Microwave power spectral density and its effects on exciting electrodeless high intensity discharge lamps

Summary form only given. The effects of a microwave source generating a spectrally dense power spectrum on the operation of an electrodeless high intensity discharge lamp were measured. Spectrally pure (monotone) sources operating within ISM bands at 915 MHz and 2.45 GHz produce stable capacitively coupled discharges useful for producing flicker-free light for numerous applications. The internal plasma temperature distribution and lamp geometry define acoustic resonance modes within the lamp which can be excited with power sidebands. The principal resonant frequencies lie in the range of 15 to 500 kHz. The resonances appear broadened owing to imperfections in the geometry of the vitreous silica tubes used to contain the discharges and sometimes shift due to condensate movement. Consequently, modulation of the carrier can produce sidebands which couple effectively to some if not all resonances. Deleterious spectral components may arise from instabilities in magnetron tubes, pulsed mode operation of magnetron tubes or improperly filtered DC supply voltages in both tube and solid-state generators. The effect of the sidebands is to perturb the discharge and cause deflection of the thermal arc from its steady state position. If the sidebands are sufficiently dense and contain sufficient power a multitude of modes are excited simultaneously causing disruption of the convective flow within the tube leading to plasma instabilities and extinguishing of the discharge. Lower order modes of acoustic resonance have been observed to perturb the discharge with as little as 0.2% of the total applied power contained within the sidebands.