Non-axisymmetric perturbation of the plasma surface in RFX: analysis of magnetic data versus CCD images of plasma-wall interaction

In the RFX experiment, as in other reversed field pinches, the sustainment of the configuration in standard conditions entails the presence of a relatively wide spectrum of internally resonant tearing modes with saturated amplitude. Such modes cause a non-axisymmetric perturbation of the magnetic configuration. In this paper a detailed comparison of the perturbed magnetic surface with the footprint of enhanced plasma wall interaction evidenced by CCD camera images of the RFX graphite first wall is presented. The comparison confirms the accuracy of the magnetic reconstruction algorithm, which computes peak-to peak perturbation of the plasma surface as large as 5 cm in some cases and a contribution from m=0 modes which can be comparable to that of m=1 modes, particularly at low currents