PROBLEM TO BE SOLVED: To provide a small and less expensive electron beam generator that generates effectively an electron beam that has large electron current and also has high acceleration energy.
SOLUTION: A thermal electron emission negative electrode quipped with a tungsten filament 14 and a cold cathode 15 connected to it, and a ring-like positive electrode 17 are arranged facing opposite to each other in a container 11, an accelerating D.C. power source 26 is connected between the cold cathode and the positive electrode. The container 11 is filled with a thin gas, for instance, argon gas at a pressure of 0.1-1,000 mTorr. Since the electron space charge of the thermal electron emission negative electrode is neutralized by positive ions produced by the discharge of the thin gas, an extremely large electron current is emitted from the thermal electron emitting negative electrode. In addition, an electric field distribution is varied by a plasma produced through the discharge of the thin gas, and a large electric field is formed in a positive electrode drop region is formed, and thereby the electrons are accelerated to high energy.