Abstract:
A drift tube linear accelerator (DTL) incorporating an improved drift tube design, wherein the DTL comprises a resonance chamber maintaining a vacuum and having an inlet port and an exit port, an RF field source producing an oscillating radio frequency field within the chamber, and a plurality of substantially cylindrical drift tubes comprising a hollow body having a low energy end and a high energy end and housing a magnet, a low energy end cap attached to the low energy end of the hollow body and a high energy end cap attached to the high energy end of the hollow body, and a stem extending from said hollow body to an inner surface of the resonance chamber.

Description:
FIELD OF THE INVENTION 
     The present invention relates to drift tube linear accelerators for charged-particle beams, and more particularly to internally cooled drift tube designs. 
     BACKGROUND OF THE INVENTION 
     Linear accelerators are devices which accelerate charged particles along a linear path through exposure of the charged particles to time-dependent electromagnetic fields. Since the first testing of linear accelerators by Rolf Wideroe in 1928, linear accelerator technology has experienced significant advancements, perhaps most dramatically following the advancements in microwave technology experienced as a result of World War II radar research. Today linear accelerators represent a powerful tool for nuclear and elementary particle research, and also have been applied to commercial applications. 
     A linear accelerator delivers energy to a beam of charged particles through application of an electrical field. An early form of linear accelerator, electrostatic linear accelerators, utilize a constant electrical field to deliver energy. Each charged particle accelerated by an electrostatic linear accelerator acquires an energy equal to the product of the potential drop across the linear accelerator and the electric charge of the accelerated particle. The energy of particles is therefore measured in units called “electron volts” (eV). The ability of electrostatic linear accelerators to deliver energy to charged particles is limited by the potential difference that can be maintained by the linear accelerator. 
     Radio frequency (RF) linear accelerators avoid this limitation by applying a time-varying electric field within a vacuum-maintaining resonance chamber to a charged-particle beam that has been modified to: arrive in “bursts” of charged particles; and only at times in which the polarity of the electrical field is appropriate to accelerate the charged particles in the desired direction. For such a linear accelerator to properly function, the charged-particle beam must be properly phased with respect to the fields, and must maintain synchronization with the fields. Particle accelerators functioning under these principles have been termed “resonance accelerators,” and come in a number of configurations, including: linacs, in which the charged particles travel in a straight line; cyclotrons, in which the charged particles travel along a spiral orbit path; and a synchrotron, in which the charged particles travel along a circular orbit path. 
     Drift tube linacs, or “DTLs,” are one form of resonance accelerator. DTLs utilize a series of drift tubes located within a resonance chamber, and through which the charged-particle beam pass, to shield the bursts of the charged-particle beam from exposure to the time-varying electric field during times when the polarity of the field would accelerate the charged particles in a direction opposite that which is intended. Due to the shielding provided by the drift tubes, the bursts of the charged-particle beam are exposed to and accelerated by the field only during passage through the gaps between the drift tubes, and only in the intended direction. Because charged particles are accelerated during passage through each gap, the velocity of the charged particles is greater in each successive drift tube through which the particles pass. The increased velocity of the charged particles in each successive drift tube requires a commensurate increase in the length of successive drift tubes to ensure shielding of the charged particles along the entire distance traveled by the charged particles while the polarity of the accelerating field is the opposite of that desired. 
     Drift tubes in a DTL generally contain focusing/defocusing magnets, such as quadrupole magnets, which maintain the size and alignment of the charged-particle beam through the DTL. One side-effect of the operation of a DTL is the generation of heat within the resonance chamber and particularly within the drift tubes. This heat can cause the expansion of drift tube components and thereby modify the geometry of the drift tubes and the length of the gaps between successive drift tubes. These modifications may affect the dynamics of the charged-particle beam, including its frequency. While small perturbations in the frequency of the beam may be compensated for, significant perturbations will impair the ability of the RF field to impart energy upon the beam. Excessive heating of the drift tubes can also prove detrimental to the magnets&#39; ability to perform its functions by altering the magnets&#39; parameters, reducing the magnets&#39; strength, or by introducing multipoles that may lead to emmittance growth. 
