RF System for Super KEKB K AKAI Jan

RF System for Super. KEKB K. AKAI Jan. 20, 2004 Super. B Factory Workshop Hawaii Contents: ◆ Base plan ◆ ARES and SC cavities ◆ High beam current and measures ◆ Crab cavity ◆ Construction Jan. 20, 2004 RF System for Super. KEKB (K. Akai)

Base plan • KEKB RF system – Excellent performance at 1. 8 A (LER) and 1. 2 A (HER) shows potential for operating beyond the design beam current of KEKB. • Strategy for Super. KEKB: – Adopt the same RF frequency as KEKB and use the existing RF system as much as possible, with improvements as necessary to meet the requirements for Super. KEKB. • If this scheme is feasible: – Construction cost is greatly reduced. – Technical uncertainties are relatively small. • Important issues: – Very high beam current (4 times as KEKB) – Short bunch length of 3 mm. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 2

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RF parameters for Super. KEKB Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 7

Problems to be solved • Strong longitudinal CBI due to a large detuning of the accelerating mode, even with ARES and/or SCC. – Growth rate = (0. 3 ms)-1 • CBI due to HOM and other parasitic modes • Large HOM power in each cavity – HOM dampers • Strengthening of RF power is required – 4 times as high as KEKB Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 8

Measures for CBI due to fundamental mode • Two measures will be taken: • (1) Modify LER-ARES – Remodel the A-C cavity of the ARES to increase the stored energy further. – The -1 mode growth rate is reduced from (0. 3 ms)-1 to (1. 6 ms)-1. • No modification on the HER-ARES: majority of the driving impedance in HER is attributed to SCC. • (2) RF feedback (-1 mode damper) – The growth time of 1. 6 ms (LER) and 1 ms (HER) is still faster than the radiation damping time by a factor of 20. – The instability will be suppressed by the -1 mode damper. – The -2 mode also needs to be suppressed by the feedback. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 9

Modification of LER-ARES • The ARES in LER will be remodeled to increase the stored energy further. • By enlarging the coupling hole between the A-C cavities, Us/Ua will be increased from 9 to 15. • Storage cavity is reused. Coupling impedance for the p/2 mode T. Kageyama, et. al. exsisting modified Energy ratio 1: 9 1: 15 Detuning (k. Hz) 65 45 Growth time (ms) 0. 3 1. 6 C-damper (k. W) 41 26 Jan. 20, 2004 Growth rate as a function of Us/Ua RF System for Super. KEKB (K. Akai) 10

The existing -1 mode damper at KEKB • The -1 mode CBI occurs at more than 1 A in KEKB-LER. • It is suppressed by the -1 mode damper. • The damping time of 1 ms is required for Super. KEKB. FB OFF FB ON S. Yoshimoto, et. al. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 11

CBI due to the 0 and p modes of ARES Top: Re [Z//] of the 0 and p modes Bottom: Re [Z//] imbalance R+-R- • The large detuning of the A-cavity gives rise to imbalance of the 0 and p mode impedance. • Longitudinal CBI can be excited. The growth time is 4 ms. • It is outside the bandwidth of the klystrons. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 12

Summary of CBI due to RF cavities Longitudinal bunch-by-bunch FB is needed. Required damping time is 4 ms. Transverse The instability can be suppressed by the present transverse bunch-by-bunch FB. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 13

ARES HOM dampers Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 14

HOM power of LER-ARES r GB Pd am pe KEKB W G da m pe r Super. KEKB Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 15

Improve the ARES-HOM dampers • The waveguide dampers – – High power tested up to 3. 3 k. W/bullet (26 k. W/cavity). Upgrade needed to 80 k. W/cavity. Will be tested at higher power with a new high power source. The number of bullets/waveguide will be increased. • The grooved beam pipe dampers – High power tested up to 0. 5 k. W/groove (2 k. W/cavity). – Upgrade needed to 20 k. W/cavity. – A new type of damper? Such as a winged chamber with Si. C bullets? Y. Suetsugu, et. al. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 16

SCC HOM power and beam pipe Beam pipe diameter 150 mm (present) 220 mm (enlarged) Loss factor for 3 mm bunch 2. 46 V/p. C 1. 69 V/p. C HOM power for 4. 1 A, 83 k. W/cavity 5000 bunches 57 k. W/cavity Influence to other groups Replace chambers Large bore magnets Develop gate valves No change • Present HOM dampers in KEKB have been operated up to 12 k. W/cavity. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 17

