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Nano-beam Scheme

Nano -Beam Scheme for SuperKEKB Andrew Hutton Slides taken from talks by Haruyo Koiso , Yukiyoshi Ohnishi & Masafumi Tawada. Nano-beam Scheme . The scheme proposed by P. Raimondi and SuperB Group. Squeeze  y * as small as possible: 0.27/0.41 mm.

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Nano-beam Scheme

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  1. Nano-Beam Scheme for SuperKEKBAndrew Hutton Slides taken from talks by HaruyoKoiso,YukiyoshiOhnishi & MasafumiTawada

  2. Nano-beam Scheme • The scheme proposed by P. Raimondi and SuperB Group. • Squeeze y* as small as possible: 0.27/0.41 mm. • Assume beam-beam parameter = 0.09 which has been already achieved at KEKB. • Change beam energies 3.5 / 8 -> 4 /7 GeV to achieve longer Touschek lifetime and mitigate the effect of intra-beam scattering in LER.

  3. Parameter Optimization Main parameters can be quickly optimized with this panel.

  4. Beam Parameters

  5. Colliding bunches Belle II New IR e- 2.6 A New superconducting /permanent final focusing quads near the IP SuperKEKB New beam pipe & bellows e+ 3.6 A Replace long TRISTAN dipoles with shorter ones (HER) Add / modify RF systems for higher beam current Low emittance positrons to inject Positron source Damping ring Redesign the HER arcs to squeeze the emittance New positron target / capture section Low emittance gun TiN-coated beam pipe with antechambers Low emittance electrons to inject x 40 Gain in Luminosity

  6. Comparison of Parameters SuperKEKB

  7. Beam Energy • In SuperKEKB, the beam energy is changed to decrease the effect of the intra-beam scattering in LER, especially tomake longer Touschek lifetime. • LER: 3.5 GeV → 4 GeV • HER: 8 GeV → 7 GeV • In HER, the lower beam energy makes lower emittance. SuperKEKB

  8. Luminosity If sx* = sx+* = sx-* and sy* = sy+* = sy-*, S = 1+sy*/sx* (aspect ratio of the beam size at IP). Three fundamental parameters to determine the luminosity. where Target Luminosity is 8x1035 cm-2s-1 in SuperKEKB. SuperKEKB

  9. Beam-Beam Parameter SuperKEKB

  10. Beam-Beam Parameter • The maximum value of the beam-beam parameter is assumed to be 0.09 in SuperKEKB. • The beam-beam parameter of 0.09 has been achieved in KEKB. • The horizontal beam-beam parameter becomes very small in the nano-beam scheme. xx~0.0028 • Dynamic effects is also small since the small horizontal beam-beam parameter and the horizontal betatron tune is far from 0.5. SuperKEKB

  11. Beam-Beam Simulation • Beam-beam simulation gives 8x1035 cm-2s-1 • Difference between with and without the crab-waist is not so large. W-S W-S L = 8x1035 LER LER SuperKEKB

  12. Vertical Beta Function at IP SuperKEKB

  13. Vertical Beta Function at IP • By using the modern technique, the vertical beta function can be squeezed to be a few hundred micron at most. • by* ~ 1/20 of KEKB • Hourglass effect (HG) is a serious problem to squeeze the beta function in the head-on collision. • In order to satisfy HG, a collision of narrow beams with a large crossing angle is adopted. This is “nano-beam scheme”. • However, dynamic aperture (DA) will be reduced because of the nonlinear fringe, and kinematic terms in the interaction region (IR), and also strong sextupoles. SuperKEKB KEKB Natural chromaticity LER: xx/xy = -104/-789 HER: xx/xy = -187/-853 LER: xx/xy = -72/-123 HER: xx/xy = -70/-124 SuperKEKB

  14. Collision Scheme High Current Scheme Nano-Beam Scheme sx* sz d 2f Head-on Half crossing angle: f overlap region = bunch length overlap region (≠ bunch length) Hourglass requirement Hourglass requirement ~5 mm ~200-300 mm SuperKEKB

