A plasma diffusion model is established to determine the optimal bias and sheath patterns in a positively biased magnetic filter of a metal arc plasma source. We determine the equation for the optimal bias on the magnetic filter. According to our model, the optimal bias is related to the electron speed, ion speed, ion mass, ion charge state, and plasma density in the filter. The optimal bias increases as these variables are increased with the exception of the electron speed. Even though the magnetic field is taken into account, it is not a variable in the final equation. Our experimental results confirm that the magnetic field has almost no influence on the optimal bias. An alternate design approach is suggested that should lead to enhanced plasma transport through the filter.