EP1198976A1 - Inductive electrodeless lamp providing rotating electric field - Google Patents

Abstract

An inductively coupled electrodeless lamp has a pair of excitation coils positioned exterior to an envelope or bulb. The pair of excitation coils are positioned and driven in a manner to produce at least one moving ring-shaped electric field within the envelope. The moving ring electric field results in formation of a correspondingly moving ring of plasma discharge within the envelope. The movement of the electric field results in a more uniformly hot plasma discharge volume within the envelope, thereby facilitating emission or re-radiation of photons. The movement of the ring-shaped electric field (and the corresponding plasma discharge) can be rotational, oscillating, wobbling, or switching. The nature of the movement depends upon such factors as coil geometry and orientation and coil excitation (driving) technique. In some embodiments, the pair of excitation coils are driven by quadrature techniques (e.g., either phase quadrature, frequency quadrature, or amplitude quadrature). Differing coil geometries and orientations are taught.

Description

INDUCTIVE ELECTRODELESS LAMP PROVIDING ROTATING ELECTRIC FIELD

[0001] The invention described herein was made with Government support under an award from the Department of Energy The government has certain rights in this invention

CROSS REFERENCE TO RELATED APPLICATIONS

[0002] This application is based on and claims the priority of U S patent application nos 60/141 ,872, filed July 2, 1999, and 09/531 ,507, filed March 21 , 2000

BACKGROUND FIELD OF THE INVENTION

[0003] The present invention pertains to discharge lamps, and particularly to the operation and structure of inductive electrodeless lamps

RELATED ART

[0004] Electrodeless lamps are capable of generating high brightness levels over more than 10,000 hours of operation without requiring replacement

Electrodeless lamps have no internal electrodes, but rather rely upon external structures to achieve breakdown and excitation of a fill material for emission of visible light Typically electrodeless lamps are classified as inductively coupled (H discharge), capacitance coupled (E discharge) microwave discharge, and traveling wave discharge Certain basic principles of electrodeless lamps, as well as each of these classifications, are discussed in the literature See, e g , Wharmby, D O "Electrodeless Lamps For Lighting A Review", IEEE Proceedings-A, Vol 140 No 6, November 1993, pp 465 - 473 [0005] An inductively coupled electrodeless lamp can be analogized to an electrical transformer In inductively coupled electrodeless lamps, the fill material (e g , plasma) in a discharge vessel (bulb) serves as a single turn secondary coil while a primary (exciter) coil is connected via suitable impedance matching to a power source There are various forms of inductively coupled electrodeless lamps the primary (exciter) coil can be outside the discharge vessel, inside the vessel within a reentrant; or wrapped around part of the tubular lamp forming a torus. The magnetic field can be provided by a coil with an air core, or a magnetic core. [0006] In an inductively coupled electrodeless lamp, an alternating current in the coil causes a changing magnetic field, which induces an electric field which drives a current in the plasma. Certain electrical properties and phenomena involved with inductively coupled electrodeless lamps have been documented, e.g., by Piejak, R.B. et al., "A Simple Analysis of An Inductive RF Discharge", Plasma Sources Sci. Technlol. 1 (1992), pages 179 - 186. [0007] Examples of structures and operating techniques for inductively coupled electrodeless lamps are provided in United States Patent Application Serial Number 09/228,230, filed January 11 , 1999, entitled "High Frequency Inductive Lamp and Power Oscillator", as well as United States Patent 5,798,611 , both of which are incorporated herein by reference in their entirety. [0008] Quadrature (e.g., 90 degree out of phase) driving techniques have been utilized for impedance matching in an electrodeless lamp, as exemplified in United States Patent 4,712,046. Employment of quadrature methods for generating an eliiptically or circularly polarized electric field are known in inductively coupled plasma generating devices utilizing a plurality of parallel conductors (see United States Patent 5,619,103 to Tobin et al.). [0009] In addition, quadrature techniques have been employed to provide more uniform electric fields, and hence more uniform radiative discharge, within bulbs of electrodeless lamps. In this regard, for a microwave electrodeless lamp United States Patent 5,227,698 to Simpson couples microwave energy to an electrodeless lamp cavity to obtain a rotating field within the cavity. In one embodiment, Simpson provides two fields which are displaced by 90 degrees, which are out of phase by 90 degrees, and which are of equal amplitude, resulting in a composite rotating field in the cavity having a circular polarization. [0010] Rotation of electric fields have been employed for other purposes in electrodeless lamps, such as avoiding an "arc attachment" phenomenon which can cause hot spots in the wall of the lamp bulb or capsule. United States Patent 5,498,928 to Lapatovich et al. proposes to reduce hot spots by positioning four electric field applicators at 90 degree positions in an excitation plane about a lamp capsule, and driving the applicators with 90 degree offset phases and equal amplitudes to create a rotating field with an electric field vector which rotates in the excitation plane.

[0011] Typically inductively coupled electrodeless lamps do not suffer from "arc attachment" phenomenon provided that the arc is sufficiently stable to sustain itself. Moreover, whereas capacitance coupled and microwave type electrodeless lamps employ electric fields that cut across the bulb, the magnetic fields created by the primary (exciter) coil of inductively coupled electrodeless lamps induce electric fields in the plasma which occur primarily inside the bulb.

SUMMARY

[0012] Driving an inductively coupled electrodeless lamp with a single inductive coil typically results in a toroidal shaped discharge with cold regions at either pole. In general, hotter regions of the discharge volume tend to radiate photons while cooler regions of the discharge volume tend to absorb photons. The cold regions at the poles limit the plasma temperature, thereby limiting the discharge volume which is hot enough to radiate visible light.

