CA1167973A - Low energy starting aid for high intensity discharge lamps - Google Patents

Abstract

D-22,650 Abstract of the Disclosure A light source, including a high pressure discharge lamp and a starting pulse generator, such as a spiral line pulse generator, utilizes a generally straight elon-gated conductor, or starting aid, in close proximity to an outer surface of the discharge tube to provide effi-cient coupling of the starting pulse to the discharge lamp. The starting aid extends between a region proxi-mate one of the electrodes and a region proximate the other of the electrodes. The starting aid provides within the discharge lamp an ionization path of minimum length free of circumferential turns when the conductor is energized by the pulse generator. The starting aid can be affixed to the discharge tube or can be mounted in one or more support brackets.

Description

LOW ENERGY STARTING AID FOR ~IGX
INTENSIT~ DISCHA~GE LAMPS

Proud et al, "Method and Apparatus F~r-5~arting ~ig~
Intensity Discharge Lamps", assignee's Canadian Application 386,752 filed concurrently with the present application and assigned to the same assignPe as the present application, contains claims to portions of the subject matter herein disclosed.
This invention relates to starting of high intensity discharge lamps and, more.particularly, to new and improved apparatus for efficiently coupling high voltage, short duration pulses to high intensity. discharge lamps.
High intensity discharge lamps, such as high pres-sure sodium lamps, commonly include nobel yases at pressures below 100 Torr. Lamps containing noble gases at pres~ures below 100 Torr can be started and operated by utilizing an igniter in conjunction with a lamp ballast~ The lamp ; ballast converts the ac line voltage to the proper ampli-tude and impedance leveI for lamp operation. The ig~iter provides pulses which assist in initiating discharge. The igniter is a rela~ively large and heavy circuit and is typically bullt into or located near the lamp ballast.
It has been found that the inclusion in high pressure sodium lamps of xenon as the noble gas at pressures well in excess of 100 Torr is beneficial to lamp performance. How-ever, the pulse energy requirements for starting of the dis-charge lamp increase as the pressure of the xenon included within ~he lamp increases and the conventional igniter des-cribed above does not, by itself, produce reliable starting~Various approaches to starting discharge lamps containing high:pressure~ xenon have been taken. A high voltage pulse is typically coupled to ~he dischar~e tube by a conductor . known as a starting aid, ~J

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~ r~3 D-22,650 -2-as shown in U.SO Patent i~O. 4,179,640 issued Decemher 18, 1979 to Larson et al. The starting aids shown in the prior art ~ave had the form of a wire wrappea around the discharge tube in a spiral con~iguration or a wire har-ness surrounding the discharge tube.
Starting aid configurations which more e~ficientlycouple the starting pulse to the discharge lamp are , , desirable for several reasons. When starting pulse energy re~uirements' are reduced by efficient coupling, the physi-cal size and cost of the starting pulse generator circuit can be reduced. Phyfiical size of the .starting circuit is of particular importance when it.is dasixed to include the starting circuit within ~he outer jacket of the lamp.
Alternativelv, more efficient coupliny of the starting pulse facilitates starting of discharye lamps having higher starting ~ulse energy requirements.

Accordingly, th~ present inventi.on~provides a li~ht .. .
source compri.si.ng: a hi.gh pressure:,discharge lamp including a discharge tube having electrodes sealed therein at opposite ends for receiving ac power and enclosing a fill material. which emits li,ght during dischal-ge; pulse generating means operative to ', provide at ~n output thereof.a high voltage, ~hort duration pulse of predetermined energy' and an elongated:collductor coupled to said output of said -pulse generating means and disposed in close proximity .~
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to an outer surface of said discharge tube in a configuration path of minimum length, free of circumferential portions, between said electrodes when said conductor is energized by said pulse genera-tin~
means.
Some embodiments of the invention will now be described, by way of example, with reference to the following drawings in which:

