BREAKDOWN CHARACTERISTICS OF Xe HID LAMPS Natalia Yu. Babaeva1, Ayumu Sato2, Nanu Brates2, Koji Noro2 and Mark J. Kushner1 1University of Michigan Department of Electrical Engineering and Computer Science Ann Arbor, MI 48109 USA 2Universal Lighting Technologies, Inc. Woburn, MA 01801 USA http://uigelz.eecs.umich.edu nbabaeva@umich.edu asato@us.pewg.panasonic.com mjkush@umich.edu 62nd Gaseous Electronics Conference, October 2009 * Work supported by Universal Lighting Technologies, Inc.

University of Michigan Institute for Plasma Science & Engr. AGENDA HID lamps – need for rapid restart Description of model for breakdown Breakdown characteristics Surface roughness Cathode field emission Work function Salt layers Concluding Remarks University of Michigan Institute for Plasma Science & Engr. GEC09_ULT

University of Michigan Institute for Plasma Science & Engr. RESTART OF HID LAMPS High Intensity Discharge Lamps (HID) are being used in a variety of non-traditional applications, such as headlamps of automobiles. Environmental concerns motivate gas mixtures devoid of Hg. For automobile headlights, “instant” restart is desired with proper spectral characteristics for safety considerations. In this talk, we discuss the breakdown properties of D4 HID lamps with Xe gas fills with results from computational investigation. Electric field emission Cathode work function Condensed salt layers on the inner walls Philips D4 Osram D4 University of Michigan Institute for Plasma Science & Engr. GEC09_ULT

MODELING PLATFORM: nonPDPSIM Poisson’s equation, continuity equations and surface charge are simultaneously solved using a Newton iteration technique. Electron energy equation Thermally enhanced electric field emission current University of Michigan Institute for Plasma Science & Engr. GEC09_ULT

MODELING PLATFORM: 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. Radiation transport and photoionization. University of Michigan Institute for Plasma Science & Engr. GEC09_ULT

POISSON TO AMBIPOLAR TRANSITION When the plasma density becomes large, the efficiency of the Poisson solution degrades due to decreasing value of the dielectric relaxation time. After breakdown Poisson solver is switched to ambipolar approximation: solution of current-conservation equation coupled with continuity equations. Poisson solver Transition to ambipolar transport is based on an assessment as to when the plasma region has conditions close to quasi-neutrality. University of Michigan Institute for Plasma Science & Engr. GEC09_ULT

GEOMETRY AND CONDITIONS Geometry based on industry standard D4 lamp as used in automobile headlamps. Implemented in model using unstructured finite-volume mesh. Negative voltage pulses applied to bottom electrode Simple circuit model - ballast resistor in series with powered electrode (R = 2.2 to 10 kΩ). Xe, 14 atm 4 cm 2.7 mm University of Michigan Institute for Plasma Science & Engr. GEC09_ULT 7

BASIC HID LAMP PARAMETERS Negative pulse to bottom electrode, - 35 kV, 260 ns rise time. Electric field emission from cathode begins at middle of the voltage ramp (130 ns). Electron densities of 4 1014 cm-3 in initial ionization wave – 1015-1016 cm-3 in near electrode regions. Te of 2.7 eV in ionization wave – sustained at 2.3 eV. Xe, 14 atm, t = 220 ns MIN MAX Log scale Potential 0 to -18.2kV [e] 5x1015 cm-3 (3 dec) Te 0 – 2.5 eV University of Michigan Institute for Plasma Science & Engr. Animation Slide GEC09_ULT 8

FIELD EMISSION vs. THERMAL EMISSION  [E-field, V/cm]  [e, cm-3] Xe, 14 atm. For cold cathodes E > 107 V/cm required for meaningful electron emission current. Smooth cathodes have E  3 x 105 V/cm. Without roughening, field emission occurs only with voltages in excess of typical operation. For warm cathodes (T = 1,000 K) emission starts at voltages > 8 kV. For hot cathode (T=3,000 K) thermal emission is significant at 6 kV. Electron density is uniformly distributed near all injection surfaces. University of Michigan Institute for Plasma Science & Engr. GEC09_ULT

ELECTRODE SURFACE ROUGHNESS 300 μm Cathodes have surface roughness and grain boundaries that produce electric field enhancement leading to thermionically enhanced electron field emission. Modeled cathode with small spikes to approximate roughness - artificially increased field by 50-150 at tip of spikes to account for unresolved enhancement. University of Michigan Institute for Plasma Science & Engr. GEC09_ULT

COLD CATHODE: SURFACE ROUGHNESS  t=0-60 ns Cold cathode with roughness can significantly lower the breakdown voltage due to electric field enhancement. Initial stage of breakdown starts from the highest spikes at a voltage between 8–16 kV. The spikes act like a jet injecting charges into the lamp gap. Animation Slide MIN MAX Log scale University of Michigan Institute for Plasma Science & Engr. GEC09_ULT