[0013] What is needed, and an object of the present invention, is method and apparatus for increasing the temperature of the cold regions in an inductively coupled electrodeless lamp. [0014] According to a first aspect of the invention, an inductively coupled electrodeless lamp has a pair of excitation coils positioned exterior to an envelope or bulb. The pair of excitation coils are positioned and driven in a manner to produce at least one moving ring-shaped electric field within the envelope. The moving electric field results in formation of a corresponding plasma discharge within the envelope. The discharge occurs over a larger portion of the bulb volume and the temperature of the cold regions is increased. The movement of the electric field results in a more uniformly hot plasma excitation volume within the envelope, thereby facilitating emission or re-radiation of photons. [0015] The movement of the ring-shaped electric field (and the corresponding plasma discharge) can be rotational, oscillating, wobbling, or switching. The nature of the movement depends upon such factors as coil geometry and orientation and coil excitation (driving) technique. In some embodiments, the pair of excitation coils are driven by quadrature techniques (e.g., either phase quadrature, frequency quadrature, or amplitude quadrature).

[0016] According to one aspect of the invention, an inductively coupled electrodeless lamp includes an envelope enclosing a fill, the fill forming a plasma discharge when excited; a pair of excitation coils positioned exterior to the envelope, the pair of excitation coils being positioned and driven in a manner to produce a moving ring electric field interior to the envelope, the moving ring electric field exciting a plasma discharge within the envelope; and a driver which applies alternating current to the pair of excitation coils, there being a phase difference between the alternating current applied to a first of the pair of excitation coils and a second of the pair of excitation coils. Preferably, the phase difference is substantially ninety degrees. For example, the driver may include a single signal source and a phase adjusting delay element which provides the phase difference between the alternating current applied to the first of the pair of excitation coils and the second of the pair of excitation coils. Alternatively, the driver has a first signal source and a second signal source, the first source being connected to apply a signal having a first phase to the first of the pair of excitation coils and the second source being connected to apply a signal having a second phase to the second of the pair of excitation coils. Generally, the pair of excitation coils are positioned to have respective centers of the pair excitation coils coincide and to have respective coil axes oriented at a predetermined angle. In most examples the predetermined angle is 90°. In some examples it is advantageous to have the axis of one coil tilted with respect to the axis of the other coil by between about 10° to 15° from perpendicular. In most examples, each of the pair of excitation coils has a wedding ring shape. In some examples each of the pair of wedding ring shaped excitation coils has a reduced width in an area where the coils cross each other. [0017] According to another aspect of the invention, an inductively coupled electrodeless lamp includes an envelope enclosing a fill, the fill forming a plasma discharge when excited; a pair of excitation coils positioned exterior to the envelope, the pair of excitation coils being positioned and driven in a manner to produce a moving ring electric field interior to the envelope, the moving ring electric field exciting a plasma discharge within the envelope; and a driver which applies alternating current to the pair of excitation coils, wherein the driver switches between applying the alternating current a first of the pair of excitation coils and a second of the pair of excitation coils For example, the switching occurs on a predetermined time constant Preferably, the predetermined time constant is in relation to a decay time of the plasma For example, the predetermined time constant is on the order of milliseconds 17 The driver may include a single signal source and a switching element which provides the switching between applying the alternating current to the first of the pair of excitation coils and the second of the pair of excitation coils The pair of excitation coils may be positioned to have respective centers of the pair excitation coils coincide and to have respective coil axes oriented at a predetermined angle (e g 90°) Alternatively, the pair of excitation coils may be positioned to be parallel to and spaced apart from each other [0018] Preferably, the alternating current is applied with a driving frequency of at least 300 MHz and each of the pair of excitation coils has an effective electrical length of less than one half wavelength of the driving frequency More preferably, the driving frequency is at least 900 MHz and each of the pair of excitation coils has an effective electrical length of less than one quarter wavelength of the driving frequency The driving techniques described herein may be particularly advantageous where the envelope is covered with a reflective material except in the region of a light emitting aperture

BRIEF DESCRIPTION OF THE DRAWINGS [0019] The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention

[0020] Fig 1 is an isometric view of an inductively coupled electrodeless lamp according to a first embodiment of the invention [0021] Fig 2 is a rear isometric view of coil and bulb components of the lamp of Fig 1 [0022] Fig. 2A is a top view of the coil and lamp components illustrated in

Fig. 2.

[0023] Fig. 2B is a front view of the coil and lamp components illustrated in

Fig. 2. [0024] Fig. 2C is a right side view of the coil and lamp components illustrated in Fig. 2.

[0025] Fig. 3 is a side view, partially sectioned, showing capacitance as well as coil and ring components of the lamp of Fig. 1.

[0026] Fig. 4A , Fig. 4B, Fig. 4C, and Fig. 4D are top views showing differing quadrature driving techniques or coil orientations for the lamp of Fig. 1.

[0027] Fig. 5 is a diagrammatic view showing a moving ring electric field created by the lamp of Fig. 1.

[0028] Fig. 5A is a diagrammatic side view showing rotating of an excited plasma fill. [0029] Fig. 5B is a diagrammatic top view of Fig. 5A.

[0030] Fig. 5C is a diagrammatic side view showing wobbling of an excited plasma fill.

