The ignition phase is a critical stage in the operation of gas discharge lamps where the neutral gas enclosed between the electrodes undergoes a transformation from the dielectric state to a conducting phase, eventually enabling the production of light. The phenomena occurring during this phase transition are not fully understood and the related experimental studies are often limited to local optical measurements in environments prone to influencing these transient phenomena. In this work unipolar ignition phenomena at sub-kilovolt levels are investigated in a 3 Torr argon discharge tube. The lamp is placed in a highly controlled environment so as to prevent any bias on the measurements. A fast intensified CCD camera and a specially-designed novel electrostatic probe are used simultaneously so as to provide with a broad array of measured and computed parameters which are displayed in space-time diagrams for cross comparisons. Experiments show that three distinct phases exist during successful ignitions: Upon the application of voltage a first ionization wave starts from the active electrode and propagates in the neutral gas toward the opposite electrode. A local front of high axial E field strength is associated with this process and causes a local ionisation to occur, leading to the electrostatic charging of the lamp. Next, a second wave propagates from the ground electrode back toward the active electrode with a higher velocity, and in this process leads to a partial discharging of the lamp. This return stroke draws a homogeneous plasma column which eventually bridges both electrodes at the end of the wave propagation. At this point both electrode sheaths are formed and the common features of a glow discharge are observed. The third phase is an increase of the light intensity of the plasma column until the lamp reaches a steady state operation. Failed ignitions present only the first phase where the first wave starts its propagation but extinguishes in the lamp, leading to a charge memory effect. It is found that the full propagation of this first wave is a requirement for a successful lamp ignition. Differences in the properties of the waves were observed depending on the voltage polarity, and it was estimated that a photoelectric effect at the wall is the most likely source of electron for the ionisation wave of positive polarity. Finally a simple model of the first ionization wave is developed and used to analyse the fundamental differences between processes occurring at negative and at positive polarity. From this study three conditions are developed for the successful unipolar ignition of lamps and the relations between them are derived.

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.

Breakdown in a discharge tube is investigated by means of a fluid model. The discharge tube is similar to a compact fluorescent lamp tube, containing argon at 3 Torr and mercury at a few millitorr. It was found that the minimum breakdown voltage is decreased substantially compared with a tube containing pure argon. Penning ionization of mercury via an argon metastable state plays an important role in this effect. This is illustrated for a lamp operated on a DC voltage, where significant Penning ionization takes place in the wake of the ionization front. Furthermore, contrary to what is suggested in earlier literature, the development of the surface potential of the lamp is shown to be not only determined by surface charges, but also by volume charges.

The effect of the frequency on the breakdown time in a straight discharge tube is investigated by means of a fluid model. The discharge tube is similar to a compact fluorescent lamp tube, containing argon at 3 Torr and mercury at a few Torr. The mechanism of breakdown at frequencies of the order of several 10 kHz is considered and related to breakdown at a dc voltage. During a negative potential on the powered electrode, an ionization wave traverses the tube in a way similar to that in a dc operated tube. During a positive potential on the powered electrode, the electric field in the part of the tube already traversed by the ionization wave is enhanced by negative charge on the inner wall of the tube. Although the ionized region does not extend during this phase, the ionization density increases substantially. Furthermore, we investigated the dependence of the breakdown time on the applied frequency and found that the breakdown voltage is independent of the frequency. This is shown to be consistent with experimental data.

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