Fast computational techniques applied to numerical modeling of RFX-mod fusion device

Fusion devices are very challenging from the computatational electromagnetics point of view. First of all, the electromagnetic interaction of the fusion plasma with the surrounding conducting structures must be taken into account, making the problem inherently multiphysics. Moreover, the geometry of the condutors may be rather complex; indeed, the most prominent fusion devices ideally have a toroidal geometry, but several deviations from this ideal situation (holes, cuts, slits, ports etc.) are necessary, e.g. to make the interior of the machine accessible. It should be also considered that the evolution of the plasma may exhibit unstable modes of evolution, thus exacerbating the aforementioned problems and requiring a feedback controller. The design of such control system requires rather accurate response model of the overall system plasma+conductors. As a consequence, the numerical models of fusion devices may be rather demanding from the computational point of view; hence, it is often required to resort to fast and parallel techniques to make the computations affordable [1,2]. In this paper, we analyze the RFX-mod device [3], a medium size (major radius R = 2m, minor radius a = 0.46 m) toroidal device particularly suited to explore innovative concepts in plasma control by means of active coils. Passive and active conductors are hence very important to determine the overall properties and performances of such feedback system and therefore they should also be adequately represented in any realistic model. Figure 1 shows some details of the 3D mesh used. In particular, RFX-mod is equipped with a state-of-the-art control system made by 192 (4 poloidally x 48 toroidally) independently fed active coils (Fig. 1), with more than 600 magnetic sensors acquired in real time. We compute the input-output transfer functions of the system, assuming as input the current or the voltage of the active coils and as output suitable magnetic measures [4]. The presence of an axisymmetric plasma evolving through equilibrium states is correctly taken into account [1].