[0031] Fig. 6A is a top view of a coil configuration according to another embodiment of the invention. [0032] Fig. 6B is an isometric view of the coil configuration of Fig. 6A.

[0033] Fig. 7 - Fig. 9 are isometric views showing examples of other coil configurations and orientations suitable for use with the lamp of Fig. 1.

[0034] Fig. 10 is a rear isometric view of coil and bulb components of a lamp of another embodiment, wherein the coils have a "split" wedding ring configuration. [0035] Fig. 11 is a sectioned view of a coil having a cross-sectional shape resembling a dumbbell.

[0036] Fig. 12A and Fig. 12B are isometric views of portions of embodiments of lamps wherein coils lie in parallel planes.

[0037] Fig. 13 is a diagrammatic view showing a bouncing or switching ring electric field created by the lamp of Fig. 12A or Fig. 12B

[0038] Fig. 14 is an isometric view of an inductively coupled electrodeless lamp according to another embodiment of the invention. [0039] Fig. 15 is a schematic view of circuitry including an electrical equivalent circuit for components shown in Fig. 14.

[0040] Fig. 16 is a rear isometric view of coil and bulb components of an aperture electrodeless lamp which utilizes the principles of the present invention. [0041] Fig. 17A is an isometric view showing another example of a coil configuration and orientation suitable for use with the lamp of Fig. 1. [0042] Fig. 17B is a top view of the coil configuration and orientation example of Fig. 17A.

[0043] Fig. 17C is a front view of the coil configuration and orientation example of Fig. 17A.

[0044] Fig. 17D is a right side view of the coil configuration and orientation example of Fig. 17A.

DETAILED DESCRIPTION [0045] In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

[0046] Fig. 1 illustrates a first example embodiment of an inductively coupled electrodeless lamp 20 configured and operated in accordance with the present invention. The inductively coupled electrodeless lamp 20 has a stationary light transmissive enclosure or bulb, herein referenced as envelope 22, which contains plasma-forming fill 24. A pair of single turn excitation coils 32, 34 are positioned exterior to envelope 22. As explained hereinafter, the pair of excitation coils 32, 34 are positioned and driven in a manner to produce a moving ring electric field which, by driving a current in the plasma, excites the fill 24 thereby causing a plasma discharge resulting in emission of visible light.

[0047] The inductively coupled electrodeless lamp 20 of Fig. 1 has a rectangular base 40 upon which two C-shaped blocks 42, 44 are translatably mounted. Each block 42, 44 carries a respective one of the pair of excitation coils 32, 34 including capacitance plates (as subsequently described) associated with the respective coils 32, 34. Pedestal 46, extending orthogonally upwardly from a rear of base 40, supports envelope 22. In particular, envelope stem 48 is held aloft by pedestal 46 and parallel to an upper surface of base 40. Envelope 22 formed at a distal end of stem 48 is situated within the single turns of the pair of excitation coils 32, 34. In fact, the turns of the excitation coils 32, 34 preferably have their centers substantially coincident with a center of envelope 22. [0048] In the particular embodiment of Fig. 1 , and as shown in Fig. 2 and Fig. 2A - Fig. 2D, each of the excitation coils 32, 34 has a "wedding ring" configuration, with the turn or semicircular segment of coil 32 having a slightly greater radius than the turn or semicircular segment of coil 34. A "wedding ring" configuration is described in the above-referenced United States Patent Application Serial Number 09/228,230, filed January 1 1 , 1999, entitled "High Frequency Inductive Lamp and Power Oscillator", which is incorporated herein by reference. Moreover, the pair of excitation coils 32, 34 are oriented so that a plane in which the turn of coil 32 lies is inclined at a predetermined angle (e.g., ninety degrees) to a plane in which the turn of coil 34 lies. As explained hereinafter, however, variations of coil configuration and orientation are within the scope of the invention. [0049] In addition to having a turn or semicircular segment, each coil 32, 34 has both a top linear segment or lead and a bottom linear segment or lead. Coil 32 has top segment 32t and bottom segment 32b; coil 34 has top segment 34t and bottom segment 34b. For each coil 32, 34, the top segment and the bottom segment are linear or straight extensions formed at opposite ends of the coil turn. For each coil the top segment and bottom segment are parallel to one another and parallel to the top of base 40. A small vertical gap exists between the top segment and bottom segment for each coil, the gap having a magnitude corresponding to circular discontinuity of the turn of the coil. [0050] The top segment and bottom segment of each coil 32, 34 are connected to, and preferably integral with, respective top and ground plane capacitance plates carried by blocks 42, 44. The top segment 32t of coil 32 is connected to capacitance plate 52t mounted in block 42; bottom segment 32b of coil 32 is connected to capacitance plate 52b mounted in block 42. Similarly, top segment 34t of coil 34 is connected to capacitance plate 54t mounted in block 44; bottom segment 34b of coil 34 is connected to capacitance plate 54b mounted in block 44. The capacitance plates are positioned or supported by dielectric posts, with plates 52t and 52b being positioned between dielectric posts 56t and 56b and plates 54t and 54b being positioned between dielectric posts 58t and 58b. The dielectric posts are sized and positioned to maintain pressure on the respective plates and thereby keep the plates in position. The heights of the dielectric posts depend on such factors as the thickness of the elements between them and the desired positioning of the coils.

