Operation of the LiFi Light Emitting

Operation of the LiFi Light Emitting Plasma in Resonant Cavity Richard P.Gilliard,Marc DeVincentis,Member,IEEE,Abdeslam Hafidi,
Daniel O’Hare,and Gregg Hollingsworth
Abstract—The LiFi lamp utilizes a microwave technology which directly delivers high-frequency power to a light-emitting plasma without the need for electrodes.A dielectric waveguide generates electromagneticfield modes within a resonator which efficiently couple the power to the high-temperature high-density plasma.
Index Terms—Light sources,microwave amplifiers,plasma devices,resonators.
I.I NTRODUCTION
T HE STANDARD way of delivering electric power to discharge light sources is through the use of electrodes by which electric current is conducted through the gas[1].The disadvantages of electrodes as a source of thermionic electrons have long been recognized.Differences in the expansion of refractory metal electrodes and accompanying feedthroughs on the one hand and the light-transmitting body material(typically fused quartz or alumina)at operating temperatures on the other generally make the seal a weak point and often the site of a nonpassive failure of the lamp.In addition,the electrodes are a source of power loss proportional to the operating current of the lamp because of the inevitable cathode and anode falls. Furthermore,the ultimate evaporation of the electrode materials at operating temperatures results in the deposition of(typi-cally)tungsten on the lamp body walls causing light absorp-tion,diminution of light transmission,and overheating.These chemical processes are exacerbated when the lamp operates at high temperature and pressure and uses chemically reactive materials(e.g.,metal halides)asfill materials.
Many attempts have been made to transfer power to plasma lamps without the use of electrodes[2].Most existing products of this type have power inductively coupled to discharges with pressures lower than1atm.These plasmas do not take advantage of the higher efficacy,better color,and more compact source possible in plasmas with significantly higher pressures (greater than5atm).Capacitively coupled discharges,on the other hand,typically result in plasmas with axially directed currents which generally enable taking advantage of the el-lipsoidal and cylindrical geometries typical of highly efficient
Manuscript received September10,2010;revised January21,2011;accepted February3,2011.Date of publication March16,2011;date of current version April13,2011.
The authors are with the Luxim Corporation,Sunnyvale,CA94089USA (e-mail:rgilliard@;mdevincentis@;ahafidi@; dohare@;ghollingsworth@).
Color versions of one or more of thefigures in this paper are available online at .
Digital Object Identifier10.1109/TPS.2011.2113358high-intensity discharge(HID)lamps.The shaping offield lines using external metal plates to form the capacitive elements, however,generally results in the overheating and oxidation of the metal elements in a compact design.
Resonators or resonant cavities in dielectric materials can be used as a means for creating a standing wave electricfield. Placing a plasma bulb within a resonant cavity or in a gap in a resonator accordingly makes possible the high-efficiency and high-power compact plasmas ideally suited as light sources with atomic metals and metal halides as radiating species. The inherent advantages of this approach include high lumi-nous efficacy of the emitter,in the range of150lm/W or greater, as well as high brightness and excellent color quality.The structure is mechanically robust without typical degradation and failure mechanisms associated with tungsten electrodes and glass-to-metal seals,resulting in a useful lamp life of 30000+h.In addition,the unique combination of high-temperature plasma and digitally controlled solid-state elec-tronics results in an economically produced family of lamps scalable in packages to over100000lm.
In this paper,we will cover the basic physics and chemistry of the operation of a specific configuration of the emitting part of the lamp(resonator and plasma).In this configuration,the resonator is cylindrically symmetric with the plasma partially embedded within the resonator.Nonsteady-state phenomena (ignition,warm-up,etc.)and operation of the driver and control hardware and software as well as other geometric configura-tions will be subjects of later publications.
