SuperKEKB Photon Monitors(SRM/XRM/LABM)
J.W. Flanagan, H. Ikeda, M. Arinaga, H. Fukuma, KEK
G.S. Varner, U. Hawaii
G. Bonvicini, Wayne State U.
SuperKEKB MAC
2012.2.21

Photon Monitors• SRM: Synchrotron Radiation Monitor– Visible light monitor. Interferometer, streak, etc.
– sz, sx (sy)• XRM: X-ray Monitor– Pinhole, URA mask, etc.
– sy (sx)• LABM: Large Angle Beamstrahlung Monitor– SR-like radiation from interaction point (~300-600 nm)– Can measure relative offsets and size ratios at collisionpoint.

• (Damping Ring monitors will be discussed by H. Ikeda.)

SRM: Extraction mirrors•Large incident power a source of heat-load induced deformations•SuperKEKB LER source bend is recycled arc bend, with bending radius twice that ofKEKB source bend.
•Incident power becomes less than that of the KEKB HER, though more than thatof the KEKB LER.
•HER bend does not change, and incident power becomes a bit higher than that of
the KEKB HER.

Energy(GeV)
Current(A)Bending radius(m)Power(W/mrad)
Distance to mirror (m)Be Mirror width (mm)Total incident power(W)LER (BSWFRP)
SuperKEKB
KEKB
43.5
3.6
2
177.4
85.7
7248
11113535161
109

HER (BSWOLE)
SuperKEKB
KEKB
78
2.6
1.4580
580
149
136
1313
3535283260

The heat deformation was already a great problem at KEKB, so would like to havea mirror that does not deform as much under the same heat load: diamondmirrors.

SRM:
Diamond
Mirrors• Monocrystalline diamond:– Being developed by Seki Technotron• 10 mm x 20 mm x 0.5 mm prototype completed.
• 20 mm x 20 mm x 1 mm prototype in fabrication– Surface: 3 mm Au, with thin Cr layer below that.
– Diamond surface is (nearly) a single crystal, so goodsurface flatness expected (Ra ~ 2 nm, <~λ/50)– Very good heat conductance and low thermal expansion
coefficient makes apparent change in magnification
smaller than that of Be mirrors used at KEKB:

Diamond mirror
temperature distribution
due to 400W of SR power• Berylllium: δmagnification = 43% @ HER full current• Diamond: δmagnification = 3% @ HER full current

Diamond mirror surface
distortion due to 400W ofSR power

Mirror surface distortion due
to 200W of SR power at
center of Be mirror

Mirror surface distortion dueto 400W of SR power at
center of Au+Dia mirror

ANSYS simulations:
M. Arinaga

SRM: Diamond Mirror &Holder• Mirror:– 10 mm x 20 mm x
0.5 mm prototype
made– 20 mm x 20 mm x1 mm prototype infabrication• Holder:– Vertically split softcopper cylinder

SRM: Interferometers
SR Source Bend Parameterex

S-LER1(BSWFRP)
3.20E-09

S-HER(BSWOLE)
4.60E-09k

0.27%

0.24%

ey

8.64E-12

1.10E-11mby

29.98

32.49msyBeam Energy
Bend effective length
Bend angle
Bend radius r
Observation wavelength l

16.1

18.9mm

40.89
5.04
179.0
4.00E-07
1.0

7
2.90
5.00
580.0
4.00E-07
0.7GeVm
mradmmmrad

0.799%

mrad

Max. Visibility (fringe depth) gmax

0.799%

Min. measurable beam size sy min

12.813.5mm

SR Opening angle qc (l)Slits opening angle D/FUnitsm•Resolution fundamentally limited bymeasurement wavelength and opening angle
between slits from beam (D/F).
•Max. slit separation determined by beam
spread and mechanical considerations.

•Vertical beam size measurement is possible with interferometers, though is near the limitof the interferometer resolution.
•Measurement wavelength needs to be lowered to fit beam size into dynamic range.
Also need to be able to measure 99% visibility. (Very difficult!)•Limitation on slit separation is due to antechamber height (14 mm).
◊Extraction mirror will be in antechamber for reduced impedance.
◊If we could increase antechamber height by factor of two for 2 m (LER) or 4 m (HER)
in front of extraction mirror, we could correspondingly reduce minimum measurable
beam size by factor of two. Appears difficult due to presence of upstream quads (bothHER and LER).

SRM: Summary•

Reuse present source bend locations– LER bend becomes longer, to reduce heat load
– HER remains the same•

Would like to move downstream ~few meters– Put mirror in antechamber for reduced impedance– More effort in HER, since no antechamber already existing•••Bases under above-ground tunnel components will be reinforced/ to reducevibrations
Transfer lines will be rebuilt to be better sealed against air flow
sz: Streak Camera– OK•

sx: Interferometer– OK (not single-shot, though)•sy: Interferometer– Will be difficult to get required resolution– Not possible to get single-shot (single bunch, single turn) measurements, which may probably
be needed for low-emittance tuning, based on experience at CesrTA.
– However, should be useful for cross-checking with x-ray monitor at larger beam sizes.

