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Hollow Cathodes 283 the tube, and orifice heating effects (especially in neutralizer cathodes), which impact the performance and life of the cathode. These effects cannot be obtained from the simple 0-D models used above. 6.7 Cathode Plume-Region Plasma The cathode insert and orifice regions were examined above with simplified models, and this information was used to provide an understanding of the plasma parameter dependence and self-heating mechanism of the cathode. The final region of the hollow cathode to cover is exterior to the cathode orifice where the cathode plume interacts with the keeper electrode and couples the cathode emission current to the thruster discharge plasma and anode. In this region, the neutral gas expands rapidly away from the cathode and is either collisionless or makes the transition to collisionless. The electrons from the cathode are accelerated by the potential difference between the cathode orifice plasma and the plasma in the discharge chamber that is at a potential usually near the anode voltage. There is usually an applied axial magnetic field on the order of 100 G in this region to provide a transition to the ring-cusp fields in ion thrusters, which produces some confinement of the cathode plume electrons. These electrons generate the cathode plume plasma, which is also rapidly expanding away from the cathode. The plasma stream exiting the hollow cathode is often reported as having various structures consisting of dark spaces, plasma balls, and brightly divergent plume shapes. Two of these cases are shown in Fig. 6-28, where the cathode is on the right and the anode on the left. The plasma stream consists of the electrons from the hollow cathode, neutral gas expanding from the keeper aperture in addition to more uniform background neutral gas from the thruster, and the plasma ball and stream generated by ionization of this gas by the electrons. The on-axis potential and temperature profiles measured by scanning probes for these two cases [1] are shown in Fig. 6-29. While the discharge current is the same in these two cases, the high gas flow reduced the discharge voltage from about 26 V at 5.5 sccm to 20 V at 10 sccm. The structure of the potential and temperature profiles is significantly different in the plume region as the gas flow and discharge voltage change. The higher gas flow case reduces the potentials and temperatures throughout the system and pushes the plasma ball observed at the cathode exit farther downstream. To provide some insight into the plasma density, temperature, and potential profiles generated in this plume, a simple 1-D model [39] will be used to examine the plume physics. A full 2-D cathode plume code for the neutral gas and plasma is discussed later in this section to more accurately predict the cathode discharge behavior. The 1-D plasma model follows the same generalPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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