Apparatus and experimental method for measurements of the potential distributions in dc glow discharges

We present an apparatus and experimental method for the measurement of the potential distributions in dc glow discharges. The apparatus employs a combination of internal and external electrodes. The internal electrodes are cylindrical or planar Langmuir probes whose collector is aligned with the inner wall of the discharge tube. The external electrodes consist of a thin Al foil wrapped around the outside of the discharge tube. The discharge is periodically switched on and off (pulsed) with a typical frequency of 10 Hz using an electronic valve controlled by optically isolated couplers. The apparatus allows measurements of the time and temperature dependence of the potentials on the electrodes. The potentials on the electrodes are sensed by two voltage followers with extremely high input impedance (5 × 10¹¹ Ω input resistance, ∼1 pF input capacitance), so that direct measurements of the potential differences between two electrodes are possible. The data acquisition and control of the experiment is accomplished using an input/output data acquisition/control board (AT-MIO from National Instruments) connected to a personal computer (PC). The maximum time resolution of the system is 10 μs, the maximum measurable voltage difference is about 2 kV. In addition to direct floating and wall potential measurements, the apparatus also allows measurements of the characteristics of a Langmuir probe by using a floating probe circuit coupled to a computer-controlled external data acquisition system using isolation amplifiers. The data acquisition/control system enables time-resolved probe characteristic and floating/wall potential measurements as a function of temperature. We present a series of measurements which demonstrate the capabilities of the new apparatus and experimental method.