To develop a high‐current electron gun for an induction linac, a prototype of a Pierce‐type electron gun using planar 12‐mm‐diam lanthanum hexaboride (LaB6) is studied as a thermionic emitter at high current densities. The cathode is heated up to temperatures of 1750 °C by electron bombardment and thermal radiation from a tungsten heater. The heater that has the highest temperature in the gun is thermally isolated from the outer vacuum chamber with heat shields. The bombardment voltage of ∼1 kV is typically applied to a gap between the cathode and the heater. The gun has been operated up to voltages of 55 kV, obtaining a maximum current density of 20 A/cm2with a pulse width of 250 ns at a cathode temperature of 1600 °C. High‐voltage pulsing results show that the gun, with applied voltages of over 40 kV, is operated in space‐charge‐limited region at temperatures of over 1600 °C; also it is operated in a temperature‐limited region at temperatures of less than 1500 °C. An effective work function of 2.68 eV is obtained. The cathode, when heated up to 1600 °C, emits over 7 A of electrons with a ∼20% reduction after 850 h of continuous operation. These measurements were made between vacuum pressures of 10−6and 10−7Torr. ©1996 American Institute of Physics.