--FIG. 1 is a schematic diagram of a li~ht source according to the present invention;
FIG. 2 is a simplified schemati.c diagram oi a spiral line pulse generator;
FIG. 3 is a partial cross-sectional view of the spiral line pulse generator shown in FIG. 2;
:, FIG. 4 is a graphic representation of t-he volta~Je.
out.put of the spiral line pulse generator of FIG. 2;
:: FIG. 5 is a schematic diagram of a light source -which provides automatic starting; ~.
F~G. 6 is an elevational view of a light source according to the present invention wherein the starting :
circuit .is included withln ~he outer jacket;
FI:G. 7 is a schematic diagram of another light source which pro~id~ aUtOLatiC starting;

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D-2~,650 -4-FIC.. ~ is a graphic representation of voltage wave-forms which occur in the light source of FIG. 7;
FIG. 9 is an elevational view, partly in cross-section, of high intensity discharge lamps illustrating starting aid configurations according to the prior a~t; and .FIG. 10 is an elevational view, partly in cross-section, of a high intensity discharge lamp illustrating a low energy starting aid configuration.
. .

A hiyh intensity light source is shown in FIG. 1 and includes a;high pressure discharge lamp 10, a spiral line pulse generator 12, a switch 14, and an elon~ated conductor 7Ø The discharge lamp 10 is a hiyh pressure sodium lamp and includes a discharge tube ~2, -typically made of alumina or other transparent ceramic material, having lectrodes 24 sealed therein at opposite ends.
The di.scharge tube 22 encloses a fill material, typica].~y including sodium or a sodium amalgam and a noble gas or ..
mi~tures of noble gases, which emits light during dis- .
charge. The electrode.s 24 receive ac power from a lamp ballast at a voItage and current suitable for operation of the discharge lam.p 10. An output 26 of the spiral l.ine pulse generator 12 is coupled to one end of the con-ductor 20, typically a fine wire, wh.ich is located i.n close proxi.mity to an outer surface of the discharge tube 22. The confiyuration of:the conductor 20 is of importan:ce in efficient starting of the light source of FIG. 1 and is described in greater detail hereinafter.
The spiral line pulse generator 12 receives electrical energ~ from a source of voltage V0 which can be the ac input to the di.scharge lamp 10. The switch 14 is coupled to the spiral line pulse generator 12. In a manner which is full~ described hereinafter, the spiral l.ine pulse generator l2, after closure of the switch 1~, :

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D-22,650 -5-pro~ides ~t its output a hiyh voltage, short duration pulse which initiates discharge in the discharge l~np 10.
The spiral line pulse yenerator 12 is shown in sim-plified form in FIG. 2 for ease of understanding. A pair of conductors 30 a.nd 32 in the form of elongated sheets of conductive material are rolled together to form a multiple turn spiral configuration. FIG. 3 is a partial cross secti'onal view of the spiral line pulse generator 12 illus~rating the layered construction of the device.
A four layered arrangement of alkernating conductors and insulators, including the conductors 30 and 32 and a pair of insulators 34 and 36, is rolled~onto a form 38 in a multiple turn spiral configuration. The form 38 provide~
mechanical rigidity. The conductors 30 and 32 are separ~
ated by dielectric material at every point in the sp.iral configuration~ ' The operation of the spiral line pulse generator 12 can be described with reference to FIG. 2, which schemat-i.cally shows the conductors 30 and 32. The conductor 30 runs from polnt 40 to point 42 while the conductor 32 runs from point 44 to point 46. In the present example, the switch 14 is coupled between the conduc'cors 30 and 3Z
at or near the points 40 and ~4. A voltage V0 is applied - between the conductors 30 and 32. Prior to the closing of the switch 14, the conductor 30 has a uniform poten-t.ial between the points 40 and 42 and the conductor 32 has a uniform potential between the psints 44 and 45 and the voltage difference between the innermost and the outermost turns of the spiral configu.ration is at most V0. This can be seell b~ summing the electric field vec-tors at time t=0 as shown in FIG~ 2. When the switch 14 is rapidly closed, a field reversing wave propagates ..
along the transmission line formed by the conductors 30 and 32. When the wave reaches the poin-ts 42 and 46, at, time t=T, the potential d.ifference between the points 42 and 46 is nV0, where n is the number o;~ turn.s in the ...
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D-G2, 650 -6-spiral. conEiguratiorl, due to the ahsence of cancelling stati.c field vectors. As is well known, the propagatin~
wavt- undexgoes an in-phase re.,,lection at -the points 42 and 46 when these points axe -terminated in a hiyh impe-dance or are open circuited as shown in FIG. 2. ~'his results in an additio~al incr~ase in the potential differ-ence between the innermost ancl outermost cond~ctors wilh ' a'maximurn occurring at time t-2~ at which time the ~ield vectors are aligned as shown in FIG. ~ .The out.put vol-tage waveform of t.he spiral line pulse generator ].2 i.5 shown in FIG. 4. The output taken. between point 42 or 46 and point 40 reaches:a maY.imum voltage of 2nV0 ~t t=2T
after the closure of the switch 1~. The operation of the spiral line pulse generator is described in further detail in U.S. Pa-tent No. 3,289,015 and in Fitch et al, Novel Principle of Transient High Voltage Generation, Proc. IEEr Vol. 111l No. 4, Ap~il 1964~
The operati.on and properties of ~ne spiral line pulse generator 12 can be expressed in terms of the fol-lowing parameters: :
VG Charginy voltage Vm Peak pulse voltage n Number of turns V(t) Transient voltage wavefo~m I Transit time in spiral line D Diameter of spiral v ~elocity of propagation i.n spiral W Width ~f line composing spiral d Thickness of dielectric c Velocity of EM waves in vacuum CO Static capacitance of li,ne C Effective output capacitance ,.
z Impedance of line composing sp.i.ral k Kelative dl.electric constant ~O Die].ectr.ic~ constant in vacuum Permeability of vacuum ' . :

