Topic/Type: 1.2 Fusion Plasmas (magnetic & inertial confinement), Poster

A numerical study of forward- and back-scattering signatures on Doppler reflectometry signals

F. da Silva1, S. Heuraux2, E. Gusakov3

1 IPFN, Instituto Superior T?cnico, Lisboa, Portugal
2 IJL Nancy-Universit?, Vandoeuvre, France
3 IOFFE Physico-Technical Institute,St Petersburg, Russia

An important development of reflectometry diagnostics is Doppler reflectometry, a technique able to convey information on the plasma turbulence rotation from the spectral shift of the return scattered signal and if applied to large size devices such as JET or JT 60 bring forth ITER relevant knowledge. In small and mid-sized devices scattering is accepted to be caused by Bragg backscattering in the vicinity of the oblique cutoff. Nevertheless, under high turbulence conditions or long plasma paths as occurs in larger machines such as JET or ITER, a distinct process of cumulative forward-scattering can appear, contributing to the Doppler frequency shift of the signal and spread-out the probing beam reducing the wavenumber resolution. In this work a numerical study of both processes is undertaken using a finite-difference time-domain (FDTD) full-wave code. The spectra modeling the plasma turbulence were chosen to isolate forward- and backscattering events allowing to segregate the two different signatures. The numerical constraints needed to simulate forward scattering, more demanding than those needed for backscattering, are also underlined. Simulations have brought forth the different mechanisms intervening. Forward scattering appears as a process of intermittent nature resulting from multiple scattering contributions while Bragg back-scattering has a fast and more continuous nature resulting essentially from a single scattering event in the vicinity of the oblique cutoff.