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Ion Thruster Accelerator Grids 225 5.6 Ion Accelerator Grid Life The most important wear mechanism in modern ion thrusters is accelerator grid erosion. Even though properly designed optics attempt to make all of the ions extracted from the discharge chamber focus through the accelerator grid apertures, a current of secondary ions generated downstream of the discharge chamber impacts the accelerator grid. These secondary ions are generated by resonant charge exchange (CEX) between beam ions and neutral propellant gas escaping from the discharge chamber. The cross section for resonant charge exchange—that is, the transfer of an electron from a propellant atom to a beamlet ion—is very large: on the order of a hundred square angstroms [48]. This process results in a fast neutral atom in the beam and a slow thermal ion. These slow ions are attracted to the negatively charged accelerator grid, and most hit with sufficient energy to sputter material from the grid. Eventually the accelerator grid apertures become too large to prevent electron backstreaming or enough material is sputtered away that the grids fail structurally. The erosion geometry is naturally divided into two regions. The first region, barrel erosion, is caused by ions generated between the screen grid aperture sheath and the downstream surface of the accelerator grid, as shown in Fig. 5-20. Charge exchange ions generated in this region impact the inside surface of the accelerator grid aperture, which results in enlargement of the aperture barrel. As the barrel diameter increases, the grid must be biased more and more negatively in order to establish the minimum potential required in the aperture to prevent neutralizer electrons from backstreaming into the discharge chamber. Thruster failure occurs when, at its maximum voltage, the accelerator grid power supply is unable to stop electron backstreaming. The second region of grid erosion is caused by charge exchange ions generated downstream of the accelerator. Since the beamlets are long and thin, inside each beamlet the radial electric forces dominate and expel the slow, charge-exchange ions into the gaps between the beamlets. Charge exchange ions generated in the region before the beamlets merge to form a continuous ion density are then attracted back to the accelerator grid by its large negative potential. This is illustrated in Fig. 5-21. On impact, these ions sputter away material from the downstream surface of the accelerator grid. Sputter erosion by these backstreaming ions results in a hexagonal “pits-and-grooves” erosion pattern on the downstream grid surface, which can lead to structural failure of the grids if the erosion penetrates all the way through the grid. Erosion of the accel grid aperture edge by backstreaming ions can also effectively enlarge the accel grid aperture diameter, leading to the onset of electron backstreaming.PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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