ITB Formation During Slow Electron and Ion Heat Pulse Propagation in Tokamaks

Understanding of properties of internal transport barrier (ITB) is of importance for the fusion research of toroidal magnetic confinement. In T-10, ITB has been recognized by means of analyses of heat pulse propagation (HPP) induced by central ECRH-onset and cold pulse propagation (CPP) by off-axis ECRH cut-off in a sawtooth-free plasma created by off-axis ECRH. The cold pulses propagate slowly and diffusively with dynamic electron heat diffusivity χe HP ~ 0.1 m/s. It has been known for many years, that in L-mode, χ e HP ≈ 2-4 χe PB (so called “enhanced” HPP). At present, this relationship is usually explained by “critical gradient model”. In the present report, we focus on the fully opposite cases with χ e HP < χe . For the first time, this case was found at T-10 in 1990 and called “self-deceleration of heat wave”, or, in contrast with L-mode, “reduced” or “slow” HPP. Non-local confinement bifurcations (jump of core transport at ~0.3-0.4r/a region inside and around ITB in a ms timescale) were found in various JT-60U normal and RS plasmas and called ITB-events. Later, ITBevents were found in T-10 plasmas. At the same time, HPP is diffusive between the non-local bifurcations of the transport. Slow outward electron and ion HPP ((χe, i HP ~ 0.1 m/s << χe,i ) induced by ITB-event was observed in JT-60U. The new method allows reconstructing a gradual reduction of χe,i (r,t) during slow electron and ion HPP in the situations described above. The values of χe,i remain higher compare with χe, i HP values. In T-10, the dynamic of ITB formation is similar with ECRH at first harmonic (1990) ant the second one. In T-10, the reconstructed decay of χe fits well the value of χe at the end of CPP (obtained independently from the power balance). level The of fluctuations measured by reflectometer falls below the Ohmic level in shots with slow CPP in T-10. In J-60U, the gradual decay of hi-e,i during HPP is accompanied by the rise of the radial electric field Er shear calculated with neoclassical velocity of the poloidal rotation. In contrast to this, Er just begins to vary slowly after ITB-event.