No abstract

Plasma polarimetry is a diagnostic technique used in nuclear fusion reactors to measure the line integral of some plasma parameters, such as the electron density and the magnetic field, and constrain, analyse and validate the equilibrium models. Despite the strong link between the plasma properties and light polarisation propagation, the interpretation of plasma polarimetry remains complex and sometimes uncertain. The type 1 approximation is the most common hypothesis used to link the polarisation effects, such as the Faraday rotation and the Cotton Mouton phase shift, with the plasma properties (electron density and magnetic fields). However, this approximation is valid only in specific conditions, which depend on both the plasma configuration and initial polarisation of the electromagnetic wave. Moreover, the uncertainty propagations of these measurements are affected by regions where their values are too high to guarantee accurate values, making the measurement unsuitable. The need to measure an unsteady physical environment, which goes from the condition of no plasma to the flat top of the discharge, makes the setting of the measurement more constrained. The aim of this work is to analyse, both analytically and numerically, the behaviours of the interpretability and uncertainty propagation of polarisation measurements, in order to give a detailed and the most general as possible description of these issues, ensuring an easier, more performant and reliable understanding and design of plasma polarimetry. The results will show that an input linear polarisation around 45° degree is the most suitable for plasma polarimetry when type 1 approximation is adopted and which the choice of the laser wavelength governs the performances of the polarimeter.

No abstract

To produce fusion reactions efficiently, thermonuclear plasmas have to reach extremely high temperatures, which is incompatible with their coming into contact with material surfaces. Confinement of plasmas using magnetic fields has progressed significantly in the last years, particularly in the tokamak configuration. Unfortunately, all tokamak devices, and particularly metallic ones, are plagued by catastrophic events called disruptions. Many disruptions are preceded by anomalies in the radiation patterns, particularly in ITER-relevant scenarios. These specific forms of radiation emission either directly cause or reveal the approaching collapse of the configuration. Detecting the localization of these radiation anomalies in real time requires an innovative and specific elaboration of bolometric measurements, confirmed by visible cameras and the inversion of sophisticated tomographic algorithms. The information derived from these measurements can be interpreted in terms of local power balances, which suggest a new quantity, the radiated power divided by the plasma internal energy, to determine the criticality of the plasma state. Combined with robust indicators of the temperature profile shape, the identified anomalous radiation patterns allow determination of the sequence of macroscopic events leading to disruptions. A systematic analysis of JET campaigns at high power in deuterium, full tritium, and DT, for a total of almost 2000 discharges, proves the effectiveness of the approach. The warning times are such that, depending on the radiation anomaly and the available actuators, the control system of future devices is expected to provide enough notice to enable deployment of effective prevention and avoidance strategies.