Mass transport in liquid gallium ion beam sources

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Abstract
The mass loss of the anode of a positive-Ga-ion source is determined from microbalance measurements, when the source is operated in a steady state. The anode consists of a short W point electrode covered by a film of liquid Ga kept at 6-9 kV with respect to an earthed cathode. From spectrograms of the light emitted by this ion source it is known that neutral Ga atoms are produced during operation, besides fast positive ions. The rate of the mass loss of the emitting Ga-covered needle is found to rise approximately linearly with the current, i.e. the mass of Ga emitted per unit charge is constant, provided the current does not exceed 100 mu A. At larger currents the mass loss rate rises faster than the current. The nature of the emission of neutral atoms is briefly discussed. It is concluded that thermally activated, classical atomic evaporation cannot be the primary mechanism responsible for the neutral component of the emission.

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