v-22,65~ ~7~

;L Inductance of ~ast switch ~ Thickness oE build-up E Energ~ available in spiral line Relationships descriptive of the output pulse are yiven by:
(1) Vm 2nVo

(2)- V(t) = (nk/l)VO 0 < t ~ 2 ~3) ` V(t) = 2n(1-t22~)VO 2c ~ t ~ 4 (~ n~/v, v = ck 1/
The capaci.tance of the spiral line and its effective out-p~lt c~pacitance are given by.
(5) CO = ~nk~ODW/d (~) C = CO/(2n)2 The stored energ~ is:
~7) E = C V 2/2 The characteristic impedance of the s-trip line composing the spiral is:
(8) z (~/f ) 1/
In optimizing performance of the spiral line pulse ~
generator 12, it is important to utilize low loss dielec- ~ -;` tric materials and conductors in order that the propaya-ting wava maintain a ~ast risetime compared to the transit time ~ of electromagnetic waves-between the innexmost turn and the outermost turn of the spiral line pulse generatox. It is additionally important to main-tain a lar~e ratio of diame-ter to ~Jinding buildup (D/~
and to provide for a vexy low inductance switch to insure that the voltaye between the conductors is switch-ed in a time interval which is much shorter than ~
The maximum permlssible value of inductance for the switch 14 is determlned from the approximation known in the ~rt that closure risetime i5 approximatel~ equal to .
L/Zo. Therefore, the followiny inequality must be met:
L<<~Zo. For a t~pical design, L, ~he inductance oE the .

