The interaction of neutral atoms with plasma ions and electrons in the edge plasma has been found to have an important effect on density limits, the L-H transition, plasma fueling and other phenomena. Thus, accurate calculation of the transport of neutral atoms and molecules recycling from the wall or divertor or injected into the plasma chamber is essential in many analyses. An accurate and computationally efficient “transmission/escape probabilities” (TEP) method, adapted from the interface current balance and escape probabilities methodology of neutron transport theory, has been developed in the Georgia Tech Fusion Research Center as an alternative to the time consuming Monte Carlo methods currently available. A 2D neutral particle transport code  based on the TEP method, GTNEUT, has been developed and benchmarked against experiment and Monte Carlo calculations and is available from NTCC Modules Library.
Friis, Z.W., “A Semi-Automated Procedure for Creating Geometry and Background Plasma Input Files for the GTNEUT 2D Neutral Particle Transport Code Using the UEDGE Plasma Edge Code”, Internal Publication (2009).
D-K. Zhang, J. Mandrekas and W. M. Stacey, “ Higher order approximations of the transmission and escape probability (TEP) method for neutral particle transport in edge plasmas”, Phys. Plasmas, 13, 062509 (2006).
W. M. Stacey, J. Mandrekas and R. Rubilar, “Interface Current Integral Transport Methods for the Calculation of Neutral Atom Transport in the Edge Region of Fusion Plasmas”, Fusion Sci. Technol. 40, 66 (2001).