Numerical investigation of ion collection by a planar probe in stationary and parallel flowing plasmas

3D hybrid particle-in-cell simulations were performed to investigate the ion current collection behavior by a thin double-sided circular planar probe in both stationary and parallel flowing plasmas with cold ions and Boltzmann electrons. Results for stationary plasma cases showed that the assumption of a constant ion density ratio at the sheath boundary to that of unperturbed plasma is inaccurate for planar probes for probe radii from 10 to 30 Debye length and probe biases from -5 to -30 times the electron temperature. The flowing plasma results showed that ion current strongly depends on the parallel flowing velocity, contrary to earlier models. The collected ion current decreases with increasing parallel velocity, reaching a minimum at around ion sound speed, then slowly recovers and even exceeds the value of the stationary case at high flow speed. The deviation of ion current due to parallel flow from stationary case could be -35% to +20% for flowing plasmas with flow speed comparable to ion sound speed. Therefore, it is important to consider the flow impact on ion collection when using planar probes in such circumstances, e.g. electric propulsion plume measurements. The mechanism for this behavior is qualitatively investigated and discussed.