II.O PERATION OF THE R ESONANT C AVITY
The resonant cavity(or more generally the resonator)is the means by which power is transferred from the power amplifier to the bulb containing the plasma[3],[4].In one of its most basic configurations,the resonant cavity can be constructed as a cylinder in which all external surfaces are coated with a conducting material.(Note that,in a practical system,a small opening is needed for an electrical probe connection. The probe serves as a monopole antenna which excites the resonant mode in the resonator.In addition,a hole must be cut in the dielectric material and coating for the placement of the plasma bulb,resulting in a small perturbation effect on thefield configuration.)When RF power is applied through a probe to the resonator interior,a standing wave in accordance with the wave equation is created within the cavity(see Fig.1)
∇2E+k2E=0.(1)
0093-3813/$26.00©2011IEEE
Fig.1.Electricfield in solid cylindrical resonant cavity.All outer surfaces are coated with conductive material.Axial electricfield E z(r)
.
Fig.2.(a)Electricfield in resonator in Fig.1in planes with and without plasma.(b)Planes forfield calculations in Fig.1.
Here,E is the electricfield,and k is the wavenumber.In this case,the solution that satisfies the boundary conditions and is unbounded is the Bessel function representing the TM010 eigenfunction of the wave equation
E z(r)=E0J0(kr).(2) Placing the plasma-containing bulb within the cavity results in the transfer of power to the conducting plasma
through Fig.3.Electricfield in cylindrical resonator.All surfaces(inner and outer)ex-
cept the front are coated with conductive material.Radial electricfield E r(r)
. Fig.4.Fringing electricfield in cylindrical resonator.All surfaces(inner and
outer)except the yellow gap region are coated with conductive material. ohmic heating.The result is a dramatic drop in the electricfield in the region which includes the plasma as shown in Fig.2. These calculations were done using the modeling program CST Microwave Studio at a frequency of1.9GHz.In the case of the plasma being present,a constant resistivity was assumed for the plasma as a function of radius.In Section IV,more detailed plasma calculations will be shown which allow plasma parameters to vary as a function of radius.
Fig.3shows a resonator in which the standing wave is a sine wave representing the TEM mode.Again,all surfaces except the one noted are coated with a conducting material.In this case,thefield lines become radial rather than axial in the simple cylinder case.The solution for Fig.3is a quarter sine wave
E r(z)=E0sin
πz
2d
(3)
Fig.5.Electricfield configuration in capacitively coupled plasma(CCP) compared to the configuration in HID lamp with currentflowing between electrodes
[11].
Fig.6.Electricfield configurations and equivalent circuits in cylindrical resonator with gap as in Fig.3.(a)Before plasma breakdown.(b)After breakdown with currentflowing
[11].
Fig.7.Electric and magneticfield intensities in resonator with gap.Bulb shown in gap region.
where d is the length of the cylinder.If a gap is created within the conducting material(with all other surfaces now coated) as shown in Fig.4,the net effect is to enablefield lines to fringe into the cylindrical region within the gap.The
placement ponents of LiFi:AC-to-DC converter,driver,and
emitter.
Fig.9.Schematic of resonator and bulb.
of the plasma bulb in this space results in generally axialfield lines similar to those in a capacitively coupled discharge and a HID lamp with electrodes as shown in Fig.5.Fig.6shows the electricfield configuration before the breakdown of the gas in the bulb and during operation along with the equivalent circuits. The intensities of the electric and magneticfields are shown in Fig.7.The electricfield is concentrated in the gap region as noted.As a result,power to the plasma is almost entirely trans-ferred through the electricfield(ohmic heating).The magnetic field,on the other hand,is concentrated at the opposite end of the puck from the gap and transfers essentially no power.
It is important to point out that thefield configurations shown (except that in Fig.2with the plasma present)are those before the breakdown of the plasma,establishment of a plasma current, and power delivery.After breakdown,the operating plasma has an electricfield.Although generally axial as shown in Fig.6(b), the amplitude diminishes at the smaller radii as a result of the skin effect(as shown in Fig.2).More details of the plasma con-figuration will be covered in the section on plasma properties.
III.E MITTER
The“emitter”is the light-emitting portion of the lamp,sepa-rate from the“driver,”which consists of the power and control electronics.The emitter specifically consists of the“bulb”(plasma-containing vessel which is the emitter of light),the “puck”(resonator),and the connector and heat sink material. These components are shown in Fig.8.A schematic of the emitter is shown in Fig.9.It is clear from Fig.9that much of
the light is blocked by the opaque puck(ceramic resonator). Inserting highly reflective material on the inside surfaces of the puck and positioning the bulb as far forward as possible ameliorate this problem.Even so,roughly30%of the light is absorbed within the resonator in this configuration.The subject of later work will be other ways of mounting the bulb and reducing the light absorption.This will be covered in subsequent publications.