     Cooling systems are frequently used in conjunction with DTLs to control drift tube heating and eliminate or reduce the effects of heating on drift tube geometry and magnets. These cooling systems typically circulate a cooling fluid, such as water, through selected components of a DTL. It is known in the prior art that cooling fluid may be circulated through the stems by which drift tubes are attached to the interior wall of a DTL&#39;s resonance chamber. U.S. Pat. No. 5,021,741 to Kornely, et al., provides another example of a drift tube cooled by the circulation of a cooling fluid. Drift tube cooling becomes especially difficult in high-energy DTLs, where the accumulation of heat may be far more acute. 
     The manufacture of drift tubes for a DTL, however, is an expensive and difficult process. Difficulties include the high cost of drift tube materials (e.g. high purity copper), the great precision which must be exercised in construction, and the need to manufacture drift tubes in a wide variety of sizes to accommodate the varying velocities achieved by the charged particles at different points within the DTL. The already expensive and difficult manufacturing process is further exacerbated by requirements to form channels for cooling fluid flow within the drift tubes. A need exists for a drift tube design incorporating channels for cooling fluid flow which can achieve desired drift tube cooling while minimizing the difficulties of drift tube construction. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved DTL design incorporating an improved drift tube design, wherein the DTL comprises a radio frequency chamber maintaining a vacuum and having an inlet port and an exit port, an RF field source producing an oscillating radio frequency field within the chamber, and a plurality of substantially cylindrical drift tubes. 
     The drift tubes comprise: a stem having inlet and outlet passages extending from the stem&#39;s inner to outer ends; a substantially cylindrical hollow body interconnected to the inner end of the stem and having a high energy end and a low energy end; a substantially cylindrical magnet disposed within and substantially co-axial with the hollow body and having a magnet orifice; a high energy end cap interconnected to the high energy end of the hollow body and having a high energy orifice; a low energy end cap interconnected to the low energy end of the hollow body and having a low energy orifice; and a substantially cylindrical bore tube co-axial with the hollow body and extending from the low energy orifice through the hollow body and the magnet orifice to the high energy orifice. 
     The hollow body, high energy end cap, low energy end cap, and bore tube are all constructed of an electrically conductive material. The central axes of the bore tubes are oriented along an line extending from the inlet port of the chamber to the exit port of the chamber. The axial length of the drift tubes increases with each successive drift tube to accommodate the increased velocity of the charged particles. The hollow body further has a first annular cooling channel and an annular return channel, each of which are enclosed within and encircling the hollow body. The first cooling channel is connected to the inlet passage of the stem, the return channel is connected to the outlet passage of the stem, and the return channel is connected to the first cooling channel through a collecting channel located on a side of said hollow body substantially opposite the inner end of the stem. 
     During operation of the DTL cooling fluid travels into the chamber and through the inlet passage of the stem to the first cooling channel, through the first cooling channel to the collecting channel, through the collecting channel to the return channel, and through the return channel to the outlet passage of the stem. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The objects and advantages of the present invention described above will be more clearly understood when considered in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a generalized diagrammatic illustration of a drift tube linear accelerator of the present invention. 
     FIG. 2 is a perspective view of a drift tube of the present invention. 
     FIG. 3 is a perspective view of a drift tube of the present invention illustrating cooling fluid channels and directions of cooling fluid flow. 
     FIG. 4 is a cross-sectional disassembled side view of a drift tube of the present invention taken along line  4 — 4  of FIG.  2 . 
