Plasma diagnosis as a tool for the determination of the parameters of electron beam evaporation and sources of ionization

The atomic vapor generated by electron beam heating is partially ionized due to atom-atom collisions (Saha ionization) and electron impact ionization, which depend upon the source temperature and area of evaporation as compared to the area of electron beam bombardment on the target. When electron beam evaporation is carried out by inserting the target inside an insulating liner to reduce conductive heat loss, it is expected that the area of evaporation becomes significantly more than the area of electron beam bombardment on the target, resulting in reduced electron impact ionization. To assess this effect and to quantify the parameters of evaporation, such as temperature and area of evaporation, we have carried out experiments using zirconium, tin and aluminum as a target. By measuring the ion content using a Langmuir probe, in addition to measuring the atomic vapor flux at a specific height, and by combining the experimental data with theoretical expressions, we have established a method for simultaneously inferring the source temperature, evaporation area and ion fraction. This assumes significance because the temperature cannot be reliably measured by an optical pyrometer due to the wavelength dependent source emissivity and reflectivity of thin film mirrors. In addition, it also cannot be inferred from only the atomic flux data at a certain height as the area of evaporation is unknown (it can be much more than the area of electron bombardment, especially when the target is placed in a liner). Finally, the reason for the lower observed electron temperatures of the plasma for all the three cases is found to be the energy loss due to electron impact excitation of the atomic vapor during its expansion from the source.