Ruhr University BochumInstitute for Electrical Engineeringand Plasma TechnologyProf. Dr.-Ing. Peter Awakowicz

Universitätstaße 150 Tel: 0234/32-2306244801 Bochum Web: www.aept.rub.de

High Intensity Discharge Lamps: Gas Phase Emitter Effect

hidl@aept.ruhr-uni-bochum.de

The lifetime of metal halide lamps is improved by a reduction of the temperature of tungstenlamp electrodes being accomplished by the gas phase emitter effect. It is generated on the electrodesurface by a monolayer of electropositive atoms of certain emitter elements (e.g. Th, Ba, Dy, Ce, La,Tm, Cs), which are added to the lamp filling. This monolayer reduces the effective work functionof tungsten by lowering the potential barrier for electrons leaving or entering the electrode. Anoptimal electrode surface coverage with emitter atoms is dependent on emitter properties (workfunction, ionisation energy, atomic mass, adsorption energy), its partial pressure, and electricallamp parameters (waveform, amplitude, frequency). The gas phase emitter effect is quantified byspace and phase resolved electrode tip temperature and particle density measurements by means ofabsolutely calibrated pyrometry as well as optical emission and absorption spectroscopy.

1 IntroductionHigh intensity discharge (HID) lamps are very efficientand economical spot light sources with high luminousefficacy, long lifetime, and good color rendering. Thus,their use is favoured in several fields of application, e.g.in street or automotive lighting as well as in video pro-jection systems. Nevertheless, the high thermal load ofthe electrodes results in high electrode temperatures,limiting the electrode lifetime and, therewith, the lamplifetime. A reduction of the electrode temperature bylowering the effective work function of the electrodecan be achieved by the emitter effect and leads to anincrease of lamp lifetime.

Doping the tungsten electrode with thorium oxide(ThO2) is the most traditional method to generate anemitter effect. As well, by a storage of an emitterelement as a compound within the windings of atungsten coil, being imposed on the electrode rod,an emitter effect is also generated. The emitter isbrought to the electrode tip by a surface diffusionalong the electrode shaft. Seeding HID lamps withmetal iodide salts results on the one hand in a multi-line emission spectrum in the visible range, which

allows to adjust a sufficient color temperature and colorrendering. On the other hand, these metal iodidesgenerate frequently an emitter effect, originating fromthe gas phase. Within the last years, the influenceof different elements on the gas phase emitter effecthas been investigated. Under certain lamp operatingconditions sodium (Na), dysprosium (Dy), cerium (Ce),lanthanum (La), thulium (Tm), and caesium (Cs) havebeen found to be suitable to generate an emitter effect.

2 Gas phase emitter effectThe gas phase emitter effect can be generated by anevaporation of metal iodide within the lamp. Withinthe cathodic phase an emitter ion current is directedtowards the cathode. The emitter ions recombine infront or at the cathode surface with electrons emittedfrom the cathode. The neutralised emitter atoms aredeposited on the electrode surface. This monolayer ofatoms, being electropositive with respect to tungsten,reduces the potential barrier at the electrode surface forelectrons leaving or entering the electrode. Thus, theelectrons have to overcome a reduced work function,which is dependent on the degree of surface coverageby emitter atoms. Therewith, the cathode temperatureis reduced and the thermionic electron emission is

High Intensity Discharge Lamps: Gas Phase Emitter Effect

increased. The cathodic emitter effect overlaps to theanodic phase at higher frequencies with increasinglifetime of the monolayer. Due to their inertia, theemitter ions and atoms remain in front of the anodeand form an atomic layer on the anode surface, whichreduces the electrode temperature furthermore. Be-cause the atomic monolayer is mainly generated by anion current towards the electrode, the ionisation energyof the emitter material and, thus, the mobility of theions has to be considered too. In addition, the lifetimeof the atomic layer is reflected by the adsorption energyof the emitter material. It determines the thermaldesorption of the layer.

