Sawamura, Masaru

Takekoshi, Hidekuni

Kyoto University Research Information Repository

Title Field Stabilization of the Alvarez Linac by Post CouplerAuthor(s)Sawamura, Masaru; Takekoshi, HidekuniCitationBulletin of the Institute for Chemical Research, KyotoUniversity (1985), 63(1): 9-15Issue Date1985-03-30URL http://hdl.handle.net/2433/77095RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityBull. Inst. Chem. Res., Kyoto Univ., Vol. 63, No. 1, 1985         Field Stabilization of the Alvarez Linac                  by Post Coupler                Masaru SAWAMURAand Hidekuni TAKEKOSHI                              Received  January 31, 1985    Field stabilization of the Alvarez drift tube linac by post couplers was studied. Post-coupler characteristic was measured on two types of the equal velocity models with different drift tube di-ameters, and with the variable number of post couplers.     KEY WORDS: Proton Linac/ Alvarez Linac/ Post Coupler/ Field Sta-                bilization/                          I. INTRODUCTION    The accelerating mode of the Alvarez structure for the proton linear accelerator at middle energy is TM,„ , so the group velocity of TM,„ wave is zero. This mode is susceptible to perturbations in electric field distributions due to manufacturing errors or beam loading. And the temperature shift of cavity causes the res-onant frequency shift at operation, so the resonant frequency has to be adjusted to the initial value by. tuners. But this adjustment tilts the flat electric field distribu-tion on the axis. These distortions of field distribution during acceleration result in the increase of beam emittance and beam loss. So it is important to stabilize the field distribution by some means or other.    For this purpose the post coupler is considered.' Post coupler is a rod extending from the outer wall opposite the dirft tube to stabilize the on-axis electric field by adjusting the coupler length.    Post coupler is in principle a quarter-wave resonator, so there is a functional relation between the operating frequency and the distance between the inner tank wall and the drift tube!) Two types of models, which have same resonant fre-quency and same inner tank diameter but different drift tube diameters, were made. Field stabilization with post couplres was investigated using these models.                 II. CONSTRUCTION OF ALVAREZ TANK   On-axis fields were measured on 9-cell tank models. Figure 1 shows the sche-matic diagram of unit cell and a drift tube. Dimensions of each cell were same for simplicity (equal velocity model). Cell length was 6 cm and tank inner diameter was 44 cm.    As the case of type 1 model, the drift tube diameter was 10 cm. In this model the distance between drift tube and tank wall was approximately equal to the quar-* Rs-j- J14, /63.  : Nuclear Science Research Facility, Institute for Chemical Research, Kyoto   University, Kyoto 606, Japan. ( 9 )                            M. SAWAMURA and H.  TAKEKOMU Cell 1 8 Stem Rc DriftDRn -----r-------------- Rb Tube....... Fig. 1. Schematic diagram of unit cell and drift tube.          Table 1. Dimensions of unit cell and drift tubeshown in Fig. 1 (unit is mm)                   Type 1Type 2   D: tank inner diameter440.0   L: cell length60.0   g: gap length21.011.2   1: drift tube length39.048.8   d: drift tube diameter 100.060.0 Rc: corner radius8.0   Rn: nose corner radius4.0   Rb: bore radius5.0  0: face angle0° ter-wave of resonant frequency 440 MHz. 6 cm diameter was chosen for type 2 to get the higher shunt impedance of cavity. The gap length between drift tubes was calculated by the computer code SUPERFISH to be resonant at 440 MHz. Table 1 shows the dimensions of Alvarez tank.    The length of half drift tube on each end plate could be changed to give per-turbation to field distribution. III. POST-COUPLER MEASUREMENTS    The field stabilization with post couplers was investigated as follows3> (1) Without post couplers the original on-axis electric field distribution was meas-ured. The bead perturbation methods were used to sample the on-axis electric fields. Figure 2 shows equipments for measurement. The bead was a plastic ball of 3 mm diameter covered with evaporated aluminum. (2) When the length of half drift tube on one end plate was changed, the resonant frequency of cavity shifted by d f Then we adjusted the length of half drift tube on another end plate to restore the resonant frequency to the original value. The