     FIG. 5 is a cross-sectional assembled side view of a drift tube of the present invention taken along line  4 — 4  of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a generalized representation of a DTL system. The system begins with a charged-particle injector  10  which extracts charged particles (e.g. H+ions) from a charged-particle source and injects the extracted charged particles into a preliminary particle accelerator  12 . The charged particles are accelerated by preliminary particle accelerator  12  to a desired speed and then injected into a DTL  14 . It should be noted that DTL systems do not require the use of preliminary particle accelerators in all applications, though in certain applications the use of such preliminary particle accelerators is preferred. DTL  14  includes a RF field chamber  16  and a plurality of substantially cylindrical hollow drift tubes  20  located within chamber  16 . Chamber  16  is maintained in a vacuum and has an inlet port  28  and an exit port  30 . An RF field generator  26  produces an oscillating RF field within chamber  16  oriented to direct charged particles along a line of acceleration  32  between inlet port  28  of chamber  16  and exit port  30  of chamber  16 . Each drift tube  20  is positioned within chamber  16  by a stem  22  extending from drift tube  20  to an inner surface  24  of chamber  16 . Bore tubes  50  co-axial with drift tube  20  extends through drift tubes  20  along line of acceleration  32 . The direction of acceleration of charged particles along line  32  within chamber  16  is dependent upon the sign of the RF field within the chamber, which changes during the field&#39;s oscillations. 
     Through means known in the prior art, charged particles enter chamber  16  not as a continuous stream of charged particles, but rather as a series of “bursts” of charged particles. The entry of each “burst” of charged particles into chamber  16  is controlled to occur at a time when the RF field is oriented to accelerate charged particles toward exit port  30  of chamber  16 . Drift tubes  20  are also positioned to shield each “burst” of charged particles from the RF field during the time when the RF field is oriented to accelerate charged particles toward inlet port  28 . In this way, the charged particles are accelerated by the RF field only as the particles pass through gaps  34  between successive drift tubes  20  (or between a drift tube  20  and a port  28  or  30 ) and only in the direction of exit port  30 . The lengths  36  of drift tubes  20  are controlled to ensure shielding of charged particles during the entire period in which the oscillating RF field would accelerate the charged particles toward inlet port  28 . Because the speed of charged particles increases with the traversing of each gap  34  between adjacent drill tubes  20 , length  36  increases with each successive drift tube  20  between inlet port  28  and exit port  30 . 
     Upon exiting chamber  16  and DTL  14 , the charged particles are directed toward and impact a target  38 . In certain applications, additional linear accelerators (or some other form of accelerator) and/or beam transport systems may be utilized between DTL  14  and target  38 . 
     Each drift tube  20  houses a cylindrical focusing/defocusing magnet  52  having a cylindrical magnet orifice  53  (see FIG.  4 ). The central axes of magnet  52  and magnet orifice  53  are substantially co-linear with line of acceleration  32 . Magnet  52  serves to maintain the size and alignment of the charged-particle beam as the beam passes through DTL  14 . One side-effect of the operation of DTL  14  is the generation of heat within chamber  16  and particularly within drift tubes  20 . This heat or the absence of this heat can cause expansion or contraction of drift tube  20  components and thereby modify the geometry of drift tube  20  and the length of gaps  34  between successive drift tubes  20 . These modifications may affect the dynamics of the charged-particle beam, such as beam frequency. While small perturbations in the frequency of the beam may be compensated for, significant perturbations will impair the ability of the RF field to impart energy upon the beam and negatively impact DTL  14  performance. The heat can also prove detrimental to the performance of magnets  52 , through the alteration of magnet parameters, the reduction of magnetic strength, or the introduction of multipoles leading to emittance growth. The present invention utilizes a cooling fluid  18  flowing from a cooling fluid reservoir  40  through stems  22  and around drift tubes  20  (and thereafter returning to reservoir  40  through stems  22 ) to regulate the temperature of drift tubes  20  when DTL  14  is in operation. Cooling fluid  18  is preferably water so as to limit cooling costs and minimize the dangers associated with more volatile or toxic cooling fluids. Magnet  52  is preferably a samarium cobalt quadrupole magnet stabilized at 100 degrees Celsius. The flow of cooling fluid  18  should be sufficient to minimize changes in drift tube  20  geometry and prevent the temperature of magnets  52  from exceeding 100 degrees Celsius. 
