Patent Publication Number: US-8125153-B2

Title: Microwave energized plasma lamp with dielectric waveguide

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 11/135,047 filed on May 23, 2005, entitled “Microwave Energized Plasma Lamp With Dielectric Waveguide,” and issued as U.S. Pat. No. 7,525,253, which is a continuation of application Ser. No. 10/356,340 filed on Jan. 31, 2003, originally entitled “Microwave Energized Plasma Lamp With Solid Dielectric Waveguide,” and issued as U.S. Pat. No. 6,922,021, which is a continuation-in-part of application Ser. No. 09/809,718 (“&#39;718”), filed on Mar. 15, 2001, entitled “Plasma Lamp With Dielectric Waveguide,” and issued as U.S. Pat. No. 6,737,809, which claimed benefit of priority of provisional patent application Ser. No. 60/222,028, filed on Jul. 31, 2000 and entitled “Plasma Lamp,” each of which are incorporated herein in their entirety by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the present invention relates to devices and methods for generating light, and more particularly to electrodeless plasma lamps. 
     2. Description of the Related Art 
     Electrodeless plasma lamps provide point-like, bright, white light sources. Because electrodes are not used, they often have longer useful lifetimes than other lamps. Some plasma lamps direct microwave energy into an air cavity, with the air cavity enclosing a bulb containing a mixture of substances that can ignite, form a plasma, and emit light. However, for many applications, light sources that are brighter, smaller, less expensive, more reliable, and have longer useful lifetimes are desired. 
     SUMMARY OF THE INVENTION 
     In a first aspect the invention provides a plasma lamp including: a waveguide body having a dielectric constant greater than about 2; first and second probes coupled to the body; a circuit, including an amplifier portion, coupled to the first probe to provide RF power to the body, and coupled to the second probe to obtain feedback from the body; and a bulb adjacent to the body, containing a fill capable of forming a plasma when RF power is provided to the body. 
     In a second aspect the invention provides a plasma lamp including: a waveguide body; first and second probes coupled to the body; a bulb adjacent to the body, containing a fill capable of forming a plasma when RF power is provided to the body; and a circuit configured to provide adjustable RF power to the first and second probes enabling at least two modes of operation. 
     In a third aspect the invention provides a method of operating a plasma lamp including: providing a waveguide body; applying RF power to the body at a first frequency causing the body to resonate in a first resonant mode; using the RF power applied to the body at the first frequency to ignite a plasma; applying RF power to the body at a second frequency causing the body to resonate in a second resonant mode; and using the RF power applied to the body at the second frequency to sustain the plasma. 
     In a fourth aspect the invention provides a method of operating a plasma lamp including: providing a waveguide body; providing RF power to a plasma through the body; causing the body to resonate in a first resonant mode; and adjusting the RF power applied to the body to cause the body to resonate in a second resonant mode. 
     In a fifth aspect the invention provides a method for controlling reflection of power from a waveguide body of a plasma lamp back to a microwave source coupling power into the body. The body is capable of resonating in at least a first resonant mode and a second resonant mode. The method includes: positioning a first probe within the body near an electric field minimum of the first resonant mode and not near an electric field minimum of the second resonant mode; positioning a second probe within the body anywhere except near an electric field minimum of the first resonant mode or the second resonant mode; connecting the first probe to an output of the source; and connecting the second probe to an input of the source. 
     A more complete understanding of the present invention and other aspects and advantages thereof will be gained from a consideration of the following description of the preferred embodiments read in conjunction with the accompanying drawing figures provided herein. In the figures and description, numerals indicate the various features of the invention, like numerals referring to like features throughout both the drawings and description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , which is FIG. 1 of the &#39;718 application, shows a sectional view of a “dielectric waveguide integrated plasma lamp” (DWIPL) including a waveguide having a body consisting essentially of a solid dielectric material, integrated with a bulb containing a light-emitting plasma. 
         FIG. 2  schematically depicts a DWIPL having a body with a lamp chamber enclosed and sealed by a ball lens. 
         FIG. 3  schematically depicts a DWIPL having a body with a lamp chamber enclosed and sealed by a window or lens aligned with an optical element attached to brackets attached to a flange extending from a heatsink surrounding the body. 
         FIG. 4  schematically depicts a DWIPL having a cylindrical body attached to a cylindrical heatsink with a bore which closely receives the body. 
         FIG. 5  schematically depicts a DWIPL having a cylindrical body enclosed within a “clamshell”-type heatsink. 
         FIG. 6  schematically depicts a DWIPL having a body with a tapped bore extending between a body side opposed to a side having a lamp chamber aperture, and the chamber bottom. A fill, including a starting gas and light emitter, in the chamber is sealed by a window over the aperture and a plug screwed into the bore. 
         FIG. 7  schematically depicts the  FIG. 6  DWIPL wherein the bore is tapered and a tapered plug is press-fitted into the bore. 
         FIG. 7A  is a detail view of the circled region “ 7 A” in  FIG. 7 , showing the plug tip and chamber bottom. 
         FIG. 8  shows first, second and third plug configurations for the  FIG. 6  DWIPL, and first, second, third and fourth plug configurations for the  FIG. 7  DWIPL. 
         FIG. 9  schematically depicts a DWIPL having a body with a narrow cylindrical bore with a glass or quartz tube inserted therein, extending between a body side opposed to a side having a lamp chamber aperture, and the chamber bottom. Fill in the chamber is sealed by a window over the aperture and a glass or quartz rod inserted into the tube. 
         FIG. 10  schematically depicts a DWIPL having a body with a lamp chamber whose aperture is covered by a window. Fill in the chamber is sealed by a glass or quartz rod inserted into a glass or quartz tube inserted into an indent in the chamber wall proximate to the window periphery, and the window. 
         FIG. 11  schematically depicts a DWIPL having a body with a side having a lamp chamber aperture circumscribed by a groove in which is disposed an O-ring. Fill in the chamber is sealed by a window maintained in pressing contact with the O-ring by a clamping mechanism. 
         FIG. 12  schematically depicts a DWIPL having a “U”-shaped body with a surface having a lamp chamber aperture circumscribed by a groove in which is disposed an O-ring. Fill in the chamber is sealed by a window maintained in pressing contact with the O-ring by a screw cap. 
         FIG. 13  schematically depicts a DWIPL having a body with a side having a lamp chamber aperture circumscribed by a preformed seal. Fill in the chamber is sealed by a heated window which melts the seal when the window is brought into pressing contact with the seal by a hot mandrel. 
         FIG. 14  schematically depicts a DWIPL having a body with a side having a lamp chamber aperture circumscribed by an attached first metallization ring and a preformed seal. Fill in the chamber is sealed when a second metallization ring attached to a window is brought into pressing contact with the first ring by a clamp and heat is applied to melt the preformed seal. 
         FIG. 15  schematically depicts the  FIG. 14  DWIPL wherein a laser is used to melt the preformed seal. 
         FIG. 16  schematically depicts the  FIG. 14  DWIPL wherein the melting of the preformed seal results from inductive heating by an RF coil. 
         FIG. 16A  is a top plan view of the  FIG. 16  DWIPL. 
         FIG. 17A  schematically depicts a DWIPL having a cylindrical body wherein a bulb and a drive probe are located at the electric field maximum of a resonant mode. 
         FIG. 17B  schematically depicts the  FIG. 17A  DWIPL wherein the bulb is located at the electric field maximum of the  FIG. 17A  resonant mode, and a drive probe is offset from the maximum. The  FIG. 17B  probe is longer than the  FIG. 17A  probe to compensate for coupling loss due to the offset. 
         FIG. 18A  schematically depicts a DWIPL having a rectangular prism-shaped body wherein are disposed a bulb, and a drive probe and a feedback probe connected by a combined amplifier and control circuit. 
         FIG. 18B  schematically depicts a DWIPL having a cylindrical body wherein are disposed a bulb, and a drive probe and a feedback probe connected by a combined amplifier and control circuit. 
