US3873884A - Electrodeless discharge lamp and power coupler therefor - Google Patents

Electrodeless discharge lamp and power coupler therefor Download PDF

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US3873884A
US3873884A US441600A US44160074A US3873884A US 3873884 A US3873884 A US 3873884A US 441600 A US441600 A US 441600A US 44160074 A US44160074 A US 44160074A US 3873884 A US3873884 A US 3873884A
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lamp
coil
power
cylinder
coupler
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Fred C Gabriel
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Applied Biosystems Inc
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Perkin Elmer Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J65/00—Lamps without any electrode inside the vessel; Lamps with at least one main electrode outside the vessel
    • H01J65/04—Lamps in which a gas filling is excited to luminesce by an external electromagnetic field or by external corpuscular radiation, e.g. for indicating plasma display panels
    • H01J65/042—Lamps in which a gas filling is excited to luminesce by an external electromagnetic field or by external corpuscular radiation, e.g. for indicating plasma display panels by an external electromagnetic field
    • H01J65/048—Lamps in which a gas filling is excited to luminesce by an external electromagnetic field or by external corpuscular radiation, e.g. for indicating plasma display panels by an external electromagnetic field the field being produced by using an excitation coil

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  • the present invention is an electrodeless discharge lamp with an improved coupler for coupling r-f (radio frequency) electric power into the lamp to start and operate the lamp.
  • Such lamps normally comprise a quartz bulb containing gas such as argon, and are operated by starting and maintaining a discharge in the ionized gas.
  • this invention is a power coupler for a electrodeless discharge lamp in which a trace of a selected metallic element is incorporated to provide a bright spectral line source of the selected metal for use in chemical analysis, particularly atomic spectroscopy.
  • a bright spectral line source provided in this manner might also be useful for optical pumping applications.
  • hollow cathode lamps For analyzing for most metallic elements hollow cathode lamps are sufficiently bright, stable, long-lived and inexpensive. However, hollow cathode lamps do not provide sufficiently bright or stable spectral lines for certain metallic elements, such as arsenic, cesium and rubidium, and for other elements such as selenium, tellurium, bismuth, antimony and tin, hollow cathode lamps are not bright enough for good spectral work.
  • Electrodeless discharge lamps in general are an alternative to hollow cathode lamps for the metallic elements with which hollow cathode lamps are now used, but more importantly electrodeless discharge lamps provide highly satisfactory sources of the spectral lines of the foregoing metallic elements which cannot be provided by hollow cathode lamps or which are barely suitable as provided by a hollow cathode lamp.
  • electrodeless discharge lamps as spectral line sources have been known for a number of years. It is known to operate electrodeless discharge lamps with high frequency power, e.g., 2,450 megacycles and also with much lower frequency power of about 27 megacycles, i.e., radio frequency.
  • high frequency power e.g. 2,450 megacycles
  • much lower frequency power of about 27 megacycles i.e., radio frequency.
  • the lower radio frequency has the particular advantage of providing more stable operation of the lamp.
  • r-f operated electrodeless discharge lamps known in the art are excited to maintain a discharge in the lamp by placing the lamp in the final tank circuit inductor of a radio transmitter or power oscillator and the r-f power is coupled magnetically to the lamp plasma once a discharge is started in the lamp.
  • Start-up of the lamp requires an initial voltage, higher than the voltage required to maintain a discharge after start-up, to ionize the gas and ignite a discharge in the ionized gas.
  • the start-up voltage supplied is typically a voltage transient from an extended circuit or from the r-f supply itself; typical arrangements are described in an article titled Miniature Device For Starting Electrodeless Discharge Tubes by J. K.
  • Previously known and used power supplies for starting and maintaining a discharge in electrodeless discharge lamps are rather complex, relatively expensive and require special switching means for supplying first the high voltage required for ionizing the gas and starting a discharge therein, and then for supplying a' reduced level of voltage to maintain the discharge.
  • An object of the present invention is to provide a spectral source electrodeless discharge lamp operating unit which includes an inductive resonator coupler, for
  • a coupling r-f electric power into the lamp for starting and operating it that provides efficient coupling, that is simple and economic to construct and operate, that provides the requisite voltage and current ratios for starting the lamp and then for maintaining it in operation without intermediate switching means or alternative power supply connections, and that has an automatic self starting capability if the lamp goes out during operation.
  • the invention is an electrodeless discharge lamp, for use as a spectral light source, and an inductive resonator coupler for coupling r-f electric power into the lamp for starting and maintaining a discharge inthe lamp.
  • the lamp is a transparent envelope, normally quartz, containing an ionizing gas, namely an inert gas such as argon, under low pressure and a minor amount of a metallic element whose spectral line is to be provided by the lamp.
  • the coupler consists of a grounded hollow cylinder of electrically conductive material open at one end, for light from the lamp to pass out, and with a grounded base member in the other end.
  • a helically coiled wire conductor is mounted concentrically within the cylinder; the coil is a smaller diameter than the cylinder so as to provide an annular space between the coil and the cylinder wall.
  • the length of wire in the coil is made nominally one quarter of the free-space wave length of the r-f electric power'to be applied.
  • One end of the coil is grounded to the base member of the cylinder and the other end is unconnected, i.e., open circuited.
  • the cylinder is longer than the coil with its open end extending beyond the open circuited end of the coil.
  • the lamp is suitably supported on the base member of the cylinder and is mounted so that the major portion of the lamp is within the coil at the grounded end of the coil.
  • R-f electric power from an r-f generator which may be of conventional design, is tapped into the coil at a point near the grounded end of the coil.
  • the tap point is selected so that when the lamp is operating the circuit formed by the coupler cylinder and coil and the lamp plasma, which is magnetically coupled by transformer action to the coil turns between the tap and ground, are tuned to the frequency of the r-f power and the impedance of the coupler and lamp match the impedance of the r-f power supply at the tap.
  • This coupler provides two distinct modes of operation successively; first it applies the electrostatic field to the gas in the lamp for starting the discharge and then it applies the electromagnetic field required for maintaining the discharge.
  • the coupler When the r-f power is first applied to the coupler, before the lamp discharge is started, the resistance and reactance reflected by the lamp are absent so that the coupler has a much higher impedance than when the lamp is operating, and the coupler has a very high electrical Q.
