Space-Charge Effects in Electron Beams and Their Reduction by Positive Ion Trapping

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Abstract
The spread of electron beams caused by space-charge forces is discussed. Under certain conditions ions formed from the residual gas may be trapped in the beam. An equilibrium condition may be established in which the electron and ion densities are equal, and then space-charge forces will be neutralized. Under these conditions high current densities may be produced at low voltages. A theory of ion trapping is discussed, and the equilibrium condition is formulated.Experimental data are presented which were obtained by the application of microsecond pulses to the beam. This technique is described, and its advantages and possibilities are mentioned. Data are given on beam spread as a function of current, voltage and pressure. Data on the improvement due to ion trapping are included. An increase of current density by a factor of 30 was observed with the structures tested. Results are included on ion build-up time as a function of pressure, and on beam noise and stability in the presence of trapped ions.

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