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Hall Thrusters 349 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Fig. 7-10. Secondary electron yield for several wall materials used in Hall thrusters, measured with a mono-energetic electron beam. Table 7-1. Fitting parameters for secondary electron yield data. 0 10 20 30 40 50 60 70 80 90 100 Electron Energy (eV) Alumina (Al2O3) Boron Nitride (BN) BNSiO2 Stainless steel a b 0.145 0.650 0.150 0.549 0.123 0.528 0.040 0.610 (2 + b) 1.49 1.38 1.36 1.44 Al2O3 Data (Gascon) Al2O3 Fit to Data BN Data (Bugeat) BN Fit to Data BNSiO2 (Gascon) BNSiO2 Fit to Data Stainless Steel that result in finite yield at low electron energy. Figure 7-12 shows the data for boron nitride and BNSiO2 with the two different fitting choices. In the evaluation of the sheath potential in the presence of the secondary electron emission below, whether one uses a linear or power fit does not make a significant difference in the ionization and acceleration regions for electron temperatures above about 10 eV. Measurements of the electron temperature in the channel of Hall thrusters by a number of authors [34–36] show electron temperatures in the channel well in excess of 20 eV. Equation (7.3-29) predicts that the sheath potential will go to zero and reverse from negative going (electron repelling) to positive going (electron attracting) as the secondary electron yield approaches unity for some of the materials. The value at which this occurs for each of the materials shown in Table 7-1 is indicated in Fig. 7-11. For boron nitride and alumina walls this occurs at electron temperatures below 20 eV, and for BN-SiO2 walls it occurs at Secondary Electron YieldPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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