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190 Chapter 5 accommodated. Finally, grid life is of critical importance and often drives thruster designers to compromises in performance or alternative grid materials. In this chapter, the factors that determine grid design and the principles of the ion accelerators used in ion thrusters will be described. 5.1 Grid Configurations To accelerate ions, a potential difference must be established between the plasma produced inside the thruster plasma generator and the ambient space plasma. As shown in Chapter 3, simply biasing the anode of a DC plasma generator or the electrodes of a radio frequency (rf) plasma generator relative to a spacecraft or plasma in contact with the space potential does not result in ion beam generation because the voltage will just appear in the sheath at the plasma boundary with the walls. If the potential is small compared to the electron temperature Te , then a Debye sheath is established, and if the potential is very large compared to Te, then a Child–Langmuir sheath exists. Therefore, to accelerate ions to high energy, it is necessary to reduce the dimension of an aperture at the plasma boundary to the order of the Child–Langmuir distance to establish a sheath that will accelerate the ions with reasonable directionality (good focusing) and reflect the electrons from the plasma. Figure 5-1 shows the Child–Langmuir length calculated from Eq. (3.7-34) for two singly charged ion current densities at an acceleration voltage of 1500 V . For xenon, the characteristic aperture dimension at this voltage is on the order of 2 to 5 mm and will decrease if the applied voltage is reduced or the current in the aperture is increased. 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 120 140 Ion Mass (AMU) Fig. 5-1. Child–Langmuir sheath length versus ion mass for two ion current densities at 1500-V acceleration voltage. 1500 V Ji = 1 mA/cm2 Ji = 10 mA/cm2 H Ne Ar Kr Xe Child Langmuir Length (cm)PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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