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Ion Thruster Accelerator Grids 191 The ion current obtainable from each grid aperture is then limited by space charge. For a 0.25-cm-diameter aperture extracting the space-charge-limited xenon current density of about 5 mA/cm2 at 1500 V [from Eq. (3.7-56)], the total ion current per aperture is only 0.25 mA. Assuming this produces a well- focused beamlet, the thrust produced by this current and voltage according to Eq. (2.3-9) is only about 16 newtons. Therefore, multiple apertures must be used to obtain higher beam currents from the ion engine to increase the thrust. For example, to extract a total of 1 A of xenon ion current for this case would require over 4000 apertures, which would produce over 60 mN of thrust. In reality, for reliable high-voltage operation, and due to non-uniformities in the plasma generator producing varying ion current densities to the boundary, the current density is usually chosen to be less than the Child–Langmuir space charge maximum, and an even larger number of apertures are required. This ultimately determines the size of the ion thruster. Figure 5-2 shows a simplified one-dimensional (1-D) view of one of these biased apertures facing the thruster plasma. The Child–Langmuir sheath is established by the bias potential between the thruster plasma and the accelerator grid and is affected by the current density of the xenon ions arriving at the sheath edge from the Bohm current. Ions that arrive on axis with the aperture are accelerated through to form the beam. However, ions that miss the aperture are accelerated into the accel grid and can erode it rapidly. For this reason, a “screen” grid with apertures aligned with the accel grid is placed upstream of the accel grid to block these ions. This is the classic two-grid accelerator system [1,2]. The screen grid is normally either allowed to float electrically or is biased to the cathode potential of the plasma generator to provide some confinement of the electrons in the plasma and so that ions that strike it have a relatively low energy and cause little sputtering. In practice, the grids are made of refractory metals or carbon-based materials, and the apertures are close-packed in a Child Langmuir Sheath Accelerator Grid Xe+ Xe+ Beam Plasma Plasma Xe+ Accelerator Aperture Fig. 5-2. Simplified 1-D view of an accelerator aperture in contact with a plasma.PDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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