IV.P ROPERTIES OF THE P LASMA
The plasma is the light source and fundamentally can use as radiating emitters the same materials used in HID lamps today.These include rare gases and atomic species dosed in either atomic or molecular forms.Many materials that are chemically incompatible with refractory metals(and therefore cannot be used in lamps with electrodes)are also candidates for fill materials in lamps of this type.
The LiFi lamp is particularly suitable for using metal halides as radiating species.The basic advantages of metal halides as dose materials in HID lamps are outlined in various references [1],and again,the absence of electrodes(highly reactive in many metal halide environments)in the LiFi lamp underscores one of the key advantages of this new technology.
Also,operating at high pressures(typically above20atm), the plasma is at local thermodynamic equilibrium throughout most of the bulb.As such,under most operating conditions,a pure mercury version of this lamp would be expected to operate similarly to the high-pressure mercury discharge,thoroughly described by Elenbaas,would operate[5].As a test of this,an experiment was carried out in which cylindrical bulbs of two arc lengths were characterized at several power loadings per unit length using the LiFi configuration.The results are shown in Fig.10.These lamps were operated horizontally.The plasmas were in cylindrical fused quartz bulbs with an ID of0.4cm.The two arc lengths were1.0and1.5cm with mercury mass densi-ties between4.5and5.0mg/cm.What is plotted is the radiation output per unit length of the arc as a function of the power input per unit length of the arc.The graph is very nearly a straight line with x-intercepts from3.3to3.5W/mm.This is consistent with the experiments of Elenbaas which found a similar linear dependence(in fact,largely independent of mercury density or tube diameter over wide ranges of these parameters).The slope of the graph represents the increment of the increase of the radiation output per unit length of the arc for the corresponding increase in the power input per unit length.The x-intercept re-presents the nonradiative power loss per unit length of the arc. For high-pressure mercury discharges with electrodes and comparable operating parameters to LiFi lamps,Elenbaas mea-sured approximately2.6W/mm of nonradiative loss for a hor-izontal operating lamp.The thermal loss in the LiFi measures slightly higher,primarily because the bulb is in virtual contact with the puck(resonator)which adds to the thermal loss.The slope of the graph indicates radiative output(between360and 1000nm)in the range of∼22%of the input power.Elenbaas’data indicated that72%of the input power of a high-pressure mercury discharge is emitted as radiation at all wavelengths (UV,visible,and IR).In light of the radiation absorption in the resonator in the case of our lamp,our data are reasonably consistent with that of Elenbaas since approximately32%of the total radiated power of a high-pressure mercury discharge is emitted in the visible range of wavelengths[6].
These data tend to substantiate our basic picture of the operation of the plasma as having an axial electricfield con-figuration and currentflow comparable to that of an HID lamp with electrodes with comparable core temperatures,but without the electrode power losses associated with conventional lamps. Although the nonradiative losses per unit length of the arc are slightly higher than that in lamps without the resonator,we are able to operate our lamp at substantially higher power loadings per unit length than standard lamps(because of the absence of glass-to-metal seals and corrosive electrode chemistry and wall darkening).As a result,a LiFi lamp operating at200W/cm (or greater)has a lower percentage of nonradiative loss than a conventional lamp operating at100W/cm.
There is one fundamental difference,however,between a discharge in which electric current isflowing between two elec-trodes and a lamp in which power is supplied by externalfields which must penetrate the plasma;specifically,in a discharge lamp with electrodes,the current isflowing primarily in the core of the discharge.On the other hand,in a lamp withfields fringing into the plasma,the current is less concentrated in the center as a result of the skin effect of the plasma.This results in a temperature distribution(as a function of radius)not nec-essarily approximately parabolic in shape nor necessarily with a peak temperature occurring at the centerline of the discharge. In fact,the peak temperature may actually occur at a nonzero radius.
