution functions in inverted hydrogen fireballs

Abstract

Electron energy distribution functions (EEDFs) play an important role in many different types of plasmas. Their shape characterises the degree of thermal equilibrium, while the EEDF mean value is proportional to the electron temperature, if the distribution function is Maxwellian. In this paper we present the first measurements of EEDFs of electrons inside and outside an inverted fireball (IFB) plasma. It turns out that the EEDF inside the IFB plasma shows larger contributions from hot electrons, while on the outside there are two distinguished electron populations, namely cold bulk electrons and hot tail electrons. The measurements were carried out at low pressure of around 5 Pa, which is typical for IFB experiments. The measurements were performed with a movable Langmuir probe system, and the obtained I-V curves were used to calculate the EEDF. The measurements indicate a spread in the EEDF inside the IFB as well as a shift to higher peak values of the electron temperature. It was shown that there are basically two populations of electrons. There are the bulk electrons, which are relatively cold and a smaller number of hot electrons in the tail of the EEDF. Particularly, the electrons close to the wall of the IFB anode show a significantly broader EEDF, which indicates that they deviate from a Maxwellian distribution function and deviate, thus, from thermal equilibrium.