Modelling of ECH modulation experiments in ASDEX Upgrade with an empirical critical temperature gradient length transport model

The relevance of the assumption of critical gradient length in electron heat transport is tested against the results of electron cyclotron heating (ECH) modulation experiments carried out on ASDEX Upgrade, using time-dependent transport simulations. A general χ e model based on a ∇ T e / T e threshold is proposed, which can provide two different heat pulse (HP) propagation regimes, with respect to the power balance (PB) diffusion coefficient χ e PB : χ e HP = χ e PB occurs if ∇ T e / T e <(∇ T e / T e ) c , while χ e HP >χ e PB if ∇ T e / T e ⩾(∇ T e / T e ) c . By a careful choice of the model parameters, the experimental T e response to ECH modulations is simulated with excellent accuracy, and the transition between the two propagation regimes is, without ambiguity, ascribed to a ∇ T e / T e threshold. Our findings are consistent with theoretical expectations, as well as with experimental observations made on TORE SUPRA. This global consistency shows the relevance of the main physical factors involved in our model, in particular the assumption of critical temperature gradient length.