Installation of disruption instability in the Uragan-2 large-shear stellarator

Instability of a single (small) disruption under weak electric fields E < Ecr and small currents [I/H] = 0.37 with strong plasma magnetization ωce > ωpe is studied in Uragan-2 racetrack stellarator in quasistationary ohmic discharges. Instability was observed at the stage of slow current decay under rather low MHD-activity of plasma. Disruption was accompained by typical perturbations of current magnetic field (m = 1, n = 0) corresponding to reconstruction of stellarator magnetic configuration and relaxation of all plasma parameters as well as plasma heating and decrease of the filament inductance. Disruption is not due to relaxation of fan-type instability. Possibility of reinitiation of small disruption plasma current being maintained near the threshold value is shown. It is most probable that disruption instability mechanism is due to loss of filament equilibrium rested on resonance surface tΣ = 1.0 or is due to loss of local equilibrium near this surface facing the plasma filament boundary. Further evolution of perturbation has many common features with the model of internal disruption instability in a tokamak suggested by Kadomtsev. 13 refs.; 7 figs