Double layer formation and its possible effects on plasma transport across the transverse magnetic filter in the context of negative ion sources*

In low temperature plasma based negative ion sources, plasma generated in the driver or source region extends in the expansion region and subsequently flows through the transverse magnetic filter field zone before encountering the plasma grid of the beam extracting system biased at a potential that is adjustable. Our 2D-3v PIC-MCC simulation of such a system, using the physical parameters of ROBIN ion source plasma during the volume mode operation when  production of negative ions are very small and neglected in the simulation. reveal that the presence of inhomogeneous magnetic and electric fields leads to the asymmetry in the plasma parameters (plasma density, plasma temperature, plasma potential) [1]. By changing the biasing voltage of the plasma grid or magnetic field value, it is possible to alter the profiles of the plasma parameters near the filter magnetic field.  The gradients in plasma parameters and associated different drifts and several collisional processes result in fluctuations or instabilities in the plasma. These instabilities are responsible for anomalous plasma transport across the transverse magnetic filter field. Different types of instabilities having different frequencies are due to density gradient, potential gradient and temperature gradient. In the simulation, instabilities are observed precisely at the location where an abrupt potential drop (double layer) occurs near the magnetic filter region under certain specific conditions. Further investigations show that the double-layer leads to ion acceleration also near the magnetic filter region in addition to ion trapping within the instability wavelengths. A systematic parametric study with different values of magnetic filter field and the bias voltage applied on the plasma grid is carried out and reported in the presentation. [1] Fusion Engineering and Design, Volume 151, February 2020, 111402.