Heaterless hollow cathode

A thermionic emitter, comprising:

• a first inner cylindrical hollow space having a first diameter that defines an upstream inlet orifice of the thermionic emitter;
• a second inner cylindrical hollow space having a second diameter that defines a downstream outlet orifice of the thermionic emitter;
• and
• a transition inner hollow space arranged between the first inner cylindrical hollow space and the second inner cylindrical hollow space,
• wherein
• the first diameter is smaller than the second diameter,
• a ratio of the first diameter to the second diameter is configured to reduce upstream penetration of electric fields when the thermionic emitter is arranged in a downstream region of a heaterless hollow cathode of a thruster, and
• at a temperature range of about 1200 to 1500 degrees Celsius, the thermionic emitter is configured to produce, via thermionic emission, sufficient electrons for operation of the thruster.

A method for reducing penetration of electric discharge upstream of a heaterless hollow cathode, the method comprising:

• providing a thermionic emitter having a constricted upstream inlet orifice compared to a downstream outlet orifice;
• fitting the thermionic insert into a downstream region of a cathode tube, thereby obtaining a heaterless hollow cathode;
• partially surrounding the heaterless hollow cathode with a keeper electrode;
• applying a potential difference between the heaterless hollow cathode and the keeper electrode;
• and
• based on the applying, generating an electric discharge that is confined to a downstream region of the heaterless hollow cathode, the electric discharge being a Paschen discharge.

A heaterless hollow cathode, comprising:

• a cathode tube having an inner cylindrical hollow space, the cathode tube comprising an upstream opening for receiving a propellant;
• and
• a thermionic emitter arranged in the inner cylindrical hollow space of the cathode tube, the thermionic emitter comprising:
• a first inner cylindrical hollow space having a first diameter that defines an upstream inlet orifice of the thermionic emitter, the upstream inlet orifice located downstream of the upstream opening of the cathode tube;
• and
• a second inner cylindrical hollow space having a second diameter that defines a downstream outlet orifice of the thermionic emitter;
• wherein the first diameter is smaller than the second diameter.