The detection of high frequency Alfven eigenmodes in ohmic discharges on spherical tokamak Globus-M2

In Globus-M2 ohmic discharges with low density, by means of Mirnov coils array, magnetic field oscillations with frequencies in 1 MHz range were detected. Frequency range of these oscillations significantly exceed the range of TAE and RSAE frequencies, which were previously observed on Globus-M and Globus-M2 tokamaks, and their amplitude, contrary, turned out to be up to an order of magnitude lower. It was found that high frequency oscillations are interrelated with suprathermal electron fraction. At the same time the observed instability seems to have Alfvenic nature, since its frequency correlates well with Alfven frequency scaling. It was also found that magnetic perturbation always forms standing wave with predominantly low toroidal wavenumbers, including $n$ = 0 structure, which makes gap (e.g. TAE) mode excitation impossible. Frequency chirping during single bursts with $\delta\omega\sim\sqrt(t)$ is consistent with hole-clump model predictions.