Beam Instrumentation for
Super-KEKB
J.W. Flanagan
KEKB Review
2004.2.17

Beam Instrumentation• Beam Position Monitors• Bunch-by-Bunch Feedback System• Synchrotron Radiation Monitors– HER and LER SR Monitors– Damping Ring SR Monitor

Beam Position Monitors• Performance of current COD BPM system is expected to be
sufficient for Super-KEKB.
• Will use the same front-end electronics.
• Need some modifications to button electrodes to accommodate
dynamic range of front-end at higher beam-currents.

KEKB BPMs
• Connector:– Ring BPMs use N-type connector.
– However, slight changes in contact pressure have led toseveral tens of µm orbit measurement error.
– Special male SMA connector feedthrough has beendeveloped, successfully used near the IP.
– This connector will be adopted for ring BPMs as well at
Super-KEKB.

KEKB BPMs
• Mounting:– Currently, electrodes are embedded in BPM blocks, which areelectron-beam welded to the beam pipe and rigidly mounted next
to each quadrupole magnet. Mapping was carried out before
welding to beam pipe.

Super-KEKB BPMs
•Change to flange-mountedelectrodes embedded in beam-pipe•Cheaper, shorter construction
time, easy replacement.
•Beam-based alignment has
proven very reliable, so preinstallation mapping unnecessary.
•R&D: Ceramic -> low-permittivity(if successfully developed for FBBPMs)•Reduce heating from HOM
losses in ceramic (may be neededfor 3 mm bunch length).

Super-KEKB BPMs
• Ring electrodes– Diameter 12→6 mm• Reduce output voltage to 1/4 for high beam-current operation• OCTPOS electrodes– Diameter 6→3 mm• To handle equivalent beam current of 13.5 A (sum of bothbeams)– Welded to beam pipe
• not enough space for flange.

BPMs, cont.
• COD BPM
– Due to new antechamber, HF cut-off frequency
at ~700 MHz → Cannot continue direct
detection at 1 GHz• 500→1000 MHz up-converter in front of detector– 2 ch x 240• Single-Pass BPM– Increase from 30 -> 120 sets

• VXI/VME: MXI-1 -> MXI->2– Bus speed-up from 6 Mb/s→10 Mb/s
– 7 x 20 locations

Super-KEKB BPMs
• Expected performance– Similar to that in KEKB, but higher minimum
measurable beam current.

Bunch-by-Bunch Feedback• Transverse feedback: design damping rateof 0.2 ms.
• Longitudinal feedback: design damping
rate of 1 ms.

Feedback Hardware• Pickup electrode– Same basic design as
OCTPOS pickups.
– Reduced dielectric constantwould reduce TE-mode
trapping and improve
higher frequency response

MAFIA simulation
σz = 3 mm
εr=9.8• R&D necessary– Present pickup chambersame inner diameter as atpresent, with material,cooling and transition
tapers modified for higher
beam currents.

εr=4

Feedback Hardware• Front-end Circuit– Transverse largely same as at present, except for detection frequency• 2 GHz (4 x fRF) → 2.5 GHz (5 x fRF) for better inter-bunch isolation.

– Automate slow-control of LO phase and DC offset tuning.
– Longitudinal similar (uses sum signal to detect phase only).

Feedback Hardware• Beam signal after 2.5 GHz BPF (upper) and detected signal (lower).
Good flatness and isolation at 2 ns separation are seen.

Feedback Hardware• Signal processing– Gboard: General-purpose feedback signal processor being developed bySLAC, KEK, INFN
– Handle transverse, longitudinal FB and diagnostics– Support bunch spacings down to 0.66 ns, sampling at 1.5 GHz– Custom digital-filter algorithms possible.
– Should give better noise reduction, beam diagnostics than present 2-tap FIRfilter.

• Back-end electronics– Almost same as atpresent. Possibly
add second
transverse kicker
system for larger FBrange.

Feedback Hardware• Power amplifiers and kickers– Need more reliable structure for transverse kickers, and durable
attenuators for higher beam currents.
• Loss factor of 0.11 V/pC at 7 mm bunch length, will worsen with 3-mmbunch length– Possible measures:• Supports with improved cooling for kicker plates using beryllia.
• Flexible longitudinal supports with good DC connection and wideband
response.
• Step back radial plate position to chamber-wall– HOM dampers needed for trapped modes inside the kicker.

– Longitudinal kicker: use DAΦNE-type low-Q-cavity
kickers, with eight-port structure for high beam
power. Better feed-throughs needed for frequency
response.

Feedback Kicker

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 (streakcamera, 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, sowould like to improve light levels before crab cavity
commissioning in 2 years.

