Influence of magnetic filter and magnetic cage in negative ion production in helicon oxygen plasma

Negative ion rich oxygen plasma at low pressure is produced in a Helicon Plasma Source setup, which is primarily designed to perform electronegative gas plasma experiments including the studies of ion-ion plasma. The negative ion fraction and hence the negative ion density are obtained by using a two probe technique in which the electron current is obtained by using an RF compensated cylindrical Langmuir probe and positive ion saturation current is obtained by using an RF compensated planar probe. By measuring the negative ion fraction, both with and without a magnetic filter, the importance of the magnetic filter field in the production of negative ions in oxygen plasma is investigated. The maximum value of negative ion fraction α (n−/ne) is calculated to be approximately around 9 when the value of temperature ratio γ (Te/T−) is taken as 10. The observed negative ion fraction and other plasma parameters are explained by considering the set of reactions that are involved in the production and loss of negative ions.