~ 73 D-22,650 -8-swi~ch, i.s on the order of one nanohenry or less.
As discussed hereina.ter, it is pre~erable to in-clude the spiral line pulse generator 12 within an outer jacket of the light source. In this situation, the spira~
line pulse generator 12 must meet certain additional requirements. It is important that the spiral line pulse generator 12 have a compact physical size. Furthermore, when the spiral line pulse generator 12 is included ' within the outer jacket o~ the l'ight source, it must be capable of withstanding the considerable heat generated by the discharge lamp. In a t~pical application, the spiral li~e pulse g~neratox 12 must be capable of Qpera-tion at 200C.
It has been determined that the eneryy content, rather than the amplitude or pulse width, of the spiral line pulse generator output pulse is the most .important ~actor in ef~ective starting of.high pressure discharge lamps. The discharge lamp can be started by output '` puls~s of less than ten kilovolts in amplitude b~ i.n-cxeasing the energy content of the puls~. Since output pulses of maximum amplitude and minimum duration are not nècessarily re~uired, the spiral line pulse generatGr design re~uirements and the switch speed requirements described hereinabove can be rel.axed.
In one example of a spi.ral line pulse generator, the conductors were aluminum foil having a thickness of 0.0007" and a width of 0.5" and the insulators were polyimide film dielectric having a thic,kness of 0.00048"
and a width of 1". The two: conductors, separated by the ; 30 two insulators, were wound on a cyl.indrical form having a diameter of 0.7". ~pproximatel~ 130 turns were to provide a capacitance of approximately 0.5 microEarad.
The insulators were wider than the conductors to prevent arcing between turns at the edye~ oE the conductors.
T~pically the voltage, yround, and output conne-,,tions are made by means of tabs which are spot welded to the - ^ .
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' ' ' D-22,650 -9--conductors du~in~3 the winding o~ the spiral line pulse gener~tor. When 200 volts is applied to this spiral line puls~ g~nerator, an OUtpllt pulse of approxirnately 3500 volts and 30 nanoseconds is provided.
The low inductance swi~ch 14, whiCh is shown in FIG. 2 connected be~ween the conductors 30 and 32 on the innermos~ turn of ~he spiral line pulse generator 1~l can alternatively be connected bet~7een the conductors 3~ and 32 on the outexmost turn at or near the points 42 and 46 or between the conductors 30 and 32 at the midpoint of the conductors 30 and 32. While the output voltage can be takerl between an~ two points on the spiral line pulse gene~ator 12, the maximum voltage multiplication facto-is obtained when the OlltpUt is taken between the inner~
most turn and the outermost turn.
A light source configuration providing automatic operation is illustrated in schematic form in FIG. 5.
A discharge lamp 50 corresponds exactly to the discharge ! lamp lO shown in FIG. 1 and described hereinabove. A
spiral l:ine pulse yenerator 52 shown symbolically in FIG. 5 corresponds to the spiral line pulse generator 12 shown in FIGS. 1, 2, and 3 and described hereinabove.
AC power is coupled to electrodes 54 at opposite ends of the discharge lamp 50 and is coupled through a current ~5 limiting resistor 56 to on~ end of one conductor of the spir~l line pulse generator S2. The output of the spiral line pulse generator 52 is coupled to one end of a con-ductor 58 located in close proximity to an outer surface of the discharge lamp 50 but not coupled to the electrode~
54. Alternatively, the output o~ the spiral line pulse generator can be coupled to the electrodes 54 of the dis-charge lamp 50 in~which case the ac power is coupled through a filter circuit to block the high voltage pulse from the source of power. A self-heated thermal switch 60 includes a bimetallic switch 62 having a normally closed coIltact 64 and a normally open contact 66 and ~:. ; ' ~ '' ' ' '''`' .-, , ; .

~-22,650 -10-further includes a heater element 68. The normall~ open contact 66 of the bimetallic switch 62 is coupled to ~he one conductor of the spiral line pulse generator 52.
The no~mally closed contact 64 of the bimetallic switch 62 is coupled through the heater element 68 and through a normally closed disabling swi-tch 70 to the ac input.
A common contact 72 of the bimetallic switch 62 and the other conductor of the spiral line pulse generator 5 are coupled to ground. The disabling switch 70 is a bimetallic switch which is located in proximity to the discharge lamp 50 ancl. senses the t~mperature o~ th~ dis-charge lamp 50. A starting circuit. 7~, comprisin~ the spiral line pulse gene.rator 52, the resistor 56, the thermal switch 60, and the disabling switch 70, has an output 78, which is the output of the spiral ~ine pulse generator 52, coupled to the conductor 58.
In operation, when ac power, typically provided by a lamp ball.ast, is appl.i.ed to the light souxce of FIG. 5, the spiral line pulse generator 52 is charged throug~l the resistor 56. At the same ti.me, current flows through the switch 70, the heater.68 and the bimetallic switch 62, thus increasing the temperature.of the heater element 68.
The heater element 68 is in close proximity to the bimetallic switch 62 and cau~es heating of the bimetallic switch 62. When the heater elemen.t 68 reaches a prede-te.rmined temperature, the bimetallic switch 62 sw.itches from normally closed contact 64 to normally open contac~
~6. The closure of normally open contact 66 provides a short circuit across the conductors of the spiral line pulse generator 52, thus prodùcing at the output of the spiral line pulse generator 52 a high voltage, short duration pulse which in~tiates discharge in the dis-charge lamp 50~ The heat produced by the discharge in the lamp 50 causes the clisabling swi~tch 70 to open, thereby disabling the thermcll switch 60.
If, for any raason, t~ie first spiral line pulse ``.. ' :