[0051] Fig. 3 shows in more detail capacitance structures for inductively coupled electrodeless lamp 20 of Fig. 1. Capacitances 52c1 and 52c2 are provided for coil 32; capacitances 54c1 and 54c2 are provided for coil 34. Capacitor 52c1 comprises the capacitance plate 52t (which is preferably integral with top segment 32t of coil 32), a power feed plate or blade 53; and dielectric 52d1. For coil 32, capacitor 52c2 comprises the capacitance plate 52b (which is preferably integral with bottom segment 32b of coil 32), the blade 53; and dielectric 52d2. Capacitor 54c1 comprises the capacitance plate 54t (which is preferably integral with top segment 34t of coil 34), a power feed plate or blade 55; and dielectric 54d1. For coil 34 the capacitor 54c2 comprises the capacitance plate 54b (which is preferably integral with bottom segment 32b of coil 32), plate or blade 55; and dielectric 54d2. The capacitances 52c1 and 52c2 are thus in a stacked formation with one another, while 54c1 and 54c2 are in a stacked formation with one another (see Fig. 3). [0052] Fig. 4A, Fig. 4B, and Fig. 4C show differing techniques for driving the inductively coupled electrodeless lamp 20 of Fig. 1. Fig. 4A illustrates a phase quadrature driving technique; Fig. 4B can illustrate either a phase quadrature or frequency quadrature driving technique; Fig. 4C illustrates an amplitude quadrature driving technique. [0053] The driver of Fig. 4A comprises one signal source 80A which supplies alternating current to the pair of excitation coils 32, 34. The driver of Fig. 4A introduces a predetermined phase difference between the alternating current as applied to coil 32 and coil 34 through provision of a phase adjusting delay element 82 which is provided between source 80A and coil 32. As understood from Fig. 3, the alternating current is applied to coil 34 via blade 55, blade 55 being located under capacitance plate 54t as shown in Fig. 4A - Fig. 4C. Similarly, the alternating current is applied to coil 32 (after being delayed by phase adjusting delay element 82) via blade 53, blade 53 being located under capacitance plate 52t as shown in Fig. 4A - Fig. 4C. An example value for signal source 80A is 915 MHz. An example phase adjusting delay element 82 is a transmission line % inch by 1 and 3/8 inch (not quite λ/4). [0054] The driver of Fig. 4B has two alternating current signal sources 80B and 80B'. The driver of Fig. 4B can be operated in either of two modes. In both modes, the signal from source 80B is applied to coil 34 and the signal from source 80B' is applied to coil 32. In the first mode, the signals from the two sources 80B and 80B' are of the same frequency but of different phase. Thus, the first mode represented by Fig. 4B essentially accomplishes the same result (differing phase) as the driver of Fig. 4A. In the second mode, the signal from source 80B applied to coil 34 is of a non-commensurate frequency than the signal from source 80B' applied to coil 32. In other words, source 80B and 80B' operate at mutually different frequencies. For example, during steady state operation of the lamp in the general range of operation of 900 MHz the degree of non-commensurateness (i.e. difference) of the frequency between the signal from source 80B and the signal from source 80B' can be between about 20 and 40 MHz (e.g. about 2% to 4% of the operating frequency range) in order to provide a wobbling electric field in envelope 22, with about 30 MHz being preferred. [0055] The driver of Fig. 4C comprises one signal source 80C which switches application of the same alternating current to the pair of excitation coils 32, 34. The switching is accomplished by a switch SW positioned between source 80C and each of coils 32, 34. The person skilled in the art can readily procure or fabricate a suitable switch SW, such as a switching circuit comprising, e.g., operational amplifiers and the like. [0056] The positioning of the pair of excitation coils 32, 34 and employment of the drivers of Fig. 4A, Fig. 4B (both modes), and Fig. 4C thus use quadrature principles to create a moving ring-shaped electric field within envelope 22. Moreover, as explained below, the ring-shaped electric field moves (e.g., rotates or spins, wobbles, or oscillates) about an axis of at least one of the coils 32, 34. The moving ring electric field in turn causes a corresponding excitation of the plasma discharge within envelope 22. [0057] The moving ring electric field R and its derivation is understood from

Fig. 5. Fig. 5 shows turns of the pair of excitation coils 32, 34 relative to orthogonal x, y, and z axes, with coil 32 now also being referred to as the coil around the y axis and coil 34 now also being referred to as the coil around the x axis. The turn of coil 32 lies in the x-z plane and is centered at x=y=0; the turn of coil 34 lies in the y-z plane and is centered at x=y=0. An axis a-b is shown as being coincident with the z axis. Coil 32 has a current l_oxcos(ωt); coil 34 has a current l_oysin(ωt). [0058] As illustrated in Fig. 5 and the expressions below, the magnetic field associated with each coil 32, 34 produces respective electric fields Ey and Ex inside the coils 32, 34. In the expressions below, x and y are unit vectors, and B₀ = B_x0 = B_y0.

