Abstract:
A superstructure for accelerating charged particles at relativistic speeds. The superstructure consists of two weakly coupled multi-cell subunits equipped with HOM couplers. A beam pipe connects the subunits and an HOM damper is included at the entrance and the exit of each of the subunits. A coupling device feeds rf power into the subunits. The subunits are constructed of niobium and maintained at cryogenic temperatures. The length of the beam pipe between the subunits is selected to provide synchronism between particles and rf fields in both subunits.

Description:
The United States of America may have certain rights to this invention under Management and Operating contract No. DE-AC05-84ER40150 from the Department of Energy. 

   FIELD OF THE INVENTION 
   This invention relates to linear accelerators and specifically to an improved accelerating structure for accelerating a high current beam of charged particles. 
   BACKGROUND OF THE INVENTION 
   High power Free Electron Lasers (FEL) with power levels of 10 kW of infrared laser light have recently been demonstrated at the Thomas Jefferson National Accelerator Facility (TJNAF) in Newport News, Va. Although 10 kW of laser light is a substantial achievement, even higher levels of power would support advanced studies of biology, chemistry, and physics and enhance manufacturing technologies. 
   A linear accelerator (linac) supplies the FEL with electrons at relativistic speeds. For higher levels of FEL power, such as 1 MW, improvements are required in the accelerating structures in the linac. To achieve a power level of 1 MW in the FEL, electron beams in the range of 500 to 1,000 mA have to be accelerated in the linac supplying the FEL. 
   Successful operation of a linac at 0.5 to 1 amperes of current will require the accelerators to be based on superconducting technology. Additionally, Higher Order Modes (HOM) excited by the beams must be effectively damped to allow stable operation of the linac. 
   Therefore, one of the requirements for increasing the power level of an FEL to the 1 MW range is an accelerating structure capable of accelerating an electron beam in the linac to the level of 0.5 to 1 ampere and having sufficient damping to suppress the HOMs excited by the beams. 
   SUMMARY OF THE INVENTION 
   The invention is a superstructure for accelerating charged particles at relativistic speeds. The superstructure is made of two weakly coupled multi-cell subunits and equipped with HOM couplers. A beam pipe connects the subunits and an HOM damper is included at the entrance and the exit of each of the subunits. A coupling device feeds rf (radio frequency) power into the subunits. The subunits are constructed of niobium and maintained at cryogenic temperatures. The length of the beam pipe between the subunits is selected to provide synchronism between particles and rf fields in both subunits. 
   OBJECTS AND ADVANTAGES 
   The superstructure beam of the present invention enables acceleration of electrons to achieve an electron beam at 0.5 to 1 A. Use of a superstructure over conventional accelerating structures advantageously increases the active cavity length as a percentage of the total length of the linac and also significantly reduces the amount of microwave components. The superstructure of the present invention reduces the number of cells per structure and therefore reduces the amount of trapped HOMs. The reduction in number of cells and microwave components leads to a significant reduction in the cost of the accelerating structures. The superstructure is very compact and will provide excellent HOM damping. The compactness of the superstructure and the reduction in the number of power feeds for the subunits reduces the number of required rf components and therefore significantly improves the economics of the linac. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a conceptual side view of a preferred embodiment of a superstructure according to the present invention including 6 HOM couplers and a coaxial input coupler. 
       FIG. 2  is a conceptual side view of a portion of the right side of the superstructure shown in  FIG. 1 . 
   

   TABLE OF NOMENCLATURE 
   The following is a listing of part numbers used in the drawings along with a brief description: 
   
     
       
             
             
           
         
             
                 
             
             
               Part Number 
               Description 
             
             
                 
             
           
           
             
               20 
               superstructure 
             
             
               22 
               input end 
             
             
               24 
               output end 
             
             
               26 
               subunit 
             
             
               28 
               cell or cavity 
             
             
               30 
               beam pipe 
             
             
               32 
               HOM coupler 
             
             
               34 
               power coupler 
             
             
               36 
               entrance opening to subunit 
             
             
               38 
               exit opening from subunit 
             
             
               40 
               beam line 
             
             
               42 
               equator 
             
             
               44 
               iris 
             
             
               D E   
               equator diameter 
             
             
               D I   
               iris diameter 
             
             
               D B   
               beam tube diameter 
             
             
               D A   
               beam tube diameter after taper 
             
             
               L C   
               cell length 
             
             
               L B   
               length of the interconnecting beam pipe 
             
             
                 
             
           
        
       
     
   
