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170 Chapter 4 microwaves can be easily lost along the field lines to the wall due to their finite parallel velocity. This means that optimal ECR designs using permanent multipole magnets will have the resonance region as far from the wall as possible and will produce a large mirror ratio approaching the wall to reflect the electrons to avoid excessive direct loss. Nevertheless, wall losses are a concern in this configuration because the plasma production is a surface effect that is confined to the boundary region, as is the loss. Electrons that are heated in the resonance zone sufficiently to ionize the propellant gas generate plasma on the near-surface magnetic field lines. Coupling the plasma from the resonance region or the surface magnetic layer into the volume of the thruster is problematic due to the reduced cross-field transport. In the other thruster designs discussed in this chapter, the ion production was a volume effect and convective loss a surface effect, so thruster efficiency scaled as the volume-to-surface ratio. This means that larger DC and rf discharge thrusters can be made more efficient than smaller ones. Microwave thrusters, on the other hand, don’t scale in the same manner with size because large amounts of plasma must be produced and transported from the surface region to fill the volume of larger thrusters, which can impact the discharge loss. In addition, the plasma density is limited by both cutoff and the magnitude of the resonant field, and so high current density ion production requires very high magnetic fields and high microwave frequencies. Therefore, microwave thrusters have been limited to date to lower current densities and smaller sizes than the other thrusters discussed here. However, work continues on scaling microwave thrusters to larger sizes and higher efficiencies. The most successful design of a microwave thruster to date is the MUSES-C 10-cm ECR thruster [53–55], which is shown schematically in Fig. 4-39 from [54]. In this case, extremely strong samarium cobalt (SmCo) magnets are used to close the resonance field at the operating frequency between the magnets. This produces heating away from the wall and traps the electrons on the field lines due to an achievable mirror ratio of 2 to 3 in this geometry. The thruster volume is also minimized, with the plasma production region close to the grids. This configuration produces over 1 mA/cm2 of xenon ions over the active grid region using a 4.2-GHz microwave source with a discharge loss of about 300 eV per ion at over 85% mass utilization efficiency [53]. Finally, there are several other components intrinsic to these thrusters that contribute to the difficulty of achieving high efficiency and compact size in a microwave thruster subsystem. Sources of microwave frequencies in the gigahertz range, such as traveling-wave tubes (TWT) and magnetrons, have efficiencies in the 50% to 70% range, and the power supply to run them is usually about 90% efficient. This represents nearly a factor of two in-line lossPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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