field                           ( 10 ) [Field Stabilization of the Alvarez Linac by Post Coupler cavity                Bead               WeightPicK-up DriveMotor             g,i  GouplerCoupler               Detector —, Standard Signal                    Amplifier Generator  Chart                    Recorder         Fig. 2. Block diagramof system of measurement by bead perturbation methods. `J ° — 1-1 MHz o ,, ---- ±2 MHz c100- P _._ -1 MHz ..o w I— 50-1\       z                                                                                   •''I      o _ _ __P_=~~        {-----2------------- °4 o's A°U  U) ~,     05 10 15 17                 POST COUPLER LENGTH (cm)      `J40-I° — -I-1 MHz o\ i ., ---- -±2 MHz    cc° - -1 MHz          H-              w'A            IQ QZi                                                                                                    P'                                                            A         I—10-:4-°Pt° A.L;"                    ~.'~'n \t-ti.o —° CI)°~A   0 1I 1      1314151617                 POST COUPLER LENGTH (cm)            Fig. 3. Distortion parameter, Dx , as a function of post coupler length for +1 MHz, +2 MHz and —1 MHz end cell perturbations. Drift                  tube diameter is 10 cm. ( 11 )                                        M.  SAWAMURA and H. TAKEKOSHI         distribution was measured for d f end cell perturbation.        (3) Post couplers were installed in the cavity at the same length. The diameter of        post coupler was 15 mm which was equal to the diameter of drift tube stem. A         tab with the area 2.37 times the post coupler cross section was mounted at the end         of post coupler. Changing the length of post couplers, we measured the on-axis        electric field distribution for d f end cell perturbation and compared with the original         distribution.            A distortion parameter,4) Dx , which indicates the effectiveness of field stabiliza-       tion with post coupler, is defined as follows                       Dx=EIE„,x-E.,olx100 (%)(1 ) x n,o         where E„,0 is the original on-axis electric field of the n-th cell, and En.x is the on-axis        electric field of the n-th cell for 4f end cell perturbation with post coupler at the        length of x. Measured field of each cell was divided by the average field of the        4th, 5th and 6th cells for normalization.                                       IV. RESULTS           For a first series of measurements post couplers were installed opposite each         drift tube of 10 cm diameter. Successive couplers were located at alternate side of        the tank. The on-axis field distributions were measured for +1 MHz, +2 MHz        and —1 MHz end cell perturbations. Figure 3 shows a plot of Dx as a function 1.2 A                                         1•1- T a' Q Lp 1.0-. LJ5Y-                                       H- A 0 0.9- /                LIJ° - original !LPerturbed Stabilized                    0'812 3 4 5 6 7 8 9                                NO. of CELL                        Fig. 4. On-axis field distribution. Drift tubediameter is 10 cm.                                   ( 12 )                 Field Stabilization of the Alvarez Linac by Post Coupler  '                   —+1 MHz  X.,----  -F2 MHzPAI            ID        f:100° -- –1 MHz;Io  w?/   wc %I Q                a. / 50 00    H.° 05 1015 19                  POST COUPLER LENGTH (cm) 0 40° — +1 MHz      oIo A ---- -I-2 MHz     w 30is'i ° --_ –1 MHz    Q'A2O-P\ab;z\PO~a'qI \G~~a•      F–10afl~,. t.o--'_ —.   ccO F—.3 d        0 o 1415 16 17                  POST COUPLER LENGTH (cm)           Fig. 5. Distortion parameter, Dx , as a function of post coupler length for +1 MHz, +2 MHz and —1 MHz end cell perturbations. Drift                  tube diameter is 6 cm. of post coupler length. At 14.5 cm post coupler length the on-axis field distribu-tion was stabilized. Figure 4 shows the on-axis electric field of each cell for the orig-inal, +2 MHz end cell perturbed and stabilized distributions respectively. For 6 cm diameter drift tube the same measurements as previously described were done. Figure 5 shows a plot of Dx as a function of post coupler length. At 15.0 cm post coupler length the on-axis electric field distribution was stabilized. Figure 6 shows the on-axis electric field of each cell same as Fig. 4.    