Estimation of charge particle concentration in uranium vapors produced by electron beam heating

A strip type electron beam is used to get a high throughput of uranium (U) vapor evaporation in electron beam evaporator. When the electrons beam incidents on the metal ingot, it heats the target locally to a high temperature and produces vapors. The vapors near the evaporating surface get partially ionized through Saha ionization at high temperature. When the vapors propagate, they interact with nearly mono-energetic primary electron beam and backscattered, secondary, thermionic electrons which have their energy distributions. The charge particles are produced by electron impact ionization. Thus the ionized vapors form the weakly ionized plasma that is embedded with the neutral vapors. To know the degree of ionization, the e-impact ionization cross section of uranium and its variation with the incident electron energy are calculated. By knowing the incident electron flux, the total ionization yield is estimated. For a given incident electron beam power of 150 kW (i.e. 60 kV x 1.5 A) and cross sectional area of 130 mm x 6 mm, the surface temperature of U target is achieved ~ 2950 K. It is estimated that the yield of Saha ionization is ~ 0.2 % whereas the cumulative contribution of electron impact ionization is ~ 0.3%. The total degree of ionization increases with the incident electron beam power density. The plasma expansion is being studied using particle in cell simulation. (author)