Surface ionization on tungsten in a varying ambient atmosphere

Experiments show that under suitable conditions the introduction of oxygen can raise the surface ionization current emitted by a heated tungsten wire by several orders of magnitude. The ions in this case are alkali ions originating from alkali silicates normally present as additives in tungsten. From the simultaneous observation, in a varying ambient atmosphere, of both the positive ion current produced by surface ionization and of the electron current due to thermionic emission, it is concluded that the large observed increase in positive ion current is not primarily a consequence of an increase in the work function of tungsten, but is mainly due to a reduction in the residence time of the ions on the ionizing surface. This appears to be caused by alkali ions being preferentially replaced as an adsorbate on tungsten by the impinging oxygen molecules. The observations suggest that the increased ion production does not occur on the outer wire surface, but at inter-phase boundaries within the wire.