PURPOSE: To lessen the consumption of an electron source, and make the structure of a cathode strong against thermal expansion and thermal contraction as well as to generate uniform electron current in a belt shape by making use of specified compounds to make the hot-cathode, and thereby making up the hot-cathode of a thermal electron emission section, and of a zigzag section absorbing thermal strain.

CONSTITUTION: A hot-cathode is formed of sintered hexaboridelanthanum used as an electron source. A filament is formed from hexaboridelanthanum, and a thermal electron emission section 1 and a thermal strain absorbing section 2 (a zigzag section) are formed out of a plate made of sintered hexaboridelanthanum. The leg sections of the filament 1 are fastened onto metallic electrodes 3A and 3B with screws 4A and 4B. The metallic electrodes 3A and 3B are fixed onto an insulator 6 in advance with the use of metallic bars 5A and 5B. The thermal electron emission section is mounted in such a way that its linear section is faced to a target.