The specific spectral features which result in emission within the plasma are,in turn,dependent upon the local temperature and number densities of the various species and their atomic or molecular structures.The net radiation output from the plasma is dependent upon the emission,reabsorption,and reemission properties throughout the plasma volume,which requires non-local radiation transport modeling[7].The complete details of the discharge can,in principle,be calculated for a cylindrically symmetric plasma with constant properties per unit length using the Elenbaas–Heller equation[8]
σ(T)E2−ε(T)
R2=−
1
ρ
∂
∂ρ
ρκ(T)
∂T
∂ρ
.(4)
Here,ρis the reduced radius(r/R),E is the(axial)electric field,and T is the local temperature(in this case,also a function ofρ).The radiation source function isε;σandκare the material functions(electrical and thermal conductivity). Solving,of course,requires that all material properties are known and all chemical and spectral features are precisely accounted for.Since there are literally hundreds to thousands of such features in any real lamp and numerous asymmetries(such as convection)and the data with regard to the materials are, in many cases,only approximately known,all existing models are,at best,approximate.For this reason,most successful commercial design models are primarily empirical based on multidimensional design space data.
Fig.10.Radiated power per unit length(360to1000nm)for mercury-only plasma as a function of input power per unit
length.
Fig.11.Electricfield and current in operating lamp.
Through private correspondence,Waymouth[9]has pro-vided microwave discharge model calculations based on some of our proprietary designs.His model is described in[10]and, in this case,allows for optical reabsorption.General parameters include1-D cylindrical symmetry,tube diameters of3to4mm, mercury pressures in the tens of atmospheres,metal halide vapor pressures on the order of1atm or less,power delivery on the order of25to40W/mm,and frequencies from150MHz to2.5GHz.Material functions for thefill materials are known only approximately,and the model calculations are therefore best used to obtain a qualitative picture of the physical effects. Fig.11shows the electricfield and current in an operating lamp.Fig.12shows the calculated temperature profiles for a lamp operated at various frequencies[9].Note that the electric field during operation is substantially altered from the static fields shown in Fig.4before the breakdown as expected from the less detailed model used for Fig.2.Note also that the radius at which the peak temperature occurs increases as the operating frequency increases.These curves most dramatically demonstrate the influence of the skin effect on the
plasma Fig.12.Plasma temperature profiles as a function of radius calculated by John Waymouth for different operating frequencies[11].
properties.In particular,instead of peaking at the arc tube axis(as shown in Figs.1and4),during operation,thefield decreases toward the core of the bulb.Similarly,power delivery is maximized at the outer radii,and the temperature profile does not peak at the core.
Several points are important to note regarding these calculations.
1)The calculations assume a radiation source function of
the form
S=A exp
−E
kT
(5)
where E is taken as the average excited state energy for mercury,indium,and rare earth discharge(assumed as
3.02eV).A is determined iteratively and is effectively
the average energy divided by the lifetime of an excited state.
2)The skin depth is
δs=
2/(ωσμ)(6)
whereωis the driving frequency,σis the electrical conductivity of the plasma,andμis the magnetic per-meability.The electricfield penetrating into the plasma externally from the resonator decreases exponentially
E=E0exp
−
R−r
δs
.(7)
3)The values of the parameters(skin depth and electricfield
penetration)vary with the radius in accordance with the temperature and electron density.The core temperature and electron density are in the ranges of5000to6000K and1015to1016/cm3,respectively.
V.P ERFORMANCE C HARACTERISTICS OF L AMPS Considerations of primary interest to any user of an actual lamp system include suchfigures of merit as spectral power distribution and the relevant quantities calculated from it,which include luminousflux,chromaticity coordinates,correlated
Fig.13.(a)Spectral power distribution for plasma containing Hg and InBr.
(b)Spectral power distribution for plasma containing Hg,NaI,and TlI.
(c)Spectral power distribution for plasma containing Hg,InBr,and DyBr3. color temperature(CCT),and color rendering index(CRI). Also of interest are the changes in the characteristics as the lamp ages,particularly the lumen maintenance.