         FIG. 19  schematically depicts a first embodiment of a DWIPL utilizing a start probe. The DWIPL has a cylindrical body wherein are disposed a bulb, a drive probe, a feedback probe, and the start probe. The feedback probe is connected to the drive probe by a combined amplifier and control circuit, and a splitter, and is connected to the start probe by the amplifier and control circuit, the splitter; and a phase shifter. 
         FIG. 20  schematically depicts a second embodiment of a DWIPL utilizing a start probe. The DWIPL has a cylindrical body wherein are disposed a bulb, a drive probe, a feedback probe, and the start probe. The feedback probe is connected to the drive probe and the start probe by a combined amplifier and control circuit, and a circulator. 
         FIG. 21A  schematically depicts a third embodiment of a DWIPL utilizing a start probe. The DWIPL has a cylindrical body wherein are disposed a bulb, a drive probe, a feedback probe, and the start probe. The feedback probe is connected to the drive probe and the start probe by a combined amplifier and control circuit, and a diplexer. 
         FIG. 21B  schematically depicts an alternative configuration of the  FIG. 21A  embodiment wherein the feedback probe is connected to the drive probe by a diplexer and a first combined amplifier and control circuit, and to the start probe by the diplexer and a second combined amplifier and control circuit. 
         FIG. 22A  schematically depicts a DWIPL wherein a start resonant mode is used before plasma formation and a drive resonant mode is used to power the plasma to steady state. The DWIPL has a cylindrical body wherein are disposed a bulb, a drive probe, and a feedback probe. A combined amplifier and control circuit connects the drive and feedback probes. 
         FIG. 22B  schematically depicts an alternative configuration of the  FIG. 22A  embodiment wherein the feedback probe is connected to the drive probe by first and second diplexers and first and second combined amplifiers and control circuits. 
         FIG. 23  schematically depicts a DWIPL having a body with a high dielectric constant. A drive feed extending into the body is surrounded by a dielectric material having a high breakdown voltage. 
         FIG. 24  is a block diagram of a first configuration of the  FIGS. 18A ,  18 B,  22 A and  22 B combined amplifier and control circuit. 
         FIG. 25  is a block diagram of a second configuration of the  FIGS. 18A ,  18 B,  22 A and  22 B combined amplifier and control circuit. 
         FIG. 26  is a block diagram of a configuration of the  FIGS. 19 ,  20 ,  21 A and  21 B combined amplifier and control circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the present invention is open to various modifications and alternative constructions, the preferred embodiments shown in the drawings will be described herein in detail. It is to be understood, however, there is no intention to limit the invention to the particular forms disclosed. On the contrary, it is intended that the invention cover all modifications, equivalences and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims. 
     As used herein, the terms “dielectric waveguide integrated plasma lamp,” “DWIPL,” “microwave energized plasma lamp with solid dielectric waveguide,” and “lamp” are synonymous, and the term “lamp body” is synonymous with “waveguide body.” The term “probe” herein is synonymous with “feed” in the &#39;718 application. The term “power”, i.e., energy per unit time, is used herein rather than “energy” as in the &#39;718 application. The terms “lamp chamber” and “hole” herein are synonymous with “cavity” in the &#39;718 application, and are used in describing construction details, such as seals and materials, of the several DWIPL embodiments disclosed. The term “cavity” is used herein when describing microwave technology-related details such as probe design, coupling and resonant modes. This change in terminology was made because from an electromagnetic point of view a DWIPL body is a resonant cavity. 
       FIG. 1 , copied from the &#39;718 application, shows a “baseline” embodiment of a dielectric waveguide integrated plasma lamp to which the embodiments disclosed herein may be compared. DWIPL  101  includes a source  115  of microwave radiation, a waveguide  103  having a body  104  consisting essentially of a solid dielectric material, and a drive feed  117  coupling the source  115  to the waveguide, which is in the shape of a rectangular prism determined by opposed sides  103 A,  103 B, and opposed sides  103 C,  103 D generally transverse to sides  103 A,  103 B. DWIPL  101  further includes a bulb  107 , disposed proximate to side  103 A and preferably generally opposed to feed  117 , containing a fill  108  including a “starting” gas, such as a noble gas, and a light emitter, which when receiving microwave power at a predetermined operating frequency and intensity forms a plasma and emits light. Source  115  provides microwave power to waveguide  103  via feed  117 . The waveguide contains and guides the energy flow to an enclosed lamp chamber  105 , depending from side  103 A into body  104 , in which bulb  107  is disposed. This energy flow frees electrons from the starting gas atoms, thereby creating a plasma. In many cases the light emitter is solid at room temperature. It may contain any one of a number of elements or compounds known in the art, such as sulfur, selenium, a compound containing sulfur or selenium, or a metal halide such as indium bromide. The starting plasma vaporizes the light emitter, and the microwave powered free electrons excite the light emitter electrons to higher energy levels. De-excitation of the light emitter electrons results in light emission. Use of a starting gas in combination with a solid light emitter is not a necessity; a gas fill alone, such as xenon, can be used to start the plasma and to emit light. The preferred operating frequency range for source  115  is from about 0.5 to about 10 GHz. Source  115  may be thermally isolated from bulb  107  which during operation typically reaches temperatures between about 700° C. and about 1000° C., thus avoiding degradation of the source due to heating. Preferably, the waveguide body provides a substantial thermal mass which aids efficient distribution and dissipation of heat and provides thermal isolation between the lamp and source. Additional thermal isolation of the source may be accomplished by using an insulating material or vacuum gap occupying an optional space  116  between source  115  and waveguide  103 . When the space  116  is included, appropriate microwave feeds are used to couple the source to the waveguide. 
     Due to mechanical and other considerations such as heat, vibration, aging and shock, contact between the feed  117  and waveguide  103  preferably is maintained using a positive contact mechanism  121 , shown in  FIG. 1  as a spring-loaded device. The mechanism provides a constant pressure by the feed on the waveguide to minimize the possibility that microwave power will be reflected back through the feed rather than entering the waveguide. In providing constant pressure, the mechanism compensates for small dimensional changes in the feed and waveguide that may occur due to thermal heating or mechanical shock. Preferably, contact is made by depositing a metallic material  123  directly on the waveguide at its point of contact with feed  117  so as to eliminate gaps that may disturb the coupling. 
     Sides  103 A,  103 B,  103 C,  103 D of waveguide  103 , with the exception of those surfaces depending from side  103 A into body  104  which form lamp chamber  105 , are coated with a thin metallic coating  119  which reflects microwaves in the operating frequency range. The overall reflectivity of the coating determines the level of energy within the waveguide. The more energy that can be stored within the waveguide, the greater the lamp efficiency. Preferably, coating  119  also suppresses evanescent radiation leakage and significantly attenuates any stray microwave field(s). Bulb  107  includes an outer wall  109  having an inner surface  110 , and a window  111 . Alternatively, the lamp chamber wall acts as the bulb outer wall. The components of bulb  107  preferably include at least one dielectric material, such as a ceramic or sapphire. The ceramic in the bulb may be the same as the material used in body  104 . Dielectric materials are preferred for bulb  107  because the bulb preferably is surrounded by the body  104 , and the dielectric materials facilitate efficient coupling of microwave power with the fill  108  in the bulb. Outer wall  109  is coupled to window  111  using a seal  113 , thereby determining a bulb envelope  127  which contains the fill. To confine the fill within the bulb, seal  113  preferably is a hermetic seal. Outer wall  109  preferably includes alumina because of its white color, temperature stability, low porosity, and low coefficient of thermal expansion. Preferably, inner surface  110  of outer wall  109  is contoured to maximize the amount of light reflected out of cavity  105  through window  111 . Preferably, window  111  includes sapphire which has high light transmissivity and a coefficient of thermal expansion which matches well with that of alumina. Window  111  may include a lens to collect and focus the emitted light. During operation when bulb  107  may reach temperatures of up to about 1000° C., body  104  acts as a heatsink for the bulb. Effective heat dissipation is achieved by attaching a plurality of heat-sinking fins  125  to sides  103 A,  103 C and  103 D. 
