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SOURCE OF A FOCUSED BEAM OF HYDROGEN AND HELIUM IONS1
On the basis of a theoretical determination of the magnitude of the potential required for the complete removal of ions from the ionization space, as discussed in the preceding note, a powerful ion source was constructed, suitable for operation with any gas that can be ionized by electrons (provided, of course, that the gas does not interfere with the operation of the oxide cathode, which is the source of the electrons).
The noteworthy features of this ion gun are: the method of feeding gas into the ionization space, distinguished by the fact that the gas is drawn through the entire length of the chamber in which it is bombarded by electrons; and also the use of a cylindrical thermionic cathode (the emitting surface is the inner surface of the cylinder), which, without having contact with the filament, makes it possible to obtain large currents (up to 600 mA, corresponding to 2.2 A/cm²), and also to obtain a focused electron beam in the ionization space. The filament current of such a cathode was 12–13 A, and the potential difference between cathode and anode was not more than 800 V. The author believes that this ion gun is the simplest and most efficient of all previously described devices of the same type2.
With this gun it was possible to obtain focused beams of hydrogen ions with a current as high as 4 mA. Mass-spectrographic analysis of the hydrogen-ion beam showed that the proton content in it could vary between 5 and 80%, depending on the gas pressure, the energy of the ionizing electrons, and also the magnitude of the electron current. The content of He++ ions in the ionized-helium beam proved to be on the order of 5% of the total beam intensity.
For beams of high intensity, focusing of the ion stream proves difficult, since the beam diverges strongly as a result of the mutual repulsion of the ions. The upper limit of readily attainable focusing is a beam current of approximately 100 μA.
N. Khlebnikov, Moscow
REFERENCES
- G. W. Scott, Phys. Rev., 55, 954, 1939.
- L. P. Smith and G. W. Scott, Phys. Rev., 51, 1025, 1937; 53, 677, 1938.