A pin–pin electrode geometry was used to study the velocities of streamers propagating over a flat dielectric surface and in gas close to the dielectric. The experiments were done in an argon atmosphere, at pressures from 0.1 to 1 bar, with repetitive voltage pulses. The dielectric surface played a noticeable role in discharge ignition and propagation. The average speed of the discharge decreased with higher pressure and lower voltage pulse rise rate. It was higher when the conductive channel between the electrodes was formed over the dielectric, rather than through the gas. Space resolved measurements revealed an increase in velocity of the discharge as it travelled towards the grounded electrode.

Abstract.AC-driven breakdown processes have been explored much less than the pulsed or DC breakdown, even though they have possible applications in industry. This paper focuses on the frequency range between 60 kHz and 1 MHz, at a pin-pin electrode geometry and gap lengths of 4 or 7 mm. The breakdown process was examined in argon and xenon at 0.3 and 0.7 bar. We used electrical and optical measurements to characterize the breakdown process, to observe the influence of frequency change and the effect of ignition enhancers -UV irradiation and radioactive material.

An optical study of pulse, dc, and ac (50–400 kHz) ignition of metal halide lamps has been performed by investigating intensified CCD camera images of the discharges. The ceramic lamp burners were filled with xenon gas at pressures of 300 and 700 mbar. In comparison with dc and pulse ignition, igniting with an ac voltage decreases the ignition voltage by up to 56% and the breakdown time scales get much longer (∼10−3 s compared with ∼10−7 s for pulse ignition). Increasing the ac frequency decreases the ignition voltages and changes the ionization channel shapes. External irradiation of UV light can have either an increasing or a decreasing effect on ignition voltages.

For energy efficiency and material cost reduction it is preferred to drive highintensity discharge lamps at frequencies of approximately 300 kHz. However, operating lamps at these high frequencies bears the risk of stimulating acoustic resonances inside the arc tube, which can result in low frequency light flicker and even lamp destruction. The acoustic streaming effect has been identified as the link between high frequency resonances and low frequency flicker. A highly coupled 3D multiphysics model has been set up to calculate the acoustic streaming velocity field inside the arc tube of high-intensity discharge lamps. It has been found that the velocity field suffers a phase transition to an asymmetrical state at a critical acoustic streaming force. In certain respects the system behaves similar to a ferromagnet near the Curie point. It is discussed how the model allows to investigate the light flicker phenomenon. Concerning computer resources the procedure is considerably less demanding than a direct approach with a transient model.

In a magnetic field a Lorentz force acts on the motion of charged particles and thus influences the electrical discharge path of a streamer. Experimental observations in a 10 T magnetic field show that the trajectory of streamers in a knife-point geometry (with 15 mm gap) in nitrogen follows near the electrode a path that always makes the same Hall angle with respect to the electric field determined by the geometry. After about 7 mm the streamer deviates from the applied electric field, showing that the electric field of the streamer itself becomes dominant. From these observations the electron scattering time during streamer propagation can be obtained. Furthermore, these results strongly suggest that in this case photoionization is not very important for streamer formation.