Demixing in high-pressure lamps

Because of their high efficiency and good colour rendering, metal-halide lamps (MH lamps) increasingly catch attention. In contrast to the fluorescent tube and the energy-saving lamp, they are small and of high pressure. Typical size is a 4 mm diameter and 32 mm length. The pressure is of the order of several bars (5-30) and is mostly determined by mercury. However, light emission is a result of electronic decay of excited atoms of rare-earth metals that are present in the hot mecury pool in low concentrations.We are dealing with plasmas that are chemically complex, of which only a minority of species is responsible for the majority of plasma properties. These rare-earth metals are not only responsible for the light emission but also for the supply of the free electrons. This implicates that these plasmas are strongly non-linear and therefore very sensitive for external factors.

For example, MH lamps are sensitive to gravitational forces. Changing the burning position from horizontal to vertical can influence the colour and colour distribution considerably. This sensitivity to the gravitational force is especially striking when realising that plasmas are determined by the charged particles (ions and electrons) and that the electric force on an ion is more than a billion times larger than the gravitational force on that ion!

This project handles various experimental and theoretical methods. In the theoretical approach we use our simulation package PLASIMO, a computer code written in the programming language C++. For the experimental studies we us the poly-diagnostic calibration methode. With this method, simple techniques such as emission spectroscopy (ES), are gauged to more complicated laser techniques and x-ray tomography. Apart from experiments under nomal gravity conditions on earth, we also investigate how the lamps burn when gravity is ¿switched off¿. This is done in the ARGES project. In that project micro-gravity is achieved in the free-fall of an aeroplane or inside a spaceship. In the first case, experiments are short-lived, while in space (in the ISS) microgravity can last many days or months.

Working on MH lamps has the large advantage that we can use the knowledge of Philips Lighting, where an enormous amount of expertise is present regarding the making of lamps of various shapes, pressures and chemical composition.

Philips Lighting also supports this project financially, very recently STW has approved the funding of this project.

This photograph shows the colour separation in the burner of an MH lamp, based on NaI and a rare-earth iodide (such as Dy, Tm, Ho, Ce, etc.). The plasma reaches a temperature of 6000 K at the core and consists of the (light-emitting) Na- and rare-earth atoms (several mbars) in a pool of high pressure mercury (about 5 bar). This burner is made of poly-cristalline aluminium and is not transparent but translucent. That means that the original direction of the light is lost leading to a more diffuse light. Nevertheless it can be seen that the discharge along the core is not uniform. The bottom is more red-like whereas the top is more blue-ish. That is the result of the segregation of the light-emitting particles, which is a demixing process that is caused by convection (analogous to ¿hot air goes up¿). The driving force of convection is gravity. In a horizontally burning lamp this segregation and colour-demixing is visible.

The photo on the left side, that shows the construction of the lamp as a whole, shows how the burner within a outer balloon is situated. The length of a burner is 36 mm and the diameter is 4 mm.

A composition that shows the transportprocesses in the vertically burning MH lamp. An electric current is driven through the lamp by the electrodes (the two staves) and light is generated. This leads to a large temperature difference of about 4500 K over a distance of 2 mm! The central temperature is 6000 K and the temperature at the wall is 1500 K. The light emitting atoms are the components of molecules that enter the plasma by the evaporation of the saltpool, the black stain on the bottom. The molecules are dragged by the convection stream of the buffergas (mercury). Going past the streamlines the molecules will dissociate, so the light emitting particles are released. The concentration gradients (the molecules M are largely at the wall and atoms at the center) will lead to diffusion; that is the speed of particles with respect to the main stream (the horizontal arrows in the picture).

The picture below shows how in a horizontally burning lamp the convection stream bends the discharge-arc.