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					SR Monitors for SuperKEKB

       J.W. Flanagan
     KEK Mini-Workshop
           SR Monitors
• SuperKEKB LER & HER SR Monitors
• Damping Ring SR Monitor
                 SR Monitors
• Problems experienced thus far:
  – Wall-current heating
     • Expanding/contracting stainless-steel beam pipe →
       mirror motion → Optical axis drift.
        – Corrected initially by optical-axis feedback.
        – Basically cured by changing stainless duct to copper.
  – SR heat loads
     • Mirror distortion
        – Measure, correct, calibrate
          SR Monitors, cont.
• Problems, cont.
  – Low light levels, especially in LER
     • Troublesome for single-bunch measurements (streak
       camera, gated camera, etc.)
     • Partial solution: reflective optics
        – Removes need for bandpass filters for imaging systems
        – Used with streak camera
            » Enables bunch-length measurement at LER
            » Still cannot resolve bunch shape or tilt very well, so
              would like to improve light levels before crab cavity
              commissioning in 2 years.
  8/2002 -
          SR Monitor Upgrade Plans
              for SuperKEKB
• Wall-current heating
   – ΔT α (I2 / N) /(σz)3/2
   – Current copper chamber:

      Bunch Spacing   σz (mm) I (A) ΔT  (℃) ΔT (℃)
      (buckets)                     [Calc]  [Meas]
      4               6       1.4   1.7     1.7 *
      4               6       9.4   78
      4               3       9.4   220
      1               3       9.4   55

                               * Measured: water-cooled pipe
                                 Calculated: no cooling assumed

 •May need increased water cooling
 SR Monitor Upgrade Plans, cont.
• SR heating:
  – Larger bending radius, longer core length magnet
    →higher λcrit.
     • Increase visible light flux for single-bunch and low-
       current measurements.
     • Lower total SR power, reducing heat load on mirror and
       resulting deformation.
        – Currently, ~10% magnification drifts at 1.5 A with new chamber
  – For adjustment freedom, may prefer to go with a
    local bump or chicane in the straight section (no
    quadrupoles inside the bump)
     • Can adjust ρ without disturbing optics.
      SR Source Bend Radius vs
Flux Density at 500 nm and Total Power

 LER                               HER
SR Monitor Upgrade Plans, cont.
 – Some HOM leakage into outer chamber was
   also indicated by arcing of thermocouple leads.
   MAFIA simulations suggested reducing
   apertures would help. Current chamber design
   expects ~100 W at 2 A, with current 4-bucket
   spacing. At 9.4 A, with 1-bucket spacing, ~500
   W would be expected. This needs to be
   measured, and possibly absorbers added outside
   light-extraction aperture.
SR Monitor Upgrade Plans, cont.
• Dynamic Beta Measurement/Compensation
  – Second monitor needed, ideally at 90 degree
    phase advance from current locations.
  – Candidate locations:
     • Weak bends (arc-section entrances, exits)
        – Relatively little disturbance to lattice, but requires new
          optics huts, and space for them
     • Arc sections near current locations
        – Can use existing huts, only need new optical transfer lines
        – Probably use a bump or chicane system to minimize
          disruption to optics
    Damping Ring SR Monitor
• Purpose: to monitor turn-by-turn emittance
  – Use gated camera for single-turn measurements
• Source bend: use main bend
Damping Ring SR Source Characteristics
     B        1.267T
     E        1GeV
     ρ        2.6 m
     λc       1.36 nm   Ec 0.84 keV
     βx        1.56 m
     βy        6.25 m
     Disp. (x) 0.1 m
     ε        1.23e-6 m → 1.22 e-8 m
     ΔE/E     6e-3      → 5.5e-4
     σx       1.5 mm    → 150 μm
     σy       2.8 mm    → 280 μm
• Bend angle: (360/N)/(1-0.35)
   = 242 mrad (N=40), 323 mrad (N=30)
• Light fan ΔΨ ~ (1/γ)(λ/λc)1/3
   =3.6 mrad [λ = 500 nm, γ= 2000, λc = 1.36 nm]
      • Can easily avoid the bend magnet fringe fields
      • Can locate extraction mirror 1-2 m downstream of bend
          – Should fit in straight section (straight section length = 6 m)
                  SR Power Load
• Magnet Core length L
  =0. 7286 m (N=40)
• Total Power P[W] = 1270 E^2[GeV] B^2[T] I[A] L[m]
  =15 W [I = 10 mA]
     • Power on mirror (mirror subtends <~ 25 mrad): <~1.5 W
           Imaging Resolution
• σPSF =~ 0.5 λ/(2ΔΨ)
  = 35 μm [λ= 500 nm, ΔΨ=3.6 mrad]
• σx[damped] =150 μm
  – Sqrt[σPSF2 + σx2]/σx = 1.03
• σy[damped] =280 μm

• ==>Imaging resolution for gated camera should
  be very good.
Damping Ring SRモニタ 予算

 取出し系 (Chamber, Mirror 他) 500万
 光路                 500万
 光学系、光学テーブル他              350万
 測定室                 500万
 高速ゲートカメラ              600万
 制御系                  150万

 合計                     2600万円
                                Old Chamber

   Temperature       °C

SUS: Resistive losses at
    1 A → ~20 W → 100°/10cm   LER Current (mA)
Cu: 1/40-1/50 of SUS           New Chamber


                              LER Current (mA)
              Extraction Mirror Orientation
                       Optical-axis feedback
                       compensation angles
          Old Chamber                          New Chamber

m                                  m
r                                  r
a                                  a
d                                  d

       LER Current (mA)                     LER Current (mA)
    •Optical-axis feedback is now unnecessary, turned off in LER
    (2003.10: Turned off in HER as well)
              SR Monitors
• Current set-up: one for each ring
Reflective Optics for Streak Camera
          Designed by T. Mitsuhashi

  Hyperbolic   Parabolic          Streak camera
    mirror      mirror            internal relay
                              optics (custom-made)
379 mA (1.35 mA/bunch)
     Tail of Train


     Hardware Improvements
• New SR extraction chambers
  – Beam-pipe section changed from stainless steel
    to copper, reducing resistive wall heating and
    improving stability of extraction mirror mount,
    and eliminating the need for optical axis
    feedback compensation with changing beam
    currents. Stable optical axis also makes study
    measurements much easier.
Old LER SR Extraction
    1998 - 7/2002

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