   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , the present invention is a superstructure  20  for acceleration of electrons in a high-energy electron beam. The superstructure  20  has an input end  22  and an output end  24 . The superstructure  20  includes two subunits  26  having two cells  28  or cavities each resonating at 750 MHz and connected by a larger diameter beam pipe  30 . The beam pipe  30  provides approximately 0.024% coupling between the subunits  26 . Two coaxial type HOM couplers  32  of approximately 70 mm diameter are located on each end  22 ,  24  of the superstructure  20  and on the interconnecting beam pipe  30 . A power coupler  34  of the coaxial variety is located on the input end  22 . Each subunit includes an entrance opening  36 , an exit opening  38 , and a beam line  40  therebetween. 
   Referring to  FIG. 2 , the cells  28  include an equator  42  at the farthest lateral extent of the cells  28  and an iris  44  at each junction with the beam pipe  30 . The geometry of the superstructure  20 , or critical dimensions as shown in  FIG. 2 , includes the equator diameter D E , the center iris diameter D I , the beam tube diameter D B , the beam tube diameter after taper D A , the cell length L C , and the length of the interconnecting beam pipe L B . The geometry of the superstructure  20  is listed in Table 1. 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Cavity Cell Geometry. 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Equator Diameter [mm] 
               362.6 
             
             
                 
               Center Iris Diameter [mm] 
               130 
             
             
                 
               Beam Tube Diameter [mm] 
               180 
             
             
                 
               Beam Tube Diameter after Taper [mm] 
               120 
             
             
                 
               Cell length [mm] 
               200 
             
             
                 
               Length of Interconnecting Beam Pipe [mm] 
               200 
             
             
                 
               HOM Coupler Body Diameter [mm] 
               70 
             
             
                 
                 
             
           
        
       
     
   
   The RF properties are summarized in Table 2. 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               RF properties of 2-cell on 2-subunit superstructure. 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Frequency [MHz] 
               749.552 
             
             
                 
               Geometry Factor [Ω] 
               280 
             
             
                 
               (R/Q)/L [Ω/m] 
               443 
             
             
                 
               (R/Q)/cell [Ω] 
               88.6 
             
             
                 
               E peak /E acc   
               2.2 
             
             
                 
               H peak /E acc  [mT/(MV/m)] 
               4.74 
             
             
                 
               Coupling between subunits [%] 
               0.024 
             
             
                 
                 
             
           
        
       
     
   
   The superstructure design features a rather large beam hole for good coupling of HOMs. The large beam hole compromises the R/Q to some extent, but still results in a reasonable ratio of E peak /E acc . With the R/Q value of ˜89 Ω/cell, the power dissipated in the superstructure at a Q-value of 8.10 9  is approximately 68 W. 
   Table 3 lists the first 20 monopole modes. Mode No. 14 (TM 020 ) is the mode with the highest impedance of 36.5Ω, which is however a factor of 10 smaller than the fundamental mode (R/Q) value (mode No. 3). The field distribution of the mode No. 14 shows sufficient field strength at the locations of the HOM couplers thereby indicating appropriate damping. The total impedance for the first 16 monopole modes is approximately 140Ω. None of the first 16 modes falls on a machine line (N*1500 MHz). 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               First 20 monopole modes for the 2-cell on 2-subunit superstructure. 
             
           
        
         
             
               MODE# 
               FREQUENCY [MHz] 
               R/Q [Ω] 
             
             
                 
             
           
        
         
             
               1 
               741.713 
               0.0001 
             
             
               2 
               741.880 
               1.467 
             
             
               3 
               749.552 
               354.5 
             
             
               4 
               749.732 
               0.005 
             
             
               5 
               1308.134 
               3.806 
             
             
               6 
               1315.955 
               0.072 
             
             
               7 
               1316.844 
               20.017 
             
             
               8 
               1360.734 
               8.713 
             
             
               9 
               1400.082 
               0.173 
             
             
               10 
               1402.311 
               13.759 
             
             
               11 
               1446.805 
               4.788 
             
             
               12 
               1506.112 
               0.268 
             
             
               13 
               1522.499 
               0.676 
             
             
               14 
               1543.080 
               36.534 
             
             
               15 
               1575.144 
               8.049 
             
             
               16 
               1646.739 
               9.335 
             
             
               17 
               1647.143 
               6.728 
             
             
               18 
               1724.096 
               4.946 
             
             
               19 
               1898.317 
               0.712 
             
             
               20 
               1900.470 
               19.676 
             
             
                 
             
           
        
       
     
   
   The dipole modes up to a frequency of 1950 MHz have been calculated with MAFIA and are listed in Table 4 below. MAFIA, an acronym for Maxwell&#39;s Equations by the Finite Integration Algorithm, is a computer program for solving problems in the simulation of electromagnetic fields. As shown in Table 4, the shunt impedances (R/Q) are favorably small. 
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 4 
             
           
           
             
                 
             
             
               Dipole Modes. 
             
