Abstract:
Standing-wave linear accelerators (linac) having a plurality of accelerating cavities and which do not have any auxiliary cavities are provided. Such linacs are useful for industrial applications such as radiography, cargo inspection and food sterilization, and also medical applications such as radiation therapy and imaging. In one embodiment, the linac includes an electron gun for generating an electron beam, and a plurality of accelerating cavities which accelerates the electron beam by applying electromagnetic fields generated by a microwave source. At least two adjacent accelerating cavities of the plurality of accelerating cavities are coupled together by at least one coupling iris. The electromagnetic fields resonate through the plurality of accelerating cavities, and the operating frequency of the electromagnetic fields is selected so that the linear accelerator is operating at a π-mode or a mode close to the π-mode. In another embodiment, the frequency of the electromagnetic fields is selected so that the linear accelerator is operating at a π/2-mode or a mode close to the π/2-mode. This more stable mode of operation is possible because at least two adjacent accelerating cavities of the plurality of accelerating cavities are coupled together by at least one coupling iris which also functions as a resonator for the electromagnetic fields, thereby achieving bi-periodic performance without requiring auxiliary cavities. In some embodiments, the linear accelerator also includes an x-ray target.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to particle accelerators. More particularly, the present invention relates to cost effective particle accelerators for applications including industrial applications such as radiography, cargo inspection and food sterilization, and also medical applications such as radiation therapy and imaging.  
         [0002]     Particle accelerators operate by generating charged particles having a particular energy depending on the application. One exemplary particle accelerator is the standing-wave (SW) electron linear accelerator (linac) used in medical and industrial applications.  FIG. 1A  shows a simplified functional representation of a linac  100  which receives electrons from an electron gun  112 . The electrons are accelerated to produce a high-energy electron beam  114  along an axial bore hole  140 . Beam  114  is focused and accelerated in a series of accelerating cavities  120   a ,  120   b  . . .  120   n  by forces exerted on the electrons by microwave fields which are fed from an external microwave source  116  such as a klystron or a magnetron. The microwave fields resonate inside accelerating cavities  120   a ,  120   b  . . .  120   n  and also resonate inside auxiliary cavities  113   a ,  113   b  . . .  113   m.    
         [0003]     For example, in some medical applications, the high-energy beam  114  produced by the linac  100  may be applied directly to a cancer therapy volume on a patient, or beam  114  may be used to generate photons (x-rays)  130  by colliding with a suitable target  122  such as tungsten or gold. The resulting x-ray beam  130  may be used to image cancerous tumors and/or to destroy the cancerous cells within the tumors by its ionizing effect (see Section 1.2, pages 29-32 of “Biomedical Particle Accelerators” by W. H. Scharf, AIP Press, 1994, ISBN 1-56396-089-3).  
         [0004]      FIG. 1B  illustrates the phases of the microwave fields along the accelerating cavities  120   a ,  120   b  . . .  120   n  and the auxiliary cavities  113   a ,  113   b  . . .  113   m . SW linacs capable of generating electron beams with an energy level of up to 25 MeV require approximately 50 resonant cavities for stable operation. In addition, in order to accelerate the electrons efficiently along an axial bore  140 , the microwave fields in the accelerating cavities  120   a ,  120   b  . . .  120   n  should have a phase difference of 180 degrees from one accelerating cavity to the adjacent accelerating cavity, e.g., from cavity  120   a  to cavity  120   b.    
         [0005]      FIG. 2  is a graph  200  showing the different resonant frequency modes for an exemplary approximate model of a linac constituting of 51 resonant cavities with attendant frequency mode separation or spacing  251 ,  255  between adjacent modes. Around the π-mode  245 , the mode frequency separation  255  is relatively small, and hence not ideal for stable operation of this exemplary linac. For this reason, the mode at the center of the graph, mode  241 , also known as the π/2-mode, is generally the preferred mode of operation. This is because the π/2-mode provides the maximum frequency spacing, as measured along the vertical frequency axis, between adjacent modes, i.e. at mode frequency separation  251 . Operation at the π/2-mode is conventionally realized through the use of a bi-periodic arrangement to ensure that the phase difference of the microwave fields is 180 degrees between adjacent accelerating cavities for efficient electron beam acceleration (see pages 76-82 of “Medical Electron Accelerators” by T. J. Karzmark, et al., McGraw-Hill, Inc., 1993, ISBN 0-07-105410-3) (see also pages 113-117 of T. P. Wangler, “Principles of RF Linear Accelerators”, John Wiley &amp; Sons, Inc., ISBN 0-471-16814-9).  
