Nonthermal electrons and small-scale plasma perturbations during density limit disruptions in the T-10 tokamak

Full text: Small-scale quasi-coherent oscillations of the non-thermal x-ray emissivity are identified in high density plasmas in the T-10 tokamak with the use of a toroidal view x-ray imaging system and in-vessel hard x-ray detector. Small-scale x-ray oscillations are associated with forward bremsstrahlung emission of the suprathermal electrons (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 at 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 produced due to modulation of the electron beams moving though an equilibrium magnetic field with ripples during rotation of the m=2,n=1 mode. (author)