NUT: A fast 3-dimensional neutral transport code

We present a fast code for calculating the steady-state transport of neutral atoms in an axially symmetric background plasma. The primary source for the neutrals is not required to have any symmetry. Due to the small momentum transfer involved in charge exchange collisions, the secondary neutral atoms emerge with the local velocity distribution of the plasma ions. Some neutrals are lost due to ionization. The neutral transport is described by an integral equation for the neutral source. With a careful choice of a three-dimensional spatial grid, the small scale features of these equations can be integrated analytically to yield a set of algebraic equations that can be solved by only a few iterations. This results in a fast and compact algorithm which can be used as a subroutine in plasma simulation codes. Comparisons with other currently used codes and with experimental measurements show good agreement.