Negative Ion Generation and Isotopic Effect in Electron Cyclotron Resonance Plasma

Abstract

The impact of the thermoelectron emission upon the efficiency of negative hydrogen ion production in an electron cyclotron resonance source with driven plasma rings is studied. The obtained data demonstrate that the negative ion production is realized in two stages. At the first stage, the hydrogen and deuterium molecules are excited to vibrational states and high-lying Rydberg levels in collisions with the plasma electrons in the discharge volume. The second stage leads to the negative ion production through the process of dissociative attachment of low-energy electrons by the excited molecules. The low-energy electrons are originated due to a bombardment of the plasma electrode by ions of a driven plasma ring and the thermoemission from heated tungsten filaments. The experiments prove that the negative hydrogen ion generation occurs predominantly in a plasma electrode superficial layer filled with thermoelectrons. The negative ion generation through the vibrational excitation channel requires the isotope effect that appears due to the difference in the velocities of the vibrational movement of the nuclei in light and heavy molecular isotopes. From the experimental data for the negative ion generation rate, it follows that the main channel for $H^{-}$ and $D^{-}$ ion production involves the process of high Rydberg states excitation of $H_{2}$ and $D_{2}$ molecules.