Dynamic mode changes of cathodic arc attachment in vertical mercury discharges

Cathodic arc attachment after a steady state anodic phase is investigated in a vertically operated 0.7 MPa mercury high intensity discharge lamp with pure tungsten electrodes. Experimental evidence of a dynamic cathodic mode change of arc attachment influenced by convective effects is documented. Finite element simulation of the temperature field in the cathode body in two and three dimensions is performed. Nonlinear surface heating using Neumann's and Benilov's models is employed. The presheath voltage drop as a function of electron temperature is proposed as a compact input parameter. For Benilov's model two alternatives are compared. All models have the ability to show transient mode changes. The models show no sharp distinction. There are strong indications that the transient spot possesses a nonzero minimum life time when it abruptly emerges at a distinct threshold. The model fairly predicts the thresholds observed in experiments for the upper and lower electrodes depending on the convection heating. The evidence is more in favour of a lower presheath voltage drop, i.e. lower electron temperatures during the spot mode. The results are applicable for the design of dimming modes of HID lamps.