Investigation of anode voltage drop in dc glow discharge in CF₄

In this work, the voltage drop U_a across the anode layer of dc glow discharge in CF₄ was determined using the moving anode method at different pressures (0.05–0.5 Torr) and discharge currents (less than 80 mA). When the anode is in the middle of the Faraday dark space or further from the cathode, the flow of fast electrons from the cathode layer becomes small, they are no longer able to make a contribution to the discharge current. Further distance of the anode from the cathode is accompanied by a rather sharp increase in the voltage between the electrodes, and an anodic glow appears near the anode surface, the thickness of which weakly depends on the gas pressure and current and is approximately 1–2 mm. A set of current-voltage characteristics (CVCs) of the discharge was measured for cases where, in the investigated current range, the anode glow does not appear (short distance between the electrodes), appears at low current and then disappears with its increase, and also when it does not disappear with increasing current (longer distances), but the positive column has not yet formed. This set of CVCs for a fixed gas pressure lies between two curves (with and without anode glow), the difference between which allows determining the positive anode voltage drop. This method allows you to quickly measure the CVCs of a discharge and determine U_a from them over a wide range of discharge current. It is shown that the anode drop at a fixed pressure of CF₄ has a non-monotonic dependence on the magnitude of the discharge current. The magnitude of the anodic drop can be 2–3 times greater than the ionization potential of CF₄ molecules due to the process of dissociative electron attachment.