Abstract:
A photoelectron linear accelerator for producing a low emittance polarized electron beam. The linear accelerator includes a tube having a cylindrical wall, said wall being perforated to allow gas to flow to a pressure chamber containing ultra high vacuum pumps located outside the accelerator. The RF accelerator cavity comprises of two concentric cylindrical regions having different outside diameters and different lengths.

Description:
GOVERNMENTAL RIGHTS IN INVENTION 
       [0001]    This invention was made with partial governmental support under Small Business Innovation Research (SBIR) Contract No. DE-FG02-06ER84460 awarded by the U.S. Department of Energy to DULY Research Inc. The government may have certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention provides a normal-conducting photoelectron linear accelerator for producing a low-emittance electron beam from a photocathode that operates in ultra high vacuum and under high heat load. 
         [0004]    2. Description of the Prior Art 
         [0005]    A polarized electron linear accelerator based on a Plane-Wave-Transformer (PWT) design was the subject of a prior U.S. Pat. No. 6,744,226, in which a plurality of iris-loaded disks are suspended by water cooling rods (or pipes) that are connected to two endplates of a cylindrical radiofrequency (RF) cavity. The electric field pattern in the cylindrical PWT cavity is such that a TEM-like mode, resembling the plane wave in free space, is sustained in the region between the outer diameter of the disks and the inner wall of the cylindrical cavity, while a TM01-like mode is sustained on and near the axis of the standing-wave PWT cavity. Because the disk(s) are not attached to any other parts of the cavity than the supporting rods, the PWT has excellent vacuum properties including a large vacuum conductance in the paths from the photocathode that is located on the back endplate to the vacuum pumps located outside the cavity. A polarized electron beam is generated from a GaAs cathode located in the center of the back endplate of the cavity when a polarized laser beam is impinged upon it. Ultra high vacuum (UHV) can be accomplished with conventional ion pumps as well as non-evaporative getters (NEG). In the previous invention, a NEG film is sputtered onto the inner surface of the cavity wall. The presence of the NEG film on the RF cavity wall, however, reduces the Q-factor of the cavity. Also in said invention the NEG-lined cavity wall is not replaceable. As the NEG pumping becomes less effective over time, the entire cavity would have to be replaced. The cooling of the disks, rods, endplates and other elements in the PWT cavity that are exposed to RF heating during electron acceleration is accomplished by water flowing through internal channels inside the disks, rods and other elements. The flow rates are determined by the external pressure head and by resistances through the pipes and orifices as well as those in the internal channels of the disks and walls of the cavity. The flow rates are predominantly limited by the flow area inside the pipes and the sizes of orifices, which in turn limit the amount of heat that can be removed from the surfaces of the cavity that are exposed to RF. Such limitations can become problematic when a high heat load such as that required when long RF pulses, a high rep rate and/or high power RF are imposed on the PWT cavity. What is desired under such circumstances is an RF cavity that operates in a UHV environment with replaceable NEG elements and if possible, without the flow restriction imposed by the rods, orifices and disks. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a method and apparatus to produce a high-quality electron beam from a photocathode which requires an ultra high vacuum for optimal operation, and to provide superior cooling in a half-cell photoelectron linear accelerator under high RF heat load. The invention provides an ultra high vacuum RF photoelectron linear accelerator design that has a perforated cavity wall through which residual gas inside the RF cavity is evacuated with ultra high vacuum pumps placed in a replaceable pressure chamber outside said perforated wall. Examples of UHV pumps are ion pumps, non-evaporative getter (NEG) modules or a NEG film sputtered on the inner surface of a pressure chamber surrounding the cavity. In one embodiment of the invention, no disks and rods are needed in a half-cell cavity, while the cavity still retains the characteristic field pattern of the PWT. This embodiment allows effective cooling of the cavity walls without the limitation imposed on the flow rate by the small pipe and orifice sizes. The characteristic field pattern of the PWT includes a hybrid mode that has a TEM-like field in the outer region of the cavity and a TM-like field on and near the axis of a cylindrical RF cavity. 
         [0007]    The invention has applications in polarized or unpolarized particle accelerators which require an ultra high vacuum. It is particularly applicable to electron accelerators in which electrons are produced from a semiconductor (such as GaAs) cathode. The method provides the UHV that is necessary in order to maintain good quantum efficiency and long life for the cathode. The embodiment of the invention of a photoelectron linac with no disks and rods, alternatively called a hybrid mode RF gun here, has particular application to electron guns that operate under a high heat load, such as a long pulse RF gun, or pulsed RF guns with a high rep rate, or continuous wave (CW) RF guns. The hybrid mode, half-cell, RF gun design is especially well matched to the features necessary for production of polarized electrons in a short, high gradient accelerator under high RF power. 