  15. Hourglass Effect for Nano-Beam With crab-waist, this term is zero e- e+ 2f d1 There are two kinds of hourglass requirements. d2 2sx- 2sx+ LER waist Hourglass (H.G) requirement without crab-waist: SuperKEKB

  16. Beta Function at IP • Machine parameters are chosen to be insensitive to the HG effect. • The vertical beta function: • The vertical beta functions in both rings satisfy the HG requirement. • To squeeze by*, low emittance and low bx* are needed. SuperKEKB

  17. Nonlinear Terms around IP K L* Phys. Rev. E47, 2010 (1993)

  18. Beam Current SuperKEKB

  19. Beam Current • In order to achieve the target luminosity, the beam current in nano-beam scheme becomes twice of KEKB. • The highest beam current is 2 A in LER and 1.4 A in HER at the KEKB operation. • 1.8 A in LER / 1.35 A in HER at physics run • In SuperKEKB, the design beam currents are 3.6 A for LER and 2.62 A for HER. • Number of bunches is 2503 to get the suitable density of the overlap region (alternate buckets filled). • Bunch current becomes 1.44 mA in LER and 1.05 mA in HER. • Luminosity gain is 1(xy) x 20(1/by*) x 2(I) = 40 times of KEKB. • Then, we get 8x1035 cm-2s-1 SuperKEKB

  20. Crossing Angle • The full crossing angle is 83 mrad in SuperKEKB. • Separation of two beams is necessary to put final focusing magnets closer to IP as much as possible. This affects the magnet design and the dynamic aperture. • The angle between the solenoid axis and the beam line is 41.5 mrad, respectively. This angle is determined by optimization of the vertical emittance generated by SR from the solenoid fringe. SuperKEKB

  21. Emittance • Redesign of arc cells to make low emittance • 2.5 p non-interleaved sextupole cell (-I’ connection) • chromaticity is corrected and nonlinear kick is cancelled. • LER (4 GeV) • Longer bending radius is adopted. • HER (7 GeV) • Number of cells increases to reduce dispersion. (hx:dispersion) SuperKEKB

  22. Emittance • Local chromaticity correction (LCC) is necessary to correct large chromaticity generated in the vicinity of IP. • However, the LCC generates some emittance to make dispersion region. • The emittance generation at the LCC should be reduced as much as possible. This is a trade-off between DA and emittance. SuperKEKB

  23. Emittance • In SuperKEKB, the horizontal emittance is 3.2 nm in LER and 2.4 nm in HER, respectively. • 2.7 nm in LER and 2.3 nm in HER at the zero beam current • Coupling parameter of 0.4 % in LER and 0.35 % in HER are assumed. (challenging!) • Vertical emittance induced by SR from solenoid fringe field can not be ignored. (suggested by E. Perevedentev) • In order to suppress it, rate of change of Bz should be reduced and/or decrease angle between the solenoid axis and the beam axis. • Machine error also should be reduced and corrected. SuperKEKB

  24. Vertical Emittance originated from Solenoid Fringe Crossing angle = LER angle + HER angle = 83 mrad proportional to q4 Solenoid axis = angle bisector Energy ratio 4/(4+7) We choose 41.5 mrad and the contribution of solenoid fringe is ~4 pm. SuperKEKB

  25. Bunch Length • In SuperKEKB, the bunch length is 6 mm in LER and 5 mm in HER which are similar to the present KEKB. • The “bunch length” means a value includes intra-beam scattering and wake field from the ring impedance at the design beam current. SuperKEKB

  26. Nano-Beam Scheme Head-on Frame narrow and long bunch with large crossing angle short bunch with head-on = Y. Ohnishi / KEK

  27. Parameter Consideration • In SuperKEKB, the luminosity performance depends on the lattice performance significantly. • Especially, Touschek lifetime becomes serious in the nano-beam scheme due to lack of DA. • Target lifetime is 600 sec in both rings. The large DA in the momentum direction is necessary. • Transverse aperture is also necessary for the “betatron injection”. If enough transverse aperture cannot be kept, “synchrotron injection” should be considered. The emittance of the injected beam should be small, therefore e+ damping ring (DR) and RF-gun are considered. Y. Ohnishi / KEK

