Benchmark of Orbit and NEMATO codes on magnetic topology reconstruction in RFPs

\textsc{Orbit} is a Hamiltonian guiding center code which describes test-particle motion in an electromagnetic field.\footnote{R. B. White and M. S. Chance, Phys. Fluids B \textbf{27} (1984) 2455} In the limit $\rho_{\parallel} \rightarrow 0$, $\rho_{\parallel} = v_{\parallel}/B$ it can be used to trace the magnetic field topology, in a way in all respects similar to simplectic codes. \textsc{NEMATO}\footnote{J. M. Finn, L. Chac\'on, Phys. of Plasma \textbf{12} (2005) 054503} is a field-line tracing code, implemented to integrate solenoidal flows for incompressible fluid dynamics, with automatic volume preservation. In a practical application, the two codes have been used to study the structure of the $q = 0$ island chain which characterizes the RFP edge and its behavior as a function of the reversal parameter $F=B_ {\phi}(a)/\langle B_{\phi} \rangle$. As input for both codes we used the snapshot of a 3D nonlinear MHD visco-resistive simulation (SpeCyl code). The first benchmarking test employs a Hamiltonian (single-mode) magnetic field configuration. Both codes successfully yield field lines which follow flux surfaces in both the $m = 1$ and $m = 0$ cases. The comparison between the codes has been successfully extended to a chaotic magnetic field configuration, including many modes.