Dublin City University. School of Physical Sciences
Abstract
Electronegative plasmas have a wide ranging applications following its primary role in the production of energetic neutral beams of hydrogen used for plasma heating in fusion devices. In technological plasma applications electronegative gases are used for plasma etching which readily form stable negative ions. Significant population of negative ions in a discharge can greatly influence the discharge properties by reducing electron density while modifying the sheath structures adjacent to the substrates. Formation of negative ions is a complex process. They are primarily formed in the bulk phase by the dissociative attachment of low energy electron to an excited molecule. However intermediate excitation of the neutral molecules requires the presence of energetic electrons in the discharge. An efficient mechanism of negative ion production is via interaction with a low work function material such as Caesium coated surface as used in the negative ion source for neutral beam extraction. However, caesium migration between the acceleration grids can be a major concern for the failure of ion source during long pulse operation. Therefore suitably tailoring the plasma properties of the source can help in improving the efficiency of negative ion production in the bulk phase rather than surface production. The thesis deals with the study of oxygen negative ion formation in the anodic glow plasma. The anodic glow is characterized by a double layer having a steep gradient in Te and electron density. This region provides a natural favourable condition for the production of negative ions. The anodic glow is created via D.C discharge between a hollow tubular-anode in conjunction with parallel plate cathodes. The results show significant fraction of negative oxygen ions near the anodic glow. Theoretical estimates of negative ions at each spatial location are obtained by providing electron density and temperature measured using plasma diagnostic probes. The results show reasonable agreement with the experimental data suggesting that anodic glow can provide a suitable breeding ground for the production of negative ions. A qualitative model is presented for observing the oscillation in the discharge current due to ionization instability at the anode sheath
