On the interpretability and uncertainty propagation of polarimetric measurements in thermonuclear plasmas as a function of the input polarisation and laser wavelength

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.

Possibility of measuring the plasma main ion mass by plasma polarimetry in tokamak similarity experiments

“…(3.4) and (3.5) in this section. The first order solution of the Stokes equations, called Type I approximation [6][7][8], can be applied for small polarimetry effects. In this case the Faraday rotation angle (𝜓) and Cotton-Mouton ellipticity(𝜀) can be expressed by the following formulas:…”

“…One can ask which wavelength must be used in a tokamak device with different major radius, taking fixed the polarimetry dynamics (this means that the Type I approximation is valid also changing the major radius, see ref. [8] for a detailed discussion on the validity of Type I approximation), for example the same values of FR and CM. The eqs.…”

“…The Polarimetry quantities like the Faraday Rotation (FR) and Cotton-Mouton (CM) ellipticity are depending on the plasma density and current: this means that changing ion mass in plasmas with the same set of dimensionless quantities, both FR and CM must follow the scaling laws of the engineering dimensional variables. These dependences are derived with simple algebra when the polarimetry effects are small in order to apply the so called Type I approximation [6][7][8].…”

A low-resolution inversion-based method to enhance poloidal polarimeter accuracy in thermonuclear plasmas

“…In poloidal polarimeters, W 2 is almost zero (W 2 ≪ W 3 , W 1 ) and thus, for a non-modulated poloidal polarimeter, it is convenient to use an input polarisation linear with a ψ 0 = 45 • [22,23], involving that the previous equations become:…”

A multi-wavelength approach to increase polarimeter diagnostic performance in nuclear fusion reactors

“…Under some assumptions, it is possible to directly correlate the Faraday rotation angle with the component of the magnetic field parallel to the propagation and the Cotton Mouton with the orthogonal one, through the type-1 approximation. This approximation is valid only if the polarisation angles vary by a small amount (of the order of 10 • -20 • ), while for large angle variations the systematic errors arise, and the measured quantities have not an easy interpretation directly correlated with the plasma parameters [3]. Regarding those future reactors will vary from weak to intense plasma scenarios in the operational lifetime, the polarimeter should be not reliable within the limits of type-1 approximation [4].…”