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196 Chapter 5 plasma generator or by ionization from the most energetic backstreaming electrons do strike the accel grid and erode it. Charge exchange ion erosion of the accel grid ultimately limits the grid life, which will be discussed in Section 5.6. 5.2 Ion Accelerator Basics The thruster ion optics assembly serves three main purposes: 1) Extract ions from the discharge chamber 2) Accelerate ions to generate thrust 3) Prevent electron backstreaming The ideal grid assembly would extract and accelerate all the ions that approach the grids from the plasma while blocking the neutral gas outflow, accelerate beams with long life and with high current densities, and produce ion trajectories that are parallel to the thruster axis with no divergence under various thermal conditions associated with changing power levels in the thruster. In reality, grids are non-ideal in each of these areas. Grids have finite transparency; thus, some of the discharge chamber ions hit the upstream “screen grid” and are not available to become part of the beam. The screen grid transparency, Ts , is the ratio of the beam current, Ib , to the total ion current, Ii , from the discharge chamber that approaches the screen grid: Ts = Ib . (5.2-1) Ii This ratio is determined by comparing the ion beam current with the screen grid current. The transparency depends on the plasma parameters in the discharge chamber because the hemispherical sheath edge is normally pushed slightly into the plasma by the applied voltage if the screen grid is relatively thin. The pre- sheath fields in the plasma edge then tend to steer some ions that would have gone to the screen grid into the beam. For this reason, the effective transparency of the screen grid typically exceeds the optical transparency for relatively large apertures and thin grid thicknesses. In addition, the screen grid current must be measured with the screen grid biased negative relative to cathode potential to reflect energetic electrons in the tail of the Maxwellian distribution in the plasma. The goal for screen grid design is to maximize the grid transparency to ions by minimizing the screen thickness and the webbing between screen grid holes to that required for structural rigidity. The maximum beam current density is limited by the ion space charge in the gap between the screen and accelerator grids [2], which was discussed abovePDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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