  • the input r-f voltage thus appears as a high voltage maximum at the open circuited end of the coil.
  • the invention comprises generally an electrodeless discharge lamp and an inductive resonant coupler 11 for coupling r-f electric power from an r-f generator 12 into the lamp for operating the lamp.
  • the lamp [0, which is mounted within the structure of the coupler 11, is a small quartz bulb enclosing a volume of from I to 10 cubic centimeters containing an ionizing gas at low pressure, and a minor amount of a metallic element whose spectral line is to be produced by the lamp.
  • the ionizing gas is one of the inert gases, argon being one that is commonly used; the pressure within the lamp is from k to about 5 torr.
  • the particular gas and pressure are not critical, the general considerations being that the gas must be ionizable and support a discharge at reasonable power levels and the pressure must be such that, when the lamp is hot, the increased pressure will not be so large as to extinguish the discharge.
  • the metallic element is present in microgram quantities; the particular amount is not critical, a minute trace will provide the spectral line desired but enough more is normally provided to make up for some of the vaporized metallic element that is lost by particles becoming embedded in the walls of the lamp.
  • the spectral lines are the lines of the metals themselves and in the usual case, as with arsenic and selenium for example, the metallic element incorporated in the lamp is the metal itself. With some metals, such as lead for which the metal itself would not provide the requisite vapor pressure to provide the spectral line in the lamp output, a salt of the metal, e.g., lead chloride is used.
  • the lamp 10 has a bulbous portion 10a and a stem portion 1017 which is utilized for mounting and positioning the lamp within the coupler 11 in the manner subsequently to be described.
  • the inductive resonant coupler 11 is made up of a hollow cylinder 14 of an electrically conductive material and a helical coil 15 wound on a ceramic coil form 16 within the cylinder.
  • One end of the cylinder 14, the right hand end, is open; its other end is closed by a base member 17 which is also of electrically conductive material.
  • the base member 17 is grounded in the r-f gen erator as indicated at 18, and is fixed in the end of the cylinder so that the cylinder is electrically connected to the base member and thus similarly connected to ground.
  • One end of the coil 15, the left hand end as shown, is grounded by being connected to the base member; its other end is unconnected and thus open circuited.
  • the length of the wire of coil 15 is made one quarter of the wave length of the rf electric power to be supplied by the r-f generator 12.
  • the diameter of the coil 15 is less than the inside diameter of the cylinder 14 so as to provide an annular air space 19 between them.
  • the coil and cylinder are thus in a capacitor relationship.
  • the relative diameters of the cylinder and coil are 2 inches and l inch respectively so that the gap between is about one-half an inch.
  • the cylinder is preferably made slightly longer than the coil to provide a suitably long ground plane relative to the field of the last turn of the coil at its open-circuited end to avoid unwanted fringe effects.
  • the open end of the cylinder extends beyond the amount necessary to avoid fringe effects and actually extends more than an inch beyond the end of the coil as means to help in suppressing unwanted transmissions.
  • the lamp 10 is mounted for its major portion, its bulbous portion 10a, to be within the portion of the coil at the grounded end.
  • the lamp is mounted by means of its stem portion 10b which fits through a bushing 20 that is fixed through the base end of the coil form 16 and the base member 17 of the cylinder.
  • the end of the bushing 20 within the coil form 16 is flanged as shown at 21 and this serves to hold the coil form in place on the base member 17.
  • the lamp stem 10b extends out through bushing 20 so that the position of the lamp bulbous portion 10a can be adjusted relative to the coil by moving the stem in or out relatively through the bushing.
  • the position of the lamp relative to the coil is thus adjustable for finding the exact position at which the energy supplied by the coil, as subsequently described, is coupled into the lamp most efficiently when the lamp is operating.
  • the position of the lamp is clamped by means of a cap 22 threaded onto the activated end of the bushing.
  • the cap 22 has a hole through it for the lamp stem 10b to extend through and clamping means is provided by an O-ring captured between the end of the bushing 20 and the inside of the end of the cap 22.
  • R-f electric power is applied to the coil 15 from the M" generator 12 by means of a connection from the generator 12 through a connector 25 mounted through the cylinder base member 17 to a tap 26 into the cell. It is normally desired to have the lamp and coupler spaced some distance from the r-f generator as a means of eliminating heat problems and for flexibility of instrument design; in this case, as illustrated, the r-f generator 12 is shown connected to the connector 25 through a coaxial cable 27.
  • the r-f generator is comparable to a conventional radio transmitter, which would be a suitable r-f source; as shown it may consist of a crystal oscillator 28 coupled respectively through a low power r-f amplifier 30 and a high power r-f amplifier 31 to apply an r-f output through the coaxial cable 27 and connector 25 to the coil tap 26.
  • the r-f generator 12 is operated from a conventional source of electric power, not shown. in practice the r-f generator is built to supply r-f electric power at 27.l2 MHz at levels up to 45 watts and having an impedance of 50 ohms.
  • the coaxial cable 27 used would of course be a 50 ohm cable.
  • the tap 26 into the coil 15 is located relative to the turns of the coil so that when the lamp is operating the coupler 11 is tuned to the frequency supplied by the r-f generator and the impedance of the coupler and lamp circuit matches the impedance of the r-f generator at the tap 26.
  • a trimming capacitor 32 may be provided between the wall of the cylinder 14 and the coil 15, as shown, to assist in providing a fine tune for adjusting the resonant frequency of the coupler and lamp circuit, when the lamp is operating, to the resonant frequency of the r-f generator. This capacitor 32 may be eliminated by adjusting the resonant frequency of the generator, or by building the coupler 11 with a precision which makes additional trimming unnecessary.
  • the length of wire in the coil 15 is nominally one quarter of the free-space wavelength of the r-f power to be supplied by the r-f generator 12.
  • the coupler and lamp configuration of this invention thus provides a compact, quarter-wave, transmission line resonator which can be proportioned to have extremely low losses.
  • the coupler 11 has, due to the general configuration shown, a high electrical O. This is enhanced by making the coupler of low loss materials and reducing losses by all practical means.