         [0006]     Referring back to  FIG. 1A , in addition to the set of accelerating cavities  120   a ,  120   b  . . .  120   n , a conventional bi-periodic structure for linac  100  requires an additional corresponding set of auxiliary cavities  113   a ,  113   b  . . .  113   m . Each auxiliary cavity couples the microwave power to an adjacent pair of accelerating cavities through a corresponding pair of coupling irises. The number of frequency modes in a bi-periodic linac is equal to the number of the combined resonant cavities, i.e., the total number of the accelerating cavities  120   a ,  120   b  . . .  120   n  and the auxiliary cavities  113   a ,  113   b  . . .  113   m . For efficient operation of the linac  100 , all the constituent resonant cavities should resonate at specific frequencies to ensure synchrony between the electrons being accelerated in the accelerating cavities and the electromagnetic field oscillating in all the resonant cavities.  
         [0007]     One conventional SW bi-periodic linac configuration is the side-coupled SW linac  300 , shown in  FIG. 3A , wherein the auxiliary cavities  313   a ,  313   b  . . .  313   m  are placed on the sides of the accelerating cavities  320   a ,  320   b , . . .  320   n , away from the axis of beam  314 . Auxiliary cavity  313   a  is coupled to the adjacent accelerating cavities  320   a ,  320   b  through a corresponding pair of coupling irises  317   a ,  317   b . Similarly, auxiliary cavity  313   b  is coupled to the adjacent accelerating cavity  320   b  through a coupling iris  318   b .  FIG. 3B  is a cross-sectional view of linac  300  illustrating accelerating cavity  320   a , auxiliary cavities  313   a ,  313   b  and coupling irises  317   a ,  318   b.    
         [0008]     Another conventional SW bi-periodic linac configuration is the on-axis SW linac  400  shown in  FIG. 4A , wherein the auxiliary cavities  413   a ,  413   b  . . .  413   m  are placed along the axis of beam  414 . The auxiliary cavity  413   a  is coupled to the adjacent accelerating cavity  420   a  through a pair of irises  416   a ,  418   a . Auxiliary cavity  413   a  is also coupled to adjacent accelerating cavity  420   b  through a pair of irises  417   a ,  419   a .  FIG. 4B  is a cross-sectional view of linac  400  illustrating accelerating cavity  420   b , auxiliary cavity  413   b  and coupling irises  416   b ,  418   b.    
         [0009]     In one conventional method of manufacturing linacs, e.g., for linacs  100 ,  300 ,  400 , constituent sub-assembly components are stacked and brazed together to ensure vacuum tight joints. These joints are also required to provide continuity of the linac inner walls hosting the microwave current associated with the electromagnetic fields hosted in the cavities. The brazing process involves the use of alloy brazing foils that are inserted into the joints between adjacent cavities. A brazing furnace provides heat to melt the brazing foils that solidify later to form the vacuum tight joints. During brazing, some of the molten brazing alloy can make its way inside the cavities, resulting in a change in the volume of the cavity(s) which in turn can change the resonant frequency characteristics of the linac.  
         [0010]     For this reason, it is a common practice to manually tune the individual cavities after the brazing step in order to bring the frequencies of individual cavities to their nominal frequencies. This is usually done by a skilled tuning technician who has to affix the linac on a fixture, perform a series of measurements, and modify the cavities as needed by deforming the physical structure of each cavity until the desired frequency is achieved. This process is a time consuming and substantially increases the manufacturing cost of the linacs.  
         [0011]     Hence there is a need for improved linacs which are less costly to manufacture, more efficient to operate and more compact in size.  
       SUMMARY OF THE INVENTION  
       [0012]     To achieve the foregoing and in accordance with the present invention, linear accelerators (linac) having a plurality of accelerating cavities and which do not have any auxiliary cavities are provided. Such linacs are useful for industrial applications such as radiography, cargo inspection and food sterilization, and also for medical applications such as radiation therapy and imaging.  