Hardware Improvements• New SR extraction chambers– Beam-pipe section changed from stainless steel
to copper, reducing resistive wall heating andimproving stability of extraction mirror mount,and eliminating the need for optical axis
feedback compensation with changing beamcurrents. Stable optical axis also makes study
measurements much easier.

HER SR Monitor Extraction Chamber
Wall current heating•Old chamber:>200°@ 700 mA•New chamber:<1°@ 1100 mA

Estimated HOM
leakage fromextraction aperture(MAFIA simulation):•Old chamber:
100-200 W @1200 mA, 5.5 mm•New Chamber:
50 W @
1200 mA, 5.5 mm

Old Chamber

New Chamber

Mirror orientation drift with beam current reduced.
•Optical axis feedback no longer needed.

SR Monitor Mirror Heating•Apparent beam-size distortion due to mirror heating with old and newchambers at HER.
•Improved, but will need further improvement for higher beam currents.

SR Monitor Upgrade Plansfor SuperKEKB• Wall-current chamber heating issues– Old chamber: wall current heating a severe problem, solved bychanging beam duct section from stainless steel to copper.
– Calculations, which agree with current beam pipe measurements,suggest an 220-degree temperature rise at 9.4 A with 3-mm bunch
length due to wall currents without cooling.
• Extra cooling will be needed.

Beam-pipe heating with the current beam chamber design

SR Monitor Upgrade Plans, cont.
• SR heating:– Larger bending radius, longer core length magnet→higher λcrit.
• Increase visible light flux for single-bunch and lowcurrent measurements.
• Lower total incident SR power, reducing heat load on
mirror and resulting deformation.
– Currently, ~10% magnification drifts at 1.5 A in LER with newchamber.

• Also increases visible light flux – desirable to help see
effect of single crab cavity

SR Source Bend Radius vsFlux Density at 500 nm and Total PowerLERHERSR Monitor Upgrade Plans, cont.
• HOM– Some HOM leakage into outer chamber wasalso indicated by arcing of thermocouple leads.
MAFIA simulations suggested reducingapertures 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 bemeasured, and possibly absorbers added outside
the light-extraction aperture.

SR Extraction Chamber

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 newoptics 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

Second SR Monitor for Dynamic
Beta Measurement• Build a second SR source in each ring
• Using known phase advance between
two locations, can measure thedynamic beta effect due to beam-beamcollisions.
– Correct beam size estimation at IP– More importantly, can monitor beambeam parameters directly, in real-time.
– Useful for luminosity tuning.

• Second source: create a local bumpnear current source– Minimize disturbance to lattice– Can use existing optics huts.

Damping Ring SR Monitor• Gated camera for
imaging turn-by-turnbunch size damping.
– Up to 4 bunches in ring at
one time, at two different
stages of damping.
– Diffraction-limited
resolution below 3% if
optical line not too long(~10 m).

Summary• BPMs:– New, smaller buttons• Feedback:– Gboard– Longitudinal Feedback• SR Monitors– More cooling of extraction chamber, longer bendingradius source bends– Second set of monitors for dynamic beta measurement– Gated camera for damping ringBunch-by-Bunch Feedback••

New BPMs for higher beam currents.
Transverse feedback similar to present design– Detection frequency 2.0 -> 2.5 GHz.
– Automated LO phase and DC offset tuning.
– Transverse kicker needs work to handle higher currents• Improved cooling, supports for kicker plates.

•Longitudinal feedback to handle ARES HOM and 0/Pi mode
instability– Use DAφNE-type (low-Q cavity) kicker.
– QPSK modulation with center frequency 1145 MHz (2.25 x RF freq.)•Digital FIR and memory board to be replaced by new GBoard underdevelopment at/with SLAC.
– Low noise, high speed (1.5 GHz), with custom filtering functions possible.
– Extensive beam diagnostics.

Old LER SR Extraction
Chamber
1998 - 7/2002

New LER SR
Extraction
Chamber
8/2002 -Old Chamber

Beam-Pipe
Temperature°C

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

LER Current (mA)Extraction Mirror Orientation
Optical-axis feedback
compensation anglesOld Chamber

mrad

New Chamber

mrad

LER Current (mA)LER Current (mA)
•Optical-axis feedback is now unnecessary, turned off inLER (expected to be unnecessary in HER as well)Beam Loss Monitors• KEKB– Originally: ~100 ionization chambers dispersed
evenly around ring. Expensive and slow, but
indestructible– Channels around masks replaced with PIN diodes forfast abort (few turns) to prevent excessive mask-headgouging. Cheap, periodic replacement needed.

• SuperKEKB– Probably move to all-PIN diode system, for reasons ofspeed. Rather than broad coverage (which hasn’t
proven necessary), aim for detailed monitoring ofaperture-restricting locations (masks, IR, etc.).

DCCT• Current design should be fine at higher
currents.