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Ion Thruster Plasma Generators 151 electric fields can be supplemented by coulomb collisions between the plasma electrons, which reduce the operating pressure requirement and permit high mass utilization efficiency to be achieved. Starting an inductive plasma discharge can also be problematic because initially there are few free electrons present to interact with the rf fields and ionize the fill gas. Prior to the plasma ignition, there is no load on the rf circuit driving the coil and the reactive power stored in the inductive components in the rf matching network grows, which increases the voltage across the coil and induces higher electric fields inside. If the minimum gas pressure is provided, the discharge will ignite when the field is either large enough to excite the few electrons naturally present in the chamber or causes field emission to occur. Another method for ignition is to inject electrons from a spark generator, small cathode, or the neutralizer cathode (with the accel voltage turned off momentarily) into the discharge chamber to provide the seed electrons for interaction with the rf electric fields. If the antenna in rf thrusters is directly exposed to the plasma, ions in the discharge can be accelerated by the rf voltage on the surfaces and sputter-erode the antenna. This can ultimately limit the life of rf thrusters. This problem is minimized by either encasing the antenna in an insulator [49] or by making the thruster body an insulating material and mounting the antenna exterior to the plasma volume [50]. In this case, the rf voltage across the coil is shielded from the plasma, and the ions are not accelerated to high energy before striking the insulator. Mounting rf antennas outside insulating-material walls such as quartz or alumina is common practice in inductive plasma generators used in the semiconductor processing industry. An example of this arrangement applied to a radio-frequency ion thruster (RIT)-XT thruster [50] is shown in Fig. 4-31. In this case, the body of the thruster is constructed of a conical (or hemispherical) alumina insulator, and a high-conductivity-material (typically copper) antenna is coiled around the insulator. As long as the alumina body is not significantly coated by conductive layers and remains an insulator, the rf fields will couple through the wall and generate plasma. This type of ion thruster is readily analyzed by particle and energy balance models because they do not have localized electron sources (hollow cathodes); the rf fields simply heat the Maxwellian electron distribution that provides the ionization, and the plasma in the discharge chamber is very uniform. In the energy balance equation, it is assumed that the power absorbed by the plasma is simply given by Pabs . Ions generated in the plasma volume drift to the interior surfaces in the thruster, and only electrons in the tail of the Maxwellian distribution have sufficient energy to overcome the potential differencePDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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