Ion energy measurements in steady state discharges

The energy distribution of ions incident on the grounded surface (cathode) of a direct current (DC) glow discharge is measured. This incident energy determines the effectiveness of glow discharge cleaning in removing gases and impurities from the vessel surface. We have found that the incident ion energy falls significantly below the anode potential when the mean free path for charge exchange is less than the width of the cathode sheath which is approximately the Child-Langmuir sheath width. The DC discharge provides a current density at the grounded surface of typically 17 micro-Amps/cm2. A gas mix of deuterium and argon at pressures up to 40 mTorr is ignited to form the plasma. A mass and energy analyzer measures the energy distribution of the ion stream which passes through an aperture in the center of the electrically grounded surface