[0059] B = x5_χo cos(wt) + yB_yo sin(wt)

[0060] where B₀ = B_x0 = B_y0 x and v are unit vectors

[0061] B = B₀ (x cos(wt) + y sin(wt))

[0062] cfine E, - dS = ^J _AREJ I - dA; E, B_l [0063] Then, neglecting usual constants,

[0064] B = ωB_oE (- x sin(wt) + y cos(wt))

[0065] Thus, Fig. 5, Fig. 5A, and Fig. 5B show that the resultant moving ring- shaped electric field R moves (e.g., rotates or spins, wobbles, or oscillates) about axis a-b (i.e., in the direction shown by arrows S). In accordance with the particular driving technique utilized, the movement of the electric field R can be a rotation or spin about axis a-b (as illustrated in Fig. 5A) or a wobble or oscillation about axis a-b (as illustrated in Fig. 5C). In all cases of the Fig. 5 geometry, the moving ring- shaped electric field moves within the envelope about axis a-b, with axis a-b and a turn of one of the excitation coils lying in a same plane (i.e., axis a-b and turn of coil 32 lie in the x-z plane of Fig. 5). The presence of the electric field R inside the envelope causes a corresponding excited plasma discharge P. The excited plasma discharge P has a bushy toroidal or donut shape which corresponds to the field R. The plasma P itself does not move in the sense of mass transfer of molecules except for effects of convection and the like. However, the ring of discharge may appear to move as different portions of the plasma P are excited. In view of factors such as differing coil geometries or coil currents, the field R and thus the excited plasma discharge P may appear to shrink and expand radially, as shown in Fig. 5B. Also, because the plasma P has some persistence related to its decay time, areas of the excited discharge may continue to emit light even when the field R is not coincident with those areas. [0066] When the inductively coupled electrodeless lamp 20 is driven using phase quadrature (as illustrated by the driver of Fig. 4A or the first mode of the driver of Fig. 4B), the ring-shaped electric field R (and the ring of inductive excitation P within envelope 22) rotates substantially 360 degrees about axis a-b at the excitation frequency (ω/2π) as shown by arrow 500A in Fig. 5A. At the moment shown in Fig. 5A, the ring of excited plasma P would have an elliptical shape as seen from above in Fig. 5B in view of its angular position about axis a-b in Fig. 5A. [0067] When the inductively coupled electrodeless lamp 20 is driven using frequency quadrature (as illustrated by the driver of Fig. 4B), the ring-shaped electric field R rotates about axis a-b also in the manner shown in Fig. 5A, but at a difference frequency, i.e., at a frequency which is the difference between the respective frequencies driving the two coils. Whether phase quadrature or frequency quadrature is being applied, the ring-shaped electric field R, and thus the ring of plasma discharge within the envelope 22, generally rotates with frequency of at least 30 MHz. Amplitude modulated switching, as hereinafter described, is preferably on a much slower time frame (e.g., milliseconds) as compared to either phase or frequency modulation (e.g., micro seconds or nano seconds). Preferably, the time frame is such that the discharge remains active and emits light between switches. In general, switching too slowly will cause the discharge to extinguish while switching too quickly may not predictably control the plasma movement, depending on the particular plasma characteristics.

[0068] In the amplitude quadrature driver of Fig. 4C, the signal is switched between coils 32. 34 at a predetermined time constant. That is, the driver of Fig. 4C applies current in alternating fashion to the pair of excitation coils 32, 34, i e , the pair of excitation coils 32, 34 are turned on and off using the predetermined time constant Such application of current in alternating (switched) fashion using the driver of Fig 4C causes a wobbling or switching of the excited plasma between positions P and P' shown in Fig 5C The positions P and P' are separated by the angle 500C The predetermined time constant employed for the turning on and off of excitation coils 32, 34 is chosen relative to a measured decay time of the plasma, and preferably is significantly faster than the decay time of the plasma (e g on the order of 10³ seconds in the illustrated embodiment) The decay time corresponds to a time period for which the plasma stays lit (i e , does not extinguish) without energy being applied to the plasma Such driving forces the ring of inductive excitation within envelope 22 of inductively coupled electrodeless lamp 20 to oscillate polarity, forcing the ring of inductive excitation to the orthogonal polarity and back again before undergoing decay [0069] Since the two coils 23, 34 may not have the same diameter (e g , in order to avoid geometric interference), in other embodiments using amplitude quadrature driving the respective excitation times of the pair of excitation coils 32, 34 may be chosen to slightly differ That is, the excitation time of each coil may be chosen to adjust for different coupling constants between the coils and the plasma [0070] The amplitude modulation driver of Fig 4C is an example situation in which a cross-talk signal can occur between an active coil and a passive coil When coils 32, 34 are precisely geometrically orthogonal to one another, their mutual inductance becomes very small But the capacitive coupling between coils 32, 34 (where their windings overlap) sees a short to ground This capacitive coupling results in significant power being coupled out of the driven coil and into the passive coil, losing power and the well defined plane of excitation By tilting the coils 32, 34 towards one another in the manner shown in Fig 4D, for example, the resulting mutual inductance can be used to minimize any cross talk between coils 32 34 Thus, to avoid cross talk, the pair of excitation coils 32, 34 are positioned exterior to the envelope so that a plane of a turn of a first of the pair of excitation coils is inclined at a slightly non-orthogonal angle with a plane of a turn of a second of the pair of excitation coils The slightly non-orthogonal angle is preferably in a range between ten degrees and fifteen degrees, and can be chosen using a network analyzer in either a cold or hot test The tilting configuration is beneficial for quadrature driving techniques other than amplitude quadrature, such as frequency quadrature, for example, and may permit operation at higher power levels [0071] For all cases discussed above, the plasma discharge volume is hotter

