No description available.

No description available.

The effect of Neodymium-in-glass laser radiation on the populations of excited atoms in a partially ionized gas is described. The radiation fluxes that exist in such plasmas are normally so small that the process of photoionization is negligible compared with other processes. However the application of external radiation via a laser source can severely perturb the populations of excited atoms. At 'moderate' photon fluxes (< 1026 photons/cm2) single photon ionization is important and is capable of perturbing the populations of excited states. The dependence of these reductions on laser power and plasma conditions has been determined for energy levels in Hydrogen. Photoionization effects due to laser radiation could not be seen in partially ionized Argon and Xenon gas discharges. This has led to an evaluation of the rate of production of excited Argon atoms. The effect of the laser beam profile on the observed reduction in the populations of excited Hydrogen atoms has been demonstrated. The radiation source used in the experiments which are described, is a Neodymium-in-glass laser emitting radiation at 1.06 Um. It produced unfocussed beam powers of about 20 MW. The results which were obtained are adequately described by a rate equation model of the plasma which has allowed a simple analysis of experimental data.

No description available.

No description available.

Rectifying and non-rectifying contacts were fabricated in n-type 4H-Silicon Carbide. To improve the characteristics of the fabricated contacts various different surface pre-treatments were used. The impact of these pre-treatments on both contacts types was evaluated, and any improvements recorded. The findings report on the improvements made to the specific contact resistance of nickel contacts fabricated to n-type 4H-SiC epilayers. Ohmic contacts with an average value of specific contact resistances as low as 1.15 x 10^-4 Ω cm² following annealing at around 900^oC were created. In addition nickel Ohmic contacts were created with similarly low specific contact resistance, which required no such annealing, a phenomenon never previously reported. Further investigation was conducted into the reasons behind this finding, and a hypothesis developed. Various different surface preparations were also experimented for use in the formation of Schottky contacts. No improvements were seen over the standard cleaning process however. High power Schottky diodes were fabricated using a single nickel Schottky contact that exhibited reverse breakdown voltages of around 600V. This figure was improved upon through the use of boron implantation as an edge termination but at the detriment of the forward I(V) characteristic. Further development of the diodes, using a multiple metal Schottky contact, yielded breakdown voltages of 1kV without the need for any further edge termination. This value is more than 85% of the theoretical value for reverse breakdown. In addition to the work on metal contacts, investigation was also performed into the use of ion implantation for the purpose of semiconductor doping. A database was developed to allow the prediction of implant profiles for both Boron and Nitrogen into SiC. This prediction compared well to experimental results. The damage created by high temperature annealing of SiC and the possible steps to prevent and repair this damage is also investigated.

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No description available.

No description available.

A new test method and finite element modelling were used to investigate how material properties of electrical steel panels affect their shielding factors. Both experiment and modelling showed an improved DC shielding factor with increasing thickness for the shields of similar magnetic properties. Enhanced shielding by the eddy currents was demonstrated by testing the same steel panel under AC and DC field conditions. Comparing to non-oriented steel panels, a decreasing shielding factor with the frequency from 50 Hz to 400 Hz was found for grain-oriented steels. This was also investigated by measuring magnetizations along rolling and transverse directions within the panels. It was found that measured shielding factors of double-layer shields with two grain-oriented steel panels could be improved significantly with orthogonally arranged rolling directions. Different shielding factors were found by placing different panel closer to the field source in the test of double-layer shields formed by one grain-oriented and one non-oriented steel panels. Although little shielding effect of aluminium panels are found at 50 Hz, adding the same aluminium panel with single electrical steel shield was dramatically improved the shielding. The magnetization of shielding sample at AC conditions has been modelled and measured. The magnetization was found very low due to the demagnetizing effect. Therefore, the permeability at very low magnetization range has a large effect on the magnetic shielding factors of the steel panels. Drilled hole as a defect in the panel and overlap of the panels have been tested with the new test method. Small hole in the panel would not cause the degradation of the overall shielding factor of the shield rooms. Overlap was proved to be an effective way to reduce the flux leakage at the joints between the panels The difference between the computed and measured shielding factors is addressed by analysing the capability of the solver used in the finite element modelling and the uncertainty of the measured B-H characteristic of the material as the input to the model.