My ongoing work inside the ITER project

ITER is a large-scale scientific experiment that aims to demonstrate that it is possible to produce commercial energy from fusion.

Transport barriers in fusion grade plasma obtained in magnetic confinement devices remain an area of active research. Indeed these barriers are key confinement structures during all the phases of ITER operation, both during the initial phase with an Edge Transport Barrier (or H-mode) to achieve a significant energy amplification factor, and during the second stage with the possible build-up of Internal Transport Barriers to achieve steady-state operation. Our work address this issue from the point of view of stochastic processes that control the interplay of the bursts with the transport barriers. One is then led to identify the key features that govern this interplay, then frame them as the result of stochastic processes.

Interplay of turbulence bursts and transport barriers
analyzed in term of combined stochastic processes

E. Floriani1 , G. Ciraolo2,3 , Ph. Ghendrih4 , R. Lima5,6 , Y. Sarazin4

1) Aix-Marseille Univ, Centre de Physique Théorique, 13009 Marseille, France.
2) Centrale Marseille, 13451 Marseille, France.
3) Aix Marseille Univ, Mécanique, Modélisation et Procédés Propres (M2P2) - 13451 Marseille, France.
4) CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France.
5) Dream & Science Factory, Marseille, France.
6) Institute for Complexity Sciences (ICC), Lisbon, Portugal.

(the pdf version of the article is here)

Abstract.

The interplay of large bursts of turbulent transport or blobs with regions of vanishing turbulence is a complex system that can eventually generate transport barriers when the majority of blobs are trapped in these weak turbulence regions. We present 2 models where stochastic processes are combined to recover the physics of this interaction. The main stochastic variables are the barrierwidth and the spreading distance of the blobs within the barrier together with their level of correlation.

We predict that for a class of Probability Distribution Function of these stochastic variables, the PDF of the escaping blobs will exhibit heavy tails, either exponential for a leaky barrier, or with power laws, for a tight barrier. Two-dimensional nonlinear fluid simulations of edge turbulence in tokamak plasmas bearing analogy with Rayleigh-Benard turbulence in neutral fluids – are used to supporte these stochastic models. The PDF of the blob penetration into the barrier is estimated as well as that of the barrier width for two different barriers generated in the plasma boundary layer. One can show that in the case of a barrier generated by external biasing – leading to an external radial electric field shear – the stochastic model predicts a leaky barrier with an exponential PDF of escaping blobs in agreement with the simulation data.