     When the waveguide body  104  consists essentially of a dielectric material which generally is unstable at high temperature, such as a titanate, waveguide  103  may be shielded from the heat generated in bulb  107  by interposing a thermal barrier between the body and bulb. Alternatively, outer wall  109  includes a material with low thermal conductivity, such as an NZP (NaZr 2 (PO 4 ) 3 ) ceramic which acts as a thermal barrier. 
     Although  FIG. 1  shows waveguide  103  in the shape of a rectangular prism, a waveguide according to the invention disclosed in the &#39;718 application may be in the shape of a cylindrical prism, a sphere, or in any other shape that can efficiently guide microwave power from a drive feed to a bulb integrated with the waveguide body, including a complex, irregular shape whose resonant frequencies preferably are determined using electromagnetic theory simulation tools. The waveguide dimensions will vary depending upon the microwave operating frequency and the dielectric constant of the waveguide body. Regardless of its shape and size, a waveguide body preferably consists essentially of a solid dielectric material having the following properties: (1) a dielectric constant greater than approximately 2.0; (2) a loss tangent less than approximately 0.01; (3) a thermal shock resistance quantified by a failure temperature greater than approximately 200° C.; (4) a DC breakdown threshold greater than approximately 200 kilovolts/inch; (5) a coefficient of thermal expansion less than approximately 10 −5 /° C.; (6) a zero or slightly negative temperature coefficient of the dielectric constant; (7) stoichiometric stability over a temperature range of about −80° C. to about 1000° C.; and (8) a thermal conductivity of approximately 2 W/mK (watts per milliKelvin). Ceramics having these properties as well as satisfactory electrical and thermo-mechanical properties include alumina, zirconia, certain titanates, and variations or combinations of these materials. 
     High resonant energy within waveguide  103 , corresponding to a high Q-value in the waveguide (where Q is the ratio of the operating frequency to the frequency bandwidth of the resonance), results in high evanescent leakage of microwave energy into lamp chamber  105 . Such leakage leads to quasi-static breakdown of the starting gas within envelope  127 , thereby generating initial free electrons. The oscillating energy of the free electrons scales as Iλ 2 , where I is the circulating intensity of the microwave energy and λ is the wavelength. Thus, the higher the microwave energy, the greater is the oscillating energy of the free electrons. By making the oscillating energy greater than the ionization potential of the gas, electron-neutral collisions result in efficient build-up of plasma density. Once a plasma is formed and the incoming power is absorbed, the waveguide&#39;s Q-value drops due to the conductivity and absorption properties of the plasma. The drop in Q-value is generally due to a change in waveguide impedance. After plasma formation, the presence of the plasma in the lamp chamber makes the chamber absorptive to the resonant energy, thus changing the impedance. The change in impedance is effectively a reduction in the overall reflectivity of the waveguide. By matching the reflectivity of the drive feed to be close to the reduced reflectivity of the waveguide, a sufficiently high Q-value may be obtained even after plasma formation so that the plasma is sustained. Consequently, a relatively low net reflection back into the energy source is realized. Much of the energy absorbed by the plasma eventually appears as heat. When the waveguide is used as a heatsink, the dimensions of the waveguide may change due to thermal expansion. If the waveguide expands, the microwave frequency that will resonate within the waveguide changes and resonance is lost. In order for resonance to be maintained, the waveguide must have at least one dimension equal to an integer multiple of the half-wavelength of the microwaves being generated by source  115 . Such dimensional changes can be compensated for by choosing a dielectric material for body  104  having a temperature coefficient for its refractive index that is approximately equal and opposite in sign to its coefficient of thermal expansion, so that expansion due to heating is at least partially offset by a change in refractive index. 
     A lamp chamber in a DWIPL is a shaped hole in the solid dielectric lamp body. The hole is covered with a transparent window or lens to keep the fill mixture inside, which typically is a noble gas or a mixture of a noble gas such as argon and a salt or halide such as indium bromide or indium iodide. The cross-section of the hole at the lamp body surface from which the hole depends is termed the “aperture.” An aperture can be circular, rectangular, or an arbitrary shape. The three-dimensional shape of the chamber hole can be: a regular prism whose cross-section has the same shape as the aperture, e.g., a cylindrical prism and a circular aperture; a regular prism whose cross-section is shaped differently than the aperture, so that there is a transition region proximate to the aperture; or an arbitrary shape. A lamp chamber bottom can be shaped to serve as a light reflector, so that light striking the bottom is reflected toward the aperture. Specifically, a bottom can be shaped as a paraboloid, an ellipsoid, a chiseled prism, or with one or more curvatures tailored for a specific application. 
     A lamp chamber can be shaped to provide desired characteristics of the emitted light. For example, the chamber can be a cylinder with a diameter optimally chosen to match the dimensions of a light collecting apparatus connected to the lamp. The diameter is constrained at a lower limit by the requirement that the mean free path of an energized electron be long enough that sufficient electron-ion collisions occur before the electron strikes the chamber wall. Otherwise, the resulting efficiency will be too low. The diameter is constrained at an upper limit dependent on the lamp operating frequency. Otherwise, microwave energy will be emitted through the aperture. 
     A typical requirement for a lamp used in an application such as a projection television set is to make the chamber have an “optical extent” (or “etendue” E) which depends on the aperture area A and an f-number (“f#”), characterizing the cone angle of the emitted light, which depends on the ratio of the diameter to the chamber depth. Specifically, E=πA/4(f#) 2 . Typically, the depth is selected to achieve a desired f#, with a greater depth resulting in a smaller f# and a smaller etendue. For a very deep chamber, light emitted toward the chamber middle or bottom may tend to hit the chamber wall and be absorbed, reducing the net efficiency of the lamp. For a very shallow chamber, light may be emitted in too broad a cone angle. 
     A lamp chamber may include a discontinuity in shape to provide an electric field concentration point (see  FIGS. 7A and 8 ) which tends to facilitate breakdown of the fill mixture when the lamp is off, resulting in easier starting. Such a discontinuity can be a cone- or cup-shape projecting from the chamber bottom or side. Alternatively, a discontinuity can be formed by a deliberately added object, such as a fill tube end extending into the chamber. 
     There can be several lamp chambers in the same lamp body. The chambers are located at electric field maxima which exist for the selected waveguide operating mode. Preferably, a mode is selected which allows the chambers to be disposed in a configuration useful for providing light to each of several different optical paths. Each chamber can contain the same fill mixture, or the mixtures can be different. Thus, the spectrum of light emitted from each chamber can be the same, or the spectra can be different. For example, a lamp having three chambers, each with a unique fill mixture, could emit from each chamber, respectively, primarily red, blue and green light, so that the light from each chamber could be used for a separate channel of a red-blue-green optical engine. Alternatively, each chamber could contain the same fill mixture so that multiple independent sources would be available for related but separate uses. 
     A lamp body can essentially consist of more than one solid dielectric material. For example, a lamp body can have a small volume around the lamp chamber made of alumina, to take advantage of its good mechanical, thermal and chemical properties, with the rest of the body made of a material with a higher dielectric constant than that of alumina but which does not have thermal, mechanical and/or electrical properties adequate to contain a plasma. Such a lamp would be a smaller than a lamp having an all-alumina body, likely would operate at a lower frequency than an all-alumina lamp of the same size, and would be less expensive to manufacture since it would require less high dielectric constant material. 
     The electromagnetic design of a lamp body having more than one solid dielectric material is performed in iterative steps. Firstly, a rough lamp shape is selected and an electromagnetic analysis and simulation performed for a lamp body consisting of the material occupying the greatest amount of body volume. Secondly, the simulation results are assessed to determine how close the lamp is to the desired operating frequency. Thirdly, the simulation is repeated with the several dielectric materials included in the simulated structure. Using the analysis results, the dimensions are adjusted and the simulation repeated until the body has the desired combination of operating frequency, size and proportions of materials. 
