 Poisson’s equation, continuity equations and surface charge are simultaneously solved using a Newton iteration technique.  Electron energy equation  Ambipolar approximation: Continuity equations with current conservation. MODELING MERCURY-FREE HID LAMPS: BREAKDOWN CHARACTERISTICS AND THERMODYNAMICS* Natalia Yu. Babaeva and Mark J. Kushner University of Michigan, Ann Arbor, MI USA Ayumu Sato, Nanu Brates, Koji Noro Universal Lighting Technologies, Inc., Woburn, MA USA * Work supported by Universal Lighting Technologies, Inc.  High Intensity Discharge (HID) lamps are used in a variety of non- traditional applications. For automobile headlights, “instant” restart is desired for safety considerations.  In Hg-free HID lamps, Hg is often replaced by ZnI 2 along with the use of conventional metal halides such as NaI and ScI 3.  We discuss the properties of D4 HID lamps with results from computer models:  Breakdown characteristics with and without condensed salt layers,  Mercury free D4 lamp thermodynamics database for Xe/NaI/ScI 3 /ZnI 2 and LTE-derived densities.  The effects of mixing, segregation and ionization of light and heavy additives. DESCRIPTION OF MODEL: nonPDPSIM  Fluid averaged values of mass density, mass momentum and thermal energy density obtained using unsteady algorithms.  Individual fluid species diffuse in the bulk fluid. GEOMETRY AND CONDITIONS  D4 lamp as used implemented in model using unstructured finite-volume mesh.  Electron emitting edges on bottom and top electrodes.  Voltage pulses are applied to bottom electrode with simple circuit model - ballast resistor in series with powered electrode.  Xe, 30 kV peak voltage, dV/dt = 150, 100, 50 V/ns, 8 atm, positive and negative 4 cm 2.7 mm  Salt layer on gravity side  Standard D4 lamp  Condensed salt layers on walls are present at breakdown. Experiments show breakdown along side with salt layers.  Salt layers (10s of μm thick) have mild electric conductivity. [e] DENSITY, NEGATIVE PULSE : dV/dt = -150 V/ns MIN MAX Log scale  Injection of seed electrons by short puff from negative electrode.  Electron cloud drifts towards the opposite electrode – intersects with high field region of opposite electrode initiating avalanche.  Symmetric discharge without salt layer.  Conductive salt layer create regions of high electric field at edges.  Avalanche initiated in these regions of higher E/N.  Tracking along salt layer as a surface discharge as charging occurs.  Multiple re-strikes to the edges of salt layer.  Surface streamer from the opposite electrode.  [e] (3 dec) CURRENT-VOLTAGE, BREAKDOWN TIMES  Without salt layer  Multiple re-strikes of the streamer during avalanche.  For large dV/dt time of flight of seed electrons is comparable with streamer formation time and the influence of salt layer is not very important.  For low dV/dt time of flight is larger than the time of streamer formation - salt layer tends to decrease the breakdown voltage and time. Xe/NaI/ScI 3 /ZnI 2 THERMODYNAMICS  Transition to arc reflects change in plasma from kinetic to thermodynamic regime. Thermodynamics of D4 mixtures are poorly understood.  Database of Xe/NaI/ScI 3 /ZnI 2 thermodynamic data produced to predict lamp performance through transition from glow to arc phase.  High degree of dissociation of ScI 3, ZnI 2 followed by dissociation of heavy dimers.  The Sc, Na, and I atoms outstrip the molecules (3000 – 6000 K).  For T >6000 K, Sc + (IP 6.54 eV) and Na + (IP=5.1 eV) dominate over neutrals..  Zn + at high temperatures.  Xe/NaI/ScI 3 /ZnI 2 = 1/ / / PLASMA COMPOSITION vs. TEMPERATURE TRANSITION TO ARC MODE Gravity  Xe/NaI/ScI 3 /ZnI 2 = 1/ / /  Alkali metal iodides gradually dissociate with appearance of free metals and free iodine.  Large special variation in the additive vapor pressure.  Temperature gradients translate into mole fraction variations.  Acoustic oscillations from rapid formation of conducting channel.  No Salt Layer  With Salt Layer No Salt Layer With Salt Layer