Topology during magnetic reconnection events in RFX-mod

This paper is devoted to the characterization of magnetohydrodynamics (MHD) tearing modes causing plasma–wall interaction (PWI) in the Reversed-Field eXperiment (RFX-mod) reversed-field pinch (RFP) device. We study an example of a magnetic reconnection event in a high plasma current discharge. The PWI is measured via a fast camera looking at the graphite-covered inner wall, showing two separated footprints of neutral carbon radiation. A first, simple analysis shows that the phase-locking of m=1 tearing modes is the principal cause of enhanced PWI, as it is well documented in literature. Many modes contribute to the phase-locking, actually more than those measured with the magnetic sensors. A more refined analysis is based on calculation of the Connection Length to the wall and of the loss time of Maxwellian ions via the Hamiltonian guiding center code Orbit. This analysis confirms the importance of the m=1 phase-locking as a loss channel of high-energy particles, which is the mechanism that dominates the PWI pattern, but an additional role of the m=0, n=7 mode is highlighted, which is a new result for the RFP. The PWI mediated by the m=0 islands is milder, which is a good outlook for the RFX-mod2 upgraded device, currently in the assembly phase.