In practical lamps,metal halidefills are generally preferable to pure mercury and other dose materials because they exhibit spectral power distributions resulting in higher luminous effi-cacy and improved color quality.Specific details of resonator and bulbfill designs are proprietary.Practical configurations
TABLE I
P ERFORMANCE AND E
FFICIENCIES
of plasma burners are fabricated from fused quartz,resulting
in cylinders with an inner diameter in the range of2to6mm
with power delivery lengths from2to10mm or more.Wall
thicknesses range from1.4to2mm.Typical operating pres-
sures are in the40–60-atm range.Wall loadings are typically 100+W/cm2,which is substantially higher than that in com-mercial HID lamps having electrodes.Typical core tempera-
tures are in the5000–6000K range with wall temperatures
in the1100–1300K range.Various metal halides have been
used,including the following:bromides and/or iodides of Li,
Na,Mg,Al,S,K,Ca,Sc,Mn,Fe,Ga,In,Cs,Tl,Pb,Ce,
Pr,Dy,Ho,and Tm.In virtually every case,all metals(except
mercury)are dosed in quantities such that there is a condensate
pool consisting of all metal halides during lamp operation.The
actual number densities of the dosed materials in the plasma
depend upon the vapor pressures of the condensed molecular
species in the condensate(which,in most cases,differ from
those in an ideal solution).The specific species(e.g.,Hg,Hg+,
Na,Na+,NaI,etc.)which exist in different regions of the arc
(i.e.,at different radii)are dependent upon the local temperature
(ranging from∼1000K to∼6000K)and the thermodynamic
properties of each species.The spectra of several sample lamps
containing variousfills are shown in Fig.13.
VI.S PECIFIC R ESULTS AND P RODUCTIZATION
The specific results achieved with this plasma/resonator de-
sign are shown in Table I(450-MHz operation).Fill materials
in addition to mercury and rare gases(described in the previous
sections)are metal bromides(including those of indium and at
least one rare earth element).The CCT is6000K,and the CRI
is80.By varying the rare earth constituent and amount,an alter-
nate spectral power distribution results in a CCT of5500K with
a CRI of94and a12%drop in the luminousflux.Recent design
enhancements have improved these numbers by positioning the
plasma vessel outside the resonator.The plasma,resonator,and
amplifier efficiencies have also been further improved.These
changes will be the subject of later work.This has resulted
in a plasma efficiency of140lm/W and component efficien-
cies as shown hereinafter(left-hand numbers).We believe the
Fig.14.Measured spectral power distribution for LiFi lamp containing rare earth dose components.The photopic curve is shown in red.The spectral power distribution shows70%of the radiated power between380and780nm(visible
range).
Fig.15.Lumen maintenance curve for a LiFi lamp.
limits of the technology are as shown on the right-hand set of numbers.
1)Plasma efficiency—140to200lm/W(depending upon
the desired spectrum).
2)Component power efficiencies.
a)Driver:
i)amplifier efficiency—82%–90%;
ii)connector efficiency—91%–95%.
b)Emitter:
i)resonator efficiency—91%–95%;
ii)emitter optical efficiency—95%–95%;
3)system efficiency—90–154lm/W.
The spectral power distribution of a metal halide dose,in-cluding rare earth metal halides,is shown in Fig.14.Life data curves to4000h are shown in Figs.15and16.
The LiFi lamp has been productized for various applications to include light sources for projection television as well as technical instrumentation.It is also used in various entertain-ment applications as well as general lighting applications to include street lighting.One particular advantage of the compact configuration and forward directionality of the source is
the T curve for a LiFi lamp with
aging.
Fig.17.Street lit using LiFi lamps.
inherent efficiency in light collection.Fig.17shows a street lit by LiFi lamps.
VII.C ONCLUSION
In summary,the physical processes involved in the operation of the LiFi lamp have been presented and described in detail. Models of the resonator and plasma profile have been presented. Lamp data have been presented indicating spectral performance and life consistent with expectations.
The general conclusions are as follows.
1)The dielectric cavity resonator is an efficient way to
deliver power to a plasma light source(90%–95% efficiencies).
2)The compactness of the source and forward directionality
of the light result in highfixture efficiencies.
3)The broad temperature profile results in high efficacy
and a broad spectrum with excellent color characteristics (140+lm/W,94CRI).
4)The absence of thermionic electrodes and glass-to-metal
seals results in long life and high system reliability (10000to30000+h).
5)The technology is economical and scalable to wide
wattage ranges with proven success in thefield.
