Modelling of a near-cathode layer in HID lamps

Summary form only given. In a recent work by M.S. Benilov and A. Marotta (1996), a model has been developed of a near-cathode plasma and cathode spots in atmospheric-pressure arc discharges. The model has been applied to the calculation of spot parameters for a series of cathode/plasma combinations in atmospheric pressure arc discharges; the calculated spot temperatures are in a reasonable agreement with the measured values. The present work is concerned with the modification of this model for the conditions of a high-pressure arc discharge in mercury, these conditions being of industrial interest due to application of such a discharge in high-intensity discharge (HID) lamps. The most important regions are the ionization layer and the space-charge sheath; these regions are treated in detail in the considered model. An important quantity to be determined as a part of a solution describing the near-cathode plasma, is the ion flux to the cathode surface. This flux is generated in the ionization layer. The distribution of the charged particles in the ionization layer is described by means of a multifluid model with account of multiply charged ions. An analytic solution taking into account multiply charged ions is developed for the space-charge sheath. The current-voltage characteristics of the near-cathode layer are obtained for a given cathode surface temperature. The heat flux coming from the plasma to the cathode surface is calculated. The calculation results are presented for a mercury plasma under pressure of 30 atm and a tungsten cathode.