         [0013]     In one embodiment, a standing-wave linear accelerator includes an electron gun for generating an electron beam, and a plurality of accelerating cavities which accelerates the electron beam by applying electromagnetic fields generated by a microwave source. At least two adjacent accelerating cavities of the plurality of accelerating cavities are coupled together by at least one coupling iris. The electromagnetic fields resonate through the plurality of accelerating cavities, and the operating frequency of the electromagnetic fields is selected so that the linear accelerator is operating at a π-mode or a mode close to the π-mode.  
         [0014]     In another embodiment, the frequency of the electromagnetic fields is selected so that the linear accelerator is operating at a π/2-mode or a mode close to the π/2-mode. This more stable mode of operation is possible because at least two adjacent accelerating cavities of the plurality of accelerating cavities are coupled together by at least one coupling iris which also functions as a resonator for the electromagnetic fields.  
         [0015]     In some embodiments, the linear accelerator also includes a target made from a suitable material such as tungsten or gold. The target produces x-rays when the electron beam collides with the target.  
         [0016]     Note that the various features of the present invention can be practiced alone or in combination. These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:  
         [0018]      FIG. 1A  shows a simplified functional representation of a bi-periodic linear accelerator.  
         [0019]      FIG. 1B  illustrates the phases of the microwave fields along the accelerating cavities and the auxiliary cavities of the linear accelerator of  FIG. 1A .  
         [0020]      FIG. 2  is a graphical model showing the different resonant frequency modes for an exemplary linear accelerator constituting of 51 resonant cavities.  
         [0021]      FIGS. 3A and 3B  illustrate a conventional standing-wave bi-periodic linac having side-coupled auxiliary cavities.  
         [0022]      FIGS. 4A and 4B  illustrate a conventional standing-wave bi-periodic linac having on-axis auxiliary cavities.  
         [0023]      FIG. 5A  is an approximate graphical model showing the frequency mode spacing for an exemplary bi-periodic linac with 13 resonant cavities.  
         [0024]      FIG. 5B  is a graphical model illustrating the approximate frequency mode spacing for a lower-energy linac with 7 resonant cavities.  
         [0025]      FIGS. 6A and 6B  show one embodiment of a lower-energy π-mode linear accelerator  600  in accordance with the present invention.  
         [0026]      FIG. 6C  is a block diagram showing the phases of the microwave fields along the accelerating cavities of linear accelerator of  FIG. 6A .  
         [0027]      FIGS. 7A, 7B  and  8 A,  8 B show two additional embodiments of lower-energy π-mode linear accelerators.  
         [0028]      FIGS. 9A and 9B  illustrate an embodiment of a higher-energy π/2-mode linear accelerator with accelerating cavities coupled to each other by resonating irises.  
         [0029]      FIG. 9C  is a microwave phase diagram for a linear accelerator operating in the relatively more stable π/2-mode without the need for auxiliary cavities.  
         [0030]      FIGS. 9D and 9E  depict the pre-assembly and the post-assembly, respectively, of the linear accelerator of  FIG. 9A .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention. The features and advantages of the present invention may be better understood with reference to the drawings and discussions that follow.  
         [0032]     To facilitate discussion,  FIGS. 5A, 5B ,  6 A- 6 C,  7 A,  7 B,  8 A,  8 B, and  9 A- 9 E include block diagrams, graphs and sectional views which illustrate several embodiments of the linear accelerators of the present invention.  
         [0033]     Historically, the prevailing approach to the design and manufacture of industrial and medical linear accelerators resulted in the commercialization of standing-wave (SW) bi-periodic linear accelerators (linacs). With the recent advent of more sensitive imaging technology and more accurate targeting technology, lower-energy compact linacs in the 4 to 8 MeV range are now in greater demand.  FIG. 5A  is an approximate graphical model showing the frequency mode spacing for an exemplary bi-periodic linac with 13 resonant cavities. A typical 13 resonant cavity bi-periodic linac may have 7 accelerating cavities and 6 auxiliary cavities.  