6)The system efficiency is90–154lm/W.
A CKNOWLEDGMENT
This paper represents the combined efforts of the Luxim technical staff.The authors would like to thank J.Waymouth for the consultation as well as T.McGettigan,Luxim CEO,for the continuing support and inputs.
R EFERENCES
[1]J. F.Waymouth,Electric Discharge Lamps.Cambridge,MA:MIT
Press,1971.
[2]D.O.Wharmby,“Electrodeless lamps,”in Lamps and Lighting,
J.R.Coaton and A.M.Marsden,Eds.,4th ed.London,U.K.:Arnold, 1997,ch.11.
[3]F.M.Espiau,C.J.Joshi,and Y.Chang,“Plasma lamp with dielectric
waveguide,”U.S.Patent6737809,May18,2004.
[4]F.M.Espiau and Y.Chang,“Microwave energized plasma lamp with solid
dielectric waveguide,”U.S.Patent6922021,Jul.26,2005.
[5]W.Elenbaas,The High Pressure Mercury Vapor Discharge.New York:
Interscience,1951.
[6]J.F.Waymouth,“Light sources,”in Encyclopedia of Physical Science and
Technology,vol.8,2nd ed.New York:Academic,1992,p.715.
[7]J.L.Giuliani,G.M.Petrov,J.P.Apruzese,and J.Davis,“Non-local
radiation transport via coupling constants for the radially inhomogeneous Hg–Ar positive column,”Plasma Sources Sci.Technol.,vol.14,no.2, pp.236–249,May2005.
[8]R.O.Shaffner,“Theoretical properties of several metal halide arcs assum-
ing LTE,”Proc.IEEE,vol.59,no.4,pp.622–628,Apr.1971.
[9]J.F.Waymouth,Consultant,Marblehead,MA,Private communication,
Mar.2009.
[10]J.F.Waymouth,“Applications of microwave discharges to high-power
light sources,”in Microwave Discharges:Fundamentals and Applica-tions,C.M.Ferreira and M.Moisan,Eds.New York:Plenum,1992, pp.427–443.
[11]R.Gilliard,M.DeVincentis,A.Hafidi,D.O’Hare,and G.Hollingsworth,
LS-WLED2010,M.Haverlag,G.M.W.Kroesen,and T.Taguchi,Eds.
Sheffield,U.K.:FAST-LS,2010,pp.
21–22.
Richard P.Gilliard received the B.S.degree from
the itary Academy,West Point,NY,the
M.S.degree from the Massachusetts Institute of
Technology,Cambridge,and the Ph.D.degree in
nuclear engineering from the University of Illinois,
Urbana,IL.
He is currently the Chief Scientist with Luxim
Corporation,Sunnyvale,CA.He has over30years
of experience in lighting technology.Prior to joining
Luxim in2005,his previous experience in discharge
lighting research and development was at General Electric,ILC Technology,and Perkin
Elmer.Marc DeVincentis(M’01)received the B.S.de-gree in electrical engineering from the University of Virginia,Charlotteville,and the M.S.and Ph.D. degrees from the University of California at Los Angeles,Los Angeles.
He is currently a Senior Staff Scientist with Luxim Corporation,Sunnyvale,CA.From2003to the present,he has been working at Luxim,developing high-power high-efficiency light
sources. Abdeslam Hafidi received the Ph.D.degree in laser diode and photonics systems from Strasbourg Uni-versity,Strasbourg,France.
He is currently the Director of Emitter Develop-ment with Luxim Corporation,Sunnyvale,CA.Prior to joining Luxim in2004,he held senior engineering positions at DiCon
Fiberoptics.
Daniel O’Hare received the M.S.degree in physics from the University of California at Davis,Davis. He is currently a Senior Staff Scientist with Luxim Corporation,Sunnyvale,CA.He has more than 20years of experience in specialty light source de-velopment,including the last four at
Luxim. Gregg Hollingsworth received the B.S.E.E.degree from Washington State University,Pullman.
He is currently with Luxim Corporation, Sunnyvale,CA,where he is the Leader of Luxim’s research and development effort concentrating on next generation and beyond technologies.He has over27years of experience in high-power RF amplifier products,including the last six at Luxim.。