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

Abstract

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 (∼0.5 THz) for the accessibility of both O and X-modes in the DTT plasma (electron density up to 1020 m−3 and magnetic field up to 9 T). To this aim, we adopt COMSOL Wave Optics® 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.

RF and microwave diagnostics for compact plasma traps and possible perspectives for fusion devices

“…The paper reports about the non-invasive microwave diagnostics, such as interferometric and polarimetric measurements, microwave imaging profilometric numerical technique, and special probes developed to characterize the plasma density and the external and/or self-generated radiowaves in the plasmas of ion sources. The gained experience and know-how is now permitting to extend the domain of applicability of such techniques to higher densities, namely fusion plasmas: in the very recent past, thanks to a collaboration with colleagues from ENEA aiming at the ICRH antenna design of the DTT project [14], we have started exploring the possibility of applying these techniques, in particular the 1D profilometry, to real fusion reactors [16].…”