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256 Chapter 6 6.4 Insert Region Plasma The insert region of the hollow cathode, as was illustrated in Fig. 6-4, usually has a cylindrical geometry with electron emission from the interior surface of a thermionic insert material. A plasma discharge is established inside the insert region, and electrons emitted from the insert surface are accelerated through the cathode sheath that forms between the insert surface and the plasma. The insert plasma must be capable of accepting the emitted electron current from the sheath and must provide heating of the insert for the cathode to operate properly. The maximum electron current density into the insert plasma is then determined by either space-charge limitations in the plasma at the sheath edge or by characteristics of the surface (work function and temperature) that limit the thermionic emission. As shown by the double sheath analysis in Chapter 3, ions flowing back from the plasma through the sheath to the cathode surface neutralize the electron space charge and increase the extracted electron current density from the insert surface. The electrons accelerated through the sheath quickly give up their energy to the dense collisional plasma inside the insert. Electrons in the tail of the Maxwellian distribution in this plasma have sufficient energy to ionize some portion of the thruster propellant injected through the cathode, which is only a small fraction of the total propellant injected into the thruster. Plasma electrons incident on the downstream end of the cathode tube flow through the orifice and into the main discharge chamber. The barium evaporated from dispenser cathode inserts is easily ionized in plasmas with this electron temperature because its ionization potential is only 5.2 eV. A calculation of the ionization mean free path in NASA Solar Electric Propulsion Technology Applications Readiness (NSTAR)-sized hollow cathodes [25] predicts about 4 10–5 m, which is much smaller than the interior dimensions of the cathode. The ionized barium then migrates upstream because the potential gradient in the hollow cathode that pulls electrons out of the cathode plasma also accelerates barium ions in the opposite direction (upstream). This means that the barium in the insert does not leave the cathode during discharge operation, but tends to travel upstream in the plasma and is deposited in the cooler sections of the hollow cathode. The pressure inside the hollow cathode is set primarily by the gas flow rate through the cathode and the orifice size and must be sufficiently high to produce a collisional plasma. This condition is required to slow ions backstreaming from the orifice region and from the peak plasma potential on axis (primarily by charge exchange) to avoid sputtering of the insert surface by high-energy ion bombardment. While this condition may not necessarily be satisfied everywhere inside a Type C cathode (with no orifice), at least some fraction of the insert is protected by the collisional processes for proper cathodePDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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