This thesis presents the results of experimental and modelling studies of breakdown processes in near-atmospheric pressure noble gasses. The motivation came from the lighting industry - our goal was to provide a better understanding of the breakdown phenomena in conditions typical for a mid-pressure high-intensity discharge (HID) lamp. However, the research can be used in a broader spectrum of applications involving breakdown, for example in high power electronics, in removal of unwanted electric charges, photocopying, handling waste, UV generation or surface treatment. We focused our research to mid-pressure (0.1 to 1 bar) discharges in argon and xenon with varying conditions that will prove to greatly influence the breakdown process. First, we examined the effect the dielectric surfaces have on the breakdown process. A pin-to-pin electrode geometry was placed in close vicinity of a flat dielectric in an argon atmosphere of pressure varying between 0.1 and 1 bar. We used positive pulsed voltages on the charged electrode (rise time varying between 47 and 100 V/ns), observed the development and measured the speed of the discharge forming on the dielectric surface and in the gas between the electrode tips. Our results prove that surface discharges use propagation and growth mechanisms that are in some aspects different from the discharges that form in the gas. The effect of the voltage form on the breakdown process was subsequently studied. Lowering of the breakdown voltage of lamps is a constant goal to be met, and it has already been observed that substituting pulsed voltages for AC in the 100-kHz range brings significant improvements. We performed electrical and optical measurements of the breakdown parameters and explained why AC breakdown works on lower voltages than pulsed breakdown. The differences between the discharges in different gasses were explained, along with the influience of voltage frequency on the breakdown process and UV- and Kr85-related effects. Statistical lag times were calculated for different parameters. A valuable contribution to the understanding of the AC ignition process was done by using computer simulations. A fluid model and a cylindrically symmetric 2D geometry with a pin-to-pin electrode configuration was used to simulate a 700 mbar argon discharge. After simulating pulsed ignition in free gas and proving the importance of metastables on the discharge growth, the AC breakdown process in a lamp-like geometry was also simulated at frequencies between 60 kHz and 1 MHz. The main finding of this part of the research was the explanation why AC breakdown requires lower voltage than pulsed breakdown. We also explained the influence of the voltage frequency observed in experiments. The final part of the research considered the influence of external structures ("antennae") on the breakdown process in AC discharges. Antennae are thin metallic formations on the outer surface of the lamp burner. An EM model was used to examine the influence of different antenna structures on the electric field enhancement in the lamp in a static case. We have also done a series of experiments on lamps, showing that the antennae significantly lower the breakdown voltage. The last part of the thesis shows how antennae work, why the active ones work better than the passive ones, and the reason behind the observed differences in the workings of the passive antennae
