Analysis of the diffuse-to-hot spot attachment mode transition for high pressure arcs on thermionic cathodes

Summary form only given. The arc attachment mode transition from a diffuse mode to a constricted, hot spot mode for high-pressure arcs on refractory (thermionic) cathodes is analyzed theoretically. The analysis confirms the hypothesis that the multidimensional problem of heat transfer through a cathode exposed to a nonlinear plasma surface heating has a nonunique solution, and that the existence of diffuse and hot spot attachment modes can be explained by such phenomenon. The model predictions are compared against recently obtained measurements for a pure argon arc (3.75 atm) on a rod tungsten cathode (diameter 1.5 mm), and information is gained on the physical conditions prevailing when the diffuse-to-hot spot transition occurs. Under such experimental conditions a transition current of /spl sim/3.47-3.81 A, above which the arc attachment is diffuse and below which the attachment is constricted into a single spot, is measured. According to the model´s predictions, the ratio of the ion-to-electron current densities (J/sub ion//J/sub c/) at the cathode surface is near unity when the transition occurs. The model also shows that the diffuse mode of arc attachment exists for a wide range of values for the ratio J/sub ion//J/sub c/ (/spl sim/1 down to <<1 values) and that this ratio remains closer to unity in the hot spot mode. The transition from a diffuse to a hot spot attachment is associated with a sudden reduction of the arc attachment diameter (from 1.5 mm down to a few hundred /spl mu/m), and a corresponding sudden increase of the current density at the arc root. The electron emission mechanism at the cathode suddenly changes from thermionic (T/spl sim/3000 K, low E-fields; /spl sim/10/sup 6/-10/sup 7/ V/m) to thermo-field emission (T>3300 K, E>10/sup 8/ V/m). From this comparison, the hypothesis that the diffuse-to-hot spot transition occurring when the arc current is reduced occurs when the ratio J/sub ion//J/sub c/ increases to a value close to unity is intro- uced.