PURPOSE: To make control possible even for rapid change of the degree of vacuum by providing electronic current control electrode between the hot cathode and the grid to control the electronic current from the hot cathode.

CONSTITUTION: Electronic current control electrode 6 is arranged between grid 5 and filament 3 and controls the electronic current led out from filament 3 by controlling the voltage applied to electronic current control electrode 6. Ion collector 4, grid 5 and electron current control electrode 6 are cylindrical desirably and are arranged on coaxial circles desirably. Voltaged applied to filament 3, grid 5 and electronic current control electrode 6 are controlled through lead 7, and the current flowed to ion collector 4 is measured through lead 8.