Dynamics of ultralow-q plasmas in the RFX-mod device

The results are presented of an experimental activity performed in the RFX-mod device aimed at characterizing plasma dynamics in the so-called Ultralow-q (ULq) magnetic configuration, which corresponds to edge safety factor values below 1. The role of the edge safety factor in determining plasma dynamics is studied. In particular, a characterization of MHD activity is performed. The results of dedicated non-linear 3D visco-resistive MHD simulations are in good quantitative agreement with the experimental observations. In particular, the predicted tendency for ULq plasmas to be characterized by magnetic spectra dominated by a single mode (either a kink or a double resonant internal mode) is confirmed by experiment. Magnetic reconnection plays a relevant role in determining the dynamics of the magnetic topology. Both almost quiescent and largely fluctuating plasmas are observed with a strong sensitivity on the edge safety factor. The main MHD properties of the ULq are compared to those of RFP and tokamak discharges, also produced in the RFX-mod device. MHD modes exhibit toroidal rotation at a frequency depending on mode amplitude. Differently from what encountered in RFP plasmas at comparable current levels, no wall locking is detected.

Automated summary

This study presents the results of an experimental activity conducted in the RFX-mod device to characterize plasma dynamics in the Ultralow-q magnetic configuration with an edge safety factor below 1. The role of the edge safety factor in determining plasma dynamics is studied, and the results are compared to nonlinear 3D visco-resistive MHD simulations, showing good quantitative agreement. Magnetic reconnection and the presence of a single dominant mode in the magnetic spectra of ULq plasmas are confirmed by the experiment.