The main elements of an electrostatic thruster are summarized in Fig. 1. Neutral propellant is injected into an ionization chamber, which may operate on a variety of principles(electron bombardment, contact ionization, radiofrequency ionization.) The gas contained in the chamber may only be weakly ionized in the steady state, but ions are extracted preferentially to neutrals, and so, to a first approximation, we may assume that only ions and electrons leave this chamber. The ions are accelerated by a strong potential difference va applied between perforated plates (grids) and this same potential keeps electrons from also leaving through these grids. The electrons from the ionization chamber are collected by an anode and in order to prevent very rapid negative charging of the spacecraft(which has very limited electrical capacity), they must be ejected to join the ions downstream of the accelerating grid. To this end the electrons must be forced to the large tive potential of the accelerator (which also prevails in the beam), and they must then be injected into the beam by some electron-emitting device(hot filament, plasma The net effect is to generate a jet of randomly mixed (but not recombined) ions and electrons, which is electrically neutral on average, and is therefore a plasma beam The reaction to the momentum flux of this beam constitutes the thrust of the device Notice in Fig. 1 that, when properly operating, the accelerator grid should collect no ions or electrons, and hence its power supply should consume no power only apply a static voltage. On the other hand, the power supply connected to the neutralizer must pass an electron current equal in magnitude to the ion beam current and must also have the full accelerating voltage across its terminals; it is therefore this power supply that consumes (ideally)all of the electrical power in the device. In summary, the main functional elements in an ion engine are the ionization chamber the accelerating grids, the neutralizer, and the various power required Most of the efforts towards design refinement have concentrated on the ionization chamber, which controls the losses, hence the efficiency of the device, and on the power supplies, which dominate the mass and parts count. the grids are, of course, an essential element too and much effort has been spent to reduce their erosion by stray ions and improve its collimation and extraction capabilities. The neutralizer was at one time thought to be a critical item but experience has shown that with good design, no problems arise from it. Following a traditional approach(1) 3), we will first discuss the ion extraction system then turn to the chamber and other elements 3 Ion Extraction and Acceleration The geometry of the region around an aligned pair of screen and accelerator holes shown schematically in Fig. 2(from Ref. 7). The electrostatic field imposed by the strongly negative accelerator grid is seen to penetrate somewhat into the plasma through the screen grid holes. This is fortunate in that the concavity of the plasma surface provides a focusing effect which helps reduce ion impingement on the accelerator. The result is an array of hundreds to thousands of individual ion beamlets which are neutralized a short distance downstream as indicated the potential diagram in Fig 2 shows that the screen grid is at somewhat lower potential than the plasma in the chamber. Typically the plasma potential is near that of the anode in the chamber, while the screen is at cathode potential(some 30-60 volts lower, as we will see). This ensures that ions which wander randomly to the vicinity 16.522, Space Propulsion Lecture 13-14 Page 3 of 2516.522, Space Propulsion Lecture 13-14 Prof. Manuel Martinez-Sanchez Page 3 of 25 The main elements of an electrostatic thruster are summarized in Fig. 1. Neutral propellant is injected into an ionization chamber, which may operate on a variety of principles (electron bombardment, contact ionization, radiofrequency ionization…). The gas contained in the chamber may only be weakly ionized in the steady state, but ions are extracted preferentially to neutrals, and so, to a first approximation, we may assume that only ions and electrons leave this chamber. The ions are accelerated by a strong potential difference Va applied between perforated plates (grids) and this same potential keeps electrons from also leaving through these grids. The electrons from the ionization chamber are collected by an anode, and in order to prevent very rapid negative charging of the spacecraft (which has very limited electrical capacity), they must be ejected to join the ions downstream of the accelerating grid. To this end, the electrons must be forced to the large negative potential of the accelerator (which also prevails in the beam), and they must then be injected into the beam by some electron-emitting device (hot filament, plasma bridge…). The net effect is to generate a jet of randomly mixed (but not recombined) ions and electrons, which is electrically neutral on average, and is therefore a plasma beam. The reaction to the momentum flux of this beam constitutes the thrust of the device. Notice in Fig. 1 that, when properly operating, the accelerator grid should collect no ions or electrons, and hence its power supply should consume no power, only apply a static voltage. On the other hand, the power supply connected to the neutralizer must pass an electron current equal in magnitude to the ion beam current, and must also have the full accelerating voltage across its terminals; it is therefore this power supply that consumes (ideally) all of the electrical power in the device. In summary, the main functional elements in an ion engine are the ionization chamber, the accelerating grids, the neutralizer, and the various power supplies required. Most of the efforts towards design refinement have concentrated on the ionization chamber, which controls the losses, hence the efficiency of the device, and on the power supplies, which dominate the mass and parts count. The grids are, of course, an essential element too, and much effort has been spent to reduce their erosion by stray ions and improve its collimation and extraction capabilities. The neutralizer was at one time thought to be a critical item, but experience has shown that, with good design, no problems arise from it. Following a traditional approach(1),(3), we will first discuss the ion extraction system, then turn to the chamber and other elements. 3 Ion Extraction and Acceleration The geometry of the region around an aligned pair of screen and accelerator holes is shown schematically in Fig. 2 (from Ref. 7). The electrostatic field imposed by the strongly negative accelerator grid is seen to penetrate somewhat into the plasma through the screen grid holes. This is fortunate, in that the concavity of the plasma surface provides a focusing effect which helps reduce ion impingement on the accelerator. The result is an array of hundreds to thousands of individual ion beamlets, which are neutralized a short distance downstream, as indicated. The potential diagram in Fig. 2 shows that the screen grid is at somewhat lower potential than the plasma in the chamber. Typically the plasma potential is near that of the anode in the chamber, while the screen is at cathode potential (some 30-60 volts lower, as we will see). This ensures that ions which wander randomly to the vicinity