Large-area uniform electron source

A method for producing a controllable electron beam of relatively large cross-sectional area, the method comprising the steps of:

• applying a selected negative voltage to a large-area cathode positioned in a vacuum chamber, to accelerate electrons toward a target also located within the vacuum chamber;
• applying a voltage to a mesh grid anode spaced from the cathode by a distance not exceeding the mean free path length of electrons leaving the cathode for the lowest selected voltage applied between the cathode and the grid anode;
• and
• initializing ionization of residual gases in the vacuum chamber, whereby positive ions are attracted to the cathode surface and cause the emission of electrons from the cathode;
• maintaining an ion plasma in the vacuum chamber principally as a result of collisions between residual gas particles and electrons that have been accelerated through the mesh grid anode and toward the target, whereby a highly uniform electron beam is obtained;
• and
• controlling the rate of electron emission by varying the voltage applied to the grid, whereby a more negative grid anode voltage will attract more positive ions through the grid anode and into the cathode, to produce more electrons.

Electron emission apparatus, comprising:

• a vacuum chamber containing residual gas atoms;
• a cathode, of which at least one surface of relatively large area is exposed to the vacuum in the chamber;
• means for applying a selected negative voltage to the cathode, to accelerate a beam of electrons to a target also located in the vacuum chamber, from a region close to the cathode, wherein positive ions formed in an ionization region located between the cathode and the target are accelerated toward the cathode and induce the emission of electrons, some of which cause more ionization;
• a mesh grid anode positioned between the cathode and the target but outside the ionization region, at a distance not exceeding the mean free path length of electrons leaving the cathode for the lowest selected voltage applied between the cathode and grid anode;
• and
• means for applying a voltage to the grid anode, to control the rate of electron emission from the cathode, whereby positive ions formed in the ionization region are selectively attracted or repelled by appropriately biasing the grid anode, and the electron beam current is controlled independently of pressure and electron accelerating voltage;
• and wherein the distance between the grid anode and the target is great enough that an ion plasma is maintained in the ionization region principally by collisions between residual gas atoms and electrons that have been accelerated through the mesh grid anode, whereby a highly uniform beam current is obtained across the entire cathode surface and over a wide range of beam intensities.