Optical Emission Investigations on Plasma between two Spherical Cathodes with Opposing Orifices

Two identical hollow spheres of stainless steel with outer diam. 96 mm and an orifice of 10 mm diam. with the orifices facing each other, were placed inside a vacuum chamber and biased identically from two independent power supplies, in Argon gas at 0.1 mbar. Plasma appears around both spheres at about –290 V as a single structure. For decreasing potential on both spheres, at –384 V the structure splits into two distinct concentric structures surrounding the spheres. Further on, the structures become eccentric with respect to their spheres, as if repelling each other. At –530 V hyperboloidic plasma structures appear inside the orifices. Increasing the voltage again from –630 V to zero, strong hysteresis behavior is seen, as the plasma structures "try to survive". At about –300 V, the hyperboloids retract inside of each own sphere until all structures disappear at –289 V. Optical spectroscopy was performed on the intersection plasma volume. The spectroscopic data were used to determine the electronic temperature of the plasma. These findings are useful for understanding the fundamental physical processes responsible for the plasma ignition and behavior of cathode systems.