Magnetic configuration at the edge of a stellarator is very complicated and contains a mixture of closed flux surfaces and stochastic field lines. At present there is no complete plasma transport code that fully accounts for all of the features of this highly complex case. It is even not very clear what kind of set of transport equations can be used for these purposes. At the same time, it is feasible that due to a strong anomalous cross-field plasma transport and convective plasma flows, a rather detailed features of stellarator magnetic topology do not matter much for averaged plasma parameters. Therefore it is worth to try a simple approach to the modeling of stellarator edge plasma based on "averaging" of edge plasma parameters along the magnetic axis and introducing effective two-dimensional flux surfaces. In this sense, we substitute the stellarator edge with that what can be called stellarator-equivalent tokamak edge.
We apply this approach to edge plasma modeling for Large Helical Device [1] as a sample case. The LHD stellarator-equivalent tokamak is simulated with 2-dimensional multi-fluid transport code UEDGE [2,3]. We consider the possible ways of construction of effective magnetic flux surface configuration that represent major features of a stellarator edge region as well as the usage of this configuration in UEDGE. We discuss the cross-field transport model adequate to ergodic edge plasma. We also discuss differences between a conventional tokamak case and the stellarator-equivalent tokamak modeling. We report some results of UEDGE modeling and their comparison with experimental data for typical LHD shot.

References