Experimental and numerical performance assessment of the Plasma Position Reflectometer of RFX-mod2

The thesis work aims at assessing the performance of the RFX-mod2 Plasma Position Reflectometry (PPR) system through both experimental and numerical procedures. Frequency Modulated Continuous Wave (FMCW) reflectometry is a well-established diagnostic technique that enables the measurement of the plasma electron density profile by exploiting its reflective properties on probing electromagnetic waves. Similar to Density Profile Reflectometry (DPR), which relies on the full reconstruction of the density profile, PPR aims to probe only its outer portion along various lines of sight (LoS) to provide accurate real-time control of the plasma boundary inside the vessel. This technique, currently under development, is crucial for monitoring plasma magnetic confinement and is being considered for future applications in large fusion devices. RFX-mod2 will benefit from an advanced PPR system involving four LoS. The upper, lower, and low-field side (LFS) are equipped with standard reflectometric set-ups, including pyramidal horn antennas and linear waveguides. The development and integration of a reflectometric system for the high-field side (HFS) that can accurately measure the nanosecond-scale delay shifts of reflected waves and reconstruct the position within a 1-cm target accuracy are the crucial focuses of the present discussion. A complex system of hoghorn antennas and bent waveguides is required to meet the severe spatial constraints imposed by the absence of a diagnostic port in this section of the machine. At the same time, it is necessary to avoid power losses and internal reflections as much as possible. The purpose of the thesis is twofold: in the first part, the performance assessment of the HFS LoS antennas and waveguides is experimentally carried out by exploiting a simple plane metallic mirror configuration as a plasma surrogate. From the corresponding group delay measurements, the actual effects of antennas and waveguides on the propagation of microwaves are investigated, and the experimental results are compared with expectations. The second part, instead, is dedicated to a numerical performance assessment of the same subsystem. The wave propagation is simulated through the Finite-Difference Time-Domain (FDTD) REFMUL3 code to investigate the effects introduced by the RFX-mod2 geometry and to obtain a comprehensive description of the physics that occur within. Initially, the diagnostic performance is assessed by replicating the mirror set-up, whose results are then compared with the experimental ones. Finally, to investigate the system’s capability of measuring the RFX-mod2 edge density profile, a set of expected RFP plasmas has been included in the simulations. As a main result, a good accordance between experimental and numerical mirror reconstruction capabilities is found, featuring a millimetre-scale accuracy well below the acceptable 1-cm one and demonstrating the correct integration of the diagnostic into the simulated system, as well as the overall validity of the current approach.