           
        
         
             
                 
               Mode 
               F [GHz] 
               Q 
               R/Q [Ω/cm 2 ] 
             
             
                 
                 
             
           
        
         
             
                 
               1 
               0.854 
               33890 
               0.095 
             
             
                 
               2 
               0.863 
               35348 
               0.147 
             
             
                 
               3 
               0.889 
               37449 
               0.226 
             
             
                 
               4 
               0.904 
               41297 
               0.432 
             
             
                 
               5 
               0.989 
               31390 
               2.817 
             
             
                 
               6 
               1.000 
               32790 
               0.003 
             
             
                 
               7 
               1.072 
               47165 
               0.097 
             
             
                 
               8 
               1.074 
               46766 
               0.550 
             
             
                 
               9 
               1.124 
               40478 
               0.020 
             
             
                 
               10 
               1.124 
               40475 
               0.165 
             
             
                 
               11 
               1.137 
               47585 
               0.421 
             
             
                 
               12 
               1.302 
               59513 
               0.024 
             
             
                 
               13 
               1.330 
               50714 
               0.001 
             
             
                 
               14 
               1.340 
               50509 
               0.157 
             
             
                 
               15 
               1.413 
               51084 
               0.261 
             
             
                 
               16 
               1.435 
               46926 
               0.281 
             
             
                 
               17 
               1.534 
               55815 
               0.752 
             
             
                 
               18 
               1.565 
               42610 
               0.521 
             
             
                 
               19 
               1.565 
               42497 
               0.007 
             
             
                 
               20 
               1.640 
               46049 
               0.028 
             
             
                 
               21 
               1.645 
               49704 
               0.017 
             
             
                 
               22 
               1.706 
               61608 
               0.792 
             
             
                 
               23 
               1.720 
               65542 
               0.003 
             
             
                 
               24 
               1.749 
               103346  
               0.054 
             
             
                 
               25 
               1.764 
               89708 
               0.008 
             
             
                 
               26 
               1.775 
               58759 
               0.082 
             
             
                 
               27 
               1.814 
               86002 
               0.074 
             
             
                 
               28 
               1.906 
               59827 
               0.011 
             
             
                 
               29 
               1.921 
               58991 
               0.125 
             
             
                 
               30 
               1.939 
               79307 
               0.054 
             
             
                 
                 
             
           
        
       
     
   
   The choice of the frequency of 750 MHz as opposed to 500 MHz was dictated by the following considerations: 
   a). Under the assumption that the beam alignment will be the same for a 1 A beam as it is for a 100 mA beam, which was the threshold current for the 1500 MHz superstructure and the achieved damping results for dipole modes, the threshold current at this frequency would be 2× higher, since the impedance (R/Q) scales with 1/r 2  where r=iris diameter, which scales with frequency. The threshold current is proportional to 1/v(R/Q). A threshold current of 1 A is therefore achievable with a reduction of the number of cells from 5 to 2 and an appropriate opening of the iris diameter. 
   b). The second consideration for proposing a 750 MHz cavity is the existing infrastructure for cavity treatments. Both the cabinets for chemical polishing and high pressure rinsing are size limited to a cavity length of ˜130 cm. The 750 MHz superstructure as shown in  FIG. 1  has an active length of 4×20 cm plus 20 cm for the interconnecting beam pipe and 15 cm on each side of the structure for beam pipes. These either must be tapered down to a smaller diameter beyond the HOM couplers to increase the damping of the fundamental mode or the beam pipes must be extended with bolted on extensions. 
   The superstructure will be fabricated of niobium. The niobium cavities will be operated at cryogenic temperatures, or a temperature below 4.2 K, so that they are superconducting. 
   Table 5 includes a detailed list of proposed assumptions and parameters for a 1 MW FEL based on the superstructure disclosed herein. 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 5 
             
           
           
             
                 
             
             
               Assumptions and parameters for a 1 MW FEL superstructure. 
             
