since the high energy ions are very effective sputtering agents, results in a very destructive mode of operation. All the perveance values reported in Ref. (7), for instance, are impingement-limited, i. e. correspond to the highest current prior to onset of direct impingement (b)Electron back-streaming: For R values near unity the barrier offered by the accelerator negative potential to the neutralizer electrons becomes weak, and beyond some threshold value of R, electrons return up the accelerator potential to the chamber. This results in screen damage, space charg distortion, and shorting of the neutralizer supply Kaufman (oa) gave the theoretical estimate 0.2 (16) D which was confirmed experimentally in Ref. 7, except that it was found to be a somewhat conservative estimate 4 Ion Production 4.1 Physical Process in Electron Bombardment Ionization Chambers In an electron bombardment ionizer, the neutral gas is partially ionized by an auxiliary dC discharge between conveniently located electrodes. Of these, the anode is the same anode which receives the electrons from the ionization process(see Fig 1). The primary electrons responsible for the ionization of the neutral gas are generated at a separate cathode which can be a simple heated tungsten filament, or for longer endurance, a hollow cathode. The cathode-anode potential difference vo is selected in the vicinity of the peak in the ionization cross-section of the propellant gas, which occurs roughly between three and four times the ionization energy i e. around 30-50 Volts for most gases). The structure of the potential distribution in the sheath near the cathode, and the body of the plasma is nearly equipotential, at a the electron saturation level, and so an electron-retarding voltage drop develops Ionization is due both to the nearly mono-energetic primary electrons(with energies of the order of evD)and to the thermalized secondary electrons themselves. These have typically temperatures(Tm)of a few ev, so that only the high energy tail of the Maxwellian energy distribution is above the ionization energy and can contribute to the process but their number density greatly exceeds that of the primaries and both contributions are, in fact, of the same order. It is therefore desirable to maximize the residence time of both types of electrons in the chamber before they are eventually evacuated by the anode this is achieved by means of a suitable distribution of confining magnetic fields. Fig 's 6(a),(b )and (c)show three types of These will be discussed in more detail later, but we note here that magnetic e nce magnetic configurations, of which only the last two are today of practical importa strengths can vary from about 10 to 1000 Guass, depending on type and location 16.522, Space P pessan Lecture 13-14 Prof. Manuel martinez Page 9 of 2516.522, Space Propulsion Lecture 13-14 Prof. Manuel Martinez-Sanchez Page 9 of 25 since the high energy ions are very effective sputtering agents, results in a very destructive mode of operation. All the perveance values reported in Ref. (7), for instance, are impingement-limited, i.e., correspond to the highest current prior to onset of direct impingement. (b) Electron back-streaming: For R values near unity, the barrier offered by the accelerator negative potential to the neutralizer electrons becomes weak, and beyond some threshold value of R, electrons return up the accelerator potential to the chamber. This results in screen damage, space charge distortion, and shorting of the neutralizer supply. Kaufman(10a) gave the theoretical estimate max a a a 0.2 R =1- le t exp D D ⎛⎞ ⎛⎞ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ (16) which was confirmed experimentally in Ref. 7, except that it was found to be a somewhat conservative estimate. 4 Ion Production 4.1 Physical Process in Electron Bombardment Ionization Chambers In an electron bombardment ionizer, the neutral gas is partially ionized by an auxiliary DC discharge between conveniently located electrodes. Of these, the anode is the same anode which receives the electrons from the ionization process (see Fig. 1). The primary electrons responsible for the ionization of the neutral gas are generated at a separate cathode, which can be a simple heated tungsten filament, or for longer endurance, a hollow cathode. The cathode-anode potential difference VD is selected in the vicinity of the peak in the ionization cross-section of the propellant gas, which occurs roughly between three and four times the ionization energy (i.e., around 30-50 Volts for most gases). The structure of the potential distribution in the discharge is very unsymmetrical: most of the potential difference VD occurs in a thin sheath near the cathode, and the body of the plasma is nearly equipotential, at a level slightly above that of the anode (typically the anode current density is below the electron saturation level, and so an electron-retarding voltage drop develops). Ionization is due both to the nearly mono-energetic primary electrons (with energies of the order of eVD) and to the thermalized secondary electrons themselves. These have typically temperatures (Tm) of a few eV, so that only the high energy tail of the Maxwellian energy distribution is above the ionization energy and can contribute to the process, but their number density greatly exceeds that of the primaries, and both contributions are, in fact, of the same order. It is therefore desirable to maximize the residence time of both types of electrons in the chamber before they are eventually evacuated by the anode. This is achieved by means of a suitable distribution of confining magnetic fields. Fig.’s 6 (a), (b) and (c) show three types of magnetic configurations, of which only the last two are today of practical importance. These will be discussed in more detail later, but we note here that magnetic field strengths can vary from about 10 to 1000 Guass, depending on type and location