Measurements of atomic state distribution functions of the Philips QL-lamp

Summary form only given. In 1992 Philips Lighting introduced the QL-lamp, an inductively coupled low pressure RF discharge containing a mixture of argon and mercury. Its main advantage is the absence of electrodes, which benefits the life-time. In order to improve our knowledge of this kind of plasmas a model to has been developed and measurements have been performed. In the near future the model will be used to optimize this lamp. In every plasma the free electrons are an important species: they control the energy transfer from the electromagnetic field to the heavy particle's. Therefore, it is important to know the spatial distribution of the electron temperature and of the electron density. These parameters can be obtained from the atomic state distribution function (ASDF), since the levels close the ionization limit are in partial local Saha equilibrium (pLSE), i.e. they are populated according to the Saha relation. From the ASDF also the densities of the mercury and argon ions can be determined. The densities of the excited states are obtained from absolute line intensity measurements. The intensities are measured using a combination of a monochromator and a two dimensional CCD-array. In one dimension of the CCD-camera the information is depicted as function of the wavelength and in the other as function of the lateral position. In this way the lateral profiles of the line and the continuum are obtained at once. The setup is calibrated by a tungsten ribbon lamp. However, it appears that the highly excited, measurable, states are not in pLSE, indicating that the QL plasma is far from Saha equilibrium. In order to obtain the electron densities and temperatures the ASDF has to be combined with either measurements of continuum radiation or a collisional radiative model (CRM).