Fundamentals of Electric Propulsion: Ion and Hall Thrusters

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Fundamentals of Electric Propulsion: Ion and Hall Thrusters ( fundamentals-electric-propulsion-ion-and-hall-thrusters )

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248 Chapter 6 then discuss the basics of the cathode insert that provides thermionic electron emission. The characteristics of the plasmas in the insert region, the orifice, and the cathode plume in the vicinity of the keeper required to extract and transmit the electrons into the thruster will then be examined. Since the neutral gas density changes all along the discharge path in hollow cathode discharges, the plasmas generated in each location (inside the insert, in the orifice, and in the cathode plume) have different properties in terms of collisionality, temperature, potential, and density. These differences determine the applicable plasma physics in each region. 6.2 Cathode Configurations The geometry and size of the hollow cathodes depend on the amount of current that they are required to emit. Discharge currents in ion thrusters are typically 5 to 10 times the beam current depending on the efficiency of the plasma generator, and discharge currents can range from a few amperes to over 100 amperes [3]. The hollow cathode used in a Hall thruster provides electrons for both ionization of the propellant gas and neutralization of the beam [4]. Hall thrusters also tend to run at lower specific impulse (Isp) than ion thrusters. Therefore, Hall thrusters require higher discharge currents from the cathode to achieve the same total power as compared to ion thrusters, and currents of the order of 10 amperes to hundreds of amperes are needed. Neutralizer cathodes in ion thrusters emit electrons at a current equal to the beam current. Therefore, they can be made smaller than discharge cathodes and must be designed to be self-heated and to run reliably at lower currents. Higher discharge currents require larger insert sizes because the thermionic emission current densities from cathode surfaces are finite. Ultimately, this determines the diameter of the insert, which will be described in the next section. The cathode orifice size depends on many parameters. Ion thruster neutralizer cathodes have been designed with very small diameter orifices (3 10–2 cm), and ion thruster discharge cathodes and small Hall thruster cathodes have been designed with orifices of less than 0.1-cm diameter to over 0.3 cm in diameter. High-current hollow cathodes for large ion thrusters and Hall thrusters will have even larger orifices. These cathodes are sometimes designed even without an orifice, where the insert inside diameter forms a tube exposed to the discharge plasma. Hollow cathodes generally fall into three categories, which will be useful later in describing the plasma characteristics in the three regions described above. The first type of hollow cathode is characterized by a small orifice with a large length-to-diameter ratio, shown schematically in Fig. 6-5 as Type A. These cathodes typically operate at low current and relatively high internal gas

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