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-, ~-22,650 -11-generator 52,output pulse did not initiate discharge in the discharge lamp 50, the'switch 70 remain~ in the closed position and the bimetallic switch 62 cools since the heater element 68 is no longer energized. When the bimetallic switch 62 cools to a predetermined temperature, it switches back to the normall~ closed contact 64 and current again flows through the heater element 68. The temperature of the heater element 68 and,the bimetallic switch 62 again rises and auses switching'of the bi- -metallic switch 62 to the normally open contact 56 and a second high voltage, short duration pulse is generated by the spiral line pulse generator 52. This process eont.inues automatically until a ~ischarge is initiated in the discharge lamp 50. At that time -the increase in temperature of the discharge lamp 50 causes the switch 70 to open and the thermal switch 60 to be disabled. As discussed hereinabove, the bimetallic switch 62 must provide a low inductanee short ci.rcuit across the spiral line pulse generator 52 ~or optimum per~ormance of'the spiral line pulse generator 52. The configuration of ; - FIG. 5 provides automatic generation of starting pulses until a discharge is initiated in the discharge lamp 50.
; A ph~sical embodiment of the light source ~hown in sehematic form in FIG. 5 is illu,.trated in,FIG. 6. r~he ~5 discharge lamp 50 is enclosed,by a light transmitting outer jacket 80. Power i.s received by a lamp base 82 and conducted through a lamp stem 84 by conductors 86 and 88 to the electrodes of the discharge lamp 50. The conductors 86 and 88 are sufficientl~ rigid to provide mechanical support for the discharge lamp 50. The start-ing circuit 76 is located in the base region of the outer jacket 80 surrounding the lamp stem 84. This location of the starting ,circuit 76 is chosen to minimize block~
age of light emitted h~ the dischargc lamp 50. The starting circui' 76 includes the spiral line pulse ge}lerator 52, the resistor 56,, the therrnal switch 60 a71d ~-) ; .
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D22,650 -12-the swi*ch 70 connectecl as shown in FIG. 5. The outpu-t 78 o the starting circuit 76 is coupled to the conduc-tor 58 which is located in close proximity to an outer sur-face of the discharge lamp 50. The location of the startirlg circuit 7~ as shown in FIG. 6 is advantageous because the generally cylindrical shape of the spiral line pulse gene~ator 52 is compatible with the annular space available in the lamp base. When very hiyh energy levels are required to start the discharge lamp 50, the lQ spiral line pulse generator 52 can become too large for inclusion within the outer jacket 80. In this instance, the starting circuit 76 can be located externa]. to the outer jacket 80, for example, in the light fixture in which the light source is mounted. The pulse energ~
requi.rements for startincJ of the discharge lamp 50 in-crease as the pressure of the noble gas included w~thin the lamp increases. For example, a lamp having a xenon pressure of about 10 Torr re~uires a starting pulse of approximately 2 to 5 millijoules while a lamp having a xenon pressure of about 300 Torr requi.res a starting pulse of approximatel~ 70 to 100 millijoules. The igni-ter commonly used in high pressure soclium lamp ballasts :~
does not provide pulses of sufficient voltage to start lamps cc,ntaining noble gases at pressures above about lOQ Torr. Therefore, such lamps cannot be used in stand-ard high pressure sodium lamp fi.xtures. In the config-uration shown in FIG. 6, the starting circuit 76 is in-cluded within the outer jacket 80 of the light source and is tailored for effective starting of the discharc~e lamp 50. Therefore~ the light source shown in FIG. 6 can be used with standard high pressure sodium lamp ballasts. Furthermore, since the starting circuit is self-contained within the light source, the configuratio of FIG. 6 can be utilized with mercur~ lamp ballasts, which do not contain an igniter.

