SR Monitors for SuperKEKB

J.W. Flanagan

KEK Mini-Workshop

2003.11.28

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.

New LER SR ExtractionChamber8/2002 -

SR Monitor Upgrade Plansfor SuperKEKB

• Wall-current heating– ΔT 　 α 　 (I2 / N) /(σ ｚ )3/2 – Current copper chamber:

Bunch Spacing(buckets)

É–z (mm) I (A) É¢TÅ@(Åé)[Calc]

É¢TÅ@(Åé)[Meas]

4 6 1.4 1.7 1.7 *4 6 9.4 784 3 9.4 2201 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 vsFlux Density at 500 nm and Total Power

LER HER

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.

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 damping– Use gated camera for single-turn measurements

• Source bend: use main bend

B 1.267T

E 1GeV

ɜ 2.6 m

É…c 1.36 nm Ec 0.84 keV

É¿x 1.56 m

É¿y 6.25 m

Disp. (x) 0.1 m

É√ 1.23e-6 m Å®1.22 e-8 m

ɢE/E 6e-3 Ů5.5e-4

ɖx 1.5 mm Ů150 ɠm

ɖy 2.8 mm Ů280 ɠm

Damping Ring SR Source Characteristics

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)

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

• σ ｙ [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

New Chamber

Beam-PipeTemperature

SUS: Resistive losses at 1 A → ~20 W → 100°/10cm

Cu: 1/40-1/50 of SUS

LER Current (mA)

LER Current (mA)

°C

°C

Extraction Mirror OrientationOptical-axis feedbackcompensation angles

LER Current (mA)LER Current (mA)

mrad

mrad

Old Chamber New Chamber

•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 CameraDesigned by T. Mitsuhashi

Parabolicmirror

Hyperbolicmirror

Streak camerainternal relay

optics (custom-made)

379 mA (1.35 mA/bunch)Tail of Train

t

y

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 ExtractionChamber

1998 - 7/2002