Preliminary evaluation of inverse scattering-based plasma-profilometry application to fusion plasmas

A new diagnostic method able to perform plasma profilometry is investigated in this work to retrieve the plasma electron density profile in front of the Ion Cyclotron Range of Frequencies (ICRF) antennas. As a reference scenario for our numerical study, the Divertor Tokamak Test (DTT [1]) ICRF antennas and plasma will be considered. Specifically, the profilometry needs to solve an inverse scattering problem, which is non-linear and ill-posed. In some recent papers [2, 3], plasma imaging profilometry in compact plasma reactors, such as the electron cyclotron ion sources (ECRIS), has been proposed and tested numerically by means of proof-of-concept examples. In particular, proper formulation of electromagnetic inverse scattering techniques have been proposed, requiring measurements of the reflection coefficient in presence of a metallic reflecting surface or, alternatively, both the reflection and transmission coefficients through two antennas facing one each other. In this paper, we would like to investigate this method also for large-size (scale-length) fusion reactors by addressing the profilometry of DTT-like plasma, assuming a very high-frequency probing regime (similar to 0.5 THz) for the accessibility of both O and X-modes in the DTT plasma (electron density up to 10(20) m(-3) and magnetic field up to 9 T). To this aim, we adopt COMSOL Wave Optics (R) module based on beam propagation method (or slowly varying envelope approximation) for simulating the wave propagation and to determine the probing frequencies. A proper formulation allowed to reconstruct a tiny scrape-off plasma layer, approximated as an isotropic medium, thanks to the high probing frequency.