  • cylinder 14 and base member are made of copper or brass and the interior of the cylinder is plated with a polished silver coating indicated at 33, and the wire of the coil 15 is solid silver or silver plated copper for the purpose of maintaining high r-f conductivity in the presence of high lamp temperatures and corrosive atmosphere. Also soldered taps are avoided, the taps preferably being welded.
  • the coupler circuit is higher than the 50 ohm mentioned above, and is typically 400 to 500 ohms. Also because the lamp is not extracting power, the coupler has a very high electrical Q and the open circuited end of the coil 15 is the location of a current node and hence a voltage maximum. This has the effect of transforming the input r-f voltage swing to a very high voltage (e.g., tens of thousands) at the open circuited end of the coil.
  • This high voltage creates a potential through the gas in the bulbous portion 10a of the lamp to the grounded base member 17 of the cylinder which ionizes the gas.
  • the electrostatic energy thus coupled into the lamp and a slight amount of additional energy applying to the lamp from the surrounding light, such as direct sunlight or light bulbs, is enough to start the discharge in the ionized gas. In any event only a slight additional amount of energy is necessary for starting the discharge and this may be supplied by such simple means as a lighted match.
  • the lamp may be operated to produce a steady output, or the output may be modulated for use with certain spectroscopy instruments, that are adapted to operate with a modulated light source, by modulating the output of the r-f generator between an upper output level and a low output level, the low output level being one at which the lamp would still be operating, but producing a light below a minimum level recognized by the spectroscopy instrument.
  • a 50 percent modulation of the output of the r-f generator is suitable for adapting the lamp and coupler of the invention for use with spectroscopy instruments intended to operate with a modulated light source.
  • the open end of the cylinder 14 is preferably provided with a safety window to prevent anyone from inadvertently sticking a finger into the high voltage end of the coil 15 at start-up, and also to keep corrosive materials out of the interior of the cylinder as much as possible.
  • This window is suitably provided by a quartz window 34 mounted in a support plug 35 fitted into the end of the cylinder 14.
  • a resonator coupler for coupling a source of r-f electric power into an electrodeless discharge lamp for starting and operating the lamp, comprising:
  • a grounded hollow cylinder of electrically conductive material open at one end, with a grounded base member at the other end;
  • a helically coiled wire conductor concentrically within the cylinder and spaced from the inner walls thereof;
  • the wire of the coil being one quarter wave long relative to the free-space wavelength of r-f power intended to be applied for operating a lamp mounted therein;
  • the end of the coil at the end portion within which a lamp is adapted to be mounted being toward said base member and being grounded, the other end of the coil being open circuited;
  • electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in a lamp mounted within the coil, said connecting means being tapped into the coil at a point near, but spaced from, the grounded end thereof, said point being selected such that, when a lamp mounted in the coil is in operation by r-f power connected to the coil, the coupler is tuned to the frequency of said r-f power and the impedance of said lamp and coupling means at said tap point substantially matches the impedance of said r-f power source, whereby when said r-f power is applied to the coil, and before a discharge is ignited in the lamp, a voltage maximum occurs at the open circuited end of the coil and creates a potential extending through the lamp portion between said open circuited end and said base member for ionizing the gas in the lamp.
  • the coupler of claim 1 including a capacitor between the wall of said cylinder and a turn of said coil for fine tuning the resonant frequency of the resonator coupler relative to the frequency of an r-f source to be applied thereto.
  • a grounded hollow cylinder of electrically conductive material open at one end with a grounded base member at the other end;
  • a helically coiled wire conductor concentrically within the cylinder and spaced from the inner. walls thereof, said wire being one quarter wave long relative to the free-space wavelength of r-f power intended to be applied for operating the lamp, the end of the wire coil at the base member end of the cylinder being grounded and the end toward the open end of the cylinder being open circuited, and the lamp being mounted with at least the major portion of the lamp within the coil at the grounded end of the coil;
  • electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in the lamp, said connecting means being tapped into the coil at a point near, but spaced from, the grounded end thereof, said point being selected such that, when the lamp is in operation by r-f power connected to the coil,
  • said discharge lamp is a transparent sealed envelope containing an ionizing gas at low pressure and a minor amount of a metallic element for producing spectral light of said metallic element.
  • said lamp being a transparent sealed envelope containing a minor amount of metallic element and an ionizing gas
  • said coupler comprising:
  • a grounded hollow cylinder of electrically conductive material open at one end with a grounded base member substantially closing the other end;
  • a helically coiled wire conductor concentrically within the cylinder and spaced from the inner walls thereof, said wire being one quarter wave long relative to the free-space wavelength of r-f power intended to be applied for operating the lamp, the end of the wire coil at the base member end of the cylinder being grounded and the end toward the open end of the cylinder being open circuited, the lamp being mounted with at least the major portion of the lamp substantially concentrically within the coil at the grounded end of the coil, and the open end of the cylinder extending beyond the open circuited end of the coil to avoid fringe effects; and
  • electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in the lamp, said connecting means being tapped into the coil at a point near, but spaced from, the grounded end thereof, said point being selected such that, when the lamp is in operating by r-f power connected to the coil, the coupler is tuned to the frequency of said r-f power and the impedance of the lamp and coupler means at said tap point substantially matches the impedance of said r-f power source, whereby when said r-f power is applied to the coil, and before a discharge is ignited in the lamp, a voltage maximum occurs at the open circuited end of the coil and creates a potential extending through the lamp portion between said open circuited end and said base member for ionizing the gas in the lamp.
  • said lamp has a bulbous portion which is said major portion of the lamp, and an elongated stem portion, said base member having a bore therethrough, said stem being received through said bore, and releasable clamping means clamping the stem in a selected longitudinal position through the bore.
  • a resonator coupler for coupling a source of r-f electric power into an electrodeless discharge lamp for starting and operating the lamp, comprising:
  • said member being grounded and being open at one end
  • the wire of the coil being one quarter wave long relative to the wavelength of r-f power intended to be cuited end of the coil and creates a potential extending through the lamp for ionizing the gas in the lamp.

Abstract

An electrodeless discharge lamp, consisting of a transparent envelope containing an ionizing gas and a minor amount of metallic element for producing spectral light, is started and operated by r-f power coupled to the lamp by a resonant coupler that consists of a helically coiled wire conductor mounted concentrically within a grounded hollow cylinder which is open at one end with a grounded base member in the other end.