     A lamp body with several materials can be designed to include a layer, such as an evacuated space, inert gas, or a solid material, between two materials to serve as a thermal barrier. An evacuated space contributes to thermal management by increasing the temperature of the chamber wall(s), and providing a region in which the net lamp thermal flow rate results in a greater temperature differential than without the evacuated space (see FIGS. 3A and 3B of the &#39;718 application). 
     One or more mechanical elements are required to enclose and seal a lamp chamber against the high thermomechanical stresses and pressures created by a plasma. Referring to  FIG. 2 , a DWIPL  200  includes a body  202  having a side  204  with a surface  204 S from which depends a lamp chamber  206  having an aperture  208 . A ball lens  210  is attached to surface  204 S by a seal  212 . Preferably, lens  210  is made of sapphire. Indicium  220  shows the direction of light emitted from chamber  206 . 
     A window or lens enclosing and sealing a chamber can be coupled to other optical elements which collect, process and direct lamp light output. Examples include a tube lined with a reflective material or coating, and a light pipe. Such optical elements can be mounted to brackets integrally attached to a heatsink around a lamp body, providing a low cost, high integrity way to mount and attach optical components to the lamp. Referring to  FIG. 3 , a DWIPL  300  includes a body  302  having a side  304  with a surface  304 S from which depends a lamp chamber  306  having an aperture  308 . Body  302  is enclosed by a “U-shaped heatsink  310  having a central portion  312  attached and generally orthogonal to opposed, generally parallel first and second portions  314 ,  316 , respectively, having, respectively, ears  314 E,  316 E generally orthogonal to portions  314 ,  316  and attached to opposed first and second lamp mounting panels  318 ,  320 , respectively. Portion  314  extends in a flange  322  to which are rigidly attached opposed, generally opposed first and second brackets  324 ,  326  generally orthogonal to the flange  322 . A window/lens  330  attached to surface  304 S and covering aperture  308  encloses and seals the chamber  306 . An optical element  332 , such as a light pipe, is rigidly attached to the brackets  324 ,  326  and aligned with window/lens  330 . Indicium  334  shows the direction of light output from element  332 . 
     A DWIPL can consist of a single integrated assembly including: a lamp body with a sealed lamp chamber; a driver circuit and driver circuit board; a thermal barrier separating the body and driver circuit; and an outer heatsink. Alternatively, separate packages are used for: (a) the lamp body and heatsink; and (b) the driver circuit and its heatsink. For a DWIPL utilizing two probes (see  FIGS. 15A and 15B , and FIG. 6 of the &#39;718 application), the body and driver circuit are connected by two RF power cables, one connecting the output of the driver circuit to the body, and the other providing feedback from the body to the driver circuit. The use of two separate packages allows greater flexibility in the distribution of lamp heat and lamp driver heat. This may enable a projection television or other device to be built without including a fan for the lamp. Such two-package configurations may also enable design of television sets having smaller depth in the critical dimension from viewing screen to back panel than heretofore has been achieved. 
     A DWIPL offers substantial advantages for heat removal because the solid dielectric material(s) used for the lamp body can be chosen for characteristics which result in heat flow along desired paths. A heatsink can have an arbitrary shape, optimized for thermal and end-use considerations. The heatsink for a cylindrical-shaped lamp body might also be cylindrical with fins and mounting details standardized for attachment to a projection television chassis, and with features for mounting optics to the lamp assembly. For a cylindrical lamp and cylindrical heatsink, a useful construction technique is to heat the heatsink until it expands, then place it around the lamp body, and let it cool and contract to form intimate mechanical contact with the body. A metallic heatsink can be used to provide a conductive outer coating of the lamp body. This technique ensures a durable and intimate connection, and satisfies both thermal and electrical requirements of the lamp, reducing its total cost. 
     Referring to  FIG. 4 , a DWIPL  400  includes a generally cylindrical lamp body  402  having a top face  404  with a surface  404 S to which is attached a window  406  covering a lamp chamber aperture  408 . Body  402  is closely received within a generally cylindrical bore  410  of a generally cylindrical metallic heatsink  412  having an annular upper face  414  with a plurality of mounting holes  416 . Preferably, a compliant, high temperature thermal interface material, e.g., grease or a silicone pad, is inserted between body  402  and heatsink  412 . 
     Another practical heatsink arrangement is a two-piece “clamshell” in which two similar or identical pieces make intimate contact with a lamp body over a large area. The pieces are held together by fasteners in compression. Referring to  FIG. 5 , a DWIPL  500  has a generally cylindrical body  502 , a top face  504  with a surface  504 S, and a window  506  attached to surface  504 S and covering a lamp chamber aperture  508 . Body  502  is enclosed by semi-cylindrical portions  510 ,  512  of a clamshell-type heatsink  514 . Portions  510  and  512  each are determined by ends  510 A,  510 B and  512 A,  512 B, respectively, attached to flanges  510 C,  510 D and  512 C,  512 D, respectively. First and second fasteners  520 ,  522  are used to connect the aligned flanges, compressing portions  510 ,  512  about the body  502 . 
     Still another heatsink arrangement is to plate a lamp body with a thermally and electrically conductive material, such as silver or nickel, and then solder or braze heatsink pieces to the plating. 
     When microwave power is applied from the driver circuit to the lamp body, it heats the fill mixture, melting and then vaporizing the salt or halide, causing a large increase in the lamp chamber pressure. Depending on the salt or halide used, this pressure can become as high as 400 atmospheres, and the bulb temperature can be as high as 1000° C. Consequently, a seal attaching a window or lens to a lamp body must be extremely robust. 
     Referring to  FIG. 6 , a DWIPL  600  includes a body  602  having a side  604  with a surface  604 S from which depends a lamp chamber  606  having an aperture  608  and a bottom  610  with a hole  610 H. A window  612 , preferably made of sapphire, is attached to surface  604 S by a seal  614 . Lamp body  602  further includes a tapped bore  616  extending between a body side  618  generally opposed to side  604 , and chamber bottom  610 , so that the bore is in communication with hole  610 H. The window  612  is sealed to surface  604 S in an inert atmosphere, using a ceramic sealing technique known in the art, such as brazing, frit, or metal sealing. Lamp body  602  and a screw-type plug  620  having a head  622  are then brought into an atmospheric chamber containing the starting gas to be used in the lamp chamber, which is at or near the desired non-operating pressure for the lamp. The light emitter is then deposited in lamp chamber  606  through bore  616  and hole  610 H. Plug  620 , which provides a mechanical and gas barrier to contain the fill mixture, is then screwed into bore  616 , and a metallic or glass material  624  deposited over head  622  to effect a final seal. 
     Referring to  FIGS. 7 and 7A , a DWIPL  700  includes a body  702  having a side  704  with a surface  704 S from which depends a lamp chamber  706  having a first aperture  708  and a lower portion  710  tapering in a neck  712  terminating in a second aperture  714 . A window  716 , preferably made of sapphire, is attached to surface  704 S by a seal  718 . Lamp body  702  further includes a tapered bore  720  extending between a body side  722  generally opposed to side  704 , and aperture  714 , so that the bore is in communication with the neck  712 , forming a lip  713 . Window  716  is sealed to surface  704 S in an inert atmosphere. Lamp body  702  and a plug  730 , tapered to match the taper of bore  720  and having a head  732 , are then brought into an atmospheric chamber containing the starting gas to be used in the lamp chamber, which is at or near the desired non-operating pressure for the lamp. The light emitter is then deposited in lamp chamber  706  through bore  720  and aperture  714 . Plug  730  is then force-fitted into bore  720  so that the plug contacts lip  713 , effecting a mechanical seal, and a metallic or glass material  734  deposited over head  732  to effect a final seal. 
