Electrical Stability and Life of the Heated Field Emission Cathode

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Abstract
The cold tungsten field cathode exhibits electrical stability and long life (12 000 hr demonstrated) at dc emission densities up to 107/cm2, when operated in vacuum tubes with a residual gas pressure below 10−12 mm Hg; with a provision for periodic reconditioning of the cathode surface by a brief flash heating, satisfactory operation of the cold cathode may be achieved at tube pressures up to about 10−9 mm Hg. Continuous heating of the field cathode is shown to yield useful electrical stability at conventional tube pressures, e.g., 10−6 mm Hg. The theory of transport phenomena in heated metals is applied to a quantitative study of the geometrical stability of the heated field cathode. Expressions are derived for the time rate of change of cathode tip radius in the presence or absence of dc and pulsed electric fields at the cathode surface, and long‐term geometrical stability of the cathode surface is predicted when the average applied field has the proper value. Large cathodes operated at high current level will only tolerate pulsed emission, and expressions are obtained for the maximum allowed duty cycle and pulse length in this case; small heated cathodes are capable of stable dc emission. Occurrence of a vacuum arc is proposed as the cause of cathode failure. Considerations based on this assumption lead to predictions concerning the dependence of average cathode life on temperature, and to quantitative expressions for the statistical distribution and average value of life for unbiased field cathodes operated at high temperatures. Experimental results yielded by the operation of 85 cathodes show good agreement with the theory. The heated cathode shows constant performance over a long period after which cathode failure occurs abruptly, and the statistical nature of cathode life is confirmed. The statistical distribution of cathode life and the measured effect of various parameters on average life show good agreement with theoretical predictions. Unbiased tungsten cathodes yielded an average life of 250 hr at an optimum cathode temperature of 2040°K. With the application of a suitable bias both long‐term stability and an average life of 487 hr at 1970°K have been achieved, and according to the theory a substantial further increase in average life may be obtained by the use of a higher cathode temperature and of cathode materials with reduced impurity content.