Three-dimensional simulation of gas conductance measurement experiments on Alcator C-Mod

Three-dimensional Monte Carlo neutral transport simulations of gas flow through the Alcator C-Mod sub-divertor yield conductances comparable to those found in dedicated experiments. All are significantly smaller than the conductance found with the previously used axisymmetric geometry. A benchmarking exercise of the code against known conductance values for gas flow through a simple pipe provides a physical basis for interpreting the comparison of the three-dimensional and experimental C-Mod conductances.