Origins of charged particles in vapor generated by electron-beam evaporation

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Abstract
The origins of electrons and ions in uranium vapor generated by electron-beam evaporation have been determined. Measurements were made for the electron emission current due to high-energy electron-beam irradiation on a uranium surface (backscattered electrons, etc.), thermionic emission current from the melt surface, and electron current due to vapor ionization. Comparison of these currents confirmed that vapor ionization was the main electron generation process at evaporation surface temperatures above 2200 K. The ionized vapor formed a weakly ionized plasma of very low electron temperature: The degree of ionization ≤1%, electron temperature ≤0.3 eV. The electron-impact ionization process contributed mainly to plasma formation. Beam electrons, their backscattered electrons, and secondary electrons from the beam-irradiated uranium surface were the source electrons for this process. Thermal ionization was the next major process. In addition to the plasma formation model, plasma behavior in vapor was described by a one-dimensional symmetric expansion model. The calculated degree of ionization was in good agreement with the measured value over a wide range of evaporation temperatures and electron-beam currents.