     FIG. 2 is a perspective view of a drift tube  20  of the present invention. Drift tube  20  comprises a substantially cylindrical stem  22  (see also FIG.  5 ), a hollow substantially cylindrical body  42 , a substantially cylindrical chimney  44  (see also FIG.  3 ), a low energy end cap  46 , a high energy end cap  48 , a bore tube  50  (see also FIG.  3 ), and a hollow substantially cylindrical magnet  52  (magnet  52  is not illustrated in FIG. 2, but is illustrated in FIG.  4 ). Stem  22  has an inner end  54  and an outer end  56 . Outer end  56  of stem  22  extends through inner surface  24  of chamber  16  (as illustrated in FIG.  1 ). Chimney  44  extends outwardly from body  42  and interconnects with inner end  54  of stem  22 . Body  42  has a energy end  58  and a high energy end  60 . Low energy end cap  46  interconnects with low energy end  58  of body  42  and high energy end cap  48  interconnects with high energy end  60  of body  42 . Bore tube  50  extends from a low energy orifice  62  (see also FIG.  3 )in low energy end cap  46  through body  42  to a high energy orifice  64  in high energy end cap  48 . Drift tube  20  is positioned so that bore tube  50  is co-axial with body  42  and is parallel to line of acceleration  32 , with low energy end cap  46  oriented toward inlet port  28  of chamber  16  (illustrated in FIG.  1 ). 
     Now referring to FIG. 3, there is shown a perspective view of the series of cooling fluid  18  channels and passages through drift tube  20  (wherein the channels and passageways are illustrated as solid figures and the general outline of drift tube  20 , cynlindrical chimney  44 , bore tube  50 , and low energy orifice  62  are illustrated with broken lines) together with indications of the direction of cooling fluid flow within those passages and channels. Stem  22  is hollow and has an inner stem surface  66 . An inner tube  68  is located coaxially with and within stem  22 . The hollow interior of inner tube  68  forms an inlet passage  70  through which cooling fluid  18  may enter chamber  16  and be introduced into drift tube  20  as shown in FIG.  1 . The area between inner tube  68  and inner stem surface  66  forms an outlet passage  72  through which cooling fluid  18  may exit drift tube  20  and chamber  16  as shown in FIG.  1 . It should be understood that this arrangement of inlet and outlet passages is not a requirement of this invention. Other acceptable arrangements include having an outlet passage located toward the interior of stem  22  and surrounded by a co-axially oriented inlet passage; or having an inlet passage adjacent to but not co-axial with an outlet passage within stem  22 . 
     Still referring to FIG. 3, inlet passage  70  terminates in a disbursing channel  74  having a substantially rectangular cross-section and extending parallel to line of acceleration  32  and towards low energy end cap  46  and high energy end cap  48  of body  42 . Disbursing channel  74  terminates in a first annular cooling channel  76  in low energy end  58  of body  42  near low energy end cap  46  and a second annular cooling channel  78  in high energy end  60  of body  42  near high energy end cap  48 . First annular cooling channel  76  is substantially rectangular in cross-section and encircles body  42  to form a cylinder having a central axis substantially co-linear with line of acceleration  32 . Second annular cooling channel  78  also is substantially rectangular in cross-section and encircles body  42  to form a cylinder having a central axis substantially co-linear with line of acceleration  32 . Collecting channel  80  is of a substantially rectangular cross-section and extends from first annular cooling channel  76  to second annular cooling channel  78 . Collecting channel  80  is substantially parallel to line of acceleration  32  and disbursing channel  74 , and is located on the side of body  42  substantially opposite disbursing channel  74 . 
     Annular return channel  82  is located within body  42  intermediate of first annular cooling channel  76  and second annular cooling channel  78 . Annular return channel  82  is substantially rectangular in cross-section and has a cross-sectional area approximately equal to the sum of the cross-sectional area of first annular cooling channel  76  and the cross-sectional area of second annular cooling channel  78 . Annular return channel  82  encircles body  42  to form a cylinder having a central axis substantially co-linear with line of acceleration  32 . Annular return channel  82  connects with collecting channel  80  and with outlet passage  72 . Annular return channel  82  is preferably located midway between high energy orifice  64  and low energy orifice  62 , and the distance between low energy orifice  62  and first annular cooling channel  76  is preferably equal to the distance between high energy orifice  64  and second annular cooling channel  78 , so as to evenly distribute the cooling capability of cooling fluid  18  flowing through channels  76 ,  78  and  82 . 
