Spatial distributions of electron density and electron temperature in direct current glow discharge

Abstract

The spatial distributions of electron temperature and density in a dc glow discharge that is created by a pair of planar electrodes were studied using double Langmuir probes. The measurement was carried out on the horizontal center plane across vertically placed circular planar parallel plate electrodes. One electrode with magnetic enhancement (magnetron) was paired with a nonmagnetron in the two possible combinations. The measurements were carried out for the following three modes: (nonmagnetron) cathode/(nonmagnetron) anode (C–A), cathode magnetron/(nonmagnetron) anode (CM–A), and (nonmagnetron) cathode/anode magnetron (C–AM). In the C–A mode, electrons gain energy from the imposed electric field, and the electron temperature (Te) rises very sharply from the cathode to the leading edge of the negative glow where Te reaches the maximum. In this region, the number of electrons (Ne) is relatively small and does not increase much. The accelerated electrons lose energy by ionizing gas atoms, and Te decreases rapidly to the plateau value, which is reached near the midpoint between two electrodes and extends to the anode. In this region, Ne increases sharply. As electrons are pulled towards the anode, a steep increase of Ne was found towards the anode and then Ne drops sharply very near the anode as electrons are captured by the anode. The value of Te increases slightly towards the edge of the cathode and the anode (radial position), and the minimum Te was found at the center. There is a clear trend that Te and Ne are inversely related (Te×Ne=constant). This trend was also found in cases in which a magnetron is used as the cathode (CM–A) or as the anode (C–AM) paired with a planar nonmagnetron electrode. In the (CM–A) mode, the magnetic entrapment of electrons is seen in the sharp increase of Ne near the cathode surface. The characteristic toroidal glow near the magnetron cathode is associated with the Ne peak, and the Te peak is found inside the circular toroidal glow where Ne is small. In the (C–AM) mode, a large Ne peak was found near the anode in the center, and the Te peak was found in the center near the cathode. A funnel shaped glow that expands from the center of the anode is distinctively different from the toroidal glow observed for the cathode magnetron glow discharge.