Neon radiation efficiency for different confinement regimes in TEXTOR-94

For neon seeded discharges the quantity [ΔP_rad/ΔZ_eff(0)]/n̄_e0², which depends linearly on the effective neon cooling rates and which can be taken as a measure of the radiation efficiency of neon, is not a constant for a given machine. When, according to the mode of operation, [ΔP_rad/ΔZ_eff(0)]/n̄_e0² is high, radiative power exhaust can be efficiently achieved at low Z_eff. The value of this ratio, which we refer to as the quality of neon cooling, is found to be correlated not only with the plasma edge temperature but also, and even more strongly, with the confinement properties of the discharges. Two different high confinement radiative regimes in TEXTOR-94 have been compared: radiative improved (RI) mode (highest confinement) and gas puff/pure radiofrequency mode (slightly lower confinement). Although the electron temperature profile is similar in the two regimes, due to feedback control of the input power level, it has been found experimentally that the quality of neon cooling is lower for the RI mode than for the gas puff/pure radiofrequency one. Similar behaviour has been observed for the quality of cooling of the residual intrinsic carbon, determined with independent diagnostics and methods. Simulations with the self-consistent RITM code confirm that particle transport, in particular the perpendicular diffusivity at the edge, is a key parameter to determine the level of the power radiated at the edge for a given impurity concentration in the plasma core.