KEKB Status and Upgrade Plan with Crab Crossing

Transcription

1 KEKB Status and Upgrade Plan with Crab Crossing Second Electron-Ion Collider Workshop March 16,24 Mika Masuzawa, KEK 1

2 Contents 1. Introduction 2. Machine Performance 3. Key Issues for High Luminosity 4. Upgrade Plan with Crab Crossing 5. Summary 2

3 1. Introduction Beam energy 8GeV (electron, HER ) 3.5GeV (positron, LER ) Circumference 316 m Use TRISTAN tunnel RF system f RF ~ 59MHz ARES (LER) ARES+SCC (HER) The construction of KEKB began in 1994, and was completed in November Commissioning started in Dec

4 Machine Parameters (12/18/23) Ring LER HER Horizontal Emittance (nm) 18 (18) 24 (18) Beam Current (ma) 153 (26) 1132 (11) Number of bunches 1281 (~5) Bunch Current (ma) 1.17 (.52).884 (.22) Number of Bunch Trains 1 Horizontal Beam Size@IP(µm) σ* x Vertical Beam Size@IP(µm) σ* y Emittance Ratio ε y /ε x Beta function@ip β x *(cm)/β y *(cm) 59/.58 (33/1) 56/.7 (33/1) Beam-beam parameters ξ x /ξ y.14/.69 (.39/.52).71/.53 (.39/.52) Beam lifetime at collision (minutes) 125 at 153mA 216 at 1132 ma Peak luminosity (/nb/s) 11.6 (1) ( ) Design values 4

5 IR Vertical focusing by a pair of superconducting Q-magnet (QCSL/QCSR). Extra vertical focusing by QC1L/R for the electron beam. One beam must go off axis due to the finite crossing angle at the IP. To minimize the flux of SR through the IP, the incoming positron (electron) beam orbit is set on the axis of QCSL (QCSR). Superconducting solenoid magnets SL/R used for compensating the detector solenoid field. 5

6 2. Machine Performance Continuous injection

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8 Best 24 hours 8

9 3. Key Issues for High Luminosity The history of the KEKB commissioning has been a struggle with issues such as: A) High beam currents Problems with hardware components Instability B) Single beam blowup due to the electron cloud instability Solenoid winding C) Beam-beam blowup Tune survey Optics corrections Other tuning knobs (X-Y coupling at IP etc.) 9

10 A) Problems due to High Beam Currents Problems with vacuum components HOM heating and damage Bellows, gate valves Arcing of components due to HOMs or wall currents Movable masks, RF shield at injection septum Direct damage by beam Movable masks, beam abort chamber Heating and vacuum leak due to SR IR chambers, Helico-flex gaskets 1

11 Damaged vacuum components Damaged RF shield fingers Damaged movable mask 11

12 A) Problems due to High Beam Currents Instability sources Fast ion (electron ring) Serious at high vacuum pressures Can be suppressed by bunch-by-bunch FB Electron cloud (positron ring) Can be suppressed by solenoid field RF cavity Beam current is not limited by instabilities due to RF cavity. We use -1 mode damper (with comb-filter) to suppress instability from the fundamental mode. We do not need longitudinal bunch-by-bunch FB. Others Dust trapping 12

13 B) Single beam blowup due to the electron cloud instability H.Fukuma ECOULD

14 Electron Cloud (EC) 1999/4 Single beam blowup was observed by SR monitor. 1999/1 Electron cloud hypothesis was proposed by K.Oide. 1999/11 C-yoke permanent magnets were installed in attempt to cure the blowup problem. 2/3 Additional C-yoke magnets were installed. 2/5 Head-tail instability model by K.Ohmi & F.Zimmermann was proposed. 2/9 C-yoke magnets were replaced by solenoid magnets (~28 sections). 2/12 Effectiveness of the solenoids in improving luminosity was confirmed. 21/1 195 additional solenoid magnet sections were installed. 21/9 345 additional solenoid magnet sections were installed. 21/11 Peak luminosity reached 5.5 nb/s. H.Fukuma ECOULD

15 C-yoke permanent magnets 15

16 Electron-cloud-suppression solenoids 16

17 Electron-cloud-suppression solenoids Total length of solenoid preliminary (very rough estimation) 32 circumference total drift length 2 4th 5th Total length(m) nd 3rd 8 1st Bz > 2 G 4 Sep. Jan. 1 Apr. 1 Sep. 1 Jan. 2 Date Effect on luminosity confirmed 17

18 LER Beam size (σ y ) 5 21 July : off 1 train, 1153 bunches, 4 rf bucket spacing Vertical beam size@ip (micron) July : on 21 Dec. : on 22 Feb. : on LER beam current (ma) 18

