Breakdown

Lighting is omnipresent in our modern world and most of the time it belongs to this category of things which are just there to make our lives easier and more comfortable. When you flick a switch the light goes on in the room you just entered. Probably this little action was done unconsciously and you do not even remember it a second later. Nevertheless, it is mildly irritating if a fluorescent lamp flashes a couple of times before burning steadily and it is the nightmare of a mayor of a big city to have the flood-lighting in an soccer stadium stay dark just before an important game.

A large group of lamps use gas discharges to generate light. A gas discharge is a neutral gas which has been made conducting by applying a sufficiently large voltage across it. This conducting medium comprises a wide range of different particles: apart from the neutral gas atoms, one can find electrons, ions and the many excited species which are responsible for the emitted light. The interplay between the different atomic and molecular constituents in this complex mixture and their interaction with electro-magnetic fields and surfaces makes gas discharge physics a challenging subject. Transforming the initially non conducting gas into a conducting medium is called breakdown and comprises an involved set of transient processes which is poorly understood even in the present day.

Gas discharges are used in many applications, ranging from, for example, surface treatment of materials to gas cleaning and indeed to lighting. Furthermore, gas discharges are omipresent in natural phenomena such as lightning, the polar light and many extraterestial processes such as stars and interstellar clouds.

In some of these occurances of gas discharges, breakdown is not an issue at all. Many, however, rely on breakdown. In light sources it is of course important that the light actually switches on. The way in which this process occurs furthermore has its influence on life time and production costs. However, in the tiny discharges that constitute a pixel in a plasma television, it is of fundamental importance how the discharge switches on: this is done many times per second.

In some of the research conducted at our institute, we ask what happens during the breakdown of the gas and why. In the following, the various interrelated projects are discussed:

Electric field measurements

• E. Wagenaars, M.D. Bowden and G.M.W. Kroesen, Plasma emission imaging of a low-pressure argon breakdown, Plasma Sources Sci. Technol. 14(2), 342-350, 2005
• M.D. Bowden, E. Wagenaars, Tao Jiang, W.J.M. Brok and M.F. Gendre, Measurements of plasma breakdown, J. of High Temp. Mat. Processes, 8(4):483-498, 2004.

Ignition of fluorescent lamps

In order to be able to separate the various mechanisms that could play a role in a real cfl, the system is simplified to a straight glass tube, operated on a DC voltage. On the left one can see a `film' of iCCD camera pictures taken of such a lamp by Maxime Gendre. The electrode at 0 cm is the cathode, the one at 14 cm is the anode. At t = 4 microseconds after the voltage has been applied, an ionisation front starts to move from cathode to anode. When it reaches the anode (at about 12 microseconds) a `return strike' fills the tube with a fairly homogeneous emission. From then on it will take some time (several tens of microseconds) before the current rises and the lamp can be said to have ignited. Depending on the conditions striations can be observed.

By modelling the system (see the false-colour map of the calculated emission on the right) we think we gained a fair understanding of what happens in the phase in which the ionisation front moves from cathode to anode.