         [0034]     In contrast, the graphical model of  FIG. 5B  illustrates the approximate frequency mode spacing for a lower-energy linac with 7 resonant cavities. Unlike the relatively narrower frequency mode spacing of the higher-energy linacs as illustrated by  FIG. 2  above, because the lower-energy linacs have fewer resonant cavities and hence fewer frequency modes, the frequency mode spacing as measured in the vertical frequency axis, e.g., mode spacing  555  of  FIG. 5B , of lower-energy linacs operating at the π-mode  545  is sufficient for stable operation because the stability of such lower-energy linacs is not seriously degraded. Note that the frequency mode spacing  555  (see  FIG. 5B ) at the π-mode  545  is comparable with the frequency mode spacing  551  (see  FIG. 5A ) at the π/2-mode  541  for a bi-periodic linac with 13 resonant cavities.  
         [0035]     In accordance with the present invention,  FIGS. 6A and 6B  show one embodiment of a lower-energy π-mode linac  600  which includes accelerating cavities  620   a ,  620   b ,  620   c  . . .  620   g  and does not include any auxiliary cavities. Linac  600  also includes a corresponding set of irises  617   a ,  617   b ,  617   c  . . .  617   f  which provides improved coupling for the microwave power between accelerating cavities  620   a ,  620   b ,  620   c  . . .  620   g , thereby allowing most of the microwave power to bypass axial bore  640 . For example, adjacent accelerating cavities  620   a ,  620   b  are coupled to each other via iris  617   a , while adjacent accelerating cavities  620   b ,  620   c  are coupled to each other via iris  617   b . Hence, irises  617   a ,  617   b ,  617   c  . . .  617   f  facilitate the overall flow of the microwave power thereby enabling linac  600  to operate efficiently in the π-mode.  
         [0036]     The simplified configuration of linac  600  is particularly useful for linacs producing electron beams with output energies less than 10 MeV where the total number of resonant cavities can be about 10 or less, thereby permitting stable operation in the π-mode. As a result, by eliminating the need for auxiliary cavities in linac  600 , the total number of resonant cavities is equal to the number of accelerating cavities. Hence, the total number of resonant cavities in exemplary linac  600  is about one-half of that needed in the conventional bi-periodic linacs of comparable energy output described above.  
         [0037]      FIG. 6C  is a block diagram showing the phases of the microwave fields along the accelerating cavities  620   a ,  620   b  . . .  620   g  of linac  600  operating in the π-mode. In this embodiment, the coupling of the microwave power between accelerating cavities  620   a ,  620   b  . . .  620   g  is accomplished mainly through irises  617   a ,  617   b ,  617   c  . . .  617   f  and partially through the axial bore  640 . Other design considerations such as high shunt impedance, may limit on how large axial bore  640  can be to maintain stable and efficient operation.  
         [0038]     In some industrial and medical applications, the electron beam of linac  600  can be directed at a suitable target such as tungsten or gold to generate x-rays. These x-rays are useful, for example, for the inspection of cargo, or for the imaging and/or treatment of medical diseases and conditions such as cancers.  
         [0039]     As illustrated by  FIG. 5B , linac  600  can operate at multiple frequency modes depending on the frequency of the microwave fields fed from the microwave source. For example, the operating frequency of linac  600  can be selected to match the frequency corresponding to the π-mode, i.e., the microwave fields should at approximately 3.01 GHz (3010 MHz). The corresponding wavelength of microwave fields operating at 3 GHz is approximately 10 cm. Accordingly, the accelerating cavities  620   a ,  620   b  . . .  620   g  should be approximately 5 cm in length in order to ensure synchrony between the electrons being accelerated and the electromagnetic fields.  
         [0040]     Note that accelerated electrons travel at a speed very close to the speed of light once these electrons attain energies higher than 1 MeV. Hence, the cavity length of the first one or two linac cavities, e.g., cavities  620   a ,  620   b , at the up-stream of the linac, where the electrons have not attained enough energy yet to be relativistic (close to the speed of light), can be between 2 and 5 cm depending on the energy of the electrons, emitted by the electron gun, as the electrons enter linac  600 . Detailed dimensions of linac configuration can be obtained using computer simulation programs such as “Maxwell&#39;s Equations by the Finite Integration Algorithm”(MAFIA) available from “Computer Simulation Technology (CTS), or “ANALYST” available from Simulation Technology and Applied Research, Inc.  