19 Effect of solenoids on luminosity 15 Effect of solenoid (after second installation) All on (7/Apr./1) Specific luminosity / bunch (1 3 cm -2 sec -1 ma -2 ) 1 5 NEG-bellows section off (9/May/1) All off (9/May/1) All on (9/May/1) L = I e 1 + 4π N f bunch Spec.Lum /bunch = I 2 σ i + i 1 xσ = 1 4πf y L N bunch e i + i N bunch 2 x y σ σ I b (LER) I b (HER) (ma 2 ) 19

20 C) Beam-Beam Blowup Tune survey Both simulations and surveys in real machine Horizontal tunes very close to the half-integer Need very fine control of tunes Optics correction (global correction) βfunctions Dispersion X-Y coupling Collision tuning knobs Waist points X-Y coupling at IP Dispersions at IP Orbit feedback around IP 2

21 Choice of Betatron Tunes & Specific Luminosity Specific luminosity was improved by 25%. 1/29/22 5/9/23 12/18/23 21 Simulations M.Tawada, et al.

22 Bunch-spacing problem Observations The specific luminosity depends on the bunch spacing. A longer bunch spacing gives a higher specific luminosity. Cause of the problem Not understood. 22

23 Comparison of specific luminosity/bunch with 3 and 4 bucket spacing More solenoids (~8m) I bunch (LER) * I bunch (HER) [ma 2 ] I bunch (LER) * I bunch (HER) [ma 2 ] L = I e 1 + 4π N f bunch Spec.Lum /bunch = I 2 σ i + i 1 xσ = 1 4πf y L N bunch i + i N bunch 2 x y e σ σ 23

24 4. Upgrade Plan with Crab Crossing Role of Crab Cavity KEKB Super-KEKB Strategy Backup scheme Adopted as baseline Ring LER HER LER HER Beam energy (GeV) Beam current (A) RF frequency (MHz) Crossing angle (mrad) ±11 ±15 βx* (m) βx, crab (m) Required kick (MV) The design luminosity of 1. x1 34 cm -2 s -1 has been achieved without crab crossing.

25 Crab crossing scheme RF Deflector ( Crab Cavity ) HER Electrons LER Positrons 1.44 MV Head-on Collision Crossing Angle (11 x 2 m rad.) 1.41 MV 1.41 MV 1.44 MV Palmer for LC (1988) Oide and Yokoya for storage rings :Phys,Rev.A4,315(1989) Recent simulations by Ohmi showed significant increase of luminosity with crab crossing. 25

26 Crossing angle Transformation from lab. Frame to head-on frame. ( ) * * * * * * * * * * 1 1 tan tan sin cos / cos / sin cos / ) tan ( ] sin [1 tan y x z z x z z z y y x x x x p p p p h h p p p x h z z p p x h y y h p p x h z x + + = + = + = = + = = + + = φ φ φ φ φ φ φ φ φ φ (φ: half crossing angle) Linear part 1 tan cos 1/ cos 1/ 1 cos 1/ tan 1 φ φ φ φ φ Oide and Yokoya for storage rings (1989) 26

27 Transverse kick by Crab Cavity x x ζ ζ 1 tan cos 1/ cos 1/ 1 cos 1/ tan 1 φ φ φ φ φ Crossing-angle term. Crab-cavity term 27 Crab cavity makes z dependent dispersion ζ x = φ at the IP, which cancels the crossing angle effect (φ << 1).

28 Simulation Head-on vs. crab-crossing (K.Ohmi) head-on crab-crossing Crab crossing restores the full luminosity of a head-on collision. 28

29 Simulation model Weak-strong K.Ohmi Many macro particles Fixed Gaussian Strong-strong Simulation of beam-beam effects in a circular e+ecollider Phys.Rev.E62 (7287)2 Many macro particles Many macro particles 29

30 Crab-crossing simulation -mrad vs. 11-mrad crossing angle (K.Ohmi) Weak-Strong model Strong-Strong model Bunch current Bunch current Beam-beam limit is ~.6 for 11 mrad half-crossing angle (both models agree well). -mrad (head-on) collision gives a higher ξ y. Beam-beam limit for -mrad crossing depends on the 3 model.

31 Parameters for 11 mrad crabbing.33m(her)/.33m(ler) V = ω rf cetanϕ β * x β x,crab 1m(HER)/2m(LER) V:voltage E:Beam energy β x* : beta-function at the IP β x : cra beta-function at the crab cavity ϕ:half crossing angle at the IP 31 (A.Morita MAC24) ω rf :the rf frequency of the cavity

32 Hardware for Crabbing (design) The TM11 mode is used to create timedependent horizontal rotational kicks to beam bunches. The TM11 mode is trapped within the cavity, while the other unwanted modes are extracted from the cavity module. 32

33 Crab Cavity Damping unwanted parasitic modes Accelerating cavities Operating mode (TM1) is the lowest frequency mode. Crab cavity Operating mode (usually TM11) is NOT the lowest frequency mode. Any parasitic mode (HOM) has higher frequency than the operating mode. Frequency of several parasitic modes can be lower than (or close to) the crabbing 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) Special cure is needed for the damping of parasitic modes. (K.Akai MAC24) 33

