Electron gun, method of controlling same, and electron beam additive manufacturing machine

An electron beam additive manufacturing machine comprising:

• an electron gun producing an electron beam;
• a support stage on which a powdered sample is tightly spread;
• an electron optical system for scanning the electron beam over the stage;
• a sample supply system for spreading the powdered sample tightly on the support stage;
• and
• a control system for controlling the electron gun,
• wherein said electron gun has:
• a cathode heated by a heating power supply and having a tip from which thermal electrons are emitted;
• a Wehnelt electrode having a central axis along which a first opening is formed, the tip of the cathode being inserted in the first opening, the Wehnelt electrode being operative to control a thermal electron emissive region of the cathode and to focus the thermal electrons by application of a bias voltage to the Wehnelt electrode;
• a control electrode having a central axis along which a second opening is formed, the control electrode being operative to cause the thermal electrons emitted from the tip of the cathode to pass into the second opening by application of a control voltage to the control electrode;
• and
• an anode having a central axis along which a third opening is formed, the anode being operative to accelerate the thermal electrons emitted from the cathode and to cause the thermal electrons passed through the second opening to pass into the third opening and impinge as an electron beam on a powdered sample by application of an accelerating voltage to the anode, and
• wherein said control system sets the bias voltage and the control voltage according to combination conditions of the bias voltage and the control voltage in order to maintain the brightness of the electron beam constant when a beam current produced by the electron beam impinging on the powdered sample is varied.

A method of controlling an electron gun, comprising the steps of:

• emitting thermal electrons from a tip of a cathode heated by a heating power supply;
• controlling a thermal electron emissive region of the cathode and focusing the thermal electrons by applying a bias voltage to a Wehnelt electrode having a central axis along which a first opening is formed, the tip of the cathode being inserted in the first opening;
• applying a control voltage to a control electrode having a central axis along which a second opening is formed to cause the thermal electrons emitted from the tip of the cathode to pass into the second opening;
• applying an accelerating voltage to an anode having a central axis along which a third opening is formed to accelerate the thermal electrons emitted from the cathode such that the thermal electrons passed through the second opening pass into the third opening and impinge as an electron beam on a powdered sample;
• and
• setting the bias voltage and the control voltage according to combination conditions of the bias voltage and the control voltage under control of a control system in order to maintain the brightness of the electron beam constant when a beam current produced by the electron beam impinging on the powdered sample is varied.