       FIG. 8  shows three configurations  630 ,  640 ,  650  of the screw-type plug  620 , and four configurations  740 ,  750 ,  760 ,  770  of the tapered plug  730 . Plugs  630 ,  640  and  650  have, respectively, a dome-shaped tip  630 T, a rod-shaped tip  640 T, and a chisel-shaped tip  650 T. Plugs  740 ,  750 ,  760  and  770  have, respectively, a conical tip  740 T, a cup-shaped tip  750 T, a chisel-shaped tip  760 T, and a rod-shaped tip  770 T having a concave end  722 . If a plug having an extended tip such as plug  650  or plug  760  is used, the tip extends well into chamber  706  creating a discontinuity which provides an electric field concentration point. 
     Referring to  FIG. 9 , a DWIPL  900  includes a body  902  having a side  904  with a surface  904 S from which depends a lamp chamber  906  having an aperture  908  and a bottom  910  with a hole  910 H. A window  912 , preferably made of sapphire, is attached to surface  904 S by a seal  914 . Lamp body  902  further includes a cylindrical bore  916  extending between a body side  918  generally opposed to side  904 , and chamber bottom  910 , so that the bore is in communication with hole  910 H. After window  912  is sealed to surface  904 S in an inert atmosphere, a glass or quartz tube  920  having an end  920 E is inserted into bore  916  so that end  920 E extends through hole  910 H into chamber  906 . The chamber is then evacuated by a vacuum pump connected to tube  920 . A fill mixture of starting gas and light emitter is then deposited into the chamber via the tube. When the fill is complete, a glass or quartz rod  930  having an outer diameter a little smaller than the inner diameter of the tube is inserted into the tube, and the tube and rod heated and pinched off. Thus, tube  920  is filled with a dielectric material which provides a reliable seal. The chamber filling and sealing process can be done without resort to a vacuum chamber, i.e., with the lamp at atmospheric pressure. Alternatively, the lamp body  902  with tube  920  inserted into bore  916  is brought into an atmospheric chamber containing the starting gas to be used in the lamp chamber, which is at or near the desired non-operating pressure for the lamp. The light emitter is then introduced into the chamber via the tube. When the fill is complete, the rod  930  is inserted into the tube, and the tube and rod heated and pinched off. 
     Referring to  FIG. 10 , a DWIPL  1000  includes a body  1002  having a side  1004  with a surface  1004 S from which depends a lamp chamber  1006  having an aperture  1008  and a bottom  1010 . Side  1004  has a hole  1004 H in communication with a hole  1006 H in chamber  1006 . A glass or quartz tube  1020  having an end  1020 E is inserted through holes  1004 H and  1006 H so that the end penetrates the chamber. A window  1030  covering aperture  1008 , preferably made of sapphire, is then attached to surface  1004 S by a frit or sealing material  1032  which melts at a temperature which will not melt the tube. After the window is sealed to surface  1004 S with the tube  1020  in place and hole  1004 H plugged by the sealing material, the chamber is evacuated by a vacuum pump connected to the tube. A fill mixture of starting gas and light emitter is then deposited into the chamber via the tube. When the fill is complete, a glass or quartz rod  1040  with an outer diameter a little smaller than the inner diameter of tube  1020  is inserted into the tube, and the tube  1020  and rod  1040  heated and pinched off. 
     Referring to  FIG. 11 , a DWIPL  1100  includes a body  1102  having a side  1104  with a surface  1104 S from which depends a lamp chamber  1106  having an aperture  1108  and a bottom  1110 . Side  1104  has an O-ring groove  1112  circumscribing the aperture  1108 . DWIPL  1100  further includes first and second clamps  1120 A,  1120 B, respectively, which can apply mechanical compression to a window  1130  covering the aperture. The lamp body  1102 , window  1130 , an O-ring  1114 , and a fill mixture of starting gas  1140  and light emitter  1150  are brought into an atmospheric chamber containing the gas  1140  at a pressure at or near the desired non-operating pressure for the lamp. The light emitter is then deposited in the chamber  1106 , the O-ring  1114  is placed into groove  1112 , the window  1130  is placed on top of the O-ring, and the clamps  1120 A,  1120 B tightened, thus forming a temporary or permanent seal. 
     Referring to  FIG. 12 , a DWIPL  1200  includes a “U”-shaped body  1202  having a central body portion  1204  attached to generally opposed first and second body portions  1206 ,  1208 , respectively, which are generally orthogonal to body portion  1204  and extend in upper portions  1206 U,  1208 U, respectively. Body portion  1204  has a side  1210  with a surface  1210 S from which depends a lamp chamber  1220  having an aperture  1222  and a bottom  1224 . Side  1210  has an O-ring groove  1212  which circumscribes aperture  1222 . Upper portions  1206 U,  1208 U have, respectively, an interior surface  1206 S,  1208 S, having a thread  1230 . The thread may be a metallic attachment to the interior surfaces or cut into the surfaces. As for the  FIG. 11  embodiment, the lamp body  1202  and a window  1240 , an O-ring  1214 , and a fill mixture of starting gas and light emitter are brought into an atmospheric chamber containing the gas at a pressure at or near the desired non-operating pressure for the lamp. The light emitter is deposited in the chamber  1220 , the O-ring  1214  is placed into groove  1212 , the window  1240  is placed on top of the O-ring, and a screw-type metallic cap  1250  is engaged with the thread  1230 . Cap  1250  has therethrough a central hole  1250 H which serves as a light tunnel. Screwing down the cap applies pressure to the window, thereby compressing the O-ring to form a temporary or permanent seal. 
     Referring to  FIG. 13 , a DWIPL  1300  includes a body  1302  having a side  1304  with a surface  1304 S from which depends a lamp chamber  1306  having an aperture  1308  and a bottom  1310 . Side  1304  has therein a detail  1312  circumscribing the aperture  1308  and adapted to closely receive a seal preform  1320 , such as a platinum or glass ring. The lamp body  1302 , a window  1330 , the seal  1320 , and a fill mixture of starting gas  1340  and light emitter  1350  are brought into an atmospheric chamber containing the gas  1340  at a pressure at or near the desired non-operating pressure for the lamp. The light emitter is deposited in the chamber  1306 , the seal  1320  placed in the detail  1312 , and the window  1330  placed on top of the seal preform. The lamp body  1302  is then placed on or clamped to a cold surface  1360 , so that the body and fill mixture remain sufficiently cool that no materials vaporize during heating of the seal preform. A hot mandrel  1370  is then applied in pressing contact to window  1330 , heating the window and melting the seal preform. Indicia  1370 A and  1370 B denote melt-through heat transfer. The seal preform material is chosen to melt and flow at a temperature below the thermal limit for the window and lamp body. When the seal preform melts and then is cooled, it forms a seal between the window and side  1304 . During the sealing operation, the gas pressure in the lamp chamber must be selected to compensate for expansion during heating. 
     Referring to  FIG. 14 , a DWIPL  1400  includes a body  1402  having a side  1404  with a surface  1404 S from which depends a lamp chamber  1406  having an aperture  1408  and a bottom  1410 . Attached to side  1404  by brazing, vacuum deposition or screening, and disposed within a detail  1404 D in side  1404  is a first metallization ring  1412  circumscribing the aperture  1408 . Within detail  1404 D is a seal preform  1420 , such as a platinum ring, superposed on ring  1412 . A window  1430  has a lower surface  1430 S to which, proximate to its periphery, is attached by brazing, vacuum deposition or screening a second metallization ring  1432 . The lamp body  1402 , the window  1430 , the seal perform  1420 , and a fill mixture of starting gas  1440  and light emitter  1450  are brought into an atmospheric chamber containing the gas  1440  at a pressure at or near the desired non-operating pressure for the lamp. The light emitter is deposited in the chamber  1406 , and the window  1430  placed on top of the seal preform  1420  so that the preform is sandwiched between rings  1412  and  1432 . Preferably, a clamp  1460  holds the window in place while a brazing flame or other heat source is applied to melt the preform and form a seal. 
