Reflectometry Simulations on Different Methods to Extract Fusion Plasma Turbulence Characteristics and Its Dynamics

Abstract

AbstractThe knowledge of turbulent transport is the key issue for the future of fusion plasma devices. A first step towards this goal rests in obtaining measurements concerning turbulence characteristics or transient events, which are required for the understanding of the transport phenomena. A possible way to obtain density fluctuations parameters is to use microwaves to probe fusion plasmas. However the interpretation of the received signals requires a model for reaching the accurate evaluation of the wanted parameters. Simulations of electromagnetic wave propagation in fluctuating plasmas permit to identify the signature of expected events and to model them. In the present work, it is shown how simulations have permitted to exhibit the role of the resonances of the probing wave induced by turbulence and to explain part of the origin of phase jumps seen during reflectometer measurements. Multi‐scattering phenomena, taking into account wave trapping, can be simulated and show that the probing wave can continue to reach in average the cut‐off layer as it is the case for a quiet plasma. These results suggest that it is possible to extract information on the turbulence at density fluctuations levels higher than allowed by usual methods using the framework of Born approximation. The software developed for reflectometry simulations is also used to compute and explain the frequency spectrum associated to Alfven's cascades seen with fixed frequency reflectometers working without a cut‐off layer in the probed tokamak plasma. To finish, possible ways to extract the properties of a turbulent wavefront and its dynamics are explored for the ultra‐fast frequency sweep reflectometer and for a reflectometer working as a backscattering diagnostics at fixed frequency. The physical aspects and the technical requirements are presented for each studied case (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)