3 Investigated HID lampsThe ceramic metal halide lamps under investigationare special research lamps, which are similar to com-mercial ones. The discharge vessel is made of yttriumalumina garnet (YAG), a transparent ceramic, which al-lows optical observations while commercial lamps havetranslucent and strongly scattering vessels, typicallymade of polycrystalline alumina (PCA). The dischargevessel is filled with a small amount of mercury (Hg)to generate an unsaturated background pressure of afew mega pascal during lamp operation and to ensurea sufficiently high power input into the arc. To adjustthe colour rendering and to generate the gas phaseemitter effect, the lamps are seeded with metal halides(e.g. CeI3). The lamps are operated with differentrectangular alternating currents at various frequencies.Nevertheless, they are built for a nominal input powerof 70W at 125Hz.

electrical connection

gas tight feed through

tungstenelectrodes

lamp vessel of YAG

arc discharge

filling gas

liquid saltpool

Figure 1: Schematic drawing of the discharge vessel ofa YAG lamp.

4 DiagnosticsHigh speed optical emission and broadband absorptionspectroscopy as well as pyrometry and high speed

photography are used to determine spatial and phaseresolved plasma temperatures, particle densities, elec-trode temperatures, electrode power losses, and thetype of arc attachment. For calibration a tungstenribbon lamp is used as a standard to obtain absolutevalues. Furthermore, electrical measurements areperformed.

5 Selected results: Gas phase emittereffect of Ce

Measurements are performed at a YAG lamp seededwith 1mg CeI3 to generate the gas phase emitter effect.The Ce-lamp is operated with switched-dc current of0.8A and operating frequencies of 1, 10, 100, 500Hz,and 1 kHz. The lamp exhibits an operating pressureof 2MPa, which is mainly formed by unsaturated Hgvapour. Phase resolved measurements are accom-plished. A distinction is made between the anodicand cathodic phase. Images of the arc attachment inthe middle of the respective phase as well as phaseresolved results of electrode tip temperature Ttip anddensity of atoms Na and ions Ni are presented independence on frequency. The images and pyrometricmeasurements are recorded at the upper electrode.The spectroscopic measurements of Na and Ni areperformed at a distance of 125µm in front of the upperelectrode while Na is additionally measured in themiddle of the discharge.

5.1 Images of the arc attachment andelectrode tip temperature measurements

Images of the arc attachment in the middle of theanodic and cathodic phase at the upper electrode ofthe Ce-lamp are given in figure 2. In the middle ofthe anodic phase the arc attachment is diffuse at lowoperating frequencies and becomes more and moreconstricted with increasing frequency. In the middleof the cathodic phase the arc attachment is spot-like ateach frequency. The spot attachment is presumablyinduced by a locally reduced work function on theelectrode surface effected by the gas phase emittereffect. This confirms that the gas phase emitter effectfavours a constricted arc attachment. Independent onthe position in phase, the arc is bended with increasingfrequency. This arc bowing is caused by acousticresonances, which occur at some high frequencies ofa few thousand hertz.

Figure 3 shows Ttip of the upper electrode in de-pendence on frequency of the Ce-lamp (solid lines)

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High Intensity Discharge Lamps: Gas Phase Emitter Effect

1 Hz 10 Hz 100 Hz 500 Hz 1 kHz

middle of anodic phase

middle of cathodic phase

Figure 2: Images of the arc attachment in the middle ofthe anodic and cathodic phase at the Ce-lampfor various frequencies, recorded by a CCDcamera with an 890nm filter in front.

