1. SR Monitors for SuperKEKB J.W. Flanagan KEK Mini-Workshop 2003.11.28

2. SR Monitors SuperKEKB LER & HER SR Monitors Damping Ring SR Monitor

3. 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

4. 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.

5. New LER SR Extraction Chamber 8/2002 -

6. SR Monitor Upgrade Plans for SuperKEKB Wall-current heating –ΔT α (I 2 / N) /(σ ｚ ) 3/2 –Current copper chamber: * Measured: water-cooled pipe Calculated: no cooling assumed May need increased water cooling

7. SR Monitor Upgrade Plans, cont. SR heating: –Larger bending radius, longer core length magnet →higher λ crit. 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.

8. SR Source Bend Radius vs Flux Density at 500 nm and Total Power LERHER

9. SR Monitor Upgrade Plans, cont. HOM –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.

11. 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

16. Damping Ring SR Monitor Purpose: to monitor turn-by-turn emittance damping –Use gated camera for single-turn measurements Source bend: use main bend

18. Damping Ring SR Source Characteristics

19. Extraction 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)

21. 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

22. 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 σ ｙ [damped] =280 μm ==>Imaging resolution for gated camera should be very good.

23. 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

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

25. Extraction Mirror Orientation Optical-axis feedback compensation angles LER Current (mA) mradmrad mradmrad Old ChamberNew Chamber Optical-axis feedback is now unnecessary, turned off in LER (2003.10: Turned off in HER as well)

26. SR Monitors Current set-up: one for each ring

30. 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.

31. Old LER SR Extraction Chamber 1998 - 7/2002