Measurements of electron energy distributions in a 14 cm diameter ring cusp ion thruster

This paper describes electron energy distributions measured in the discharge chamber of a 14 cm ring cusp xenon ion thruster. The electron energy distributions were acquired by processing Langmuir probe characteristics measured at more than twenty points in the discharge chamber. The results show that a narrow electron beam is emitted from the hollow cathode and flows along the center line of the discharge chamber. The energies of the beam electrons (or primary electrons) are far from mono-energetic and their density is higher than that of the thermalized or Maxwellian electrons near the hollow cathode. The electron beam abruptly spreads in the downstream region of the discharge chamber, however, a small fraction of these electrons have energies greater than that associated with the cathode-to-plasma potential difference and they are able to reach the screen grid which is held at cathode potential. Many ions, which appear to be produced in the electron beam, concentrate near the center line of the discharge chamber. 7 refs.