Calculation of current density and temperature distributions at the cathode surface using a collisional sheath model

Abstract

Summary form only given. Thoriated-tungsten is a widely used cathode material in HID (high intensity density) lamps. The degree of thorium coverage, as defined on the tungsten surface may change the current density and temperature distributions at the cathode/plasma interface considerably. This paper presents these distributions that are calculated assuming a collision-dominated sheath. A 1-D model for the micron-order cathode sheath-region of the plasma is developed first to yield heat-flux and current density lookup tables whose entries are a function of cathode surface temperature for a given combined parametric sheath and pre-sheath potential drop. The lookup tables, generated for atmospheric pressure mercury, are used to solve the energy balance and current continuity equations in the cathode body in order to get the current density and temperature distributions at the cathode/plasma interface. Figure 1 shows the current density distributions obtained for a total current of 10 A and for different degrees of surface coverage. Thorium coverage on tungsten changes the work function and Richardson's constant of the surface and therefore, affects the current density distribution.