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.~ ~2,650 -13-~ n alternative light source configuratiorl providing automatic operation is illustrated in schematic forrn in FIG. 7. The discharge lamp 50 and khe spiral line pulse ger.erator 52 are connected as shown in FIG. 5 and de-scribed hereinabove except that the thermal swil:ch 60 andthe disabling switch 70 of FIG. ~ are replaced by a spark gap 90. The spark gap 90 is a two terminal device ~-hich is connected directly across the conductors.of the spiral line pulse generator 52. The spark gap 90 is normally an open circuit but switches to a short circuit when a voltage greater than a pred~termi~.ed value is applied to the device. In FI~ 7, the predetermined firing voltage of the spark gap 90 is selected to be slightly less than the peak ac input voltage so that the spiral l~ne pulse generator 52 achieves maxirnum output voltage. A starting circuit 92, including the spiral line pulse generator 52.
the resistor 58, and the spark gap gO, has an ou~put 94 coupled to the conductor 58. The s-tarting circuit 9.2 can replace the starting circui.t 76 shown in the light source 20, of FIG. 6.
In operation, an ac voltage, typically provided by a - lamp ballast, is applied to the configuration of FIG. 7.
The voltage across the spiral line pulse generator 52, illustrated in FIG. 8A, i.ncreases until the firing vol-tagQ of the spark gap 90 is reached at time To~ Thespark gap 90 rapidly short,circuits the spiral line pulse generator 52 and a high voltage, short duration pulse, illustrated ;n FIG. 8B, i.s provided at the OlltpUt of the spiral line pulse generator 52 at time To a.s described hereinabove. By repetition of this process, a high vol-tage pulse is produced by the spiral line pulse generator on each half ~yc].e of the ac input voltage, as shown in FIG. ~B, until starting of the discharge lamp 50. After the discharge lamp 50 is started, the voltage supplied by the lamp ballast to the light source is reduced and the spark gap 90 does not fire.

D-22,650 -14-The canfig~l~atior~ of FIG. 7 provides several advan-tages. (1) Starting pulses are produced when maximum potential exists across the discharge lamp 50, thus maxi-mizing the probability of starting. (2) Starting pulses S are produced at 120 Hz until starting occurs. (3~ The starting circuit stops functioning automatically after the discharge lamp 50 starts. ~4~ The nurnber of cixcuit components is minimal.
As noted hereinabove, the configuration OL the con-ductor 20 in FIG. 1 and the conductor 58 in FIGS~ 5-7 is of importance in efficient starting of the light source described herein. Conductors, such as the conductors 20 and 58, used for starting of discharge lamps are ~ommonl~
re~erred to as starting aids. By providing ef~icient transfer of energy from the spiral line pulse generator to the discharge lamp, the energy required in the output pulse of the spiral line pulse generator can be reduced.
A reduction in energv requirements is beneficial in two ways. For a given discharge lamp, the size of the spiral line pulse generator can be reduced, thus resulting in easier packaging of the spiral line pulse generato:r and lower cost. Second, a given spiral line pulse generator can be used to start discharge lamps with higher nobl2 gas pressures.
Various starting aid confiyurations are known in the prior art. Referring now to FIG. gA, there is shown a discharge lamp 100, corresponding to the discharge ~amp 10 shown in FIG. 1 and described hereinabove. The dis-charge la~lp 100 includes a light transmitting discharge tube 102 having electrodes loa sealed therein at opposite ends. A starting aid 106, in the form of a fine wire, is wrapped around the outer surface of the discharge tube 102 in a spiral configuration having several turns. The starting aid 106 is coupled at its ends to a pulse gener-ator. Upon application of a high ~701tage, short duration pulse to the startiny aid 106l ~n ionization path lOg is : .