University of Michigan Institute for Plasma Science & Engr. HOT CATHODE: THERMAL EMISSION DOMINATES  [e]  t=0-40 ns With hot cathode (T=3,000 K) with roughness, electron emission is largely thermal. Electron density is nearly uniformly distributed along the cathode surface in spite of electric field enhancement at roughness. Animation Slide MIN MAX Log scale University of Michigan Institute for Plasma Science & Engr. GEC09_ULT

University of Michigan Institute for Plasma Science & Engr. INITIAL GAS HEATING NEAR CATHODE: Tgas  Smooth electrode Uniform heating near smooth electrodes. Roughness produces non-uniform current injection. Higher local current densities produce gas hot-spots and rarefaction earlier. Xe, 14 atm  0-200 ns  Rough electrode  0-60 ns Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale GEC09_ULT

University of Michigan Institute for Plasma Science & Engr. LONGER TIMES: FLUID MOVEMENT DOMINATES Tgas (300-610 K) Te (0.1-2.1 eV)  t=0-68 µs  0-60 ns Tgas and Te after breakdown as arc channel forms for Xe 1 atm. Temperature rise starts from cathode and anode, and propagates into the plasma channel. Acoustic waves due to heating and channel rarefaction. Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale GEC09_ULT

W (4.6 eV) vs Th (2.3 eV): WORK FUNCTION W cathodes often have Th inclusions to lower work function. Charge injection depends on E-field enhancement by roughness (spikes) and work function. W spikes: Higher field enhancement for emission. Th spikes: Inject charge at lower voltage. Injected negative charges can prevent emission from other spikes. [e] (2 ·1014 cm-3) Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale GEC09_ULT

CATHODE ROUGHNESS AND GAS HEATING Tgas (350 K) Se 5x1021 cm-3 s-1 (4 dec) Non-uniform current injection due to roughness results in non-uniform gas heating. Higher local current densities produce gas hot-spots Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale GEC09_ULT

University of Michigan Institute for Plasma Science & Engr. BREAKDOWN VOLTAGE VS CATHODE WORK FUNCTION W W-Th W W-Th [e] (2 ·1014 cm-3) Animation Slide Electron density immediately after breakdown for cathodes of pure W and W with Th inclusions. Pure W cathode: Underdeveloped discharge due to potential shield by injected charges. Thoriated cathode: Th spikes inject more charge at earlier stages of the voltage pulse resulting in earlier breakdown. MIN MAX Log scale University of Michigan Institute for Plasma Science & Engr. GEC09_ULT

University of Michigan Institute for Plasma Science & Engr. CONDENSED SALT LAYERS In lamps using metal-halide fills, condensed salt layers on walls are present at breakdown. Due to physical orientation, salt layers are on one side of lamp. Salt layers (10s of μm thick) have mild electric conductivity. Experimental observations show preferential breakdown along side of lamp with salt layers. D4 arc tube with salt layers University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale GEC09_ULT

University of Michigan Institute for Plasma Science & Engr. SYMMETRIC BREAKDOWN WITHOUT SALT LAYERS  [e]  t=0-180 ns Symmetric ionization wave (IW) initially propagates along the walls due to proximity of ground planes. After bridging of gap, discharge occurs along axis. Xe, 14 atm, -35 kV, 300 ns. Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale GEC09_ULT

University of Michigan Institute for Plasma Science & Engr. EFFECT OF SALT ISLANDS: BEFORE BREAKDOWN +ρ -ρ Te 0-2.7 eV E/N 0-170 Td Se 4x1022 cm-3 s-1 Charges accumulate on salt layers prior breakdown due to displacement currents. Mildly conducting salt layers (10-3 – 10-5 /Ohm-cm) short out electric field. Electric field enhancement at edge of salt layers with subsequent enhancement of electron ionization source. Path for ionization wave is formed before the breakdown occurs. University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale GEC09_ULT

EFFECT OF SALT ISLANDS: BREAKDOWN (4 ·1014 cm-3) Animation Slide The ionization wave tends to propagate towards the edge of the nearest salt layer. The pattern of discharge propagation is governed by regions of electric field depletion and/or enhancement. The discharge path becomes asymmetric. [e] E/N Se University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale GEC09_ULT

University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS Pure field emission, field-enhanced thermal emission and thermal emission have been studied for cold, hot and warm cathodes. Fields > 107 V/cm are needed for cold field emission. Typical E-fields near smooth cathodes are much lower. Hot cathodes can emit currents at voltages of a few kV. Cathode roughness decreases breakdown voltage and increases local gas heating. Modes of charge injection depend on electric field enhancement by cathode roughness. Lower work function of thoriated cathodes results in earlier breakdown. With condensed conducting salt layer on the tube wall ionization wave tends to propagates along the inner wall. University of Michigan Institute for Plasma Science & Engr. GEC09_ULT