Electron density profile diagnostics and plasma wall interaction characterization in fusion plasmas

In the framework of magnetically confined plasmas for fusion experiments, the electron density (ne) is a very important physical quantity to better understand the overall plasma behavior. Accurate measurements of this parameter are extremely important in the characterization of fusion plasmas, since ne is linked to many of the parameters that define the experimental limits and the objectives of the scenarios studied in the fusion research context. Moreover, a deep understanding of this physical quantity is needed for plasma control and machine protection. In magnetically confined plasmas, ne varies from 10^18 to 10^21 m−3 with different temporal and spatial characteristic scales (1 − 10 ms and 1 − 10 mm respectively) in the various areas of the plasma (core, edge, scrape-off-layer, divertor). For these reasons, a single diagnostic is not able to offer all the information needed. This thesis is a study of three different diagnostics, observing in three different plasma regions, that together offer a complete representation of the evolution of the electron density in fusion plasmas, both in time and space: the interferometer, the Thermal Helium Beam, and a visible camera system. The innermost region of the plasma in a toroidal device is the core: ne profiles in this section can be obtained using interferometric measurements. In this work, the two-color interferometer of RFX-mod has been upgraded, and the design of the diagnostic for the RFX-mod2 machine is presented. The new system will be equipped with a new, more stable laser, with wavelength λ = 1.55 nm. The optical system has been simulated with the purpose of optimizing it for optical losses and displacements. Moreover, an inversion algorithm of the line-integrated density based on the tomographic pixel technique has been implemented. The edge region of the plasma is characterized by fast turbulent phenomena (in the order of a few μs) and higher ne gradients; thus, the interferometer is not the right diagnostic to investigate this region. For this purpose, the Thermal Helium Beam (THB) was chosen: this diagnostic, installed at the TCV experiment, has proven to be a useful tool in a variety of plasma scenarios. The THB is characterized by fast acquisition (1 MHz) and good spatial resolution (4 mm). The diagnostic was installed, calibrated, and used for the evaluation of edge ne profiles, the detection of turbulent events and of rapid fluctuations of ne. Where the plasma encounters the first wall, the electron density is heavily influenced by the plasma-wall interactions. For the purpose of investigating them, the diagnostic chosen is a visible camera. In particular, a camera system composed of 7 cameras looking at the carbon emissions from the walls has been developed for the RFX-mod2 experiment. A mathematical description of the projective geometry governing the image reconstruction of the cameras has been studied. The system has been tested to characterize its capability in the identification of modes from the interactions of the plasma with the first wall. Some experimental values from RFX-mod have also been used to prove the ability of the system, defining the starting point for more detailed methods to detect the edge modes. The electron density profiles and their fluctuations have been the center of the investigation of some H-mode scenarios at TCV. The high confinement mode will be a fundamental regime for future fusion reactors, and is undergoing a vast study to better understand and control the abrupt releases of energy that often come with it, called Edge Localized Modes (ELMs). Two of the selected diagnostics (interferometer and THB), installed at the TCV tokamak, have been used to study ne in both Type-I ELMy scenarios and QCE, an H-mode that does not present ELMs. The fast fluctuations of density have been detected and characterized for a variety of parameters.