Dewa, Hideki

Fujita, Hirokazu

Ikegami, Masanori

Inoue, Makoto

Iwashita, Yoshihisa

Kakigi, Shigeru

Kando, Masaki

Noda, Akira

Shirai, Toshiyuki

Kyoto University Research Information Repository

Title Space Charge Dominated Beam Transport and MatchingSection between Ion Source and LinacAuthor(s)Noda, Akira; Ikegami, Masanori; Iwashita, Yoshihisa; Kakigi,Shigeru; Kando, Masaki; Shirai, Toshiyuki; Dewa, Hideki;Fujita, Hirokazu; Inoue, MakotoCitationBulletin of the Institute for Chemical Research, KyotoUniversity (1994), 72(1): 12-19Issue Date1994-03-31URL http://hdl.handle.net/2433/77554RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityBull. Inst. Chem. Res., Kyoto Univ., Vol.72, No.1, 1994         Space Charge Dominated Beam Transport and         Matching Section between Ion Source and Linac         Akira  NODA*, Masanori IKEGAMI*, Yoshihisa IWASHITA*,Shigeru KAKIGI*,          Masaki KANDO*, Toshiyuki SHIRAI*, Hideki DEWA*, Hirokazu FUJITA*                              and Makoto INouE*                               Received February9, 1994   The low energy (50 keV) beam transport line from the ion source to the RFQ whose beam intensity had been limited at —1 mA by space charge repulsion has been redesigned to attain the beam current up to —20 mA. The main modification is to utilize the smooth focusing with rather large beam size, replacing some hardware with wider beam aperture. Perfect matching in transverse phase space is also pursued with the use of permanent solenoidal fields, which is expected to improve the beam transmission through the RFQ linac. KEY WORDS: Space Charge Repulsion / Phase Space Matching / Axial Symmetry / Twiss              Parameter                                   1. Introduction    The low energy beam transport line from the ion source to the entrance of the RFQ has been operated since 1991 at the ion linear - accelerator of Institute for Chemical Research, Kyoto University1 . In the existing system, the transverse phase space matching is incomplete and beam transmission through the RFQ is limited below 74%. Further, the system utilizes such an ion optics as be focused into a very small beam size in order to cope with the small gap (35 mm) of the Mixing Magnet, which increases the space charge repulsion and, therefore, results in beam blow up. From the reasons above mentioned, the beam line has an upper limit of transmittable beam current of —1 mA2). In order to improve this situation, upgrading of the transport line is started assuming the design current of 20 mA for the time being. At this in-tensity, beam behavior affected by a non-linear effect due to space charge force is expected to become apparent in the linear accelerator. In the present paper, design of the ion optics for this upgrading is presented together with the hardware modifications.                                 2. Beam Optics    From the design of the RFQ, Twiss parameters at the entrance of the RFQ are required to have the values as listed up in Table 1. As the starting values of these parameters at the position of the Einzel lens shown in Fig. 1, we used the ones also given in Table 1 assuming *RP® IN E. fia, F 71c N. ViPEJ Mc, #;;,Z, "77XE, ~fF~(A #~ 4A  Nuclear Science Facility, Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan ( 12 )                        Space Charge dominated Beam Transport the parallel beam both in horizontal and vertical directions.    The four vanes of the  RFQ are tilted by 45° for easiness of attaching the evacuation port to the cavity and transverse focusing by the RF electric field is decomposed into x and y directions shown in Fig. 2. On the other hand, existence of the Mixing Magnet defines the hori-zontal plane as the orbital plane. In order to avoid the mixing between betatron oscillations in horizontal and vertical directions, it is needed to make a round (i.e. axially symmetric) beam            433 MHz ---- KLYSTRCN ----------------------------------- ,o .                                               o•0 0 O .0 BeamO              Profile, Einzel          Monitor, LENS ION         ~~'&Faraday              Cup 4 i I °Faraday                  aSOURCE                  Faraday Cup 1 '~%A      i r Cup 3~^a'o.     r--~ ---'           r 7r:yvih,!,•                              XElectrostatic Quadrupoles     .aa'l1sins.'Qm,I."