PURPOSE: To prevent a gap between hot shell and collector from changing even by an axial thermal expansion difference by making a thermionic element be in a double end-retained state.

CONSTITUTION: An emitter 4 is formed on the inner surface of a hot shell 1. More inwardly of this emitter 4, a collector 6 is placed with a gap 5 between. These emitter 4 and collector 6 are arranged on a concentric circle to form a coaxial cylinder-type thermionic element 31. This thermionic element 31 has no tip because it is retained at its both ends. Externally of a furnace wall 32, a bellows 34 arranged on a circle concentric with the hot shell 1 is placed along the outer periphery of the hot shell 1. The tip of this bellows 34 is brazed to an emitter flange 11 provided at the rear end of the hot shell 1. Also, the rear end of a bellows 34' is EBW-joined to the outer peripheral surface of the collector 6 so that an insulating zone 35 will exist between metal bellows 34, 34'.