A technique which employs a laser-excited optical resonator has been developed to measure the electron density distribution, both temporal and spatial, in the collapse stage of a fast Z-pinch discharge. The resonator device has an experimentally determined time resolution of better than 0.05 µsec and is suitable for measuring electron densities in excess of 5 • 10¹⁶ /Lʎ cm⁻³, where L is the length of the plasma in cm and ʎ is the wavelength of the laser in microns. A novel feature of this instrument is the use of an unstable optical resonator. The technique is applied to a discharge in argon at filling pressures of 100 and 1000 µHg. The temporal and radial electron density distributions obtained exhibit several interesting features which are discussed from the point of view of the collapse process. / Science, Faculty of / Physics and Astronomy, Department of / Graduate