Spontaneous drop of the Li-containing flake as a trigger of ITB-event in plasmas of the T-10 tokamak during simultaneous co+counter injection of EC waves

Abstract

Various triggers of ITB-events (ITB-event is an abrupt and non-local reduction of the heat flux within a radius of ∼50% of the minor radius) have been previously found at the JT-60U, the T-10 tokamaks and the LHD stellarator. This short paper describes a new trigger of ITB-events discovered in the T-10 plasmas in experiments on simultaneous co+counter axial EC current drive performed using the X-wave with a frequency equal to the second ECR harmonic. In these experiments, the tungsten limiter and lithium wall coating were used. The ionization of atoms in the Li-containing flake, which occurs at the plasma edge, generally causes the rise of the electron temperature and density in the core of the plasma column. The electron heat flux reduces abruptly within the radial range of 0.2 < r/a < 0.8, just as it was at the LHD in experiments on the injection of small C8H8 pellets. The energy confinement time abruptly increases by ∼10%. After the transient phase, the ITB appears, which is located between the radii corresponding to the safety factors of q = 1 and q ≈ 1.4. Typically, the ITB lifetime is equal to the plasma energy lifetime. As far as we know, the triggering of ITB-events by the drop of Li-containing flake has been never reported in tokamaks and stellarators.