As default mesh use 5 mesh lines on sigma in vacuum and “NStepsInConductive=10” in metal. It means that the skin depth in the metal will be meshed with 10 mesh lines. If you increase the mesh density in vacuum by factor 2 (10 mesh lines on sigma) you should simultaneously increase by the same factor “NStepsInConductive” in metal: “NStepsInConductive=20”. It means that the calculation depth in the metal remains the same. For 20 mesh lines on sigma use ”NStepsInConductive=40” and so on.  In this case the calculation domain remains the same and the wake converges.