Poisson’s equation, continuity equations and surface charge are simultaneously solved using a Newton iteration technique

1. MINMAX Log scale MODELING MERCURY-FREE HID LAMPS: BREAKDOWN CHARACTERISTICS AND THERMODYNAMICS* * Work supported by Universal Lighting Technologies, Inc. Ayumu Sato, Nanu Brates, Koji Noro Universal Lighting Technologies, Inc., Woburn, MA 01801 USA Natalia Yu. Babaeva and Mark J. Kushner University of Michigan, Ann Arbor, MI 48109 USA CURRENT-VOLTAGE, BREAKDOWN TIMES • 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 ZnI2 along with the use of conventional metal halides such as NaI and ScI3. • 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/ScI3/ZnI2 and LTE-derived densities. • The effects of mixing, segregation and ionization of light and heavy additives. • 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. DESCRIPTION OF MODEL: nonPDPSIM • 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. Xe/NaI/ScI3/ZnI2THERMODYNAMICS • Transition to arc reflects change in plasma from kinetic to thermodynamic regime. Thermodynamics of D4 mixtures are poorly understood. • Database of Xe/NaI/ScI3/ZnI2 thermodynamic data produced to predict lamp performance through transition from glow to arc phase. • 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 • Salt layer on gravity side • Standard D4 lamp 2.7 mm 4 cm • 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. PLASMA COMPOSITION vs. TEMPERATURE • 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 • High degree of dissociation of ScI3, ZnI2 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/ScI3/ZnI2 = 1/0.000316/ 0.0000463/0.0000448 [e] DENSITY, NEGATIVE PULSE : dV/dt = -150 V/ns  No Salt Layer [e] (3 dec) TRANSITION TO ARC MODE Gravity  With Salt Layer • Xe/NaI/ScI3/ZnI2 = 1/0.000316/0.0000463/0.0000448 • 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. • 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. No Salt Layer With Salt Layer