           
        
         
             
                 
               Units 
               Value 
             
             
                 
                 
             
           
        
         
             
               LINAC: 
                 
                 
             
             
               Assumptions: 
             
             
               Energy gain in linac 
               MeV 
               145 
             
             
               Real Estate Gradient 
               MV/m 
               8.7 
             
             
               F 
               MHz 
               750 
             
             
               Operating Temperature 
               K 
               &lt;4.2 
             
             
               Operation Mode 
               — 
               cw 
             
             
               Fill factor 
               — 
               0.5 
             
             
               Energy Recovery Efficiency 
               % 
               99 
             
             
               Resulting Parameters 
             
             
               Length of the linac 
               M 
               16.7 
             
             
               Active length 
               M 
               8.3 
             
             
               Gradient in cavities 
               MV/m 
               17.4 
             
             
               BEAMS: 
             
             
               Assumptions: 
             
             
               I beam /beam 
               A 
               1 
             
             
               Bunch frequency: acceleration 
               MHz 
               750 
             
             
               Bunch frequency: deceleration 
               MHz 
               750 
             
             
               2 x beams current 
               A 
               2 
             
             
               Resulting Parameters 
             
             
               Charge/bunch 
               C 
               1.3E−09 
             
             
               CAVITIES: 
             
             
               Assumptions: 
             
             
               F 
               MHz 
               750 
             
             
               Q 0   
               — 
               8.0E+09 
             
             
               Number of cells/unit 
               — 
               4 
             
             
               (R/Q) 
               Ω 
               354 
             
             
               Lcav active 
               M 
               0.8 
             
             
               Number of cavities 
               — 
               10 
             
             
               Number of cavities per cryomodule 
               — 
               5 
             
             
               Number of HOM couplers 
               — 
               6 
             
             
               Resulting Parameters 
             
             
               Voltage/cavity 
               MV 
               13.92 
             
             
               Cryo-Loss/cavity 
               W 
               6.84E+01 
             
             
               Power/cavity with energy recovery 
               W 
               139200 
             
             
               Total Dynamic Cryo-Loss 
               W 
               6.84E+02 
             
             
               Plug in power for cryo 
               kW 
               4.79E+02 
             
             
               HOM (resonant mode No. 14): 
             
             
               (R/Q) 
               Ω 
               36 
             
             
               Tolerable Power/HOM Coupler 
               W 
               300 
             
             
               Total Tolerable HOM Power 
               W 
               1800 
             
             
               Qext/HOM coupler needed for the tolerable power 
               — 
               75 
             
             
                 
             
           
        
       
     
   
   The present invention as described herein is a superstructure for a 1 Amp beam, resonating at 750 MHz and consisting of two 2-cell subunits coupled weakly by a beam pipe. This superstructure features a total of 6 coaxial type HOM couplers. Two HOM couplers are located at the end of each subunit and two at the interconnecting beam pipe. The superstructure also includes a high power coaxial input coupler of the KIK (SNS) type, as described in “Superconducting Cavities for HERA”, B. Dwersteg et al., Proceedings of the 3. Workshop on RF Superconductivity, Report ANL-PHY-88-1, p. 81ff, ANL, 1987. 
   Simulation calculations indicate that the ratio of peak surface fields and accelerating gradients are reasonable, whereas the shunt impedance suffers somewhat from the large iris diameter. The HOM spectrum of this structure is quite favorable as the highest parasitic shunt impedance is only 10% of the fundamental mode shunt impedance indicating that the HOMs would be damped to Q ext  values of less than 1000. In this case the HOM power generated by the beam would be only 60 W, distributed over 6 HOM couplers. 
   As described herein, the present invention describes a method of developing a superstructure for accelerating charged particles in a high energy particle beam. The method for developing a superstructure includes 1) selecting a resonant frequency, 2) providing a cell including a cell length, an equator diameter, and an iris diameter, 3) connecting a plurality of the cells into a multi-cell subunit, 4) connecting the subunits with a beam tube, 5) weakly coupling the subunits, 6) providing a plurality of couplers for feeding rf power into the subunits to define a superstructure, 7) identifying the monopole modes for the superstructure, 8) adjusting the field strength of each monopole mode to achieve appropriate damping, 9) determining the dipole modes at a range of frequencies, and 10) verifying that the impedances for each dipole mode are small. 
   Although the specific embodiment described herein is comprised of 2-cell subunits, the superstructure for high current applications can be constructed of subunits having between one and nine cells per subunit. 
   Having thus described the invention with reference to a preferred embodiment, it is to be understood that the invention is not so limited by the description herein but is defined as follows by the appended claims.