Multigrid Method for Numerical Simulation of Faraday Probe Measurements

An axisymmetric hybrid electron fluid particle-in-cell computational method is used extensively to simulate Faraday probe measurements for a range of plasma conditions and probe techniques. A new multigrid (MG) method is developed and implemented to speed up the simulations and allow larger domains to be simulated in a practical amount of time. The first study considers how different flowing ion distributions affect the current measurements at a planar Faraday probe surface. The second study considers variations of the standard probe technique and includes varying a uniform bias voltage and varying the guard-ring bias relative to the collecting surface bias. These studies indicate that the standard Faraday probe technique is very robust, and measurements accurately reflect the ion current for a broad range of conditions. The new MG method obtains an overall speedup slightly better than a factor of two, enabling a study of a reversed Faraday probe configuration. This study indicates that the plasma flow behind a reversed probe has strong gradients and complex structure. It is recommended that the probe-body potential should be allowed to float, in order to minimize the focusing effects of sheaths along the side of the probe.