Field emission electron gun

Claim 1

In an electron gun device of the field emission type including a cathode tip of needle shape, a first anode spaced from said cathode tip, first voltage source means for applying a first potential between said cathode tip and said first anode so as to produce an electric field therebetween capable of causing electron emission from said cathode tip, a second anode spaced from said first anode, and second voltage source means for applying a second potential between said first and second anodes for accelerating a beam of electrons emitted from said cathode tip, the improvement which comprises a control electrode disposed in the neighborhood of the cathode tip for controlling the amount of electrons emitted from said cathode tip; control voltage source means connected between said cathode tip and said control electrode for applying a variable control voltage therebetween having the same polarity as said first and second potentials; detector means for deriving a detecting signal responsive to the intensity of said electron beam from an object subjected to electron beam impingement; comparator means including at least a reference voltage source for producing a control signal representing the difference between a reference signal from said reference voltage source and said detecting signal; and control means for varying said control voltage of said control voltage source means in response to said control signal, thereby reducing said difference between said reference signal and said detecting signal to zero; whereby said electron beam can be maintained constant.