Patent ID: 8330344

Claim:
An electron gun comprising: an electron source for emitting electrons; an accelerating electrode, placed facing an electron emission surface of the electron source, for accelerating the electrons; an extraction electrode, placed between the electron emission surface and the accelerating electrode, for drawing the electrons from the electron emission surface; a suppressor electrode, placed in an opposite side of the electron emission surface from the extraction electrode, for suppressing emission of the electrons from a side surface of the electron source; and an electron beam converging unit for converging an electron beam of thermal field emission electrons emitted from the electron emission surface by applying an electric field to the electron emission surface while keeping the electron source at a temperature low enough for a material for the electron source not to sublime; wherein the electron beam converging unit is a magnetic field generator placed in a vicinity of the accelerating electrode, and wherein the magnetic field generator is a permanent magnet having any one of north and south poles on the accelerating electrode side facing the electron source, which generates the magnetic field that solely decreases from the accelerating electrode toward the suppressor electrode.