34 Analogy with rectangular cavity f n,m,l = c n 2a 2 + m 2b 2 + l 2d 2 a b d Rectangular (n-m-l) Cylindrical Beam coupling Mode TM1-1- TM1 monopole-like Accelerating mode TM2-1- TM11(H) dipole-like Crabbing mode TM1-2- TM11(V) dipole-like Unwanted crabbing TE-1-1 TE111(H) dipole-like Lowest TE-like TE1--1 TE111(V) dipole-like Lowest TE-like TM monopole-like TM dipole-like TE-2-1 TE211 quadrupole-like (K.Akai MAC24) 34

35 Squashed cell Extremely polarized cell Large eccentricity where horizontal size is about twice the vertical size (a ~2b). Frequency of the unwanted crabbing mode increases. Relatively short cell length (small d) The frequencies of lowest TE-like parasitic modes increase. Frequencies of all parasitic modes except the accelerating mode can be made higher than the crabbing mode. (K.Akai MAC24) 35

36 Crab Cavity Design for KEKB Squashed cell Coaxial coupler is used as a beam pipe (axial view) Absorbing material Notch filter Absorbing material Coaxial beam pipe Cooling for inner conductor inner conductor "Squashed cell" Squashed Crab cavity for B-factories 36 (K. Akai et al., Proc. B-factories, SLAC-4 p.181 (1992).)

37 Coaxial coupler with notch filter Monopole mode (including the Lower Freq. Mode ) Couples strongly and propagates in the coaxial line as TEM wave and can be guided out. Crabbing mode Couples as dipole-like, but does not propagate in the coaxial line, if the cut-off frequency of TE11 mode is higher than the crabbing frequency. Possible asymmetry or misalignment causes monopole-like coupling, which propagates in the coaxial line as TEM. A notch filter is attached to reject the TEM-coupled crabbing mode back to the cavity. absorber notch filter Possible location to attach coaxial coupler (K.Akai MAC24) 37

38 Crab crossing experiment at KEKB KEKB (LER) β cra b=4m V kick =1.47MV KEKB (HER) β cra b=2m V kick =1.51MV Or ig inal cavit y No. of caviti es 1 Grow t h ti me (hori zon t al) 36ms Grow th time (longitudinal) 96ms Total HOM pow er 23k W No. of caviti es 1 Grow t h ti me (hori zon t al) 33ms Grow t h t ime (longit ud inal) 415ms Total HOM pow er 6kW The crab crossing experiment in KEKB is planned in FY 25. The original crab cavity will be used for the experiment. (K.Akai MAC24) 38

39 Superconducting Crab Cavity Crab Cavity Group KEK Crab Cavity R&D Group K. Hosoyama, K. Hara, A. Kabe, Y. Kojima, Y. Morita, H. Nakai A. Honma, A. Terashima, K. Nakanishi MHI S. Matsuoka, T. Yanagisawa I.D. 24 Input Coupler I.D. 12 I.R. 9 I.D. 188 I.R I.R. 2 Coaxial Coupler 866 I.D. 3 Monitor Port scale (cm) K.Hosoyama (MAC 24) 39

40 Test Result of KEKB Crab Cavity #1 Design Esp (Surface peak electric field) 1 1 without coaxial coupler without coaxial coupler 2.8 K 4.2 K Q 1 9 with coaxial coupler Esp [MV/m] K.Hosoyama (MAC 24) 4

41 Multipacting in Crab Cavity with Coaxial Coupler K.Hosoyama (MAC 24) 41

42 Crab cavity R&D, production Fabrication and Surface Treatment RF Performance Test with a Coaxial Coupler Multipacting could be overcome by RF process. We have established these techniques! K.Hosoyama (MAC 24) 42

43 Crab Cavity Installation Plan Both crab cavities will be installed in the Nikko straight section because: There is some space for both rings. Cryogenic system is available. LER D1 D11 HER Crab Cavity for HER Crab Cavity for LER D1 Straight Section Transfer Line Connection Port LER e + e - HER Acc. Cavities K.Hosoyama (MAC 24) Crab Cavity 43

44 Crab Cavity Installation in JFY 25 (April 25-March 26) Test Plan Install one cavity in either the HER or the LER at the end of 25. Check hardware and modify if necessary. Install another crab cavity in the other ring in the summer of 26. Beam will be crabbed throughout the entire ring. What will happen? HOM, RF cavity? If everything is OK, luminosity doubles (?) 44

45 5. Summary KEKB has achieved its design luminosity of cm -2 s -1 without crab crossing. Crab cavities are planned to be installed in both rings by summer 26. Hardware preparation is going well. A doubling of the luminosity is hoped for. 45

46 KEKB Luminosity Projection 1 /ab appears on the horizon! Crab Cavity 18 /fb/mo. Beam Test (K.Oide MAC24) 46

47 H.Fukuma ECOULD

48 H.Fukuma ECOULD

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