and a pure Hg-lamp without any metal iodide salt asreference (dotted lines). Ttip within the anodic phaseis represented by red lines while the cathodic phaseis illustrated by blue lines. Within both lamps theelectrode temperature is higher when the electrode actsas an anode than as a cathode. This change withpolarity diminishes with increasing frequency. Phaseresolved results of Ttip, given in [1], demonstrate aheating of the electrode in the anodic phase and areduction of its heating in the cathodic phase. Within apure Hg-lamp the maximal value of Ttip is at least only100K below the melting point of tungsten, whereasit is 450K lower within the Ce-lamp. In comparisonwith the pure Hg-lamp, a general reduction of Ttip ofat least 350K within the Ce-lamp is confirmed. This iscorrelated with the presence of Ce atoms and ions infront of the electrode. Additionally, within the Ce-lampa decrease of Ttip is observed with increasing frequencywhile Ttip stays constant within the pure Hg lamp.

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Figure 3: Anodic and cathodic electrode tip tempera-ture of the Ce-lamp Ttip,Ce and a pure Hg-lamp Ttip in dependence on frequency, deter-mined by pyrometry at 890nm.

5.2 Particle density measurements

The courses of the Ce atom density Na is given infigure 4 while the courses of the Ce ion density Ni isshown in figure 5 in dependence on frequency. Na andNi are given 125µm in front of the upper electrode.Additionally, Na is also presented in the middle ofthe discharge as reference. Na and Ni are higherin the cathodic than in the anodic phase. Both Cedensities decrease with increasing frequency in thecathodic phase and the courses within both phasesconverge with increasing frequency. Also, the coursesin front of the electrode approach to the measurementin the discharge middle with increasing frequency.Phase resolved Ce density results, already given in[1], demonstrate an increase of Ni within the cathodicphase with a time constant of the order of 10ms and ofNa with a higher time constant of 20ms. Accordingly,the maximum values of Na and Ni are lowered withincreasing frequency at operating frequencies above afew tens of hertz. The variation of Ni and the delayedvariation of Na in dependence on phase and operatingfrequency demonstrate a cataphoretic transport of Cein front of the electrode. Thus, an atomic monolayeron the cathode surface is generated by a Ce ion currentfrom the arc towards the electrode. The low densityvalues in the anodic phase with respect to the cathodicphase is produced by a reversal of the Ce ion current.Na and Ni are nearly constant over time for higherfrequency, already shown in [1]. This indicates that

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Natom,Ce,c

(in front of cathode)

Natom,Ce

(discharge middle)

Figure 4: Anodic and cathodic Ce atom density, mea-sured 125µm in front of the upper electrodeand in the middle of the discharge within theCe-lamp in dependence on frequency, deter-mined by absorption spectroscopy at 577nm.

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High Intensity Discharge Lamps: Gas Phase Emitter Effect

the Ce ions are too immobile to follow the frequencyabove 500Hz. They remain directly in front of theelectrode, recombine, and form a layer of Ce atomson the electrode surface even during the anodic phase.Thus, the emitter effect is extended to the anodic phase.

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Figure 5: Anodic and cathodic Ce ion density, mea-sured 125µm in front of the upper elec-trode within the Ce-lamp in dependenceon frequency, determined by emission spec-troscopy at 457nm.

6 Conclusion

The gas phase emitter effect is proven by higher emitteratom and ion densities in the cathodic than in theanodic phase. Images of the arc attachment showan emitter spot in the middle of the cathodic phase,while the arc attachment is diffuse in the middle ofthe anodic phase. The deposition of emitter materialon the surface of the tungsten cathode reduces thework function of tungsten and, therefore, the electrodetemperature and electrode power losses.

References

[1] C. Ruhrmann et al 2011 J. Phys. D: Appl. Phys. 44355202

[2] C. Ruhrmann et al 2013 Proceedings of the 18thPlansee-Seminar, Reutte (Austria) RM123

[3] C. Ruhrmann 2014 Investigation of the emitter effectinduced by rare earth elements in HID lamps, PhDthesis, Ruhr-Universität BochumThis essay is generally taken from [2], but is short-ened. Further detailed investigations of the gas phaseemitter effect can be found in [3].

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