         [0041]     Another embodiment of a lower-energy π-mode linac  700  useful for industrial and medical application is depicted in  FIGS. 7A and 7B . A set of staggered coupling irises  718   a ,  717   b ,  718   c  . . .  717   f  provides improved coupling for the microwave power between accelerating cavities  720   a ,  720   b ,  720   c  . . .  720   g , thereby allowing most of the microwave power to bypass axial bore  740 . For example, adjacent accelerating cavities  720   a ,  720   b  are coupled to each other via iris  718   a , while adjacent accelerating cavities  720   b ,  720   c  are coupled to each other via iris  717   b . Hence, irises  718   a ,  717   b ,  718   c  . . .  717   f  facilitate the overall flow of the microwave power thereby enabling linac  700  to operate efficiently in the π-mode. Note that the microwave fields phases for linac  700  are similar to that shown in  FIG. 6C  for linac  600 . Accordingly, by operating in the π-mode, the total number of cavities in linac  700  is advantageously reduced to about one-half of that needed in a conventional bi-periodic linac of equivalent power.  
         [0042]     Yet another embodiment of lower-energy π-mode linac  800  useful for industrial and medical application is depicted in  FIGS. 8A and 8B . A set of paired coupling irises  817   a  &amp;  818   a ,  817   b  &amp;  818   b  . . .  817   f  &amp;  818   f  provides improved coupling for the microwave power between accelerating cavities  820   a ,  820   b ,  820   c  . . .  820   g , thereby allowing most of the microwave power to bypass axial bore  840 . For example, adjacent accelerating cavities  820   a ,  820   b  are coupled to each other via a pair of irises  817   a ,  818   a . Irises  817   a  &amp;  818   a ,  817   b  &amp;  818   b  . . .  817   f  &amp;  818   f  greatly facilitate the overall flow of the microwave power thereby enabling linac  800  to operate more efficiently in the π-mode. The microwave fields phases for linac  800  are also similar to that shown in  FIG. 6C  for linac  600 . As discussed above, the total number of cavities in π-mode linac  800  needs to be about one-half of that required in an equivalent conventional bi-periodic linac.  
         [0043]     In some industrial and medical applications, higher-energy SW linacs are needed to produce electron beams with output energies greater than 10 MeV. Such higher-energy linacs would require a relative large number of accelerating cavities, and it may not be feasible to operate these higher-energy linacs in a stable π-mode because of insufficient frequency mode spacing as illustrated by  FIG. 2 . Conventionally, the solution for designing stable higher-energy linacs is to incorporate auxiliary cavities as described above for bi-periodic linacs  300 ,  400 .  
         [0044]     In contrast,  FIGS. 9A and 9B  illustrate an exemplary linac  900  with a relatively large number of accelerating cavities  920   a ,  920   b ,  920   c  . . .  920   n  capable of generating output energies substantially greater than 10 MeV, and without the need for auxiliary cavities. Accelerating cavities  920   a ,  920   b ,  920   c  . . .  920   n  of linac  900  are coupled to each other by a set of staggered coupling irises  918   a ,  917   b  . . .  917   m . For example, adjacent accelerating cavities  920   a ,  920   b  are coupled to each other via an iris  918   a.    
         [0045]     In accordance with the present invention, in addition to coupling the microwave fields between accelerating cavities, coupling irises  918   a ,  917   b  . . .  917   m  also function as microwave resonators thereby enabling linac  900  to operate in the relatively more stable π/2-mode, as shown in the microwave phase diagram of  FIG. 9C . In other words, coupling irises  918   a ,  917   b  . . .  917   m  also advantageously function as resonant irises. Thus, resonant irises  918   a ,  917   b  . . .  917   m  enable linac  900  to achieve bi-periodic performance without the need for a corresponding set of auxiliary cavities, thereby reducing the total number of cavities by half the number of that needed for a conventional bi-periodic linac with a similar energy output.  
