Multi-directional optical diagnostics of thermal plasma jets

Abstract

An optical system consisting of optical fibers and photodiodes arranged in four detection directions and in two planes perpendicular to the jet axis is used for the diagnostics of a thermal plasma jet where the application of one-directional observation is insufficient due to a strong disturbance of cylindrical symmetry. The experimental data are processed using the inverse Radon transform where the number of input profiles is extended by means of interpolation. The results show that the method application can bring new findings, e.g., in detection of coherent structures or evaluations of velocity distributions in thermal plasma jet under turbulent conditions.

Plasma airflow jets diagnosis by means of time-resolved tomography

“…Knowing J(x, y), projections at any angle θ are obtained by mean of Radon transform and reciprocally, the local emission distribution is reconstructed applying inverse Radon transform to side-on intensity profiles, known in practice for a limited number K of viewing directions. In the present study, the local emission distribution J(x, y) is reconstructed adopting the approach described in [9], based on the iradon function under MATLAB environment to calculate the inverse Radon transform starting from K experimental intensity profiles recorded at equidistant angle. Assuming the plasma is optically thin, the intensity profiles recorded at two opposite directions are mutually flipped, reducing then the useful domain of angular scanning from [0 − 2π[ to [0 − π[.…”

“…can be expressed analytically for fast evaluation [9]. Such procedure leads to double the number of available intensity profiles, henceforth well determined at θ k = π(k−1) 2K , k = 1, 2, ..., 2K.…”

“…The effective enhancement when interpolated profiles are taken into account into the rebuilding procedure has been assessed in [9] through qualitative comparisons between a reference distribution J ref and the distribution J sim reconstructed with iradon applied to intensity profiles computed for K = 4 initial projection angles. The improvement with increasing numbers of input profiles was quantitatively estimated evaluating the Root Mean Square Error (RMSE) between a reference emission distribution J ref and the distribution J sim (K, P ) rebuilt using an initial set of K intensity profiles and doubling P times the initial set.…”

“…If tomography technique has already been used in the past to document thermal plasma jet local emission distribution [9], the present contribution addresses the development of tomography based technique using a single CCD camera. The new diagnostic method has been applied to the investigation of air plasma jets produced with a Microwave Plasma Torch (MPT).…”

Time-resolved tomographic measurements of temperatures in a thermal plasma jet

“…The values of radiation intensities in these elements were adjusted by a normalization procedure ensuring that the integral radiation of the cross section (represented by the sum of the matrix) was equal to the integral radiation measured in a single direction. Tests on models have shown the ability of this method to reconstruct rather complicated structures with more than one maximums [26]. Therefore we assume a correct reconstruction of more simple, single-maximum distributions representing the radiation in the plasma jet cross-sections.…”