Description

United States Patent Gabriel Mar. 25, 1975 ELECTRODELESS DISCHARGE LAMP AND POWER COUPLER THEREFOR Fred C. Gabriel, Stamford, Conn.
The Perkins-Elmer Corporation, Norwalk, Conn.
Filed: Feb. 11, 1974 Appl. No.: 441,600
Related U.S. Application Data Continuation of Ser. No. 337,060, March 1, 1973, abandoned.
Inventor:
Assignee:
U.S. Cl. 315/267, 315/344 Int. Cl. H05b 41/24 Field of Search 315/248, 267, 344, 348;
References Cited UNITED STATES PATENTS 4/1966 Booth et a1. 315/248 X .HM J
warms Primary Examiner-James B. Mullins Attorney, Agent, or Firm-John K. Conant 12 Claims, 1 Drawing Figure ELECTRODELESS DISCHARGE LAMP AND POWER COUPLER THEREFOR This is a continuation, of application Ser. No. 337,060, filed Mar. 1, 1973, and now abandoned.
BACKGROUND AND PRIOR ART The present invention is an electrodeless discharge lamp with an improved coupler for coupling r-f (radio frequency) electric power into the lamp to start and operate the lamp. Such lamps normally comprise a quartz bulb containing gas such as argon, and are operated by starting and maintaining a discharge in the ionized gas.
In particular this invention is a power coupler for a electrodeless discharge lamp in which a trace of a selected metallic element is incorporated to provide a bright spectral line source of the selected metal for use in chemical analysis, particularly atomic spectroscopy. A bright spectral line source provided in this manner might also be useful for optical pumping applications.
At present the basic light source for atomic spectroscopy are the hollow cathode lamps. For analyzing for most metallic elements hollow cathode lamps are sufficiently bright, stable, long-lived and inexpensive. However, hollow cathode lamps do not provide sufficiently bright or stable spectral lines for certain metallic elements, such as arsenic, cesium and rubidium, and for other elements such as selenium, tellurium, bismuth, antimony and tin, hollow cathode lamps are not bright enough for good spectral work. Electrodeless discharge lamps in general are an alternative to hollow cathode lamps for the metallic elements with which hollow cathode lamps are now used, but more importantly electrodeless discharge lamps provide highly satisfactory sources of the spectral lines of the foregoing metallic elements which cannot be provided by hollow cathode lamps or which are barely suitable as provided by a hollow cathode lamp.
Moreover, when operated efficiently electrodeless discharge lamps are capable of providing far brighter spectral line sources of metallic elements than hollow cathode lamps. While this is not of great significance for present atomic spectroscopy techniques with which brightness above a level of sufficiency is not especially useful, the practical availability of brighter spectral line sources might well make improved analytical techniques possible or enhance the usefulness of known techniques. For example, atomic fluorescence spectroscopy has little analytical usefulness at present, but since it has the characteristic that its detection limits improve almost linearly withincreased source interisity, the practical availability of brighter sources might transform it into a commercial useful analytical tool.
The use of electrodeless discharge lamps as spectral line sources have been known for a number of years. It is known to operate electrodeless discharge lamps with high frequency power, e.g., 2,450 megacycles and also with much lower frequency power of about 27 megacycles, i.e., radio frequency. The lower radio frequency has the particular advantage of providing more stable operation of the lamp.
Typically r-f operated electrodeless discharge lamps known in the art are excited to maintain a discharge in the lamp by placing the lamp in the final tank circuit inductor of a radio transmitter or power oscillator and the r-f power is coupled magnetically to the lamp plasma once a discharge is started in the lamp. Start-up of the lamp requires an initial voltage, higher than the voltage required to maintain a discharge after start-up, to ionize the gas and ignite a discharge in the ionized gas. The start-up voltage supplied is typically a voltage transient from an extended circuit or from the r-f supply itself; typical arrangements are described in an article titled Miniature Device For Starting Electrodeless Discharge Tubes by J. K. Brady in the Review of Scientific Instruments, Volume 36, 1965, Page 710, and in an article by M. M. Katsman, V. l. Konstantinov and S. M. Sutovskii, in the Russian Journal Zhurnol Priklodnoi Spectroskopii, Volume 6, Number 2, 1967, Pages 279 to 281.
Previously known and used power supplies for starting and maintaining a discharge in electrodeless discharge lamps are rather complex, relatively expensive and require special switching means for supplying first the high voltage required for ionizing the gas and starting a discharge therein, and then for supplying a' reduced level of voltage to maintain the discharge.
An object of the present invention is to provide a spectral source electrodeless discharge lamp operating unit which includes an inductive resonator coupler, for
a coupling r-f electric power into the lamp for starting and operating it,,that provides efficient coupling, that is simple and economic to construct and operate, that provides the requisite voltage and current ratios for starting the lamp and then for maintaining it in operation without intermediate switching means or alternative power supply connections, and that has an automatic self starting capability if the lamp goes out during operation.
BRIEF SUMMARY OF THE lNVENTlON The invention is an electrodeless discharge lamp, for use as a spectral light source, and an inductive resonator coupler for coupling r-f electric power into the lamp for starting and maintaining a discharge inthe lamp. The lamp is a transparent envelope, normally quartz, containing an ionizing gas, namely an inert gas such as argon, under low pressure and a minor amount of a metallic element whose spectral line is to be provided by the lamp. The coupler consists of a grounded hollow cylinder of electrically conductive material open at one end, for light from the lamp to pass out, and with a grounded base member in the other end. A helically coiled wire conductor, is mounted concentrically within the cylinder; the coil is a smaller diameter than the cylinder so as to provide an annular space between the coil and the cylinder wall. The length of wire in the coil is made nominally one quarter of the free-space wave length of the r-f electric power'to be applied. One end of the coil is grounded to the base member of the cylinder and the other end is unconnected, i.e., open circuited. The cylinder is longer than the coil with its open end extending beyond the open circuited end of the coil. The lamp is suitably supported on the base member of the cylinder and is mounted so that the major portion of the lamp is within the coil at the grounded end of the coil. R-f electric power from an r-f generator, which may be of conventional design, is tapped into the coil at a point near the grounded end of the coil. The tap point is selected so that when the lamp is operating the circuit formed by the coupler cylinder and coil and the lamp plasma, which is magnetically coupled by transformer action to the coil turns between the tap and ground, are tuned to the frequency of the r-f power and the impedance of the coupler and lamp match the impedance of the r-f power supply at the tap.
