Bichromator for electron density measurements

A bichromator system consisting of two interference filters with different bandwidths centered on the H-alpha line and photomultiplier detectors for measuring the time evolution of the electron density in hydrogen-containing plasmas is described. This method is based on the time variation in the width of the Stark broadened line, as a result of which the ratio of the signals in the two detector channels is a function of the plasma electron density. Various aspects of this system are discussed briefly, including background light (line and continuum), self-absorption, choice of filter bandwidths, and detection limits. The apparatus is inexpensive, easy to calibrate, and comparatively insensitive to minor changes in optical setup. It has been used for measuring the electron density in laser spark plasmas in room air, hydrogen, and dry air plus hydrogen. (C) 2002 American Institute of Physics.