(i.e has a higher minimum temperature) and more uniform than would otherwise be the case for a conventional single coil inductive lamp in view of the fact that the moving ring-shaped electric field R couples more power at poles of the axis a-b than at an intermediate point on the axis a-b The moving ring-shaped electric field R does not cut the surface of bulb or envelope 22, and tends to distribute power more uniformly through the bulb than other coupling schemes For all cases discussed above, the excited plasma discharge P appears to have a bushy toroidal or donut shape as it moves about the axis a-b. [0072] As indicated above, the coils 32, 34 can assume other numerous configurations and orientations. For example, as shown in Fig. 6A and Fig 6B, coils 32₆ and 34₆ have a reduced width in the region of their overlap The reduced coil width of coils 32₆ and 34₆ tends to reduce capacitive coupling between coils 32₆ and 34₆ [0073] As another example shown in Fig 7, the pair of excitation coils need not have turns that are exactly circular, but instead can have essentially elliptical turns as shown by coil 32₇. Whereas the Fig 1 embodiment had the turn of coil 32 with greater radius than co-centered turn of coil 34, the Fig. 8 and Fig 9 embodiments facilitate essentially equal radii for the turns of the pair of coils In Fig 8 the radii of turns of coils 32₈ and 34₈ are substantially the same by virtue of positioning of each coil so that its turn extends through a circular discontinuity (at top and bottom coil segments) of the turn of the other coil In the Fig. 8 embodiment, the top and bottom segments 32t₈ and 32b₈, respectively, of coil 32₈ are not orthogonally oriented about envelope 22 with respect to top and bottom segments 34t₈ and 34b₈ of coil 34₈, but are instead oriented at 180 degrees about envelope 22 In the Fig. 9 embodiment, equal radii are achieved for coils 32₉ and 34₉ by situating the center of the turns of the coils slightly offset from one another so that a turn of each coil is partially outside and partially inside an imaginary spherical surface in which the turn of the other coil lies

[0074] Fig 10 shows an example wherein coil 32₁₀ and coil 34₁₀ are formed as

"split" wedding rings which are orthogonal to one another In the "split" wedding ring coils 32₁₀ and 34₁₀ of Fig 10 a middle section of each coil has been removed, since in some circumstances removal of the middle section of the coils does not effectively change performance Thus, each of coil 32₁₀ and coil 34₁₀ are, in structural terms, two coil components which are closely spaced apart in view of the removed middle section or gap Advantageously, intra-coil capacitance is reduced compared to the corresponding coils of Fig 2 because the surface area of the coils 32₁₀ and 34₁₀ is reduced Functionally, however, the two coil components of each coil achieve essentially the same performance as the corresponding coils of Fig 2 [0075] Fig 17A - Fιg 17B show an example wherein coil 32₁₇ is formed as

"split" wedding rings, having members 32A₁₇ and 32B₁₇ which are separated by a greater distance than the coils shown in Fig 10 Coil 34₁₇, on the other hand, is not split but is orthogonal to coil members 32A₁₇ and 32B₁₇ For wedding ring type coils generally, it is preferable that coil widths be in a range from just slightly smaller than the bulb inner diameter to about one half the bulb inner diameter For example, in a situation in which a bulb has a 6 mm inner diameter a coil has a 9 mm inner diameter, the coil should preferably have a width in the range from approximately 2 5 mm to 5 5 mm and a thickness of about 1/6 mm to 2/3 mm For the particular configuration shown in Fig 17A - Fig 17B, the coil members 32A₁₇ and 32B₁₇ each have widths at the lower end of this range (e g 2 5 mm, as indicated by 32W₁₇ in Fig 17B), while coil 34₁₇ has a width 34W₁₇ at the higher end of this range (e g 5 5 mm)

[0076] Fig 11 shows coil 32_Ή having a cross-sectional shape resembling a dumbbell In other words, the ring edges of the coil 32^ of Fig 11 are somewhat bulbous relative to a thinner intermediate or middle section of the coil Both coils 23 and 34 in suitable ones of the foregoing embodiments may have the dumbbell cross section of the coil illustrated in Fig 11

[0077] Fig 12A and Fιg 12B illustrate embodiments in which coils e in parallel planes (rather than being orthogonal with one another in the manner of various preceding embodiments) In Fig 12A coil 32_12A and coil 34_12A lie in parallel planes, e g , planes which are both perpendicular to axis 1200A with the flat or linear portions of the coils 32_12A and 34_12A being oriented in a same direction relative to axis 1200A In Fig 12B coil 32_12B and coil 34_12B also lie in parallel planes, but the flat or linear portions of the coils 32_12A and 34_12A are oriented at a non-zero angle about axis 1200B (e g , oriented at a 180 degree angle about axis 1200B) Both the embodiment of Fig 12A and the embodiment of Fig 12B are preferably driven using the quadrature techniques For example, the amplitude quadrature driving technique of Fig 4C can be utilized, with alternating current (supplied from source 80) being switched between the coils 32, 34 via switch SW

[0078] The embodiments of the invention, such as those shown in Fig 12A and Fig 12B, in which the coils e in parallel planes, produce a moving ring electric field R which switches between the planes of the coils in the manner understood from Fig 13 Fig 13 shows (from the side) the bushy toroidal or donut-shaped excited plasma discharge switching between positions P_13T and P_13B Depending on the relaxation time of the plasma, the excited plasma discharge may appear to bounce or move in a continuum back and forth between the positions P_13T and P_13B Such bouncing appearance results from the fact that, in the switching between the coils, there is a tendency also to heat up the regions in between the coils [0079] Fig 14 shows another embodiment of an inductively coupled electrodeless lamp, particularly lamp 120 As in the case of the embodiment of Fig 1 , inductively coupled electrodeless lamp 120 has envelope 122 and a pair of single turn excitation coils 132, 134 As in the Fig 1 embodiment, the turns of coils 132, 134 of inductively coupled electrodeless lamp 120 have a center of curvature coincident with the center of envelope 122 The coils 132 and 134 are oriented to have their axes ninety degrees apart The envelope 122 is supported stationary on a support 146, the support 146 being anchored in base 140 [0080] For inductively coupled electrodeless lamp 120, capacitors 152 and