     The flow of cooling fluid  18  within the channels and passages of body  42  may be summarized as follows: cooling fluid  18  travels through inlet passage  70  to disbursing channel  74 ; through disbursing channel  74  to first annular cooling channel  76  and second annular cooling channel  78 ; through first annular cooling channel  76  and second annular cooling channel  78  to collecting channel  80 ; through collecting channel  80  to return channel  82 ; and through return channel  82  to outlet passage  72 , from which cooling fluid  18  exits drift tube  20 . The flow of cooling fluid  18  through first cooling channel  76  is approximately equal to the flow of cooling fluid  18  through second cooling channel  78 . 
     For the purposes of this invention, to flow “through” an annular channel means to flow from the entry point of the annular channel to the exit point of the annular channel by all available routes. For example, to flow “through” first cooling channel  76  means to flow from dispersing channel  74  to collecting channel  80  through both first semi-annular  84  and second semi-annular cooling channel  86 . To flow “through” second cooling channel  78  and return channel  82  implies a similar flow pattern. 
     The location of first cooling channel  76  and second cooling channel  78  within low and high energy ends  58  and  60  respectively, and near low and high energy end caps  46  and  48  respectively, advantageously facilitates the cooling of low and high energy end caps  46  and  48  without utilization of cooling channels within end caps  46  and  48 . 
     Now referring to FIGS. 4 and 5, there are shown cross-sectional views taken through line  4 — 4  of FIG. 2 illustrating the particular components through which the preferred embodiment of s drift tube  20  is constructed, and the co-axial alignment of a bore tube  50  (see FIG.  5 ), magnet orifice  53  (see FIG.  4 ), magnet  52 , and body  42 . FIG. 4 specifically provides an exploded cross-sectional view of drift tube  20 , and FIG. 5 provides an cross-sectional view of an assembled drift tube  20  including stem  22 . Hollow cylindrical body  42  comprises a substantially cylindrical inner shell  90 , a low energy Z-ring  92 , a high energy Z-ring  94 , a hollow spacer cylinder  88 , and a substantially cylindrical cover  96 . Low and high energy Z-rings  92  and  94 , cover  96 , shell  90 , spacer  88 , and chimney  44  are preferably constructed of copper, as are low and high energy end caps  46  and  48 . When these elements are constructed from copper, and cooling fluid  18  (see FIG. 1) is water, the flow rates of cooling fluid  18  within channels  74 ,  76 ,  78 ,  80  and  82  (see FIG. 3) should be limited to less than 10 feet per second to avoid erosion/corrosion of the elements. 
     As shown in FIG. 4, inner shell  90  has a low energy side wall  110  and a high energy side wall  112 , an inner surface  116  and an outer surface  117 . From the low energy side wall  110  to the high energy side wall  112 , inner surface  116  comprises a spacer contacting surface  118 , a first shell shoulder  120 , a magnet contacting surface  122 , a second shell shoulder  124 , and a vacuum contacting surface  126 . Contacting surfaces  118 ,  122 , and  126  are all substantially parallel to line of acceleration  32 . The lengths of vacuum contacting surface  122  and spacer contacting surface  118  when measured parallel to line of acceleration  32  are about equal, as are the lengths of magnet  52  and magnet contacting surface  122  when measured parallel to line of acceleration  32 . In assembling drift tube  20  magnet  52  is inserted into inner shell  90  and along magnet contacting surface  122  from the direction of low energy end cap  46  until magnet  52  abuts second shell shoulder  124 . The diameter  123  of the cylinder formed by magnet contacting surface  122  is controlled to ensure a tight engagement between magnet  52  and magnet contacting surface  122 . Spacer  88  is then inserted into inner shell  90  and along spacer contacting surface  118  from the direction of low energy end cap  46  until spacer  88  abuts first shell shoulder  120  and magnet  52 . The diameter  119  of the cylinder formed by spacer contacting surface  118  and the outer diameter  89  of spacer  88  are controlled to ensure a tight engagement between spacer  88  and spacer contacting surface  118 . 
