Analysis of the small-scale plasma perturbations during density limit disruptions in the T-10 tokamak

Small-scale quasi-coherent oscillations of the x-ray emissivity and poloidal magnetic field perturbations are identified in high density plasmas in the T-10 tokamak with the use of a toroidal view x-ray imaging system, in-vessel hard x-ray detector and fast magnetic probes. Small-scale x-ray oscillations are associated with forward bremsstrahlung emission of the suprathermal electrons (Eγ ∼ 25–100 keV) from the plasma region where the m = 2, n = 1 magnetohydrodynamic (MHD) mode is localized. The oscillations are typically represented by a solitary high frequency (15–85 kHz) harmonic in the spectrum of perturbations accompanying MHD modes during density limit disruptions. During a disruption energy quench, the small-scale oscillations are often transformed to repetitive bursts of the nonthermal x-ray radiation localized in the outer part of the plasma. Analysis indicates a possible connection of the oscillations with nonthermal electron beams induced during magnetic reconnection around X-points of the m = 2, n = 1 magnetic islands. It is argued that the small-scale oscillations can be generated due to the modulation of the electron beams moving through an equilibrium magnetic field with ripples during rotation of the m = 2, n = 1 mode.