154 are associated with respective coils 132, 134 The capacitors 152 and 154 are realized by concentric and spaced apart conductive cylinders 152c, 154c, and 160 The cylinder 152c connects to one end of coil 132 cylinder 154c connects to one end of coil 134 The remaining ends of coils 132 and 134 are connected by conductive rod 164 to ground Capacitor 152 is thus formed by cylinder 152c and cylinder 160 with an air dielectric therebetween, capacitor 154 is formed by cylinder 154c and cylinder 160 with an air dielectric therebetween

[0081] An inductor 164 is shown as a series input inductance which may also be a blade connected between a center terminal of an RF input terminal 165 and the cylinder 160 Cylinder 160 forms a common plate for capacitors 152 and 154 A capacitor 166 is connected between cylinder 160 and ground The capacitor 166 may be a discrete capacitor having a large capacitance value and provides impedance matching to a resonant circuit including coils 132 and 134 In this regard, capacitors 152 and 154 form parallel resonant circuits with coils 132 and 134 (ignoring the much higher value of capacitor 166)

[0082] An equivalent coupling circuit 170 for the inductively coupled electrodeless lamp 120 of Fig 14 is shown in Fig 15 Fig 15 specifically shows single turn coils 132 and 134 connected in series to respective capacitors 152 and 154 Impedance matching form the 50 ohm input of connector 165 is provided by inductance 164 and capacitor 166 As mentioned above, the capacitance of capacitor 16 is much greater than that of either capacitor 152 or 154 The capacitances of capacitor 152 and 154 are approximately, but not exactly, equal [0083] Fig 15 also shows an example power circuit that can be used for powering, as well as testing and analyzing, the inductively coupled electrodeless lamp 120 Terminal 165 is connected via directional couplers 180 and 182, as well as through linear RF amplifier 184, to an RF oscillator 185 Directional coupler 182 interfaces with network analyzer 186, directional coupler applies the RF signal to forward power meter 187 and reverse power meter 188 The person skilled in the art will appreciate that the power source may take forms other than the circuit shown in Fig 15

[0084] In operation inductively coupled electrodeless lamp 120 sees two parallel resonant circuits A first resonant circuit is formed by the series connection of coil 132 and capacitor 152 in parallel with capacitor 162 A second resonant circuit is formed by the series connection of coil 134 and capacitor 154 in parallel with capacitor 162 The two resonant circuits are tuned so that their resonant frequencies are slightly apart such that when the applied power (at terminal 165 to inductor 164) is half way between the two resonant frequencies, there is a ninety degree phase difference between the current in coil 132 and the current in coil 134. [0085] Given the quadrature operation of coils 132 and 134 of inductively coupled electrodeless lamp 120 in the manner described above, there is also produced a rotating ring-shaped electric field R interior to envelope 122 for lamp 120. Thus, the depiction of moving ring-shaped electric field R in Fig. 5 and the principles discussed in connection therewith are applicable to the embodiment of Fig. 14 as well as to other embodiments such as the embodiment of Fig. 1. [0086] The lamps described herein and lamps within the scope of the invention can operate in low, mid, and high power ranges. The signals applied to the lamps illustrated herein are preferably in the 200 MHz to 2000 MHz band, with 300 MHz to 900 MHz band being the most preferred frequency band. [0087] The present invention is not fill-dependent. In this regard, various types of fills can be employed for the lamps described herein. For example, the envelope 22 of inductively coupled electrodeless lamp 20 of Fig. 1 can be a 6 mm inner diameter x 7 mm outer diameter bulb having 0.05 mg Selenium and less than 0.01 mg Cesium Bromide (CsBr) at 500 Torr Xe. Other fill materials and fill material combinations are possible, including high impedance electronegative fills such as Sulfur, Selenium, Tellurium, and Indium Bromide, and combinations thereof (for example). Mercury-based and other metal halide fills may also be utilized.

[0088] It should be understood that materials comprising the capacitance structures described above are not critical to the present invention, and that the person skilled in the art can select suitable materials. For example, as the insulation layers materials such as polyimide or Kapton™ can be utilized. [0089] The envelope 22 may be formed of any suitable material, such as quartz (for example). While the envelope 22 has a spherical shape in the illustrated embodiments, other shapes for envelope 22 are possible, such as cylindricaliy shaped and pill-box shaped bulbs, for example. [0090] In the illustrated embodiments the coils (e.g., 32, 34 and 132, 134) have been illustrated as having single turns. It should be understood, however, that the phenomena described herein and principles of the invention are equally applicable to coils having plural turns. Moreover, coils of differing types of geometries may be employed where advantageous (e.g., to minimize arcing, optimum sizing for quadrature driving, etc.). Preferably, the effective electrical length of each excitation coil is less than one half wavelength of the exciting frequency applied thereto and most preferably less than one quarter wavelength of the exciting frequency in order to avoid adverse effects on the desired field / plasma movement.

