Résumé

This study deals with a new type of light source for indoor luminaries of high luminous efficiency. Unlike most low-energy lamps used nowadays, this one is free of pollutant. The light is emitted by a vapour of sulphur heated by microwave and enclosed in a bulb of fused quartz. In the present day commercialized versions, the bulb is rotating in order to avoid melting. In a recent study we have succeeded to avoid any mechanical motion as well as to reduce the nominal power without degrading the efficiency. Our innovation consists in fitting the power supply with a RF modulator. The system is designed to make vibrate the plasma in the ultrasonic frequency range. In the present study, new evidences of the ultrasonic vibrations of the plasma have been accumulated. First, the existence of a standing acoustic wave at particular tunings explains the effect of sustention that occurs then. Second, oscillations of the light have been observed at the very frequency of modulation. Making use of the conditions of acoustic resonance, we have also assessed that the mean electronic temperature lies around 8700 K. In addition, it is now clear that the cut-off frequency increases with pressure in large proportion. Hence the rise of pressure expands out of the centre only when it is lift enough so that the cut-off frequency exceeds the frequency of modulation, which happens with a certain delay. This analysis opens technical routes for improving the acoustic quality factor; this should enhance the luminous efficiency and also allow scaling down the bulb in order to decrease the wattage of the luminary.

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