Variation of discharge parameters versus cold spot temperature in a 50 Hz AC operated fluorescent lamp

Abstract

In 50Hz AC operated fluorescent lamps, cold spot temperature of the lamp tube decides mercury vapor pressure; thus decides almost all the discharge parameters such as electric field, electron temperature, electron density etc. In the experiment of Liu and Zissis, the lamp tube is partially cooled by cooling spray. Discharge parameters of this cooled part deviates from their initial values. However for other parts of the tube, no obvious variation is found. In this session, we will look into how discharge parameters in the cooled region vary against cold spot temperature of the tube. We establish a 0D model of the fluorescent lamp, which includes excitation and ionization of small amount of Hg and 3 Torr Ar. We lump 6- <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and 6- <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> levels of Hg together for the sake of simplification. We only take into account the lowest metastable state of Ar. Stepwise ionization from excited states of both species and Penning effect between metastable Ar and Hg atoms are also included in the model. Excitation and ionization rates are obtained with Bolsig+ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . In the model, we assume that cold spot temperature of the lamp falls down from 40 C to -5 C in 5 seconds and then rises back to 40 C in 15 seconds. This process resembles that in experiment of Liu and Zissis. Simulation indicates that when cold spot temperature comes down, electric field rises from about 120V/m to 220V/m; electron temperature rises from about 2.3 eV to 3.4 eV. The reason why the calculated electron temperature is overestimated goes that in the calculation of rate coefficients, we do not include coulomb collision. Therefore, higher electron temperature is necessary for the balance of ionization. Electron density, excited Hg atom density and ionized Hg atom density also decrease with cold spot temperature. This results from the decreased Hg vapor pressure. On the contrary, excitation and ionization of Ar become more frequent, which leads to higher metastable Ar density and Ar ion density. The increase of Ar participation in inelastic collision results in the increase of electric field and electron temperature because of the higher threshold energy of Ar excitation (11.5 eV) and ionization (15.8eV).