For a second series of measurements we installed eight, four or two post couplers for +2 MHz end cell perturbation. As the case of eight post couplers they were installed opposite each drift tube, the case of four opposite the odd numbered drift tubes, and the case of two opposite the 3rd and 6th drift tubes. Successive couplers were located at alternate side of the tank for all the cases. Figure 7 and Figure 8                            ( 13 )                             M.  SAWAMURA and H. TAKEKOSHI                      1.2 L_G L CD G/ G 1.0a E— a Z a u) 0 J G'                                0.9°.'                                   •— 0riginal                                         •---- Perturbedo - Stabilized               0.81 2 3 4 5 6 7 8 9                        NO. of CELL               Fig. 6. On-axis field distribution. Drift tube diameter is 6 cm.              cp40-                             o-~l  ~30 G     20-°-L         qo 4 ~'                                                               tr              10°-8 A ----4 Post Couplers ---2 U) -----------------------------------------------------      13 14 15 16                  POST COUPLER LENGTH (cm)          Fig. 7. Distortion parameter, Dx , as a function of post coupler length with                  eight, four or two post couplers for +2 MHz end cell perturbation.                  Drift tube diameter is 10 cm. show polts of Dx in the case of drift tube diameters of 10 cm and 6 cm respectively. For eight, four, and two post couplers of 10 cm diameter drift tube field stabilizations were achieved at the post coupler length of 14.5 cm, 14.3 cm and 13.2 cm respec-tively, and for 6 cm diameter drift tube at the post coupler length of 15.0 cm, 14.3 cm and 13.4 cm respectively. ( 14 )                    Field Stabilization of the Alvarez Linac by Post Coupler  ~40_c - C` '30I~,1 s   Q u L:                                                                     Li Y o~~,/\\ ~,4-a of ~10 a\ o 8                                                    ----4 Post Couplers --- 2       13 14 15 16                   POST COUPLER LENGTH (cm)               Fig. 8. Distortion parameter, Dx , as a functionofpost coupler length with                     eight, four or two post couplers for +2 MHz endcellperturbation.                     Drift tube diameter is 6 cm.                              V. DISCUSSION      It was reported that the stabilization should be possible if the distance between   the drift tube and the tank wall is between and e1.03 times quarter-wave   length of operating frequency in safety.2) In our measurements the distance was   1.00 time quarter-wave length in type 1 and 1.12 times in type 2. So in both types   stabilization was achieved, that N1.1 times quarter-wave length could be chosen for   stabilization in the case of short tank length, but it is not sure in the case of long     one.       As the number of post couplers was decreased, the post coupler length for sta-  bilization became shorter and the range of stabilization was more limited.      Although the drift tubes of 10 cm and 6 cm diameters were used in the experi-  ments, the post coupler length for stabilization was nearly equal in both cases re-  gardless of the number of post couplers. When the stabilization was achieved in these   experiments, the capacitance between post coupler and drift tube changed much but   the inductance of post coupler changed not much. And so to stabilize the field dis-  tribution by post couplers the inductance of post coupler might be more important   than the capacitance between post coupler and drift tube. But if the capacitance   is too small i.e. the end of post coupler is too far from the drift tube, the radial electric   field between the end of post coupler and drift tube is too small to cause the enough   energy flow along the axis. So if the distance between the drift tube and the tank   wall is much different from the quarter-wave length, field stabilization might not be   possible.                               REFERENCES (1) E.A. Knapp, J.M. Potter, E.J. Schneider and D.A. Swenson, unpublished. (2) J.H. Billen, LANL memorandum. (3) Y. Katayama and H. Takekoshi, Bull. Inst. Chem. Res., Kyoto Univ., 61, 1 (1983). (4) J. Ungrin, S.O. Schriber and R.A. Vokes, IEEE Trans. on Wad. Sci., 30, 3013 (1983). ( 15 )

Field Stabilization of the Alvarez Linac by Post Coupler

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Field Stabilization of the Alvarez Linac by Post Coupler

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