     Referring to  FIG. 15 , a DWIPL  1500  includes a body  1502  having a side  1504  with a surface  1504 S from which depends a lamp chamber  1506  having an aperture  1508  and a bottom  1510 . Attached to side  1504  by brazing, vacuum deposition or screening, and disposed within a detail  1504 D in side  1504  is a first metallization ring  1512  circumscribing the aperture  1508 . Within detail  1504 D is a seal preform  1520 , such as a platinum or glass ring, superposed on ring  1512 . A window  1530  has a lower surface  1530 S to which, proximate to its periphery, is attached by brazing, vacuum deposition or screening a second metallization ring  1532 . The lamp body  1502 , the window  1530 , the seal preform  1520 , and a fill mixture of starting gas  1540  and light emitter  1550  are brought into an atmospheric chamber containing the gas  1540  at a pressure at or near the desired non-operating pressure for the lamp. The mixture is deposited in the chamber  1506 , and the window  1530  placed on top of the seal preform  1520  so that the preform is sandwiched between rings  1512  and  1532 . Preferably, a clamp  1560  holds the window in place while a laser  1570  is focused and moved in a controlled pattern to melt an then permit cooling of the seal preform material. Laser sealing can be done at atmospheric or partial pressure. 
     Referring to  FIGS. 16 and 16A , a DWIPL  1600  includes a body  1602  having a side  1604  with a surface  1604 S from which depends a lamp chamber  1606  having an aperture  1608  and a bottom  1610 . Attached to side  1604  by brazing, vacuum deposition or screening, and disposed within a detail  1604 D of side  1604  is a first metallization ring  1612  circumscribing the aperture  1608 . Within detail  1604 D is a seal perform  1620 , such as a platinum or other conductive material, superposed on ring  1612 . A window  1630  has a lower surface  1630 S to which, proximate to its periphery, is attached by brazing, vacuum deposition or screening a second metallization ring  1632 . The lamp body  1602 , the window  1630 , the seal preform  1620 , and a fill mixture of starting gas  1640  and light emitter  1650  are brought into an atmospheric chamber containing the gas  1640  at a pressure at or near the desired non-operating pressure for the lamp. The light emitter  1650  is deposited in the chamber  1606 , and the window  1630  placed on top of the seal preform  1620  so that the preform is sandwiched between rings  1612  and  1632 . Preferably, a clamp  1660  holds the window in place while a radio frequency (RF) coil  1670  is moved close to the seal preform. The coil heats and melts the preform which, after cooling, forms a seal between the window and side  1604 . RF sealing can be done at atmospheric or partial pressure. 
     Electromagnetically, a DWIPL is a resonant cavity having at least one drive probe supplying microwave power for energizing a plasma contained in at least one bulb. In the following portion of the detailed description “cavity” denotes a DWIPL body. As disclosed in the &#39;718 application, a “bulb” may be a separate enclosure containing a fill mixture disposed within a lamp chamber, or the chamber itself may be the bulb. To provide optimal efficiency, a bulb preferably is located at an electric field maximum of the resonant cavity mode being used. However the bulb can be moved away from a field maximum at the cost of additional power dissipated by the wall and cavity. The location of the drive probe is not critical, as long as it is not at a field minimum, because the desired coupling efficiency can be achieved by varying probe design parameters, particularly length and shape.  FIGS. 17A and 17B  schematically show two cylindrical lamp configurations  130 A,  130 B, respectively, both operating at the fundamental cylindrical cavity mode, commonly known as TM 0,1,0 , and having a bulb  132 A,  132 B, respectively, located at the single electric field maximum. Dashed curves  131 A,  131 B show, respectively, the electric field distribution in the cavity. In  FIG. 17A , a drive probe  134 A is located at the field maximum. In  FIG. 17B , drive probe  134 B is not located at the field maximum; however, it contains a longer probe which provides the same coupling efficiency as probe  134 A. Although the TM 0,1,0  mode is used here as an example, higher order cavity modes, including but not limited to transverse electric field (“TE”) and transverse magnetic field (“TM”) modes, can also be used. 
     Drive probe design is critical for proper lamp operation. The probe must provide the correct amount of coupling between the microwave source and lamp chamber to maximize light emitting efficiency and protect the source. There are four major cavity loss mechanisms reducing efficiency: chamber wall dissipation, dielectric body dissipation, plasma dissipation, and probe coupling loss. As defined herein, probe coupling loss is the power coupled out by the drive probe and other probes in the cavity. Probe coupling loss is a major design consideration because any probe can couple power both into and out of the cavity. If the coupling between the source and cavity is too small, commonly known as “under-coupling”, much of the power coming from the source will not enter the cavity but be reflected back to the source. This will reduce light emission efficiency and microwave source lifetime. If initially the coupling between the source and cavity is too large, commonly known as “over-coupling”, most of the power from the source will enter the cavity. However, the cavity loss mechanisms will not be able to consume all of the power and the excess will be coupled out by the drive probe and other probes in the cavity. Again, light emission efficiency and microwave source lifetime will be reduced. In order to maximize light emission efficiency and protect the source, the drive probe must provide an appropriate amount of coupling such that reflection from the cavity back to the source is minimized at the resonant frequency. This condition, commonly known as “critical coupling,” can be achieved by adjusting the configuration and location of the drive probe. Probe design parameters depend on the losses in the cavity, which depend on the state of the plasma and the temperature of the lamp body. As the plasma state and/or body temperature change, the coupling and resonant frequency will also change. Moreover, inevitable inaccuracies during DWIPL manufacture will cause increased uncertainty in the coupling and resonant frequency. 
     It is not practical to adjust probe physical parameters while a lamp is operating. In order to maintain as close to critical coupling as possible under all conditions, a feedback configuration is required (see FIG. 6 of the &#39;718 application), such as lamp configurations  140 A,  140 B shown, respectively, in  FIGS. 18A and 18B  for a rectangular prism-shaped cavity and a cylindrical cavity. A second “feedback” probe  142 A,  142 B, respectively, is introduced into a cavity  144 A,  144 B, respectively. Feedback probe  142 A,  142 B, respectively, is connected to input port  146 A,  146 B, respectively, of a combined amplifier and control circuit (ACC)  148 A,  148 B, respectively, and a drive probe  150 A,  150 B, respectively, is connected to ACC output port  152 A,  152 B, respectively. Each configuration forms an oscillator. Resonance in the cavity enhances the electric field strength needed to create the plasma and increases the coupling efficiency between the drive probe and bulb. Both the drive probe and feedback probe may be located anywhere in the cavity except near an electric field minimum for electric field coupling, or a magnetic field minimum for magnetic field coupling. Generally, the feedback probe has a lesser amount of coupling than the drive probe because it samples the electric field in the cavity with minimum increase in coupling loss. 
     From a circuit perspective, a cavity behaves as a lossy narrow bandpass filter. The cavity selects its resonant frequency to pass from the feedback probe to the drive probe. The ACC amplifies this preferred frequency and puts it back into the cavity. If the amplifier gain is greater than the insertion loss at the drive probe entry port vis-a-vis insertion loss at the feedback probe entry port, commonly known as S 21 , oscillation will start at the resonant frequency. This is done automatically and continuously even when conditions, such as plasma state and temperature, change continuously or discontinuously. Feedback enables manufacturing tolerances to be relaxed because the cavity continually “informs” the amplifier of the preferred frequency, so accurate prediction of eventual operating frequency is not needed for amplifier design or DWIPL manufacture. All the amplifier needs to provide is sufficient gain in the general frequency band in which the lamp is operating. This design ensures that the amplifier will deliver maximum power to the bulb under all conditions. 
     In order to maximize light emission efficiency, a drive probe is optimized for a plasma that has reached its steady state operating point. This means that prior to plasma formation, when losses in a cavity are low, the cavity is over-coupled. Therefore, a portion of the power coming from the microwave source does not enter the cavity and is reflected back to the source. The amount of reflected power depends on the loss difference before and after plasma formation. If this difference is small, the power reflection before plasma formation will be small and the cavity will be near critical coupling. Feedback configurations such as shown in  FIG. 18A  or  18 B will be sufficient to break down the gas in the bulb and start the plasma formation process. However, in most cases the loss difference before and after plasma formation is significant and the drive probe becomes greatly over-coupled prior to plasma formation. Because much of the power is reflected back to the amplifier, the electric field strength may not be large enough to cause gas breakdown. Also, the large amount of reflected power may damage the amplifier or reduce its lifetime. 