This coupler provides two distinct modes of operation successively; first it applies the electrostatic field to the gas in the lamp for starting the discharge and then it applies the electromagnetic field required for maintaining the discharge.
When the r-f power is first applied to the coupler, before the lamp discharge is started, the resistance and reactance reflected by the lamp are absent so that the coupler has a much higher impedance than when the lamp is operating, and the coupler has a very high electrical Q. The input r-f voltage thus appears as a high voltage maximum at the open circuited end of the coil.
This results in a high potential through the gas in they lamp between the open circuited end of the coil and the grounded base member of the cylinder. This ionizes the gas to the point at which a discharge can start. Sometimes the lamp will start by itself at this point, apparently due to stray energy reaching the lamp from surrounding light or the sun. In any event, very little extra energy is then required to initiate the discharge; the extra light from a lighted match is sufficient. When the discharge starts the coupler loses energy to the lamp the Q drops and the impedance of the lamp and coupler circuit drops to the level at which there is an impedance match (resistive) with the r-f power supply.
BRIEF DESCRIPTION OF THE DRAWINGS The invention is described in more detail below with reference to an illustrative embodiment shown in the accompaning drawing which is a side elevation, partly in section, partly broken away and partly schematic, of a preferred form of the lamp and inductive resonant coupler in accordance with the invention.
DETAILED DESCRIPTION Referring to the drawing the invention comprises generally an electrodeless discharge lamp and an inductive resonant coupler 11 for coupling r-f electric power from an r-f generator 12 into the lamp for operating the lamp.
The lamp [0, which is mounted within the structure of the coupler 11, is a small quartz bulb enclosing a volume of from I to 10 cubic centimeters containing an ionizing gas at low pressure, and a minor amount of a metallic element whose spectral line is to be produced by the lamp. The ionizing gas is one of the inert gases, argon being one that is commonly used; the pressure within the lamp is from k to about 5 torr. The particular gas and pressure are not critical, the general considerations being that the gas must be ionizable and support a discharge at reasonable power levels and the pressure must be such that, when the lamp is hot, the increased pressure will not be so large as to extinguish the discharge. The metallic element is present in microgram quantities; the particular amount is not critical, a minute trace will provide the spectral line desired but enough more is normally provided to make up for some of the vaporized metallic element that is lost by particles becoming embedded in the walls of the lamp. The spectral lines are the lines of the metals themselves and in the usual case, as with arsenic and selenium for example, the metallic element incorporated in the lamp is the metal itself. With some metals, such as lead for which the metal itself would not provide the requisite vapor pressure to provide the spectral line in the lamp output, a salt of the metal, e.g., lead chloride is used.
The lamp 10 has a bulbous portion 10a and a stem portion 1017 which is utilized for mounting and positioning the lamp within the coupler 11 in the manner subsequently to be described.
The inductive resonant coupler 11 is made up of a hollow cylinder 14 of an electrically conductive material and a helical coil 15 wound on a ceramic coil form 16 within the cylinder. One end of the cylinder 14, the right hand end, is open; its other end is closed by a base member 17 which is also of electrically conductive material. The base member 17 is grounded in the r-f gen erator as indicated at 18, and is fixed in the end of the cylinder so that the cylinder is electrically connected to the base member and thus similarly connected to ground.
One end of the coil 15, the left hand end as shown, is grounded by being connected to the base member; its other end is unconnected and thus open circuited. The length of the wire of coil 15 is made one quarter of the wave length of the rf electric power to be supplied by the r-f generator 12. The diameter of the coil 15 is less than the inside diameter of the cylinder 14 so as to provide an annular air space 19 between them. The coil and cylinder are thus in a capacitor relationship. In practice, the relative diameters of the cylinder and coil are 2 inches and l inch respectively so that the gap between is about one-half an inch. The cylinder is preferably made slightly longer than the coil to provide a suitably long ground plane relative to the field of the last turn of the coil at its open-circuited end to avoid unwanted fringe effects. In the embodiment shown the open end of the cylinder extends beyond the amount necessary to avoid fringe effects and actually extends more than an inch beyond the end of the coil as means to help in suppressing unwanted transmissions.
The lamp 10 is mounted for its major portion, its bulbous portion 10a, to be within the portion of the coil at the grounded end. The lamp is mounted by means of its stem portion 10b which fits through a bushing 20 that is fixed through the base end of the coil form 16 and the base member 17 of the cylinder. The end of the bushing 20 within the coil form 16 is flanged as shown at 21 and this serves to hold the coil form in place on the base member 17. The lamp stem 10b extends out through bushing 20 so that the position of the lamp bulbous portion 10a can be adjusted relative to the coil by moving the stem in or out relatively through the bushing. The position of the lamp relative to the coil is thus adjustable for finding the exact position at which the energy supplied by the coil, as subsequently described, is coupled into the lamp most efficiently when the lamp is operating. The position of the lamp is clamped by means of a cap 22 threaded onto the activated end of the bushing. The cap 22 has a hole through it for the lamp stem 10b to extend through and clamping means is provided by an O-ring captured between the end of the bushing 20 and the inside of the end of the cap 22. When the lamp position has been selected, the lamp is clamped by screwing the cap 22 in to squeeze the O to frictionally clamp the lamp stem 10b.
R-f electric power is applied to the coil 15 from the M" generator 12 by means of a connection from the generator 12 through a connector 25 mounted through the cylinder base member 17 to a tap 26 into the cell. It is normally desired to have the lamp and coupler spaced some distance from the r-f generator as a means of eliminating heat problems and for flexibility of instrument design; in this case, as illustrated, the r-f generator 12 is shown connected to the connector 25 through a coaxial cable 27. The r-f generator is comparable to a conventional radio transmitter, which would be a suitable r-f source; as shown it may consist of a crystal oscillator 28 coupled respectively through a low power r-f amplifier 30 and a high power r-f amplifier 31 to apply an r-f output through the coaxial cable 27 and connector 25 to the coil tap 26. The r-f generator 12 is operated from a conventional source of electric power, not shown. in practice the r-f generator is built to supply r-f electric power at 27.l2 MHz at levels up to 45 watts and having an impedance of 50 ohms. The coaxial cable 27 used would of course be a 50 ohm cable.
