Time-resolved measurements of electron number density and collision frequency for a fluorescent lamp plasma using microwave diagnostics

Abstract

Microwave interferometry is an established nonperturbing plasma diagnostic technique to measure plasma number density that is simple, accurate, robust, and reliable. This paper reports a related microwave diagnostic method that measures both the electron number density and the electron-neutral collision frequency, which are crucial to understanding the behavior and transport coefficients of plasma. This method measures the attenuation and phase shift of a microwave signal propagating through a plasma with a network analyzer. These measured quantities are related to the real and imaginary parts of the plasma index of refraction by Appleton's equations, which contain the electron number density and collision frequency. Since the electron number density and collision frequency can be obtained directly from measured quantities, one need not know the electron energy distribution function, the electron kinetic temperature, or the electron energy-dependent cross section for the collision process to determine the electron collision frequency. The experimental measurements used to illustrate the method are a paired comparison of the time-resolved electron number density and collision frequency of two types of commercial fluorescent lamps: the standard mercury-based lamp, and the recently introduced green low mercury lamp. Since the plasma properties are periodic at 60 Hz, time-resolved measurements could be made by using the external triggering feature of the network analyzer. Data were taken to illustrate the variation of electron number density and collision frequency during one 60-Hz cycle of the fluorescent lamp plasmas.