Evolution of a Two-Temperature Plasma Expanding With Metal Vapor Generated by Electron-Beam Heating

During the electron-beam evaporation of metals, a weakly ionized plasma is formed, which consists of two different groups of electrons characterized by different energy spreads (or temperature). While this plasma expands along with the metal vapor, a thermodynamic equilibrium between these two groups of electrons is gradually established by electron–electron Coulomb collisions and electron–atom inelastic collisions. The evolution of this two-temperature plasma was experimentally observed by a Langmuir probe during an electron-beam evaporation of zirconium. Mathematical expressions for the effect of different interactions on the evolution of the electron temperatures of the plasma were derived and applied to our experimental observations. Taking the initial temperature of the plasma at the source of vapor, the total cross section for electron–atom inelastic collisions was calculated, the order of which agreed well with the reported values. Finally, the contributions of each type of interaction (electron–electron and electron–atom) on the cooling of the high-temperature group of electrons in the plasma are quantified.