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D-22,650 -15-form~d in t~lO in~erior o~ ~he discharcJe lamp 100 be~een the electrodes 104. The ionization path 10~ follows the path of the starting aid 106 and therefore is spiral in configuralion.
A similar configuration of a startiny aid according to the prior art is shown in FIG. 9B. A discharge lamp 110, corresponding to the discharge lamp 10 shown in FIG. 1 and described hereinabove, includes a discharge tube 112 having electxodes 114 sealed therein at opposite ends. ~ starting aid 116, in the form of a conductive wire harness, is disposed around the outer surface of the discharge tube 11?. The startin~ aid 116 includes a number of circumferential portions~118 which surround the discharge tube 112 and a number of interconnecting por-tions 120 ~hich connect the circumferential portions 118, thus forming a harness. When a high voltage, short dura-tion pulse is applied to the starting aid 116, an ioniza-tion path 122 is formed wi-thin the discharge tuke 112 between the electrodes 114. The ionization path 122 fo]lows the path of the conductor which forms the star-t-ing aid 116~ Thus, the ionization path 122 includes portions 124 which follow the circumferential portions 118 of the starting aid~116, and portions 126 which fol-low the interconnecting portions 120 of the startlng aid ; 25 116.
It has been found that the use of a straight wire starting aid results in superior starting of high inten-sit~ discharge lamps. Referring now to FIG. 10, there is shown a c~ischarge lamp~130~ corresponding to the dischar~e lamp 10 shown in FIG. 1 and described hereinabove. The discharge lamp 130 includes a transparent discharge tube 13~ having electrodes 134 and 136 sealed therein at oppo-site ends. A starting aid 138, in the form of an elon-gated conductor in a generally straight configuration, is located in proximity to an outer surface of the discharge tube 132 . The starting aid 138 is coupled to a generator .

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;J~ 3 D--2 2 , 6 5 0 1 6--of hi.gh voltage; short duration pulses and runs in a gen-erally straight path between a region 140 proximate ~he electrode 134 and a region 142 proximate the electrode 136A
The starting aid 138 can be mounted in proximity 1:o the discharge tube 138 in any convenien-t manner which does not appreciably block the li;ght ou-tput of the discharge lamp 130. For example, insu].at.ing support brackets can be located at opposi.te ends o~ the discharge larnp 130, When the conductor which forms the starting ~id 138 is of sufficient diameter to have mechanical rigidity, a single insulating support bracket can be used. Alternatively, the starting aid 138 can be affi~ed to thP oliter surface of the discharge tube 132 b~r cement capable of withstand-ing the heat generated by the discharge lamp 130, When a hiyh voltage, short duration pulse,, such as that generated by the spi,ral line pulse generator describ-ed hereinabove, is applied to the start~ng aid 138, an ionization path 144 is formed in the interior of the dis-charge lamp 130 between ~he electrodes 134 and 136. The ionization-path 144 follows the path of the starting aid 138 and thus runs in a yenerall~ straight path between the electxodes 134 and 136. rrhe formatJon of the ioniza-tion path 144 is dependent upon ths peak pulse voltage applied to the startiny aid 138. Whether the degree of ionization develops further to form an arc discharge between the electrodes 13~ and 136 depends upon the ini-tial conductivity of the ionization path 144. Conductiv-ity :in turn depends on the degree of ionization and elec-tron temperature and is directly related to the energy ,initia.~1~7 supplied b~ the st:arting pulse. Thus very narrow high voltage pulses can, in some cases, produce ionization but can fail to produce sufficient conductivit~
in the iani7ation path 144 to induce further deve'lopment of a self-sustained discharge. In contrast to the ion.iza-tion path 108 in FIG. 9A and t,he ioni2ation path 122 in .
E~IG. 9B, the ionization path 14~ in E~IG. ;0 is free O:e .` '.

