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Hollow Cathodes 249 Fig. 6-5. Schematics of the three characteristic types of hollow cathodes (A, B, and C) depending on the orifice geometry. pressures, and are heated primarily by orifice heating. The second type of cathode features has an orifice diameter typically larger than the length, shown in Fig. 6-5 as TypeB, and operates at lower internal gas pressures. The heating mechanism in these cathodes can be due to electron or ion bombardment of the insert, or a combination of the two depending on the orifice size and operating conditions. The third type of cathode, typically used in high-current cathodes and shown in Fig. 6-5 as Type C, has essentially no orifice at all. These cathodes have a large neutral density gradient in the insert region, but they typically have a reduced internal pressure overall as compared to orificed cathodes. The heating mechanism for Type C cathodes is normally ion bombardment of the insert. The value of the neutral gas pressure inside the hollow cathode affects both the plasma density and plasma profile due to collisional effects [5]. Figure 6-6 shows examples of axial plasma density profiles measured with fast scanning probes [6] inside a 0.38-cm inside- diameter (I.D.) cathode insert operating at 13 A of discharge current and a xenon flow of 3.7 standard cubic centimeters per minute (sccm) for two different orifice diameters and the case of no orifice plate at all. Small orifices, characteristic of Type A cathodes, have high internal pressures that produce high plasma densities but constrain the axial extent of the plasma to the order of a few millimeters. For a given emission current density, this can restrict the discharge current that is available. As the orifice is enlarged, the pressure decreases and the plasma extends farther into the insert, resulting in utilization of more of the insert surface area for electron emission. The electron current density in the orifice is higher than anywhere else in the system and, depending on the orifice size, can easily exceed 1 kA/cm2. If the orifice is long compared with its radius, as is the case in most Type A neutralizer hollow cathodes, the physics are the same as for a classical positivePDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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