       FIG. 19  shows a lamp configuration  160  which solves the drive probe over-coupling problem wherein a third “start” probe  162 , optimized for critical coupling before plasma formation, is inserted into a cavity  164 . Start probe  162 , drive probe  166 , and feedback probe  168  can be located anywhere in the cavity except near a field minimum. Power from output port  170 B of an ACC  170  is split into two portions by a splitter  172 : one portion is delivered to drive probe  166 ; the other portion is delivered to start probe  162  through a phase shifter  174 . Probe  168  is connected to input port  170 A of ACC  170 . Both the start and drive probes are designed to couple power into the same cavity mode, e.g., TM 0,1,0  for a cylindrical cavity as shown in  FIG. 19 . The splitting ratio and amount of phase shift between probes  166  and  162  are selected to minimize reflection back to the amplifier. Values for these parameters are determined by network analyzer S-parameter measurements and/or simulation software such as High Frequency Structure Simulator (HFSS) available from Ansoft Corporation of Pittsburgh, Pa. In summary, the start probe is critically coupled before plasma formation and the drive probe is critically coupled when the plasma reaches steady state. The splitter and phase shifter are designed to minimize reflection back to the combined amplifier and control circuit. 
       FIG. 20  shows a second lamp configuration  180  which solves the drive probe over-coupling problem. Both start probe  182  and drive probe  184  are designed to couple power into the same cavity mode, e.g., TM 0,1,0  for a cylindrical cavity such as cavity  186 . Configuration  180  further includes a feedback probe  188  connected to input port  190 A of an ACC  190 . The three probes can be located anywhere in the cavity except near a field minimum. Power from output port  190 B of ACC  190  is delivered to a first port  192 A of a circulator  192  which directs power from port  192 A to a second port  192 B which feeds drive probe  184 . Prior to plasma formation, there is a significant amount of reflection coming out of the drive probe because it is over-coupled before the plasma reaches steady state. Such reflection is redirected by circulator  192  to a third port  192 C which feeds the start probe  182 . Before plasma formation, the start probe is critically coupled so that most of the power is delivered into the cavity  186  and start probe reflection is minimized. Only an insignificant amount of power goes into port  192 C and travels back to ACC output port  190 B. Power in the cavity increases until the fill mixture breaks down and begins forming a plasma. Once the plasma reaches steady state, the drive probe  184  is critically coupled so reflection from the drive probe is minimized. At that time, only an insignificant amount of power reaches the now under-coupled start probe  182 . Although the start probe now has a high reflection coefficient, the total amount of reflected power is negligible because the incident power is insignificant. In summary, the start probe is critically coupled before plasma formation and the drive probe is critically coupled when the plasma reaches steady state. The circulator directs power from port  192 A to  192 B, from port  192 B to port  192 C, and from port  192 C to port  192 A. 
       FIGS. 21A and 21B  show third and fourth lamp configurations  240 A,  240 B which solve the drive probe over-coupling problem. A “start” cavity mode is used before plasma formation, and a separate “drive” cavity mode is used to power the plasma to its steady state and maintain that state. Start probe  242 A,  242 B, respectively, operates in the start cavity mode, and drive probe  244 A,  244 B, respectively, operates in the drive cavity mode. As indicated by dashed curves  241 A and  241 B, preferably the drive cavity mode is the fundamental cavity mode and the start cavity mode is a higher order cavity mode. This is because normally it requires more power to maintain the steady state plasma with the desired light output than to break down the gas for plasma formation. Therefore it is more economical to design a DWIPL so the high power microwave source operates at a lower frequency. For a cylindrical cavity such as cavities  246 A and  246 B, the start probe  242 A,  242 B, respectively, can be critically coupled at the resonant frequency of the TM 0,2,0  mode before plasma formation, and the drive probe  244 A,  244 B, respectively, can be coupled at the resonant frequency of the TM 0,1,0  mode after the plasma reaches steady state. The feedback probe can be located anywhere in the cavity except near a field minimum of the drive cavity mode or a field minimum of the start cavity mode. The start probe can be located anywhere in the cavity except near any field minima of the start cavity mode. The drive probe should be located near or at a field minimum of the start cavity mode but not near a field minimum of the drive cavity mode. This minimizes the coupling loss of the drive probe before plasma formation so that the electric field in the cavity can reach a higher value to break down the gas. A diplexer  248 A,  248 B, respectively, is used to separate the two resonant frequencies. In  FIG. 21A , a single ACC  250  is used to power both cavity modes. The two frequencies are separated by diplexer  248 A and fed to the start probe  242 A and drive probe  244 A. Feedback probe  252 A is connected to input port  250 A of ACC  250 . In  FIG. 21B , two separate amplifiers  260 ,  262  are used to power the two cavity modes independently. Diplexer  248 B separates the two frequencies coming out of feedback probe  252 B. In summary, the start probe operates in one cavity mode and the drive probe operates in a different mode. The feedback probe can be located anywhere in the cavity except near a field minimum of either mode. The start probe can be located anywhere in the cavity except near a field minimum of the start cavity mode. The drive probe should be located near or at a field minimum of the start cavity mode but not near a field minimum of the drive cavity mode. 
     An alternative approach is to add a second feedback probe, which eliminates the need for a diplexer. The first feedback probe is located at a field minimum of the start cavity mode to couple out only the drive cavity mode. The second feedback probe is located at a field minimum of the drive cavity mode to couple out only the start cavity mode. 
       FIGS. 22A and 22B  show lamp configurations  280 A,  280 B, respectively, which do not include a start probe but utilize two separate cavity modes. As indicated by curves  281 A and  281 B, respectively, in cavities  282 A and  282 B, a relatively high order start cavity mode is used before plasma formation and a relatively low order drive cavity mode is used to power the plasma to steady state and maintain the state. Preferably, for economy and efficiency, the drive cavity mode again is the fundamental cavity mode and the start cavity mode is a higher order cavity mode. For example, the TM 0,2,0  mode of a cylindrical lamp cavity can be used before plasma formation, and the TM 0,1,0  mode can be used to maintain the plasma in steady state. By utilizing two cavity modes, it is possible to design a single drive probe that is critically coupled both before plasma formation and after the plasma reaches steady state, thereby eliminating the need for a start probe. The feedback probe  284 A,  284 B, respectively, can be located anywhere in the cavity except near a field minimum of either cavity mode. The drive probe  286 A,  286 B, respectively, should be located near a field minimum of the start cavity mode but not near a field minimum of the drive cavity mode. By placing the drive probe near but not at a field minimum of the start cavity mode, the drive probe can be designed to provide the small amount of coupling needed before plasma formation and the large amount of coupling required after the plasma reaches steady state. In  FIG. 22A , a single ACC  290  is used to power both cavity modes. In  FIG. 22B , two separate ACC&#39;s  292 ,  294  are used to power the two cavity modes independently. A first diplexer  296 B separates the two frequencies coming out of feedback probe  284 B and a second diplexer  298 B combines the two frequencies going into drive probe  286 B. In summary, the drive probe is critically coupled at the start cavity mode resonant frequency before plasma formation and critically coupled at the drive cavity mode resonant frequency when the plasma reaches steady state. The feedback probe can be located anywhere in the cavity except near a field minimum of either cavity mode. The drive probe should be located near a field minimum of the start cavity mode but not near a field minimum of the drive cavity mode. 
     The &#39;718 application disclosed a technique for drive probe construction wherein a metallic microwave feed is in intimate contact with the high dielectric material of the lamp body. This method has a drawback in that the amount of coupling is very sensitive to the exact dimensions of the probe. A further drawback is that due to the large temperature variation before plasma formation and after the plasma reaches steady state, a mechanism such as a spring is needed to maintain contact between the feed and body. These constraints complicate the manufacturing process and consequently increase production cost. 
