Optical and electrostatic potential investigations of electrical breakdown phenomena in a low-pressure gas discharge lamp

Abstract

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.

Collision of ionization waves in long discharge tubes

“…It was found that the discharge length cannot be less than 4 cm. Similar result was found for IW of both polarities in [22]. The authors of this work stressed that IW forms at some distance from the active electrode which is the minimal value of At some distance ahead the collision area the ionization wave attenuates quickly and becomes lowobservable then passing into the total suppression.…”

“…IW appears as a result of electric field non-uniformity specific for the tube which length exceeds its diameter (long discharge tube). The most common examples are fluorescent lamps [2] and tubes of gas discharge lasers [3]. In such geometry, the electric field before breakdown rapidly decreases from electrodes toward the tube center, and becomes zero at distances in some tube diameters [4].…”

“…Examples of x-t diagrams for collided IWs (1),(2) and one IW (3). Ne p = 1 Torr, U = 3.0 kV, f = 100 HzIV.…”

“…Plots for the collided waves accompanied with line (3) which corresponds to the propagation of one IW from the right electrode. The approximation gives constant negative accelerations for both opposite waves: -2.1 cm/(μs)2…”

Зависимость Потенциала Пробоя От Скорости Роста Напряжения В Длинной Разрядной Трубке При Низком Давлении

“…Стенка заряжается током, который протекает через плазменный след, оставленный волной, и далее через емкость трубка−земля. Плотность поверхностного заряда измерялась в работах [4,5], а потенциал стенки в работах [5,6]. Эти измерения проводились с помощью емкостного (электростатического) зонда.…”