         [0046]     To achieve efficient resonating microwave fields, the dimensions of resonant irises  918   a ,  917   b  . . .  917   m  can be mathematical functions of operating microwave frequency of linac  900 . In this embodiment, the length of resonant irises  918   a ,  917   b  . . .  917   m  is a function of the microwave wavelength such as one half or one quarter of the wavelength of the operating microwave. For example, for operation at 3 GHz, the iris length is approximately 5 cm. The width and the thickness of resonant irises  918   a ,  917   b  . . .  917   m  are design parameters that can be selected to optimize the efficiency of linac  900 . Hence, linac  900  is capable of operating in a stable bi-periodic manner without the need for auxiliary cavities. By operating in this bi-periodic manner, i.e., in the π/2-mode, linac  900  is able to generate upwards of about 25 MeV, while operating in a stable manner and permitting relaxation of manufacturing tolerances.  
         [0047]     By eliminating the need for auxiliary cavities, linacs  600 ,  700 ,  800  and  900  advantageously maintain a simplified structure and a cylindrical cross-sectional shape.  FIGS. 9D and 9E  depict the pre-assembly and the post-assembly, respectively, of exemplary linac  900 . While linacs  600 ,  700 ,  800  and  900  can be assembled using the brazing process described above, the cylindrical cross-sectional shape of linacs  600 ,  700 ,  800  and  900  makes assembly easier, enabling linacs  600 ,  700 ,  800  and  900  to be manufactured using a more cost effective diffusion bonding described below for exemplary linac  900 .  
         [0048]     First, the constituent sub-assembly components  900   a ,  900   b ,  900   c  . . .  900   y  are machined to the nominal design dimensions. The joining surfaces of components  900   a ,  900   b ,  900   c  . . .  900   y  are also machined to the required flatness and surface roughness. In linac  900 , joints  950   a ,  950   b  . . .  950   x  should be vacuum tight to ensure linac vacuum integrity. Joints  950   a ,  950   b  . . .  950   x  are also required to provide the microwave continuity for the inner cavity walls of linac  900  hosting the microwave currents associated with the electromagnetic fields. In diffusion bonding, the stacked assembly for linac  900 , comprising of components  900   a ,  900   b ,  900   c  . . .  900   y , is placed in a furnace which provides the heat for bonding joints  950   a ,  950   b  . . .  950   x  at a temperature close to, but lower than, the melting point of the linac material, e.g., copper. During the heating process, the stacked assembly for linac  900  is kept under the required pressure for proper bonding.  
         [0049]     Since diffusion bonding does not involve the melting of a brazing alloy, the problem of having foreign material deposited inside the cavity walls of linacs  600 ,  700 ,  800  and  900  has now been eliminated. For this reason, post assembly tuning of the individual accelerating cavities of linacs  600 ,  700 ,  800  and  900  should no longer be needed. Hence, the simpler and more cost effective diffusion bonding process can replace the more expensive brazing and tuning steps. This result in advantageous savings associated with cost of material, manufacturing time, and capital and operating cost of multiple brazing furnaces. In addition, this cylindrical cross-sectional configuration allows for potential robotic stacking of cavities and automated assembly of linacs  600 ,  700 ,  800  and  900 .  
         [0050]     The cylindrical cross-sectional profile of linacs  600 ,  700 ,  800 ,  900  also advantageously allows for the easy placement of magnetic coils around linacs  500 ,  700 ,  800 ,  900 . This is because for some applications, linacs  600 ,  700 ,  800 ,  900  may require magnetic coils for beam focusing and/or beam steering as to better control of the beam spot size and beam position. A well-controlled electron beam colliding on the x-ray target will result in more accurate x-rays.  
         [0051]     Many modifications to linacs  600 ,  700 ,  800 ,  900  are also possible. For example, instead of operating at the π-mode, the exemplary 7 cavity linac  600  can operate at a mode adjacent to the π-mode such as the 6/7 π-mode.  
         [0052]     Although the above description uses exemplary microwave frequencies, exemplary linac energy levels, exemplary linac dimensions, and exemplary industrial and medical applications, these examples are not intended to limit the scope of the invention. For example, while assembly techniques such as brazing and diffusion bonding has been described in this application, it is possible to use other assembly techniques known to one skilled in the art.  
         [0053]     While this invention has been described in terms of several preferred embodiments, there are alterations, modifications, permutations, and substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, modifications, permutations, and substitute equivalents as fall within the true spirit and scope of the present invention.