  28. IR design QC2LP Horz. F PM LER QC2RE Horz. F PM HER QC1RE Vert. f QC1LP Vert. f 41.5 mrad Detector solenoid axis IP 41.5 mrad QC2LE Horz. F PM QC1RP Vert. f QC1LE Vert. f QC2RP Horz. F LER and HER beam are focused by separated quadrupole doublet, respectively SuperKEKB

  29. IR Magnets design • Use both of superconducting magnets and permanent magnets • Superconducting magnets • Leakage fields of superconducting magnets are canceled by correction windings on the other beam pipe • Warm bore • Permanent magnets for QC2LE, QC2LP, QC2RE • Pros. • Cryostats are small • Assembly of vacuum chamber can be simple • Vacuum pump can be located near IP • Cons. • Need to develop a permanent magnet • Fine temperature control is required • Extra magnets are required to change beam energies SuperKEKB

  30. IR design with PM version N. Ohuchi Cryostat Anti solenoid-R QC1LP +corr. coil QC1RE +corr coil QC2RE PM QC2LP PM IP QC2LE PM QC2RP +corr. coil QC1LE +corr. coil QC1RP +corr. coil Anti solenoid-L Cryostat QC1RE SuperKEKB

  31. The magnet design of QC1R/L-P N. Ohuchi Vacch ID 20mm Magnet Parameters • Magnet inner radius=22 mm, outer radius=27.55 mm • Magnet current=1511 A • Field gradient=75.61 T/m • Current density, JSC= 1615.8 A/mm2 • Magnetic lengthLeff= 0.2967 m • Peak field w/o solenoids = 2.24 T • Operation temperature = 4.4 K Cable Parameters • Size =2.5mm 0.93mm • Number of SC strand=10 • Diameter of the strand=0.5mm • Cu ratio=1.8 • Ic= 2000 A @ 5T and 4.2 K Beam envelop Coolant path SC correctors: Normal Octupole Skew Quadrupole Normal Dipole Skew Dipole SC correctors: Normal Sextupole Normal Quadrupole Normal Dipole Cross section of QC1 RP 1.123 1.013 Designed new SC cablefor main quadrupole 0.93 7.0 3.9 2.5 Leakage fields are canceled by the correction windings on the other beam pipe 0.967 S-KEKB H-Current 0.9 S-KEKB NanoBeam 1.057 KEKB QCS

  32. ○ Belle Detector 8m(W)×8m(L)×10m(H) Total weight: 1300tons 650tons (Barrel Yoke) 600tons (End Yoke) 50tons (Others) H. Yamaoka 32 15th KEKB Review

  33. Anti solenoid H. Koiso Vertical emittance degradation depends on the anti-solenoid field profile Anti-solenoid Ver.2 HER:53mrad Bz SuperKEKB

  34. IR vacuum chamber K. Kanazawa Positron • Vacuum level will be worse due to poor vacuum conductance. • IR assembly is a big issue • BPM can’t be fixed rigidly on the quadrupole magnet SuperKEKB

  35. Physical Aperture • Required acceptance for injection • 2Jx=5x10-7 (m), 2Jy=2x10-8(m), 4% coupling • Dynamic effects are ignored because nominal nx = 0.53 • Physical aperture is not enough for the design β-functions • COD is not taken into account • No margin for error SuperKEKB

  36. A. Morita COD (Belle) COD in Belle detector depends on the solenoid field profile COD(LER 41.5 mrad) COD(HER 41.5 mrad) 100um 200um 4mm 1mm IP IP QC2RE QC1RE QC2LE QC2RP QC1RP QC1LP QC2LP QC1LE SuperKEKB

  37. Machine Parameters of SuperKEKB 2010/Feb/08 (): zero current parameters

  38. http://www-kekb.kek.jp/MAC/2010/ SuperKEKB

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