Patent Number: 042740355
Section: claims

1. A field emission electron gun comprising a cathode, a control electrode disposed in the vicinity of said cathode, and anode disposed along a beam path extending from said cathode for accelerating electrons emitted from said cathode, a variable D.C. voltage source connected between said cathode and said anode, switch means connected between said voltage source and said control electrode for changing-over the potential of said control electrode between ground potential and the potential of said cathode, and means for varying the position of said cathode relative to said control electrode said anode being fixed in position relative to one of said cathode and said control electrode at a position downstream of said control electrode along said beam path. 2. A field emission electron gun according to claim 1, wherein said means for varying comprises means for moving said cathode whereby the relative positions of said cathode and said control electrode are variable. 3. A field emission electron gun according to claim 1, wherein said means for varying comprises means for moving said control electrode whereby the relative positions of said cathode and said control electrode are variable. 4. A field emission electron gun according to claim 1, wherein said control electrode has a cylindrical shape, and said cathode is arranged in a central part of the cylindrical control electrode. 5. A field emission electron gun according to claim 1, 2, 3, or 4, wherein the D.C. voltage source comprises a single variable high-voltage source. 6. A field emission electron gun according to claim 1, further comprising a vacuum vessel containing said cathode, said control electrode and said anode, said D.C. voltage source, said switching means and said means for varying being arranged exteriorly of said vacuum vessel. 7. A field emission electron gun according to claim 1, wherein said means for varying includes a vertical moving means for supporting at least one of said cathode and said control electrode for enabling varying of the relative positions thereof. 8. A field emission electron gun according to claim 7, wherein the vertical moving means includes a bellows. 9. A field emission electron gun comprising a cathode, a control electrode disposed in the vicinity of said cathode, an anode disposed along a beam path extending from said cathode for accelerating a beam of electrons emitted from said cathode, a D.C. voltage source connected between said cathode and said anode, and switch means connected between said voltage source and said control electrode for changing-over the potential of said control electrode between ground potential and the potential of said cathode, said voltage source being variable between a low energy voltage level while said control electrode is connected to ground potential and a high energy voltage level when said control electrode is connected to the same potential as said cathode such that the energy of said beam of electrons can be changed from a low level to a high level without significant increase in the beam current.