High current density electron gun with a LaB6 cathode

To develop a high current electron gun for the induction linac, a small prototype of a Pierce-type electron gun using a planar 12 mm-diameter lanthanum hexaboride(LaB6) is being studied as a thermionic emitter. The cathode is heated up to a temperature of 1,750degC, by electron bombardment and thermal radiation from a tungsten heater. The gun has been operated up to a voltage of 55kV, obtaining beam current of 18A with 250 ns width. The obtained perveance of the gun is 1.5 x 10-6. High-voltage pulsing results show that the gun is operated in space-charge-limited region at temperatures greater than 1,600degC, while it is operated in temperature-limited at temperatures less than 1,540degC. The measured work function of the LaB6 cathode is 2.51-2.55 eV. The cathode emits over 5A of electrons with no reduction of the emission after 420 hours of operation. Lifetimes of the cathodes are being measured as a function of the vacuum pressure and the cathode temperature. (author)