         [0008]    The features of the RF linac of the present invention include a cavity wall (or sieve) that has built-in, through-the-wall, longitudinal slots that are open to a replaceable pressure chamber surrounding the cavity. The pressure chamber contains non-evaporative getters either in the form or fabricated modules, available for example through SAES, or as a thin film comprising of NEG such as TiZrV that is directly deposited onto the inner surface of the said pressure chamber. The pumping through the slots and through the cavity is capable of providing the ultra-high vacuum condition especially needed for the survivability of the semiconductor photocathode such as GaAs. The size of said slotted openings in the cavity wall is specified so that RF waves are attenuated inside the slots while residual gases inside the cavity are allowed to flow through the slots to the pumps located outside the cavity. Additional pumps may be used to pump the cavity at locations other than the pressure chamber. 
         [0009]    In one embodiment of the present invention, the hybrid mode cavity has no disks or rods but comprises instead of two concentric cylindrical regions of different outer diameters and different lengths to achieve the characteristic electrical field pattern of the PWT. The electrical field pattern comprises a TEM-like mode in the larger cylindrical cavity and a TM-like mode in the smaller cylindrical cavity close to the axis of the cavity. 
         [0010]    In one embodiment of the rodless and diskless hybrid mode cavity, the RF coupler is coaxial with the cylindrical cavity. The coaxial coupler has an outer conductor and an inner conductor whose shape and dimensions are designed to allow the external RF power to critically couple into the standing wave RF cavity coaxially. 
         [0011]    Having no rods and disks, the hybrid mode cavity is cooled efficiently by ordinary liquid such as water that flows through internal channels embedded in cavity walls. The slotted outer wall (sieve) of the cavity has separate longitudinal internal channels that carry flowing water. Pressurized deionized water is fed into the internal channels via external pipes. Having no rods and orifices that incur high pressure drops, the cooling of the hybrid mode cavity is thus highly efficient. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0012]    For a better understanding of the present invention as well as other objects and further features thereof, reference is made to the following descriptions which are to be read in conjunction with the accompanying drawing wherein: 
           [0013]      FIG. 1  is a schematic diagram of the ultra high vacuum, PWT photoelectron linear accelerator with rods and one disk, with a replaceable pressure chamber surrounding the cavity; 
           [0014]      FIG. 2   a  is a schematic diagram of the ultra high vacuum, hybrid mode cavity without rods and disks; 
           [0015]      FIG. 2   b  is a two dimensional electric field map from Superfish for the RF cavity shown in  FIG. 2   a;    
           [0016]      FIG. 3   a  is a cross-sectional view along line  2 - 2  of  FIG. 1 ; 
           [0017]      FIG. 3   b  is a cross-sectional view along line  3 - 3  of  FIG. 1 ; 
           [0018]      FIG. 4  illustrates the slotted wall or sieve of the hybrid mode cavity or the modified PWT; 
           [0019]      FIG. 5  illustrates an alternative design of the replaceable pressure chamber that houses the NEG pump. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0020]    The ultra high vacuum (UHV) photoelectron linear accelerator (linac) of the present invention with the modified PWT design    110   , or hybrid mode design    120   , comprises a radiofrequency cavity having a porous outer wall  12  through which is connected a pressure chamber  10  that houses non-evaporative getter (NEG) material  14  for ultra high vacuum pumping. The NEG pumps may be commercially available NEG modules (for example, SAES)  14  mounted on the inside wall of the pressure chamber  10 , or a layer of NEG film sputtered directly onto the inside wall of the pressure chamber  10 . The removable pressure chamber  10  is attached to the body of the linac    110    or    120    via a standard Conflat flange  24 , and a second Conflat flange  26  that is inverted from the standard design. The standard Conflat flange  24  has a bolt circle on the outside of the knife edge. The inverted Conflat flange  26  has a bolt circle on the inside of the knife edge. The mating inverted Conflat flange  26  is optionally connected to a bellows or an eyelet  38  that has both vertical and horizontal degrees of freedom. The porous cavity wall  12 , or “sieve”, has longitudinal slots through it. The width of the slot is smaller than the cutoff dimension of the RF wave in order to prevent the RF power inside the RF cavity from leaking into the pressure chamber  10 . In one embodiment of the UHV linac    110    of the plane wave transformer (PWT) design, illustrated in  FIG. 1  and  FIG. 3 , the RF cavity is formed by one or more iris-loaded disk(s)  35  that is (are) supported by rods (or pipes)  22  that are anchored to the endplates of the cavity. The pipes  22  carry liquid coolant, for example water, that flows into channels  32  imbedded inside the disk(s)  35  and the first endplate. Cooling of the RF cavity