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Ion and Hall Thruster Plumes 419 with frequency f passing through a plasma with density ne will undergo a phase shift according to the following formula: f 0L n e d , ( 8 . 5 - 1 ) c nc where c is the speed of light and nc is the critical density at which the plasma density has a plasma frequency equal to the microwave frequency. Since the plasma density drops rapidly with distance from the thruster, the scale length over which the plasma frequency is comparable to the wave frequency is usually small. 8.5.2 Plume Plasma Optical Emission The optical emissions from ion and Hall thrusters are very weak but can be measured by sensitive instruments. The only in-space measurement of the optical emissions from a xenon plasma plume generated by an electric propulsion device is from a shuttle flight that had a “plasma contactor” as part of the Space Experiments with Particle Accelerators (SEPAC) [18] flown on the NASA Space Shuttle Mission STS-45. The “plasma contactor” was actually a Xenon Ion Propulsion System (XIPS) 25-cm thruster without accelerator grids or a neutralizer hollow cathode. Plasma and electron current from the discharge chamber were allowed to escape into space, unimpeded by an ion accelerator grid set. The absolute-intensity optical emission spectrum measured in space of the xenon plasma plume from the operating plasma source is shown in Fig. 8-18. The spectrum was measured by the Atmospheric Emissions Photometric Imaging (AEPI) spectrographic cameras. The source was the SEPAC plasma contactor [18,19] that generated about 2 A of singly-charged xenon ions in a ring-cusp discharge chamber. The plasma density was about 1017 m–3, and its temperature was about 5 eV. Upon leaving the discharge chamber, the quasi- neutral plasma expanded into the much less dense surrounding ionosphere. The spectrum was taken about 15 meters from the contactor plume, focusing on the plume about 1.5 m downstream of the contactor exit plane. The apparent broadness of the lines is due to the spectrograph’s relatively wide slit [20]. Optical emissions from the SEPAC plasma contactor are higher than the emissions expected from a similarly sized ion thruster for two reasons. First, the plasma contactor ion density is higher since the contactor ions are traveling about a quarter as fast as thruster beam ions. Second, the electrons in the SEPAC plasma contactor plume originate in the discharge chamber and are much hotter than the neutralizer cathode electrons in an ion thruster plume,PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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