Numerical analysis of electronegative plasma near the extraction grid in negative ion sources

The effects of plasma electrode (PE) bias on the extraction process in a negative ion source are studied with a series of two-dimensional (2D) electrostatic particle simulations. Motion of charged particles in their self-consistent electric field is modeled by the Particle-In-Cell (PIC) method. The effect of a weak transverse magnetic field is also taken into account. Extracted electron current density gradually decreases as the PE bias increases, while the absolute value of PE net current density initially increases and is saturated for higher values of PE bias. The extracted negative ion current density reaches a weak peak when the PE bias approaches the bulk plasma potential (Vpf) in the case without the PE bias, but the decrease afterwards is mild. Physical mechanisms leading to these results are discussed.