[0091] Advantageously, the embodiments of the present invention provide a more uniform and hotter excited plasma discharge volume within the envelope or bulb as compared to conventional signal coil inductive lamps, even at the poles of an axis such as axis a-b in Fig. 5. It is believed that, the more uniformly hotter is the plasma discharge volume, the more likely is the volume to emit photons uniformly and provide more light. Moreover, the hotter the plasma discharge volume, the less opportunity for cold regions of the volume to absorb photons and more likely for photons to be re-radiated. In other words, a more uniform high temperature plasma increases the likelihood that an absorbed photon is not lost to the thermal sea but will re-radiate as governed by the plasma's temperature.

[0092] Enhanced re-radiation is particularly important for embodiments of the invention wherein light is emitted through an aperture. In an aperture bulb configuration, an emitted photon samples a large range of the bulb volume as it makes the necessary multiple passes through the plasma before being emitted through the aperture. One example aperture lamp embodiment of the present invention is shown in Fig. 16. The lamp of Fig. 16 resembles lamp 20 of Fig. 1 with the exception that envelope 22 (not visible in Fig. 16) is enclosed by a reflective jacket 1600. The jacket 1600 has a light transmissive aperture 1602 being provided therein through which light is emitted. Other examples of aperture electrodeless lamps are provided, for example, in United States Patent 5,903,091 to MacLennan et al., which is incorporated herein by reference.

[0093] In embodiments of the present invention, the moving ring electric field and excited plasma volume may seem to shrink and expand (in a radial sense of the toroid). Such shrink and expanding, i.e., fluctuating radius of the excited plasma volume, occurs for various reasons, such as lack of symmetry in the coils and/or differing current flows in the coils. [0094] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

What is claimed is

1 An inductively coupled electrodeless lamp comprising an envelope enclosing a fill, the fill forming a plasma discharge when excited, a pair of excitation coils positioned exterior to the envelope, the pair of excitation coils being positioned and driven in a manner to produce a moving ring electric field interior to the envelope, the moving ring electric field exciting a plasma discharge within the envelope, and a driver which applies alternating current to the pair of excitation coils, there being a phase difference between the alternating current applied to a first of the pair of excitation coils and a second of the pair of excitation coils

2 The apparatus of claim 1 , wherein the phase difference is substantially ninety degrees

3 The apparatus of claim 1 , wherein the driver includes a single signal source and a phase adjusting delay element which provides the phase difference between the alternating current applied to the first of the pair of excitation coils and the second of the pair of excitation coils

4 The apparatus of claim 1 , wherein the driver has a first signal source and a second signal source, the first source being connected to apply a signal having a first phase to the first of the pair of excitation coils and the second source being connected to apply a signal having a second phase to the second of the pair of excitation coils

5 The apparatus of claim 1 wherein the pair of excitation coils are positioned to have respective centers of the pair excitation coils coincide and to have respective coil axes oriented at a predetermined angle

6. The apparatus of claim 5, wherein the predetermined angle is 90°.

7. The apparatus of claim 5, wherein the axis of one coil is tilted with respect to the axis of the other coil by between about 10° to 15° from perpendicular.

8. The apparatus of claim 1 , wherein each of the pair of excitation coils has a wedding ring shape.

9. The apparatus of claim 8, wherein each of the pair of wedding ring shaped excitation coils has a reduced width in an area where the coils cross each other.

10. The apparatus of claim 1 , wherein the alternating current is applied with a driving frequency of at least 300 MHz and each of the pair of excitation coils has an effective electrical length of less than one half wavelength of the driving frequency.

11. The apparatus of claim 10, wherein the driving frequency is at least 900 MHz and each of the pair of excitation coils has an effective electrical length of less than one quarter wavelength of the driving frequency.

12. The apparatus of claim 1 , wherein the envelope is covered with a reflective material except in the region of a light emitting aperture.

13. An inductively coupled electrodeless lamp comprising: an envelope enclosing a fill, the fill forming a plasma discharge when excited; a pair of excitation coils positioned exterior to the envelope, the pair of excitation coils being positioned and driven in a manner to produce a moving ring electric field interior to the envelope, the moving ring electric field exciting a plasma discharge within the envelope; and a driver which applies alternating current to the pair of excitation coils, wherein the driver switches between applying the alternating current to a first of the pair of excitation coils and a second of the pair of excitation coils

14. The apparatus of claim 13, wherein the switching occurs on a predetermined time constant.

15 The apparatus of claim 14, wherein the predetermined time constant is in relation to a decay time of the plasma.

16. The apparatus of claim 15, wherein the predetermined time constant is on the order of milliseconds.

17. The apparatus of claim 13, wherein the driver includes a single signal source and a switching element which provides the switching between applying the alternating current to the first of the pair of excitation coils and the second of the pair of excitation coils.

18. The apparatus of claim 13, wherein the pair of excitation coils are positioned to have respective centers of the pair excitation coils coincide and to have respective coil axes oriented at a predetermined angle.

19 The apparatus of claim 18, wherein the predetermined angle is 90°.

20. The apparatus of claim 13, wherein the pair of excitation coils are positioned to be parallel to and spaced apart from each other.

21 The apparatus of claim 13, wherein each of the pair of excitation coils has a wedding ring shape.

22 The apparatus of claim 13, wherein the alternating current is applied with a driving frequency of at least 300 MHz and each of the pair of excitation coils has an effective electrical length of less than one half wavelength of the driving frequency.

23. The apparatus of claim 22, wherein the driving frequency is at least 900 MHz and each of the pair of excitation coils has an effective electrical length of less than one quarter wavelength of the driving frequency.

24. The apparatus of claim 23, wherein the envelope is covered with a reflective material except in the region of a light emitting aperture.