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Electric power systems could benefit from a device that would limit the magnitude of short-circuit current. A switched resistor fault current limiter requires a commutating switch capable of interrupting current without the benefit of a natural current zero. The unstable diffuse plasma discharge between electrodes in a cylindrical geometry might be capable of such a task but the phenomena involved in this discharge is not completely understood. To develop a detailed model of this unstable discharge, measurements of the plasma density as a function of position and time were made using a double probe. A maximum density of 5 x 1020 m-3 was measured 90 mm from the cathode of this diffuse copper vapor discharge in vacuum.