Coupling transport fluid codes in 1D and 2D

The modeling of diffusive transport between the core and the edge of a magnetically confined fusion plasma by means of independent codes that operate on different space and time scales is addressed. Two 1D codes are run simultaneously, one representing the edge and one representing the core. The open-quotes edge codeclose quotes is typically run with a finer space resolution than the open-quotes core-code,close quotes because of the short spatial scale introduced by rapid parallel loss to the divertor or limiter in the scrapeoff layer (modeled here by a simple loss term). The open-quotes code mergingclose quotes approach, already developed for electrostatic particle simulation is here extended to the solution of the diffusion equation, providing a full self-consistent simulation while preserving a substantial degree of independence of the codes. Different models have been tested for the density and heat transport equation. The original open-quotes code mergingclose quotes scheme is also applied to couple non-linear models solved via the Newton iteration method. In particular it is shown (analytically and numerically) that if the self-consistent solution scheme is applied at each iteration stage, then the exact solution, i.e. that which would have been found by solving the whole system in a single code, is obtained. Simulation examples are shown and compared with the most straightforward iterative approach for code coupling where the self-consistency is obtained by iterating the separate solutions to achieve continuity of the field variable and its derivative at the interface. A model is also presented in which a 2D transport code for the edge is coupled to a simple 1D transport code for the core plasma. In this case the 1D code provides, in a self-consistent fashion, essentially a time-varying boundary condition for the average density, temperatures, etc. on the inner boundary (core-edge interface) of the 2D domain