The tap 26 into the coil 15 is located relative to the turns of the coil so that when the lamp is operating the coupler 11 is tuned to the frequency supplied by the r-f generator and the impedance of the coupler and lamp circuit matches the impedance of the r-f generator at the tap 26.
A trimming capacitor 32 may be provided between the wall of the cylinder 14 and the coil 15, as shown, to assist in providing a fine tune for adjusting the resonant frequency of the coupler and lamp circuit, when the lamp is operating, to the resonant frequency of the r-f generator. This capacitor 32 may be eliminated by adjusting the resonant frequency of the generator, or by building the coupler 11 with a precision which makes additional trimming unnecessary.
As noted above, the length of wire in the coil 15 is nominally one quarter of the free-space wavelength of the r-f power to be supplied by the r-f generator 12. The coupler and lamp configuration of this invention thus provides a compact, quarter-wave, transmission line resonator which can be proportioned to have extremely low losses.
The coupler 11 has, due to the general configuration shown, a high electrical O. This is enhanced by making the coupler of low loss materials and reducing losses by all practical means. In practice the, cylinder 14 and base member are made of copper or brass and the interior of the cylinder is plated with a polished silver coating indicated at 33, and the wire of the coil 15 is solid silver or silver plated copper for the purpose of maintaining high r-f conductivity in the presence of high lamp temperatures and corrosive atmosphere. Also soldered taps are avoided, the taps preferably being welded.
When the power from the r-f generator 12 is first applied to the coupler 11 and the lamp has not yet started a discharge, the resistance and reactance reflected by the lamp discharge when the lamp is operating are absent so that the impedance. of the coupler circuit is higher than the 50 ohm mentioned above, and is typically 400 to 500 ohms. Also because the lamp is not extracting power, the coupler has a very high electrical Q and the open circuited end of the coil 15 is the location of a current node and hence a voltage maximum. This has the effect of transforming the input r-f voltage swing to a very high voltage (e.g., tens of thousands) at the open circuited end of the coil. This high voltage creates a potential through the gas in the bulbous portion 10a of the lamp to the grounded base member 17 of the cylinder which ionizes the gas. in many instances the electrostatic energy thus coupled into the lamp and a slight amount of additional energy applying to the lamp from the surrounding light, such as direct sunlight or light bulbs, is enough to start the discharge in the ionized gas. In any event only a slight additional amount of energy is necessary for starting the discharge and this may be supplied by such simple means as a lighted match.
When the discharge starts, the Q of the coupler drops, the high voltage subsides and the lamp is thereafter maintained in operation by power in the turns of the coil 15 between the grounded end and the input tap 26 magnetically coupling into the lamp. Ifthe discharge should extinguish, voltage builds up in the open circuited end of the coil again, and discharge will then automatically reignite. When the lamp is operating the voltage drop back as just described and the impedance of the circuit of the coupler 11 and lamp 10 then matches the impedance of the r-f supply at the tap 26 thereby providing highly efficient operation of the lamp. I
The lamp may be operated to produce a steady output, or the output may be modulated for use with certain spectroscopy instruments, that are adapted to operate with a modulated light source, by modulating the output of the r-f generator between an upper output level and a low output level, the low output level being one at which the lamp would still be operating, but producing a light below a minimum level recognized by the spectroscopy instrument. In practice a 50 percent modulation of the output of the r-f generator is suitable for adapting the lamp and coupler of the invention for use with spectroscopy instruments intended to operate with a modulated light source.
As shown in the drawing, in practice the open end of the cylinder 14, the right hand end, is preferably provided with a safety window to prevent anyone from inadvertently sticking a finger into the high voltage end of the coil 15 at start-up, and also to keep corrosive materials out of the interior of the cylinder as much as possible. This window is suitably provided by a quartz window 34 mounted in a support plug 35 fitted into the end of the cylinder 14.
What is claimed is:
1. A resonator coupler for coupling a source of r-f electric power into an electrodeless discharge lamp for starting and operating the lamp, comprising:
a grounded hollow cylinder of electrically conductive material open at one end, with a grounded base member at the other end;
a helically coiled wire conductor concentrically within the cylinder and spaced from the inner walls thereof;
means for mounting a discharge lamp substantially concentrically within one end position of the coil;
the wire of the coil being one quarter wave long relative to the free-space wavelength of r-f power intended to be applied for operating a lamp mounted therein;
the end of the coil at the end portion within which a lamp is adapted to be mounted being toward said base member and being grounded, the other end of the coil being open circuited; and
electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in a lamp mounted within the coil, said connecting means being tapped into the coil at a point near, but spaced from, the grounded end thereof, said point being selected such that, when a lamp mounted in the coil is in operation by r-f power connected to the coil, the coupler is tuned to the frequency of said r-f power and the impedance of said lamp and coupling means at said tap point substantially matches the impedance of said r-f power source, whereby when said r-f power is applied to the coil, and before a discharge is ignited in the lamp, a voltage maximum occurs at the open circuited end of the coil and creates a potential extending through the lamp portion between said open circuited end and said base member for ionizing the gas in the lamp.
2. The coupler of claim 1 including a capacitor between the wall of said cylinder and a turn of said coil for fine tuning the resonant frequency of the resonator coupler relative to the frequency of an r-f source to be applied thereto.
3. The resonator of claim 1 in which the outside diameter of the coil is substantially one-half the inside diameter of the cylinder, the coil being of silver wire and the interior of the cylinder being polished silver plating.
4. The resonator of claim 1 which the open end of the cylinder extends beyond the open circuited end of the coil at least sufficiently to avoid fringe effects.
5. The resonator of claim 1 in which said base member extends across and substantially closes an end of the cylinder, the outside diameter of the coil and inside diameter of the cylinder being substantially 1 inch and 2 inches respectively, the wire of said coil being silver, and the interior walls of the cylinder and base member being polished silver plating.