-(Triplet--ESQ)     IumismaluuI%r1.Mixing                ~~M agnet              Electric•Faraday Cup 2   Solenoid with Solenoid,/ Electrostatic Quadrupoles    Permanent Magnets(doublet--QF1,001)   (PSOL1, PSOL2)                                                                  .                                                                       °         nCElectrostatic Quadrupoles      RF )(d oublet--QF2,QD2) Fig. 1. Layout of the Low Energy Beam Transport System. Transverse Phase Space              matching is made between the center of the Einzel lens and the entrance of the              RFQ.            Table 1. Twiss Parameters at the starting and end points of the transport line            Starting Point (Center of Einzel)End Point (Entrance of RFQ)'        Beta-functionAlphaBeta-functionAlpha 3.1 m00.057 m2.508  ( 13 )                  A. NODA, M. IKEGAMI, Y. IWASHITA, S. KAKIGI, M. KANDO, T. SHIRAI, H.DEWA                             Vertical                               Direction                             450           Y X Beam  Direction 45°                                   Horizontal                    45°N Direction                          _tt Fig. 2. Definition of x and y directions with respect to horizontal and vertical direc-                 tions. If the horizontal and vertical directions are 45 degrees rotated counter                 clockwise around the beam direction viewing from the unstream, they will                 coincide with x and y directions respectively.    downstream from the electric solenoid to the entrance of the RFQ utilizing only axially sym-   metric optical elements in this region. For this purpose, the transverse phase space acceptance    is traced back to the entrance of the electric solenoid as shown in Fig. 3 (a) for zero beam cur-   rent with use of the computer code TRACE-3D3', where the strengths of the solenoidal fields    made by permanent magnets, PSOL1 and PSOL2 (both 60 mm in length) , are assumed to be    4600 G and 6600 G, respectively.        In order to make matching throughout the entire beam transport line, transverse matching    is pursued with two doublets of the electrostatic quadrupoles (4 parameters) . As it is difficult    to install focusing element just upstream the Mixing Magnet, edge focusing of 23° in vertical    direction is adopted. In Fig. 3 (b) , the transverse phase space matching between the positions    of the Einzel lens and the electric solenoid for zero beam current is shown. Combining these    two parts in real beam direction, the entire beam transport line is composed and matching condition for this section is searched for various beam intensities below 20 mA and such an    ion optics is searched, as is not changed so largely and beam blow up does not occur for these    beam currents. The beam envelope of the entire "Low Energy Beam Transport" is shown in    Fig. 3 (c) for the beam current of 20 mA as a typical example.                               3. Improvement of the Hard-Ware        So as to realize the ion optics described in the previous section, the aperture of the ion-                             ( 14 )                     Space Charge dominated  Beam  Transport NP1- 1NP2- 10 50.000 mm 200.0 DesHoriz, Longitude OLSOL 1__2AM78 910                      VertLength- 475.00mm                     (a) NP1- 1NP2= 26 50.000 mm 200.0 DegHoriz, Longitude      G 0E B Er11410 °I 12 2436 9 11011112131415 161_17_ Si 21 228~b 26 VertLength- 2701.69mm                    (b) NP1- 1 NP2- 34  50.000 mm 200.0 DegHoriz, Longitude 1 • 1.1 air -S e                        VertLength- 3176.69mm  (C) Fig. 3. Beam envelopes at (a) the axially symmetric final stage, (b) other region than          (a) and (c) the entire Low Energy Transport line. (a) and (b) are calculated           at the designing stage and beam current of 0 mA is assumed to make the de-          sign process easy. (c) is calculated after the compositionof the ion optics is           fixed and beam current of 20 mA is assumed. ( 15 )             A. NODA, M. IKEGAMI, Y. IWASHITA, S. KAKIGI, M. KANDO, T. SHIRAI, H. DEWA optical elements are checked and the Mixing Magnet is decided to be replaced by the one with wider gap size (60 mm) . The power supply for the magnet is not to be changed and the new magnet is required to be excited to the necessary magnetic field (1.67 kG and 2.37 kG are re-quired to bend H+ and H2+ as large as 45°, respectively) with the old power supply (20 A, 15 V). The magnet is required to be excited up to the level which can deflect H2+ (molecular hydrogen) as large as 45° so as to observe abundance of H+ relative to Hz+.    