A free-burning thermal plasma jet is a poorly characterised medium. It is far hotter than a flame, chemically active, strongly non-uniform, and unsteady on timescales set by the motion of the arc and by the turbulence of the jet, and almost none of its interior is accessible to a physical probe. Basic questions therefore remain open: what the thermochemical state of the jet actually is, how the gas temperature and composition are distributed and how they respond to the way the torch is operated, how strongly the jet fluctuates and how that unsteadiness is coupled to the electrical behaviour of the discharge, and which measurements can reach these quantities at all in an environment that is at once intensely luminous, spectrally congested and steeply non-uniform. These questions are of interest in their own right, and they are also what currently limits the design and scale-up of plasma burners intended to replace fossil-fuel burners in high-temperature process industries, in processes such as iron-ore pelletising, cement and lime production and steel reheating, where the associated nitrogen-oxide formation is a further open problem.

This licentiate thesis addresses these questions by developing optical diagnostics for thermal plasma jets and for the high-temperature gases they produce, and applying them to a non-transferred plasma torch. Mid-infrared laser absorption spectroscopy was first established for a target species, nitric oxide, in plasma-heated and combusting gases, where the medium is hot and rich in water vapour but comparatively well behaved; the same approach was then carried into the plasma jet itself, where carbon monoxide serves both as the dominant carbon-bearing species of a carbon-dioxide-fed discharge and as an indicator of the dissociation, and hence the thermal, state of the gas, and where the measured absorption feature, formed by two transitions whose lower states differ in energy by 3065 cm⁻¹, yields the gas temperature simultaneously with the concentration through the temperature dependence of the two line strengths. The same line pair was recorded in premixed flat flames of known and computable state, which fixes the accuracy attainable when the medium does not vary along the beam. In parallel, spontaneous Raman scattering was used to obtain the gas temperature at a point inside the luminous core of the jet, which is spatially resolved and therefore complementary to the line-of-sight character of absorption. Because absorption necessarily reports a quantity averaged along the beam, while the jet possesses steep gradients in temperature and composition, a central concern throughout is what such an averaged value represents and how it can be related to the local thermodynamic properties of the plasma. The high repetition rate of the laser scan, together with synchronous recording of the discharge current and voltage, additionally samples the fluctuation of the jet rather than averaging over it, which is the precondition for relating its thermochemical unsteadiness to the behaviour of the arc.

Taken together, the appended papers establish a common measurement chain that extends from plasma-heated gases at pilot scale to the plasma jet at torch scale, and they deliver absolute, calibration-free temperatures and species concentrations of a kind that has so far been reported only to a very limited extent for this class of jet. They also identify what such measurements can and cannot resolve, in particular the interpretation of path-averaged quantities in a strongly non-uniform medium and the accuracy limits imposed by high-temperature spectroscopic parameters. The resulting methods and data provide the experimental basis against which magnetohydrodynamic and chemical-kinetic models of the torch can be validated, and thereby an initial step toward the design of large-scale, low-emission plasma burners for energy-intensive industry.