3D magnetic surfaces reconstruction in RFX-mod

The Reversed Field Pinch configurations are characterized by strong asymmetries [1]; in order to prevent or mitigate possible consequent instabilities, suitable control systems are required. In RFX-mod (Padua, Italy), such a system includes a number of 192 saddle coils, independently controlled, fully covering the toroidal surface and operating in a coordinate strategy. An equal number of saddle probes provides the control system with the signals needed to close the feedback loop. Of course, the effectiveness of the control action depends on the capability to identify the actual 3D behaviour of the plasma column and to detect possible plasma-wall interactions [2]. Therefore, the classical identification procedures based on the axisymmetric assumption can be unsatisfactory and new methodologies able to detect the main 3D plasma characteristics are required. The paper shows the results achieved in a purely electromagnetic analysis of the experimental RFX-mod shots. The methodology is based on the equivalent 3D representation of the plasma current by means of stream functions and an analytical description of 3D magnetic surfaces based on the 3D interpolation of magnetic field lines. An effective use of high-performance computing architectures [3] is able to strongly reduce the required computational burden. [1] Bolzonella, T., and D. Terranova. "Magnetic fluctuation spectra and non-linear MHD mode interaction in RFX." Plasma physics and controlled fusion 44.12 (2002): 2569. [2] Zanca, P., et al. "Plasma wall interactions in RFX-mod with virtual magnetic boundary." Journal of nuclear materials 363 (2007): 733-737. [3] Chiariello, A. G., et al. "Effectiveness in 3-D magnetic field evaluation of complex magnets." IEEE Trans. Mag 51.3 (2015).