6. The combination of an electrodeless discharge lamp and a resonator coupler for coupling the lamp to a source of r-f electric power for starting and operating the lamp, said coupler comprising:
a grounded hollow cylinder of electrically conductive material open at one end with a grounded base member at the other end;
a helically coiled wire conductor concentrically within the cylinder and spaced from the inner. walls thereof, said wire being one quarter wave long relative to the free-space wavelength of r-f power intended to be applied for operating the lamp, the end of the wire coil at the base member end of the cylinder being grounded and the end toward the open end of the cylinder being open circuited, and the lamp being mounted with at least the major portion of the lamp within the coil at the grounded end of the coil; and
electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in the lamp, said connecting means being tapped into the coil at a point near, but spaced from, the grounded end thereof, said point being selected such that, when the lamp is in operation by r-f power connected to the coil,
7. The combination of claim 6 in which said discharge lamp is a transparent sealed envelope containing an ionizing gas at low pressure and a minor amount of a metallic element for producing spectral light of said metallic element.
8. The combination of an electrodeless discharge lamp for providing spectral light and resonator coupler for coupling the lamp to a source of r-f electric power for starting and operating the lamp;
said lamp being a transparent sealed envelope containing a minor amount of metallic element and an ionizing gas;
said coupler comprising:
a grounded hollow cylinder of electrically conductive material open at one end with a grounded base member substantially closing the other end;
a helically coiled wire conductor concentrically within the cylinder and spaced from the inner walls thereof, said wire being one quarter wave long relative to the free-space wavelength of r-f power intended to be applied for operating the lamp, the end of the wire coil at the base member end of the cylinder being grounded and the end toward the open end of the cylinder being open circuited, the lamp being mounted with at least the major portion of the lamp substantially concentrically within the coil at the grounded end of the coil, and the open end of the cylinder extending beyond the open circuited end of the coil to avoid fringe effects; and
electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in the lamp, said connecting means being tapped into the coil at a point near, but spaced from, the grounded end thereof, said point being selected such that, when the lamp is in operating by r-f power connected to the coil, the coupler is tuned to the frequency of said r-f power and the impedance of the lamp and coupler means at said tap point substantially matches the impedance of said r-f power source, whereby when said r-f power is applied to the coil, and before a discharge is ignited in the lamp, a voltage maximum occurs at the open circuited end of the coil and creates a potential extending through the lamp portion between said open circuited end and said base member for ionizing the gas in the lamp.
9. The combination of claim 8 in which said lamp has a bulbous portion which is said major portion of the lamp, and an elongated stem portion, said base member having a bore therethrough, said stem being received through said bore, and releasable clamping means clamping the stem in a selected longitudinal position through the bore.
10. The combination of claim 8 in which the outside diameter of the coil is on the order of one-half the inside diameter of the cylinder, in which the interior surfaces of the cylinder and base member are polished silver and in which the wire of the coil is essentially silver.
11. A resonator coupler for coupling a source of r-f electric power into an electrodeless discharge lamp for starting and operating the lamp, comprising:
a coiled wire conductor grounded at one end and open circuited at the other,
means for mounting a discharge lamp within the coil,
a generally tubularmember of electrically conductive material surrounding and spaced from the coil so as to be in a capacitor relationship therewith,
said member being grounded and being open at one end,
the wire of the coil being one quarter wave long relative to the wavelength of r-f power intended to be cuited end of the coil and creates a potential extending through the lamp for ionizing the gas in the lamp.
12. The combination of an electrodeless discharge open circuited at the other, a discharge lamp mounted with at least a major portion within the coil, a generally tubular shield of electrically conductive pp for operating a p mounted in the Coil, material surrounding and spaced from the coil so as to be in a capacitor relationship therewith, said electrical connecting means for connecting to the Shield being grounded and being Open at one end coil a source of r-f electrical power that is sufficient the wire of the coil being one quarter wave long rc|a to malfltalrl a dlscharge m a lamp mounted W'thin tive to the wavelength of r-f power intended to be the coil, said connecting means being tapped into applied for Operating Said lamp and s f g ir g f i thelreof electrical connecting means for connecting to the Sal emg 9 Sue w en a amp coil a source of r-f electrical power that is sufficient mounted in the coil IS in operation by r-f power to maintain a discharge in the lamp, said connectconnected to the COll, the coupler lS- tuned -to the mg means being tapped mto the coil at a pomt infrequency of said r-f power and the impedance of termediate the ends thereot, said point being sesaid lamp and coupling means at said tap pomt sub- 1 d h h h h l I b stantially matches the impedance of said r-f power ecte Sue t W S t e amp l mhoperdnlon source, whereby when said r-f power is applied to M power connecte to h col t e Coup l ls the coil, and before a discharge is ignited in the tuned to the frequency ofsald r'fpower and h lamp, a voltage maximum Occurs at the open cip pedance of the lamp and coupler means at said tap point substantially matches the impedance of said r-f power source, whereby when said r-f power is applied to the coil, and before a discharge is ignited in the lamp, a voltage maximum occurs at the open lamp and a resonator coupler for coupling the lamp to a source of r-f electric power for starting and operating tending through the lamp for ionizing the gas in the the lamp, said coupler comprising: lamp.
a coiled wire conductor grounded at one end and circuited end of the coil and creates a potential ex-

Claims (12)

1. A resonator coupler for coupling a source of r-f electric power into an electrodeless discharge lamp for starting and operating the lamp, comprising: a grounded hollow cylinder of electrically conductive material open at one end, with a grounded base member at the other end; a helically coiled wire conductor concentrically within the cylinder and spaced from the inner walls thereof; means for mounting a discharge lamp substantially concentrically within one end position of the coil; the wire of the coil being one quarter wave long relative to the free-space wavelength of r-f power intended to be applied for operating a lamp mounted therein; the end of the coil at the end portion within which a lamp is adapted to be mounted being toward said base member and being grounded, the other end of the coil being open circuited; and electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in a lamp mounted within the coil, said connecting means being tapped into the coil at a point near, but spaced from, the grounded end thereof, said point being selected such that, when a lamp mounted in the coil is in operation by r-f power connected to the coil, the coupler is tuned to the frequency of said r-f power and the impedance of said lamp and coupling means at said tap point substantially matches the impedance of said r-f power source, whereby when said r-f power is applied to the coil, and before a discharge is ignited in the lamp, a voltage maximum occurs at the open circuited end of the coil and creates a potential extending through the lamp portion between said open circuited end and said base member for ionizing the gas in the lamp.
