First observations of Alfvén cascades on the Globus-M2 tokamak and their application for minimal safety factor value analysis

On the spherical tokamak Globus-M2 in discharges with toroidal magnetic field of 0.7 T by means of magnetic probes and a number of other diagnostics during neutral beam injection at the current ramp up stage, magnetic field oscillations in the range of 100 – 300 kHz having their frequency increasing in time, had been observed. Eigenmodes with wavenumbers $n$ = 1 – 3 and $m$ = 2 – 4 were being registered. These oscillations were identified as Alfvén cascades. By means of multi-channel fluctuation reflectometer, observed eigenmodes were found to be localized near the magnetic shear reversal radius. Application of MHD-spectroscopy technique allowed us to determine safety factor temporal evolution and experimental values are in well agreement with modelling results, provided by ASTRA transport code.