Starting mechanisms for high pressure metal halide lamps

Abstract

Ultra compact, short pulse, high voltage and current pulsers are needed for a variety of non-linear electrical and optical applications. With a fast rise time and short pulse width, these drivers are capable of producing sub-nanosecond electrical and optical pulses by gain switching semiconductor laser diodes. Gain-switching of laser diodes requires a sub-nanosecond pulser with low output impedance (5 W or less) to generate a fast (<loops) optical pulse. The designed pulsers also could be used for triggering photo-conductive semiconductor switches, gating high speed optical imaging systems, and providing electrical and optical sources for fast transient.sensor applications. Building on concepts from Lawrence Livermore National Laboratory [ 11, the development of pulsers based on solid state avalanche transistors was adapted to low impedances. As each successive stage is avalanched in the circuit, the amount of overvoltage increases, increasing the switching speed and improving the turn on time of the output pulse at the last stage. The output of the pulser is coupled into the load using a parallel plate charge line. The impedance and associated discharge time of this transmission line is tailored to the impedance and pulse duration requirements of the load. With the adapted circuit modifications, we have designed and evaluated a driver pulser capable of delivering 200 Vdc into a 5 W load with a 250 ps risetime and a 500 ps pulsewidth. Results from transmission line, resistive, and semiconductor laser diode test loads will be presented. Re fe rences :High pressure metal halide lamps typically have cold fills of a rare gas (usually Ar) of 10s Torr. Starting of the lamp involves applying a high voltage pulse to the electrodes, breaking down the gas, and upon heating produces a thermal arc. Lamps often include auxiliary electrodes to produce preionization, either by electron impact or by photoionization. To investigate these processes, a 2-dimensional lamp model has been developed. The model uses a conformal triangle-based mesh to resolve fine structures. Charged particle transport is based on a drift-diffusion formalism with Sharfetter-Gummel fluxes, solved implicitly coincident with Poisson's equation using a modified Newton's method. Neutral particle transport is separately implicitly solved using the method of successive-over-relaxation. Electron transport coefficients are obtained from either a local-field approximation or by solving the electron energy equation. Plasma properties during startup will be discussed for a selection of electrode geometries, voltage formats and impurity concentrations. References:Uiiivrrsity of'lilinois, Drporfinent c$ Elec,tric,ul uiid Coiriprrfer Eii,girieeriti,g W o k was supported by General Electric RBD Center and the National Science Foundation. 244