PURPOSE: To facilifate the way of operation as well as to shorten the time required for the thermal stability from the start of the operation, by increasing the heating power at the start and applying the steady heating power after reaching the thermal stability.

CONSTITUTION: A heating member 2 is provided around a cathode chip 1 while said member 2 is supported by a supporter 3 which is fixed to a base 4 and a connector or a pin 5 is provided on said base. The heating power is supplied from a heating power source 7 while a temperature detecting element 8 composed of a thermo-couple is provided in the proximity of the base of the supporter 3 and connected to a temperature detecting circuit 9 which is connected to a heating power source voltage control circuit 10 which is then connected to a heating power source 7. Consequently the thermal stability can be achieved quickly at the start of the operation resulting in the easiness of the way of operation.