Edge transport properties of RFX-mod approaching the Greenwald density Limit

As in Tokamaks RFX-mod discharges are subject to the Greenwald density limit. The similarity between Reversed Field Pinches (RFP) and Tokamaks extends to many properties of the edge transport physics, including the electrostatic nature of turbulence, the presence of highly sheared ExB flows - sustained by the Reynolds’ stress - and the presence of coherent structures emerging out of the turbulent background. The origin of the density limit in RFX-mod is sought along the legitimate idea that it should be similar to the Tokamak case. It is proposed that the poloidally symmetric and toroidally asymmetric strong radiation emission that develops where the plasma interacts with the wall, at high densities triggers a thermal instability that cools the plasma core. The process resembles the evolution of a MARFE in a Tokamak, but in the RFP the effect on the plasma core occurs well before the total radiated power approaches the (ohmic) power input and the result is a soft landing of the discharge.