Acoustic resonance in high pressure discharge lamps

Summary form only given. Nowadays, the rational use of energy is getting a priority for most countries. As the lighting often accounts for an important part of the total energy consumption, much research is needed to improve its management. The design of new generation electronic ballasts offers interesting possibilities in this field but the coupling with discharge lamps and its characteristic electrical behavior is critical. S. Epron (1999) experimentally underlined the effect of the power supply on the acoustic resonance in the bulk of high pressure mercury and sodium lamps. The propagation of acoustic waves in the plasma can cause the deformation of the arc and, in extreme situations, the destruction of the vessel. The aim of that work is to determine the influence of the cathode sheath on the excitation of the acoustic modes. Assuming that a high pressure mercury lamp is supplied by a constant current on which is superimposed a sine wave perturbation at various frequencies, the momentum exchanged by electrons accelerated in the sheath can cause an overpressure of 100Pa as an order of magnitude. This overpressure is located in the near cathode region and implies the propagation of an acoustic wave in the bulk. This magnitude of overpressure is generally too low to produce an arc modification but if the excitation frequency corresponds to an acoustic resonance mode, then the pressure achieves high values at antinodes and arc modifications can be observed. Another influence of the power excitation on the acoustic resonance is the transition of the cathode mode from the diffuse to hot spot mode. Experimentally, in the cathode diffuse mode, an excitation of acoustic resonance is generally reversible but as soon as the arc switches to the hot spot cathode mode, the high speed random motion of the spot results an important excitation of several azimuthal modes and an unstable deformation of the arc can be observed. This nonlinear phenomenon is characterized by a hyst- resis as the excitation threshold differs from the deexcitation one. The study of the acoustic resonance and the understanding of its stimulation are very important steps to improve the coupling with new power supplies. Indeed, lamps are more and more submitted to various frequencies or wave forms.