2. The coupler of claim 1 including a capacitor between the wall of said cylinder and a turn of said coil for fine tuning the resonant frequency of the resonator coupler relative to the frequency of an r-f source to be applied thereto.
3. The resonator of claim 1 in which the outside diameter of the coil is substantially one-half the inside diameter of the cylinder, the coil being of silver wire and the interior of the cylinder being polished silver plating.
4. The resonator of claim 1 which the open end of the cylinder extends beyond the open circuited end of the coil at least sufficiently to avoid fringe effects.
5. The resonator of claim 1 in which said base member extends across and substantially closes an end of the cylinder, the outside diameter of the coil and inside diameter of the cylinder being substantially 1 inch and 2 inches respectively, the wire of said coil being silver, and the interior walls of the cylinder and base member being polished silver plating.
6. The combination of an electrodeless discharge lamp and a resonator coupler for coupling the lamp to a source of r-f electric power for starting and operating the lamp, said coupler comprising: a grounded hollow cylinder of electrically conductive material open at one end with a grounded base member at the other end; a helically coiled wire conductor concentrically within the cylinder and spaced from the inner walls thereof, said wire being one quarter wave long relative to the free-space wavelength of r-f power intended to be applied for operating the lamp, the end of the wire coil at the base member end of the cylinder being grounded and the end toward the open end of the cylinder being open circuited, and the lamp being mounted with at leaSt the major portion of the lamp within the coil at the grounded end of the coil; and electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in the lamp, said connecting means being tapped into the coil at a point near, but spaced from, the grounded end thereof, said point being selected such that, when the lamp is in operation by r-f power connected to the coil, the coupler is tuned to the frequency of said r-f power and the impedance of the lamp and coupler means at said tap point substantially matches the impedance of said r-f power source, whereby when said r-f power is applied to the coil, and before a discharge is ignited in the lamp, a voltage maximum occurs at the open circuited end of the coil and creates a potential extending through the lamp portion between said open circuited end and said base member for ionizing the gas in the lamp.
7. The combination of claim 6 in which said discharge lamp is a transparent sealed envelope containing an ionizing gas at low pressure and a minor amount of a metallic element for producing spectral light of said metallic element.
8. The combination of an electrodeless discharge lamp for providing spectral light and resonator coupler for coupling the lamp to a source of r-f electric power for starting and operating the lamp; said lamp being a transparent sealed envelope containing a minor amount of metallic element and an ionizing gas; said coupler comprising: a grounded hollow cylinder of electrically conductive material open at one end with a grounded base member substantially closing the other end; a helically coiled wire conductor concentrically within the cylinder and spaced from the inner walls thereof, said wire being one quarter wave long relative to the free-space wavelength of r-f power intended to be applied for operating the lamp, the end of the wire coil at the base member end of the cylinder being grounded and the end toward the open end of the cylinder being open circuited, the lamp being mounted with at least the major portion of the lamp substantially concentrically within the coil at the grounded end of the coil, and the open end of the cylinder extending beyond the open circuited end of the coil to avoid fringe effects; and electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in the lamp, said connecting means being tapped into the coil at a point near, but spaced from, the grounded end thereof, said point being selected such that, when the lamp is in operating by r-f power connected to the coil, the coupler is tuned to the frequency of said r-f power and the impedance of the lamp and coupler means at said tap point substantially matches the impedance of said r-f power source, whereby when said r-f power is applied to the coil, and before a discharge is ignited in the lamp, a voltage maximum occurs at the open circuited end of the coil and creates a potential extending through the lamp portion between said open circuited end and said base member for ionizing the gas in the lamp.
9. The combination of claim 8 in which said lamp has a bulbous portion which is said major portion of the lamp, and an elongated stem portion, said base member having a bore therethrough, said stem being received through said bore, and releasable clamping means clamping the stem in a selected longitudinal position through the bore.
10. The combination of claim 8 in which the outside diameter of the coil is on the order of one-half the inside diameter of the cylinder, in which the interior surfaces of the cylinder and base member are polished silver and in which the wire of the coil is essentially silver.
11. A resonator coupler for coupling a source of r-f electric power into an electrodeless discharge lamp for starting and operating the lamp, comprising: a coiled wire conductor grounded at one end and open circuited at the other, means for mounting a discharge lamp within the coil, a generally tubular member of electrically conductive material surrounding and spaced from the coil so as to be in a capacitor relationship therewith, said member being grounded and being open at one end, the wire of the coil being one quarter wave long relative to the wavelength of r-f power intended to be applied for operating a lamp mounted in the coil, and electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in a lamp mounted within the coil, said connecting means being tapped into the coil at a point intermediate the ends thereof, said point being selected such that, when a lamp mounted in the coil is in operation by r-f power connected to the coil, the coupler is tuned to the frequency of said r-f power and the impedance of said lamp and coupling means at said tap point substantially matches the impedance of said r-f power source, whereby when said r-f power is applied to the coil, and before a discharge is ignited in the lamp, a voltage maximum occurs at the open circuited end of the coil and creates a potential extending through the lamp for ionizing the gas in the lamp.
12. The combination of an electrodeless discharge lamp and a resonator coupler for coupling the lamp to a source of r-f electric power for starting and operating the lamp, said coupler comprising: a coiled wire conductor grounded at one end and open circuited at the other, a discharge lamp mounted with at least a major portion within the coil, a generally tubular shield of electrically conductive material surrounding and spaced from the coil so as to be in a capacitor relationship therewith, said shield being grounded and being open at one end, the wire of the coil being one quarter wave long relative to the wavelength of r-f power intended to be applied for operating said lamp, and electrical connecting means for connecting to the coil a source of r-f electrical power that is sufficient to maintain a discharge in the lamp, said connecting means being tapped into the coil at a point intermediate the ends thereof, said point being selected such that, when the lamp is in operation by r-f power connected to the coil, the coupler is tuned to the frequency of said r-f power and the impedance of the lamp and coupler means at said tap point substantially matches the impedance of said r-f power source, whereby when said r-f power is applied to the coil, and before a discharge is ignited in the lamp, a voltage maximum occurs at the open circuited end of the coil and creates a potential extending through the lamp for ionizing the gas in the lamp.
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