of the linac    110    is additionally provided by a water circuit comprising pipes  40  and channels  32  imbedded inside the second endplate of the cavity, and by longitudinal channels inside the sieve  12 . The inlet and outlet flows in the cooling circuit in the endplate  27  are separated by flow dividers  29  which direct flow through internal compartments into flow channels in the sieve  12 , said flow is connected by a circumferential channel or reservoir  31  in the opposite endplate  30 . The UHV PWT    110    has a demountable photocathode  28  located at the center of the back endplate  30 . Electrons are produced from the photocathode  28  when a laser pulse is directed into the cavity nearly along the axis of the cavity by an optical system located outside the cavity. An RF seal  20  is inserted between a cathode puck (not shown) that holds the cathode  28  in place and the back endplate  30  to prevent the RF power from leaking out of the cavity. For a short RF cavity where is insufficient room for an RF side coupler, RF power is fed into the cavity by means of a coaxial coupler  50  which is connected to an external RF coupler  55 , for example, a doorknob coupler of the DESY design. Additional pumping devices such as ion pumps, may be connected to the external RF coupler  55  or the pressure vessel  10  to further improve the vacuum in the cavity. The electromagnetic field in the PWT cavity is characterized by two modes present respectively in two distinct regions of the standing wave cavity: An inner region  16  in which a TM-like mode is present to provide an axial electric field, typically that of the “π” mode, for acceleration of the electron beam; and an outer region  18  in which a TEM-like mode is present. In one embodiment of the UHV PWT linac with disks, the inner region  16  occupies a cylindrical volume extending from one endplate to the other, with a diameter approximately the same as the outer diameter of the disk(s), and the outer region  18  occupies the rest of the cavity volume outside the disk(s). A PWT cavity of this invention with a single disk design operating in the “π” mode is illustrated in  FIG. 1 , where the distance between the back endplate  30  and the disk  35 , as well as that between the disk  35  and the front endplate  27 , is approximately one-quarter wavelength long in the longitudinal direction. If no RF side coupler is used so that the entire porous cavity wall (sieve) provides the maximum vacuum conductance through said wall, the PWT cavity    110    is critically coupled via a coaxial coupler  50  to an external RF power source. The electron beam accelerated in the PWT cavity    110    is focused by means of emittance-compensating magnets comprising a main solenoid  42  and a bucking solenoid  44 . 
         [0021]    A second embodiment of the UHV linac    120    with a modified PWT design is shown in  FIG. 2 , for which no disk or supporting pipes are needed. The hybrid mode cavity    120    is formed instead by two conjoined and concentric cylindrical regions  16  and  18  with different axial lengths. The inner region  16  occupies a cylindrical volume approximately one-quarter of a wavelength long. The outer region  18  occupies a longer coaxial volume immediately outside the inner region  16 . Its outer wall comprises the porous wall or sieve of the UHV PWT linac. In this variant of the rodless and diskless UHV PWT, the endplates of the UHV PWT    120    are cooled with flow inside imbedded channels  32 . A photocathode  28  is placed at the center of the first endplate of the integrated PWT linac    120   . The front endplate  33  has a top hat shape, shown in  FIG. 2 , that defines the lengths of the PWT cavity regions  16  and  18 . The iris of the front endplate  33  can further be shaped with a nose to increase the shunt impedance of the cavity. External pipes  40  feed coolant into imbedded channels inside the endplates. The pipes  40  can be as large as needed to provide the desired flow to cool the endplates. The sieve  12 , of which a three dimensional rendering is shown in  FIG. 4 , is cooled by coolant inside longitudinal flow channels fed by separate external pipes  40 . In this embodiment, RF power is critically coupled into the UHV PWT cavity    120    via a coaxial coupler  50  and an external RF coupler  55 . 
         [0022]    The replaceable pressure chamber  12 , shown in  FIG. 1  and  FIG. 2  includes an inverted Conflat flange  26 , optionally connected to a flexible eyelet  38 , to allow adequate compression of the gasket between the two knife edges and proper alignment of the bolt holes between the pair of inverted Conflat flanges in order to provide a good vacuum seal. An alternative design of the replaceable pressure chamber  12  is shown in  FIG. 5 . In this design, standard Conflat flanges are used on both ends of the pressure chamber  12 . One of the Conflat flanges  24  is connected to the body of the RF cavity as in the aforementioned design, while the other standard Conflat flange  23  is connected to a mating flange on a circular cover plate  60  that forms part of the pressure chamber which is brazed to the cathode tube  19 . Pins  75  may be used to align the pressure chamber cover plate  60  with the endplate  70  in the body of the RF cavity. 
         [0023]    While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.