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Measurements of the effect of increasing the discharge column voltage gradient were investigated using argon based mixtures with nitrogen, oxygen and sulphur hexafluoride in a plasma torch. The theoretical calculation of the voltage gradient and the electron number density was based on the Saha equation which was modified for application to the gas mixtures. The investigations showed that a mixture of Ar and SF6 was most effective and increased the voltage gradient to 0.5 V/mm from 0.3 V/mm. The best mixture was 89% Ar, 10% N2 , 1% SF6 based on the highest increase of the voltage gradient and the least added gas. A model has been developed to illustrate the effects of dissociation, excitation, ionisation of gases and their effects on the discharge column voltage gradient: The mode of an electric discharge in Ar was investigated using spectroscopy. The study showed that for a glow discharge the 520.0 nm line and for an arc discharge the 427.1 nm line were unique. These lines were used to investigate a Glydarc electric discharge which was shown to be a mixture of the glow and the arc discharges. Measurements of the transition of the glow to arc in Ar with discharge current ranging from 0.1 A to 1.0 A at atmospheric pressure showed that at the lower value of discharge current (O. 25 A) the spectral lines were dominated by the near infra-red lines whereas at the higher value of discharge current (1.0 A) the spectral lines were included from the near infra-red to the near UV. The Glydarc electric discharge has been studied in still and fast air flows at atmospheric pressure over a range of discharge currents from 100 mA to 3 A. The results showed that the increase of the discharge voltage with increasing discharge current was due to increase of the discharge column length which varied with time and the air flow rate and was not due to a positive dynamic characteristic.

Total charge exchange and excitation cross-sections are presented for collisions between He(^2+) ions and helium atoms, the laboratory frame collision energy being in the range 16-800keV. Cross-sections from the helium ground state and the He(ls2s(^1)S) and He(ls2s(^3)S) metastable states are calculated. The calculations were performed using the semiclassical impact parameter model where wave-functions are expressed as an expansion of two-electron atomic basis states. Transfer channels included plane-wave translation factors. The transition He(^+)(n = 4) → He(^+)(n = 3) results in the emission of visible light (468.5nm) which is observed in the JET fusion device. Helium beam injection into the JET device introduces some of the atoms in initially excited states. Even a small fraction of metastable helium effects the observed 468.5nm spectrum significantly because of the very large charge exchange cross-sections. Accurate charge exchange cross-sections to the He(^+)(n = 4) states are required to analyse the spectroscopic data used in the diagnosis of the plasma. The cross-sections produced are therefore also presented in a form suitable for direct inclusion into the plasma analysis database. Where possible the current results are compared to previous calculations. Total charge exchange cross-sections from the He(ls2s(^1)S) state are in good agreement with the previous one-electron calculation. Charge exchange cross-sections from the triplet state are found to differ with those from the singlet. The results are discussed in the context of their usefulness in the diagnosis of fusion plasmas. Suggestions for further work are made.

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This thesis describes the design and application of a new array of Langmuir probes to measure the plasma parameters in the divertor region of the Joint European Torus (JET) tokamak. A novel probe tip design was developed to withstand the high heat fluxes in the divertor region following thermal analysis and beam tests to identify the most suitable material. The probes operate in the "very strong magnetic field" regime with a large projected area (4mm2) and incidence angle to the magnetic field (> 10°) to avoid the sheath expansion effects associated with flush mounted divertor probes. The probes may be remotely configured as single, triple or floating probes. The combination of triple probe measurements and strike point sweeping has provided unique high resolution profiles of plasma parameters across the divertor target. Emphasis is placed upon the evaluation of probe interpretation assumptions and comparison with non-disturbing diagnostics to assess the reliability of the measurements. The distortion of the probe current-voltage characteristics is strongest for high recycling and detached divertor conditions. Single probe analysis methods overestimate the electron temperature while the virtual asymmetric double probe assumption matches well the functional form of the current-voltage characteristics and is more consistent with independent measurements and numerical modelling than single probe interpretation. However, the effects of plasma resistivity remain difficult to evaluate quantitatively. Analysis of the divertor plasma parameters over a wide range of conditions is presented. The behavior of the ion fluxes and parallel gradients during high recycling and detached discharges is studied and compared with predictions from simple analytical models of detachment. Numerical models are used to study the impurity production from the divertor target plate. A new technique of evaluating the plasma parameters both between and during Edge Localized Modes (ELMs) using the triple probes is applied to study the influence of active pumping on the divertor parameters.

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