Basic plasma parameters and physical properties of inverted He fireballs

In this paper, we present systematic measurements of the basic plasma parameters of inverted fireballs (FBs) in a helium plasma for the first time. Inverted FBs in helium with a diameter of 10 cm are generated in a highly transparent hollow electrode via impact ionisation of accelerated electrons from an existing background plasma. The measured quantities are the plasma potential, the electron density and the electron temperature. The dependence of the plasma parameters on the discharge current was studied experimentally inside as well as outside the inverted FB. Furthermore, estimations of the neutral gas temperature inside the inverted FBs are made and possible explanations of the creation of macroscopic gas flows from the inverted FBs are given. It is shown in this paper that due to efficient thermalisation of electrons with neutrals the gas temperature inside the inverted FB can reach several thousand Kelvin, which also leads to a thermal equilibrium of ions and electrons inside this plasma structure. (paper)