PURPOSE: To reduce the thermal capacity of a whole cathode and to improve the rapid operation property by providing a potting material only between the cathode and heaters.

CONSTITUTION: To reduce the thermal capacity of a cathode 6 including heater parts, a potting material 4 is furnished only between heaters 1W3 and the cathode 6. As the potting material 4, the sinter of a metal powder with a high thermal conductive coefficient is used. And to suppress a crack owing to the heat cycle, coarse particles of alumina is used on the surface of the cathode 6 or a cathode holding tube 5 facing the heaters. In such a composition, the heat from the heaters can be transmitted in a large quantity, and, since the thermal capacity of the cathode 6 part including the heaters can be reduced, the cathode 6 including the heaters can be heated rapidly up to the operational temperature.