Small- scale modes in T-10 tokamak

Smallnscale plasma perturbations and localised x-ray bursts are identified in T-10 tokamak with the use of toroidally viewing x-ray imaging system, in- vessel hard x-ray detector, and fast magnetic probes. Non-thermal x-ray spikes placed around X-points of the m=2 and m=1 magnetic islands are observed during the energy quench at density limit disruption and sawtooth crash. Analysis of the plasma perturbations indicates possible role of the supra-thermal electrons in the relaxation events. INTRODUCTION Abrupt trigger of the sawtooth crash and energy quench during major disruption (see (1,2)) indicates that the relaxation events can not be described self- consistently in term of large-scale MHD modes with low poloidal wave numbers (m=1-4). Moreover, extended theoretical studies (see references in (2)) indicted that low-m MHD modes in some conditions can be accompanied by small-scale perturbations characterised by high-m wave numbers (m=5-30). One of the possible diagnostic for identification of the high-m modes is based on idea of the soft x-ray imaging system viewing in toroidal direction (see (3)). Integration of the soft x-ray intensity perturbations is provided in the case along the lines of sight of detector tangential to the helical mode. Calculations indicated that tangential measurements can provide considerably higher resolution of helical modes than in the case of conventional x-ray arrays (4,5). Installation of the toroidally viewing x-ray imaging system in T-10 (5,6) allows identification of the small-scale modes in a variety of ohmically and auxiliary heated plasma.