When an electron beam produces a uranium vapor in an electron beam evaporator, a uranium plasma is formed on the evaporating surface which expands with the vapor. To clarify the effect of the polarity of the applied voltage to the electrodes and also the effect of electrode areas when the plasma is removed by surrounding electrodes, the relationship between the plasma reaction to the applied voltage and the amount of plasma removed was investigated by using a boxhyphen;type electrode with inner fins which involves the plasma. When a positive bias, compared to the plasma potential, was applied to extract electrons from the plasma, the plasma was effectively removed while increasing the bias potential. The plasma became more diffusive due to increases in electron temperature, attributable increases in plasma sheath instabilities. However, when the vapor density increased, the instability resulted in ionization of the vapor to form an additional plasma. Therefore, under this condition, it became difficult to remove the plasma by increasing the bias potential. However, when a negative bias potential was applied to extract ions, the instability did not arise, because an ion sheath was formed in front of the hot electrode. However, a high absolute potential was required to remove the plasma. A simple plasma removal model showed thatSithinsp;/Se=470 was an optimum electrode area ratio when removing a uranium plasma, whereSiis the ion removal electrode area andSeis the electron removal electrode area; this effect was confirmed experimentally.
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