The newly fabricated Mixing Magnet is shown in Photo 1. The main specifications of the magnet are given in Table 2. The excitation characteristics of the magnet is shown in Fig. 4 and the field distributions along the central line at the entrance and the exit of the magnet are shown in Fig. 5 (a) and Fig. 5 (b) , respectively.                             4. Present Status and Summary    The low energy transport line with transverse phase space matching which can deliver the beam with the intensity of 20 mA is newly designed based on the existing ion optics in 1, 4`f •                                                                         gi.                            ~yi,        Photo. 1. An overall view of the newly fabricated mixing magnet with wider gap (60 mm) .                    Table 2. M in Specifications of the New Mixing Magnet       Magnet Gap60 mm               Maximum Excitation Current20 A        Maximum Field2.7 kG         Coil Impedance0.65 f2         Deflection Angle45°             Radius of Curvature0.193 m                 Flow Rate of Cooling Water4 //min.     Weight192 kg  ( 16 )                        Space Charge dominated Beam Transport  4.0 -------------------------------------------------------------------------- I I I I - 3.5 ——    ,_3.0 —— N-- N--               7                    co Y 2.5 ——     v -            d                Jr.                  2.0 _  y --                c              •1.5 — 1.0 — 0.g  I I I I -                  0.0 10.0 20.0 30.0 40.0 50.0                            Excitation Current (A) Fig. 4. Excitation characteristics of the newly fabricated Mixing Magnet with wider             gap of 60 mm. most part. The main change for the new line is replacement of the Mixing Magnet and its chamber to enlarge the beam aperture and addition of solenoidal fields made by permanent magnets to realize transverse matching with the acceptance of the RFQ.    The mixing magnet has been just fabricated and is found that it can safely excite the needed field with the existing power supply. The new vacuum chamber is to be fabricated based on the new magnet to accommodate the beam of larger size as shown in Fig. 3.    Among the solenoids made by permanent magnets, the one to be used just before the entr-ance of the RFQ (PSOL2) has already been fabricated and is reported elsewhere. Another one (PSOL1) is to be fabricated from now on based on the experience of the first fabrication and the ion optics described here.    Before the completion of the new high current beam transport line, another a few months will be needed.                                Acknowledgement    The authors would like to thank Mr. K. Takano of Takano Giken Co. Ltd., for his sincere collaboration during the process of fabrication of the new Mixing Magnet. They are also grate-ful to Mr. I Kazama for his continuous assistance during the work. ( 17 )               A. NODA, M.  IKEGAMI, Y. IWASIIITA, S. KAKIGI, M. KANDO, T. SHIRAI, H. DEWA     B(kG) 3 --      20 AI Boundary of                                      Iron Pole 15AI _I    2 10 A 5A 1 •  I1 ~I -500 50100s(mm) If ii//!//11              1J(a)   B(kG) Il CQQ 3 Boundary of       20 A  Iron Pole15 A I 2 10 A 1 -SA    -50 0 50100s(mm) Y/ / ' /                      (b)         Fig. 5. Fringing field distribution of the new Mixing Magnet at the entrance (a) and              the exit (b).                                         References (1) A. Noda, et al., "Improvement of the Proton Accelerator System", Bull. Inst. Chem. Res., Kyoto Univ.,       Vol. 70, No. 1, pp37-44 (1992) . ( 18 )                          Space Charge dominated Beam Transport  (2) A. Noda, et al., "Characteristics of the Ion Source and Low Energy Beam Transport of the Proton        Linac at ICR", Bull. Inst. Chem. Res., Kyoto Univ., Vol. 71, No. 1, pp6-14 (1993).(3) K. R. Crandall, "TRACE 3-D Documentation", Los Alamos National Laboratory preprint, LA-11054-MS, UC-32 and UC-28 (1987). (4) Y. Iwashita, "Axial Magnetic Field Lens with Permanent Magnet", to be published in Proceedings of        1993 Particle Accelerator Conference, Washington D.C., U.S.A. ( 19 )

Space Charge Dominated Beam Transport and Matching Section between Ion Source and Linac

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Space Charge Dominated Beam Transport and Matching Section between Ion Source and Linac

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