       FIG. 23  shows a technique which avoids both problems. A metallic microwave feed  350  extending into a lamp body  352  is surrounded by a dielectric material  354  having a high breakdown voltage. Body  352  includes a lamp chamber  356 . Due to the large amount of power delivered within a limited space, the electric field strength near tip  350 T of feed  350  is very high; therefore a high breakdown voltage material is required. Typically, material  354  has a lower dielectric constant than that of the dielectric material forming body  352 . Material  354  acts as a “buffer” which desensitizes the dependency of coupling on probe dimensions, thereby simplifying fabrication and reducing cost. 
     The amount of coupling between the microwave source and body can be adjusted by varying the location and dimensions of the probe, and the dielectric constant of material  354 . In general, if the probe length is less than a quarter of the operating wavelength, a longer probe will provide greater coupling than a shorter probe. Also, a probe placed at a location with a higher field will provide greater coupling than a probe placed at a location where the field is relatively low. This technique is also applicable to a start probe or a feedback probe. The probe location, shape and dimensions can be determined using network analyzer S-parameter measurements and/or simulation software such as HFSS. 
       FIG. 24  shows a circuit  430  including an amplifier  432  and a control circuit  434 , suitable for DWIPL&#39;s having only a drive probe  436  and feedback probe  438  such as shown in  FIGS. 18A ,  18 B,  22 A and  22 B. The function of amplifier  432  is to convert DC power into microwave power of an appropriate frequency and power level so that sufficient power can be coupled into lamp body  440  and lamp chamber  442  to energize a fill mixture and form a light-emitting plasma. 
     Preferably, amplifier  432  includes a preamplifier stage  450  with 20 to 30 dBm of gain, a medium power amplifier stage  452  with 10 to 20 dB of gain, and a high power amplifier stage  454  with 10 to 18 dB of gain. Preferably, stage  450  uses the Motorola MHL21336, 3G Band RF Linear LDMOS Amplifier, stage  452  uses the Motorola MRF21030 Lateral N-Channel RF Power MOSFET; and stage  454  uses the Motorola MRF21125 Lateral N-Channel RF Power MOSFET. These devices as well as complete information for support and bias circuits are available from Motorola Semiconductor Products Sector in Austin, Tex. Alternatively, stages  450 ,  452  and  454  are contained in a single integrated circuit. Alternatively, stages  450  and  452 , and control circuit  434  are packaged together, and high power stage  454  is packaged separately. 
     Amplifier  432  further includes a PIN diode attenuator  460  in series with stages  450 ,  452  and  454 , preferably connected to preamplifier stage  450  to limit the amount of power which the attenuator must handle. Attenuator  460  provides power control for regulating the amount of power supplied to lamp body  440  appropriate for starting the lamp, operating the lamp, and controlling lamp brightness. Since the amplifier chain formed by stages  450 ,  452  and  454  has a fixed gain, varying the attenuation during lamp operation varies the power delivered to body  440 . Preferably, the attenuator  460  acts in combination with control circuit  434 , which may be analog or digital, and an optical power detector  462  which monitors the intensity of the light emitted and controls attenuator  460  to maintain a desired illumination level during lamp operation, even if power conditions and/or lamp emission characteristics change over time. Alternatively, an RF power detector  464  connected to drive probe  436 , amplifier stage  454  and control circuit  434  is used to control the attenuator  460 . Additionally, circuit  434  can be used to control brightness, i.e., controlling the lamp illumination level to meet end-application requirements. Circuit  434  includes protection circuits and connects to appropriate sensing circuits to provide the functions of over-temperature shutdown, over-current shutdown, and over-voltage shutdown. Circuit  434  can also provide a low power mode in which the plasma is maintained at a very low power level, insufficient for light emission but sufficient to keep the fill mixture gas ionized. Circuit  434  also can shut down the lamp slowly by increasing the attenuation. This feature limits the thermal shock a lamp repeatedly experiences and allows the fill mixture to condense in the coolest portion of the lamp chamber, promoting easier lamp starting. 
     Alternatively, attenuator  460  is combined with an analog or digital control circuit to control the output power at a high level during the early part of the lamp operating cycle, in order to vaporize the fill mixture more quickly than can be achieved at normal operating power. Alternatively, attenuator  460  is combined with an analog or digital control circuit which monitors transmitted and/or reflected microwave power levels through an RF power detector and controls the attenuator to maintain the desired power level during normal lamp operation, even if the incoming power supply voltage changes due to variations in the AC supply or other loads. 
       FIG. 25  shows an alternative circuit  540  including an amplifier  542  and a control circuit  544 , suitable for supplying and controlling power to the body  546  and lamp chamber  548  of a DWIPL  550  having a drive probe  552  and feedback probe  554 , such as shown in  FIGS. 18A ,  18 B,  22 A and  22 B. A “starting” bandpass filter  560 A and an “operating” bandpass filter  560 B, in parallel and independently selectable and switchable, are in series with the  FIG. 24  amplifier chain and preferably, as in  FIG. 24 , on the input side of the chain. Filters  560 A and  560 B filter out frequencies corresponding to undesired resonance modes of body  546 . By selecting and switching into the circuit a suitable filter bandpass using first and second PIN diode switches  562 A,  562 B, the DWIPL  550  can operate only in the cavity mode corresponding to the selected frequency band, so that all of the amplifier power is directed into this mode. By switching in filter  560 A, a preselected first cavity mode is enabled for starting the lamp. Once the fill mixture gas has ionized and the plasma begun to form, a preselected second cavity mode is enabled by switching in filter  560 B. For a short time, both filters provide power to the lamp to ensure that the fill mixture remains a plasma. During the period when both filters are switched in, both cavity modes propagate through body  546  and the amplifier chain. When a predetermined condition has been met, such as a fixed time delay or a minimum power level, filter  560 A is switched out, so that only the cavity mode for lamp operation can propagate through the amplifier chain. Control circuit  544  selects, deselects, switches in, and switches out filters  560 A and  560 B, following a predetermined operating sequence. An optical power detector  564  connected to control circuit  544  performs the same function as detector  462  in the  FIG. 24  embodiment. 
       FIG. 26  shows a circuit  570  including an amplifier  572  and an analog or digital control circuit  574 , suitable for supplying and controlling power to the body  576  and lamp chamber  578  of a DWIPL  580  having a drive probe  582 , a feedback probe  586  and a start probe  584 , such as shown in  FIGS. 19 ,  20 ,  21 A and  21 B. The feedback probe  586  is connected to input  450 A of preamplifier  450  through a PIN diode attenuator  588  and a filter  589 . The start probe  584  is designed to be critically coupled when lamp  580  is off. To start the lamp, a small amount of microwave power is directed into start probe  584  from preamplifier stage  450  or medium power stage  452  of the amplifier chain. The power is routed through a bipolar PIN diode switch  590  controlled by control circuit  574 . Switch  590  is controlled to send RF microwave power to start probe  584  until the fill mixture gas becomes ionized. A sensor  592 A monitors power usage within body  576 , and/or a sensor  592 B monitors light intensity indicative of gas ionization. A separate timer control circuit, which is part of control circuit  574 , allocates an adequate time for gas breakdown. Once the gas has been ionized, control circuit  574  turns on switch  590  which routes microwave power to high power stage  454  which provides microwave power to drive probe  582 . For a short time, start probe  584  and drive probe  582  both provide power to the lamp to ensure that the fill mixture remains a plasma. When a predetermined condition has been met, such as a fixed time period or an expected power level, control circuit  574  turns off switch  590  thereby removing power to start probe  584  so that the plasma is powered only by drive probe  582 . This provides maximum efficiency. 
     To enhance the Q-value (i.e., the ratio of the operating frequency to the resonant frequency bandwidth) of the DWIPL  580  during starting, the control circuit  574  can bias the transistors of high power stage  454  to an impedance that minimizes leakage out of probe  582  into stage  454 . To accomplish this, circuit  574  applies a DC voltage to the gates of the transistors to control them to the appropriate starting impedance.