Interfacing \textsc{vacuum} to \textsc{m3d-c1} and other nonlinear codes

In order to interface the linear 2-D (in equilibrium) \textsc {vacuum} code to nonlinear 3-D codes a buffer zone is assumed which separates the fully nonlinear region from the intrinsically linear vacuum region. Within the buffer zone the plasma can: 1) gradually transition radially to a vacuum-like (e.g., high resistive) virtual layer where it matches directly to the outer fields calculated by the \textsc{vacuum} code, or 2) abruptly transition through a thin resistive layer beyond which the fields are again calculated by the \textsc{vacuum} code. The latter solves for the magnetic scalar potential response to the normal field at the layer, which in both cases is assumed continuous. In case 1) the tangential fields are also continuous, but in case 2) the apropriate dicontinuities of the tangential fields across the resistive shell are accounted for. In the outer vacuum region the fields satisfy the outer boundary conditions and is Fourier analyzed in the toroidal angle $\phi$. Although the method is not restiicted to this, an example is presented where the scalar potential of the \textsc{vacuum} code matches on to the magnetic field decomposition used in the nonlinear \textsc{m3d-c1} code currently under development.