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extraneous circum~eren~ial turns. As a result, -the length o~ the .ionization path 144 is less than either of the ionization paths 108 or 122, and less pulse eneryy is required to establish conditions suitable or arc for-mation or starting of the discharge lamp 130.
The reduction in requisite pulse energy h~s beenshown by experiment to be roughly a factor of two for the starting aid 138, shown in FIG..lO, as compared with the starting aids shown in FIGS. 9A and 9B. This is genera].ly consistent with the reduction achieved ln the leng-th of the i.onization path b~ utilizing a straight skarting ~id~
U5ing the prior art starting aid configuration illustrated in FTG. 9B, i.t has been found that high pressure sodium lamps contaîning 203 Torr xenon pressure require 35 kilo--volt, 20 millijoules pulses, when the pulses are approxi-mately 10 nanoseconds in width. A high pressure sodium lamp containing 300 Torr xenon cannot be started ~ithin a --reasonable voltage ranye using the starting aid shown in FIG. 9B. When the starting aid 138, as shown in FIG. lO, ~0 is uti.lized, experiment has shown that a discharge tube containing 200 Torr xenon can be started with a 25 kilo-volt J 10 millijoules pulse of lO nanosecond pulse width.
The straight star-ting aid 138, shown in FIG. lO, enables reliable starting o~ high pressure sodium discharge lamps containing 300 Torr xenon with 33 kilovolt, 15 millijol.lles pulses at a pulse width of lO nanoseconds.
It is to be understood that while the starting aid 138, shown in FIG. lO, has been described in connection with a spiral line pulse generator, a starting aid having 3n a generally straight config~lration can be usea with any pulse generator capable of generating the requisite high voltage, short duration pulses. The starting aid 138 is of particular i.mportance when it is desired to minimi~e the si7.e of the pulse generator or when it is desi.red to start discharge lamps having hi~h energy starting re~uire-ments.

` `' ' .

- : :
~ ' 7~ 3 D--22, 650 -18--Thus there is provided by the present invention a light source in which a spiral line pulse generator pro-vides starting pulses of sufficient ene~y to start a discharge lamp containi~g high pressure noble gases. The spiral line pulse generator reduces the mass and volume associated with inductive starting circuits. In addition, tha spiral line pulse generator has a physicai conigura~
tion which can advantageously be included within a dis~
charge lamp envelope.
While there has been shown and described what is at present considered the preferred embodiments of the inven~
tion, it will ~e obvious to those skilled in the art that various changes and modifications may be made therein without departiny from the scope of the invention as defined kv the appended claims.

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Claims (2)

24,312 THE EMBODIMENTS OF THE INVENTION FOR WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A light source comprising:
a high pressure discharge lamp including a discharge tube having first and second electrodes sealed therein at opposite ends for receiving ac power and enclosing a fill material which emits light during discharge;
pulse generating means operative to provide at an output thereof a high voltage, short duration pulse of predetermined energy; and a conductor including a generally straight portion extending from a region proximate to one of said electrodes towards a region proximate to the other of said electrodes coupled to said output of said pulse generating means and disposed in close proximity to an outer surface of said discharge tube for providing within said discharge tube an ionization path between said electrodes when said conductor is energized by said pulse generating means at which time said first electrode is at a first voltage potential, said second electrode is at a second voltage potential, and said conductor is at a third voltage potential higher than said first and second potential, wherein:
said high pressure discharge lamp is a high pressure sodium discharge lamp;
said fill material contains approximately 200 torr xenon pressure; and said pulse generator means provides a pulse of approximately 25 kilovolts for a duration of approximately 10 nanoseconds of approximately 10 millijoules energy.

24,312

2. A light source comprising:
a high pressure discharge lamp including a discharge tube having first and second electrodes sealed therein at opposite ends for receiving ac power and enclosing a fill material which exits light during discharge;
pulse generating means operative to provide at an output thereof a high voltage, short duration pulse of predetermined energy; and a conductor including a generally straight portion extending from a region proximate to one of said electrodes towards a region proximate to the other of said electrodes coupled to said output of said pulse generating means and disposed in close proximity to an outer surface of said discharge tube for providing within said discharge tube an ionization path between said electrodes when said conductor is energized by said pulse generating means at which time said first electrode is at a first voltage potential, said second electrode is at a second voltage potential, and said conductor is at a third voltage potential higher than said first and second potential, wherein:
said high pressure discharge lamp is a high pressure sodium discharge lamp;
said fill material contains approximately 300 torr xenon pressure; and said pulse generator means provides a pulse of approximately 33 kilovolts for a duration of approximately 10 nanoseconds of approximately 15 millijoules energy.