Improve the SCC-HOM damper • The present HOM damper will be bench tested to see its performance limit. • The points are effective cooling, surface temperature, and outgas. • If SBP side damper works at 20 k. W, the present dampers may be used for Super-KEKB. If not, modification or new design of dampers is necessary. • In addition, the beam pipe diameter will be changed to 220 mm to reduce the loss factor. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 18

Strengthening of RF power • Required RF power provided to beam is 18 MW (LER) and 16 MW (HER), four times as high as those of KEKB. • The required RF voltage is relatively low. • The number of cavities should be kept as small as possible to reduce the total impedance in the ring. • Change to one ARES/klystron configuration. – KEKB: two ARES/one klystron • The input power to each cavity will be nearly doubled. • The number of klystrons will be more than doubled. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 19

Loss factor and Number of RF units • Required number of RF units is expressed as: 28 unit kothers is loss factor except cavities, and Pbo is beam power by each unit. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 20

Input couplers • Performance at KEKB – Operating typically at 300 -350 k. W/coupler (ARES, SCC). – The ARES coupler tested up to 950 k. W (through). – The SCC coupler tested up to 800 k. W (through), 300 k. W (total reflection). • Requirement for Super. KEKB – Operation at 900 k. W/coupler (ARES), 500 k. W/coupler (SCC). • Plans and prospect – The present couplers can be probably used in Super. KEKB. – Improvements and tests planned. • A new high power test setup is being constructed. • R&D to suppress multipactoring (Ti. N coating) is going on. • Increasing the operating power of SCC > 400 k. W in 2004. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 21

Number of RF units KEKB LER Oho Fuji Nikko HER D 4 3 D 5 3 LER HER 14 8 D 7 5 10 D 8 5 10 2 D 10 4 6 D 11 4 6 Total Jan. 20, 2004 Super. KEKB 24 RF System for Super. KEKB (K. Akai) 56 22

New buildings needed Building for Power Supply (hight=5 m) Control room Schedule D 4 455 m 2 (35 m× 13 m) 170 m 2 2005〜 06 D 5 - - D 7 273 m 2 (21 m× 13 m) 100 m 2 2005〜 06 D 8 304 m 2 - 2005〜 06 〜 2009 (16 m× 13 m+12 m× 8 m) D 10 81 m 2 (9 m× 9 m) 50 m 2 D 11 - - Total 1113 m 2 320 m 2 Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 23

Construction plan • Before 2008 – Construct 14 units of RF system • To change to 1 ARES/1 klystron configuration – 2 RF stations for Crab crossing experiment @Nikko • After 2008 – – Construct 18 units of RF system Fabricate 10 more ARES’s Fabricate 4 SCC’s Construct RF system for Crab cavities Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 24

Schedule Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 25

Crab crossing Palmer for LC (1988) Oide and Yokoya for storage rings (1989) Recent simulations by Ohmi showed significant increase of luminosity by several times by the crab crossing. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 26

Parameters for the crab crossing Plan Ring KEKB Super. KEKB Backup scheme Adopted as baseline LER HER Beam energy (Ge. V) 3. 5 8. 0 Beam current (A) 2. 6 1. 1 9. 4 4. 1 RF frequency (MHz) Crossing angle (mrad) βx* (m) βx, crab (m) Required kick (MV) Jan. 20, 2004 508. 887 ± 11 ± 15 0. 33 0. 2 20 100〜 200 300〜 400 1. 41 1. 44 1. 10 〜 0. 78 1. 46 〜 1. 26 RF System for Super. KEKB (K. Akai) 27

Damping parasitic modes • Accelerating cavities • Crab cavity – Operating mode (TM 010) is the lowest frequency mode. – Any parasitic mode (HOM) has higher frequency than the operating mode. – Wave guides or beam pipe with cut-off frequency higher than the operating mode can damp all HOM’s. (ARES, SCC, PEP-II cavity, etc) Jan. 20, 2004 – Operating mode (usually TM 110) is NOT the lowest frequency mode. – There exists lower frequency parasitic mode(s). – Wave guides can damp higher frequency modes. However, special care is needed for the lower frequency modes. RF System for Super. KEKB (K. Akai) 28

Original crab cavity • Squashed cell operating in TM 2 -1 -0 (x-y-z) • Coaxial coupler is used as a beam pipe • Designed for B-factories (1〜 2 A) Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 29

