Analysis of the Hg-Na arc-W cathode interactions at increasingly high Na contents

Summary form only given, as follows. The effect of increasingly high sodium contents on the attachment conditions of a DC mercury-sodium (Hg-Na) arc discharge on a tungsten (W) cathode is studied. A physical model based on the collisionless assumption is used for the description of the cathode sheath. The solution of this model; i.e. the plasma heat flux versus the cathode surface temperature, is coupled to a thermal model for the cathode bulk for a self-consistent solution of the problem under diffuse attachment conditions. Domains of existence for self-sustaining operation of the system in the electron temperature at the sheath edge - cathode surface temperatures space (T/sub e/-T/sub s/ space) are obtained versus the Na content in the arc discharge and reveal the higher cathode surface temperature requirements as the Na content increases. For a typical set of HPS lamp operating conditions (p=1 atm, p/sub Hg/=0.9P, p/sub Na/=0.1P, I=3.2 A), the model predicts melting of the pure W cathode tip. The substantial increase of the operating tip temperature at increasingly high Na content is attributed to the increasingly high density of Na/sup +/ ions, itself associated with the much lower ionization potential of Na with respect to Hg, which must be sustained by the electron emission processes. Lowering the W cathode tip work function brings the operating temperatures to lower values in agreement with well-known experimental facts.