Confinement loss during dynamo relaxation event in RFX-mod

The magnetic configuration reversed field pinch is sustained by the MHD dynamo process that provides the poloidal plasma current needed for the field reversal. In the virtual shell discharges of the RFX-mod experiment the dynamo process occurs in a discrete way, during the so-called dynamo relaxation events (DREs). In this paper the dynamo effect on the confinement is studied through the electron density behaviour during the DRE. The analyses show that the occurrence of a DRE is accompanied by a local, non-axisymmetric loss of particle confinement. Such local loss occurs at the toroidal angle lock where the MHD tearing modes lock in phase between themselves and at the wall. The analyses performed with a field line tracing code show that nearby lock several magnetic lines lying in the plasma core punch through the confinement barrier located in the proximity of the field reversal and directly connect the plasma core to the wall. The formation of this path towards the wall agrees with the local particle loss observed experimentally. © 2008 IOP Publishing Ltd.