     The insertion of magnet  52  into inner shell  90  along magnet contacting surface  122  may be difficult due to the intended tight tolerances between the two elements. It should be understood that shoulders  120  and  124  and spacer  88  are not required elements of the present invention, and that magnet  52  may also engage inner surface  116  of inner shell  90  solely through friction or through a third method. However, the use of spacer  88  is preferred in that spacer  88  permits magnet  52  to be locked into place between two physical barriers (spacer  88  and second shell shoulder  124 ), and the use of spacer  88  reduces the difficulty of inserting magnet  52  into inner shell  90  by reducing the distance over which magnet  52  must be slid, while in contact with inner surface  1   16  of inner shell  90 , before reaching its desired position. 
     Outer surface  117  comprises a first channel surface  130 , a second channel surface  132 , and a return channel surface  134 . A first elevated ring  140  having a first side surface  142 , a cover contacting surface  144  and a return side surface  146  substantially encircles outer surface  117  intermediate of first channel surface  130  and return channel surface  134 . Similarly, a second elevated ring  150  having a second side surface  152 , a cover contacting surface  154 , and a return side surface  156  substantially encircles outer surface  117  intermediate of second channel surface  132  and return channel surface  134 . First and second elevated rings  140  and  150  may not completely encircle outer surface  117  due to the presence of chimney  44  and stem  22 , under which first and second elevated rings  140  and  150  may not extend. Channel surfaces  130 ,  132 , and  134  and cover contacting surfaces  144  and  154  are all substantially parallel to line of acceleration  32 . The lengths of first channel surface  130  and second channel surface  132  are about equal when measured parallel to line of acceleration  32 , and are each about one-half the length of return channel surface  134  when measured parallel to line of acceleration  32  (see FIG.  5 ). 
     When drift tube  20  is assembled, cover  96  is disposed over and engages cover contacting surfaces  144  and  154 . Inner surface  97  of cover  96 , return side surfaces  146  and  156 , and return channel surface  134  thereby form annular return channel  82  (see FIG.  5 ). Cover  96  preferably engages cover contacting surfaces  144  and  154  through brazing in which a copper-gold alloy brazing material is utilized. 
     Low energy Z-ring  92  comprises a central element  160 , an outer flange  162  extending parallel to line of acceleration  32  and toward cover  96 , and an inner flange  164  extending parallel to line of acceleration  32  and toward low energy end cap  46 . When assembled outer flange  162  of low energy Z-ring  92  abuts cover  96  and chimney  44  and contacts cover contacting surface  144  of first elevated ring  140 ; central element  160  of low energy Z-ring  92  abuts low energy side wall  110 ; and inner flange  164  contacts spacer  88 . First cooling channel  76  (see FIG. 5) is thereby defined by first channel surface  130 , first side surface  142 , and central element  160  and outer flange  162  of low energy Z-ring  94 . 
     Similarly, high energy Z-ring  94  comprises a central element  170 , an outer flange  172  extending parallel to line of acceleration  32  and toward cover  96 , and an inner flange  174  extending parallel to line of acceleration  32  and toward high energy end cap  48 . When assembled outer flange  172  of high energy Z-ring  92  abuts against cover  96  and chimney  44  and contacts cover contacting surface  154  of second elevated ring  150 ; and central element  170  of high energy Z-ring  94  abuts high energy side wall  112 . Second cooling channel  78  (see FIG. 5) is thereby defined by second channel surface  132 , second surface  152 , and central element  170  and outer flange  172  of high energy Z-ring  94 . Due to the absence of a structure comparable to spacer  88  adjacent to high energy Z-ring  94 , central element  170  and inner flange  174  are larger than central element  160  and inner flange  164  of low energy Z-ring  92 . 
     Low and high energy Z-rings  92  and  94  are preferably engaged to chimney  44 , cover  96 , and inner shell  90  through brazing in which a copper-gold alloy brazing material is utilized. It should be understood that the use of Z-rings, spacers, covers, and inner shells is but one method of forming the cooling channels within body  42  and that other methods of forming cooling channels within body  42  are also acceptable. 
