On the use of critical gradient models in fusion plasma transport studies

Transport models for tokamak devices are often based on transport coefficients involving a critical threshold condition. In this paper, it is argued that the validation of such models against experimental data requires special care when the system profiles are close to this threshold (at some locations), due to the contribution of fluctuations to transport. The arguments presented here could have implications for the understanding and modeling of heat transport in tokamaks, since the large stiffness of the temperature profile observed in experimental points to a near-critical situation over much of the radius. The difficulties are illustrated by means of a simplified transport model, and a possible way to ameliorate this issue is proposed.