Observation of double layer formation in low-temperature E x B plasma based negative ion sources

In this paper, formation of a double layer (DL) is studied in the background plasma (considering only electrons and ions) in the presence of a magnetic field in the context of the Rf operated beam source in India for negative ion research negative ion source by using 2D-3V particle-in-cell Monte Carlo collision kinetic simulations. A rectangular geometry to model the source, expansion, and magnetic filter regions in the negative ion source is considered in the XY plane. A Gaussian shaped magnetic field is applied in the Z-direction (perpendicular to the simulation plane), and a bias voltage is applied on the extraction boundary. Our simulations show the presence of instabilities as reported in the literature. We confirm the formation of a weak DL in the magnetic filter region under some specific combinations of the magnetic field and bias voltage. Initiation of DL leads to ion acceleration in the magnetic filter region. Split in ion velocities is also visible in the phase space plots due to the presence of free and trapped ions in the DL region. Our study highlights the effect of DL on plasma transport, which should be taken into account when interpreting the experimental observations associated with E x B plasma based negative ion sources.