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Hollow Cathodes 247 Fig. 6-4. Hollow cathode schematic showing the cathode tube, insert, and heater enclosed in a keeper electrode. Hollow cathodes operate in a “self-heating” mode, in which the external heater is turned off during operation and the cathode insert is heated by plasma bombardment. There are three self-heating mechanisms possible in hollow cathodes: (1) orifice heating, (2) ion heating, and (3) electron heating. In orifice heating, the cathode is designed with a small, restrictive orifice, which produces a high internal pressure in both the insert and orifice regions. The plasma discharge passing through the orifice is then very resistive, causing a significant amount of power to be deposited in the orifice plasma and transferred to the orifice walls by convection. This power deposition then heats the insert by conduction and radiation. Orifice heating is used primarily in neutralizer cathodes where the discharge currents are very low. The classic mechanism for cathode heating is ion heating, where ions in the cathode insert region plasma fall through the sheath potential at the insert surface and heat the surface by ion bombardment. Electron heating occurs in a regime where both the cathode internal pressure and the discharge current are relatively high, resulting in the very high plasma densities (>1015 cm–3) generated in the insert region. The low electron temperatures and low sheath voltages produced in this situation result in the energetic tail of the Maxwellian electron distribution having sufficient energy to exceed the sheath potential and reach the insert surface. These electrons then deposit their energy on the insert and heat it to emission temperatures. The heating mechanism that dominates in any hollow cathode design depends on the geometry of the cathode, the internal neutral gas pressure in the insert and orifice regions, and the discharge current. This chapter will start with a simple classification of different hollow cathode geometries to aid in the discussion of the important effects in the system, andPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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