Full-Wave simulations of the enhanced Upper-Hybrid Resonance Scattering
Résumé
Introduction:The aim of this work is to improve the simple linear interpretative model usually applied to extract
turbulence characteristics of tokamak plasmas using data of Upper-Hybrid Resonant Scattering
(UHRS) experiments [1]. To evaluate the limits of such model, full-wave simulations have to be
performed. These computations are complex due to the fact that the spatial scales change a lot
especially as the wave propagates in vicinity of the resonance. Close to the resonance the mode
conversion should be taken into account, which requires considering both components of the wave
electric field together with the thermal effects. The amplitude of the electromagnetic component Ey,
remains finite overall the propagative zone, which can be used in computational scheme. However
the group velocity decreases drastically and can reach in principle a velocity close to zero as it was
shown numerically [2] and experimentally [3-4]. This cold plasma approximation is inapplicable for
our problem, and the thermal effects have to be included to describe the wave conversion from
extraordinary mode (X-mode) to a warm plasma mode as mentioned in [1]. However the possible
mode conversion described by the warm plasma model used corresponds to a warm wave
propagating in an opposite direction to the one expected in tokamaks for the probing frequency
close to the fundamental electron cyclotron harmonic due to constraints of the numerical scheme.
Assuming that the probing wave propagates along the resonance cone which assumes to have a
parallel wavenumber, ignored here, giving more or less a propagation at constant density, so 1Dstudies
on the Doppler shift induced by moving structures near the resonance were performed to
evaluate if the effective Doppler shift obtained is directly connected to the velocity of the scattering
structure. The scattering efficiency above the limitations of the [1] model is also provided for
different cases. To finish an estimation of the parameters required to simulate realistic cases is
provided and corresponds to the existing devices such as FT2, WEST, and what could be expected
from the full-wave simulation to interpret more accurately the UHRS diagnostic measurements.
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