Abstract
A directly heated Ta rod cathode diam = 0.8 mm, which was fed dc current through small contact areas of a few knife‐edge, fan‐shaped W current feeders symmetrically supported near the emitting end of the rod, is studied. An electron beam from a gun made of such cathode structure is free from the effects of the magnetic fields produced by the heating current when the cathode is heated to 2000 K and the current through two feeders is 26 A. An optimum contact area at the supporting point is obtained from a one‐dimensional heat equation in the current feeder. A typical brightness value for this gun is of the order of 5×102 A/cm2 for the electron beam of 2 mA at 4 kV. The average lifetime of the cathode is not less than 60 h.
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