     Low and high energy end caps  46  and  48  may be interconnected with body  42  and bore tube  50  (see FIG. 5) after insertion of bore tube  50  through low energy Z-ring  92 , spacer  88 , magnet orifice  53 , inner shell  90  and high energy Z-ring  94 . High energy end cap  48  has a substantially semi-spherical outer surface  180  that is pierced by centrally located high energy orifice  64 . End cap  48  further has a bore tube contacting surface  182 , a first shoulder  184 , a z-ring contacting surface  186 , and a second shoulder  188 . When drift tube  20  is assembled, inner flange  174  of high energy z-ring  94  contacts z-ring contacting surface  186  and abuts second shoulder  188 , and bore tube  50  contacts bore tube contacting surface  182  and abuts first shoulder  184 . The interface between semi-spherical outer surface  180  and orifice  64  is rounded to aid in the prevention of electrical arcing. For similar reasons, chimney  44 , cover  96 , high energy z-ring  94  and end cap  48  are configured to form a smooth cylindrical surface  192  (see also FIG.  5 ). During operation of DTL  14  the area  190  (also see FIG. 5) between magnet  52  and inner surface  194  of end cap  48  and is exposed to vacuum. 
     Low energy end cap  46  has a substantially semi-spherical outer surface  200  that is pierced by centrally located high energy orifice  62 . End cap  46  further has a bore tube contacting surface  202 , a first shoulder  204 , a z-ring contacting surface  206 , a second shoulder  208 , a spacer contacting surface  207 , and a third shoulder  209 . When drift tube  20  is assembled, inner flange  164  of low energy z-ring  92  contacts z-ring contacting surface  206  and abuts second shoulder  208 ; bore tube  50  contacts bore tube contacting surface  202  and abuts first shoulder  204 ; and spacer  88  contacts spacer contacting surface  207  and abuts third shoulder  209 . The interface between semi-spherical outer surface  200  and orifice  62  is rounded to aid in the prevention of electrical arcing. For similar reasons, chimney  44 , cover  96 , low energy z-ring  92  and end cap  46  are configured to form a smooth cylindrical surface  212  (also see FIG.  5 ). During operation of DTL  14  the area  210  (also see FIG. 5) between magnet  52  and inner surface  214  of end cap  46  and is exposed to vacuum. 
     Low and high energy end caps  46  and  48  are preferably attached to low and high energy z-rings  92  and  94  respectively through high energy electron beam welding. Low and high energy end caps  46  and  48  are also preferably attached to bore tube  50  through high energy electron beam welding. Electron beam welding is preferred based upon the ability of electron beam welding to achieve relatively deep “penetration” and thereby achieve an integrally attached relationship between the welded elements over a greater area. An integrally attached relationship between end caps  46  and  48  and their respective z-rings  92  and  94  and bore tube  50  is preferably achieved to a depth of 100 mils. The larger area of integral attachment achieved through electron beam welding facilitates heat transfer from the end caps  46  and  48  to body  42 , and helps achieve the desired cooling of drift tube  20  without resort to cooling channels located within end caps  46  and  48 . The utilization of simpler end caps  46  and  48  in turn permits significant reductions in the manufacturing costs of end caps  46  and  48 . 
     Low and high energy end caps  46  and  48  have a axial lengths  47  and  48  respectively. Axial length  47  is about equal to axial length  49 . Length  36  of drift tube  20  may be increased for successive drift tubes  20  within chamber  16  by increasing axial lengths  47  and  49  while maintaining the size of hollow body  42 . However, the larger axial lengths  47  and  49  become, the more difficult it becomes to cool end caps  46  and  48  using first cooling channel  76  and second cooling channel  78 . In high energy DTL applications, where cooling requirements may be especially high, this difficulty in cooling end caps  46  and  48  may require the use of hollow bodies  42  of greater sizes, to reduce axial lengths  47  and  49  while maintaining desired length  36  of drift tube  20 . 
     It should be understood that the invention is not limited to the exact details of construction shown and described herein for obvious modifications will occur to persons skilled in the art.