Application of a 150 GHz interferometer to fluorescent lamps

A 150 GHz interferometer system has been developed, and is being used, to gain new insights into many aspects of fluorescent lamps discharges. This presentation describes results of the application of the interferometer to studies of the negative glow plasma, especially with regard to plasma-electrode coupling. In a related presentation, details are provided of the interferometer's setup, function, and performance. Through measurements of the electron density in the vicinity of the electrode, the interferometer can sense effects of the beam electrons. These are a subset of the electrons which, after having been emitted from the electrode, acquire significant energy when traversing the electrode sheath, and emerge into the bulk plasma where they excite and ionize mercury atoms. Through this process, they create and sustain the near-electrode plasma. When lamp current is time-varying, the interferometer senses variations in bulk electron density that are, in part, related to variations in beam electron energy, a consequence of the variations in electrode sheath potential that are necessary for supporting the current changes. Thus, inferences can be drawn from interferometer measurements which provide insight into the this key plasma-electrode coupling mechanism. Augmentation by additional diagnostics (e.g., floating probes and electrode temperature), provides additional insight. In the course of these studies, electron density measurements were acquired for different modes of lamp operation, as characterized by variations in the driving current (e.g., waveform shape, frequency, amplitude) and by the presence or absence of auxiliary electrode heating. In addition, measurements were acquired over a range of positions in the lamp in order to see the contrasting behaviors of the near- electrode and positive column plasmas. Measurements were acquired during the plasma build-up time following ignition, the steady-state plasma, and the afterglow plasma. Many of these measurements are shown, and interpretations gleaned from these measurements, are discussed.