Crab cavity with 10 A beam • The original cavity is designed for 1 〜 2 A beam – Simple structure, suitable for SC → High kick voltage is obtained by one cavity. – Sufficient damping of parasitic modes. – Not necessarily optimized for a 10 A beam. • Possible problems at 10 A – Large HOM power (200 k. W) • Loss factor is not very small, because the radius of coaxial beam pipe can not be widely opened. • Additional loss factor comes from the absorber on wide beam pipe. – Much heavier damping of HOM’s may be needed, particularly for horizontal polarization of transverse modes (large bx_crab). • A new crab cavity has been designed, which can be used at 10 A. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 30

Design concept of new crab cavity • Squashed cell operating in TM 2 -1 -0 (same). • Waveguide dampers are attached. – Much heavier damping of parasitic modes can be obtained. – Larger HOM power is allowed for the dampers. • Coaxial dampers are used, but not as a beam pipe. – The TM 1 -1 -0 is well damped. – Loss factor is reduced since widely opened beam pipes can be used. • Optimization has been carried out for both the SC and NC versions independently. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 31

Schematic drawing of new crab cavity • • (Left) The cross-shaped waveguide dampers and coaxial dampers are attached at the squashed-cell. (Right) Cross section of the coaxial damper at the cut plane of Y-Y’. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 32

New crab cavity (1/4 cell) K. Akai and Y. Morita, to be submitted. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 33

Coupling impedance Ohm/m Ohm Crabbing mode Lower frequency mode Horizontal Longitudinal Original New(SC) New(NC) unit Highest value of ZT (H) 25. 0 5. 6 4. 4 k. W/m Highest value of ZT (V) 15. 0 3. 2 10. 1 k. W/m Highest (Z// x freq. /GHz) 2070 1020 760 W GHz Kloss@3 mm 0. 73 0. 56 V/p. C Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 34

Comparison of cavities for Super. KEKB New crab cavity has advantages for Super-KEKB, especially in LER. Original crab cavity could be used in HER, if HOM absorber is OK with 50 k. W. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 35

Summary of new crab cavity • Parasitic modes are sufficiently damped. – The lower frequency mode is also damped. – Instability can be suppressed with the present FB system, even at a beam current of 10 A. • The problem for the dampers due to a large parasitic power is greatly eased: – Loss factor of the cell is reduced. – The absorbers do not contribute to the loss factor. – The parasitic power to each damper is further reduced since it is divided into the waveguide and coaxial dampers. • The new crab cavity can be used in Super. KEKB. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 36

Summary of Super. KEKB RF system • Base plan: – The existing RF system will be used as much as possible, with improvements as necessary. – The ARES (LER, HER) and SCC (HER) will be used. • CBI due to the accelerating mode – LER-ARES will be modified, that eases the growth time from 0. 3 ms to 1. 6 ms. – The -1 mode damper will suppress the CBI with a growth time of 1 ms. • HOM dampers – Performance limit of the present HOM dampers will be tested. – A new HOM damper may by necessary, particularly for the GBP damper. • Large RF power – Improvement of the couplers will continue to double the operating power. – The number of RF unit will be doubled. • Crab cavity – A new crab cavity is proposed, which can be used at 10 A. – The design is completed. It has sufficient property for Super. KEKB. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 37

RF system for Damping Ring • Base plan assumed • RF-related parameters – Same RF frequency as KEKB – Use ARES (full set) • Construction Bunch charge Number of bunches – Fabricate a klystron and an ARES cavity. – An existing power supply (B-type) will be moved. – High-power and low-level system: partly new, partly reused. – Total cost is about 2. 4 Oku-en. (Building is not included. ) Circumference Beam current 4 (2 x 2) 131. 3 m 23 m. A Energy loss/turn 0. 073 MV RF frequency 508. 9 MHz RF voltage 0. 261 MV Wall dissipation 42 k. W Beam power 1. 7 k. W Number of cavity Jan. 20, 2004 2. 5 n. C RF System for Super. KEKB (K. Akai) 1 38

Impedance-related issues Loss factor of LER (Suetsugu, Shibata, Stanic, Kageyama, Akai) Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 39

Cost estimation • Total cost = 111. 2 Oku-yen, including – – – – – 32 klystrons 15 power supplies Evaporative cooling system for klystron collector 32 High-power and Low-level systems 20 existing ARES’s to be modified 10 new ARES’s for LER 4 additional SCC’s for HER RF system for Crab cavities R&D and Beam tests • Cost for related infrastructures such as buildings, electricity, cooling system are not included. Jan. 20, 2004 RF System for Super. KEKB (K. Akai) 40