XRM: LER X-ray beamline(Fuji D8)Magnetpassage

Opticsbox

X-ray beamline

Detector Box

XRM: HER X-ray monitor beamline(Oho D4)

Detector Box

X-ray beamline

Opticsbox

Magnetpassage

XRM: Coded Aperture Imaging
Technique developed by x-ray astronomers using a mask tomodulate incoming light. Resulting image must be deconvolved
through mask response (including diffraction and spectralwidth) to reconstruct object. Open aperture of 50% gives highflux throughput for bunch-by-bunch measurements. Heatsensitive and flux-limiting monochromator not needed.
We need such a wide aperture, wide spectrum technique forshot-by-shot (single bunch, single turn) measurements.

Source distribution:+Kirchhoff integral over mask(+ detector response) Detected pattern:

Measured slow-scan detector
image (red) at CesrTA, used tovalidate simulation (blue)XRM: Single-shot resolution estimation• Want to know, what is chance that a beam of a certain size
is misfit as one of a different size?
• Tend to be photon statistics limited. (Thus coded aperture.)• So:– Calculate simulated detector images for beams of different sizes– “Fit” images pairwise against each other:• One image represents true beam size, one the measured beam size
• Calculate c2/n residuals differences between images:
N = # pixels/channels
n = # fit parameters (=1, normalization)
Si = expected number of photons in channel i• Weighting function for channel i:

N[ si  si ]2c2
1,2
 N  n  1 i 1 s i

s i  si .

– Value of c2/n that corresponds to a confidence interval of 68%
is chosen to represent the 1-s confidence intervalXRM: Coded Aperture tests at CesrTA
14 ns spacing

Example of turn-by-turn size andposition data (one bunch out of train)
Examples of bunch-by-bunch data
(electron-cloud blow-up study data)
Single-shot data average for each bunch

4 ns spacing

Single-shot resolution (simulation+ measured spread)XRM: SuperKEKB x-ray monitor
Xray Source Bend Par. S-LER(BS2FRP.1)
3.20E-09
exk0.27%
8.64E-12ey
50.0byS-HER (BS2E.82) Units
4.60E-09
0.24%
1.10E-1111.5msy
Beam Energy
Effective length
Bend angler
Critical Energy

11.3
75.9
55.7
105.9
7.1mm
GeVmmradmkeV•

20.840.89
28.0
31.7
4.4mm

Uniformly Redundant Array(URA) for x-ray imaging to be
used at SuperKEKB

Mask:

59-element, 10 mm/element URA
High-power design• 10 mm Au mask
• 625 mm Si substrate• Test at CesrTA• Other patterns, materials under study
Detector:• 64-channel, 50 mm• More channels desirable, for backgroundsubtraction and to accommodate beam
deviations.
•••

Simulated detector response
for various beam sizes atSuperKEKB LERXRM: SuperKEKB estimated single-shot resolutions(SuperKEKB full current)
Beam to mask: 12 m
Mask to detector: 36 m

•Red points: using 64-pixel
detector of same type as at CesrTA(Fermionics)•Green points: using detector withimproved photon detection
efficiency at higher x-ray energies(to be developed)
Detected spectrum(Fermionics detector)

HER

Beam to mask: 8 m
Mask to detector: 32 m

LER

XRM: Detector• Tests with Fermionics detector at Photon Factory showedthat detection efficiency at high energies is very low– Active pixel depth is only 3.5 microns.

• Detector development needed for the future– Higher efficiency at hard x-ray spectrum seen at SuperKEKB– Faster response for being able to directly diagnose head-tailinstabilities at SuperKEKB.
– Trench diode design being pursued:• Deep but narrow pixels• First prototype fabricated (SLAC/UH)XRM: Mask
• Tests at CesrTA show URA mask gives predicted single-shotresolution.
• High-power mask testing started in Fall, 2011
– Thick Si mask installed at CesrTA for high-power testing.
• CesrTA at 5.3 GeV can duplicate power load expected at SuperKEKB• SuperKEKB full current power load test successful!
– Big thanks to folks at Cornell for their assistance and efforts.

• Plan to test at ~20% higher power load

– Diamond substrate mask also fabricated and installed at CesrTA,and ready to be tested.

Si+Au mask

Diamond+Aumask

XRM:
Digitizer
STURM2ASIC
STURM ASIC for high-speed
readout (G. Varner).
Ver. 1 tested at KEK PF,
March 2009.
Ver. 2 fabricated.
Ver. 2 specs:
8 channels/STURM sampling
1 monitor channel
4 TSA sample buffers
8 samples/TSA buffer (32x channel)
288 Wilkinson conversion cells
1-200 GSa/s effective (5ps - 1ns Tstep)
1 word (RAM) sample readout
1+n*0.02 us to read n samples
100 kHz sustained readout (orbit)

XRM: 64-channel system• 64-channel system for testing at ATF2
– Using Fermionics (low-energy) detector.

Preampboard

Digitizer
daughterboard

Digitizer ASIC
(STURM2)

Motherboard

I/O BoardATF2 x-ray beamline
Duringconstruction

Rotatingmask holder

Detectortable

XRM: Summary• Source bends upstream of SRM source bends– LER bend is reused KEKB LER bend magnet (at half strength)– HER uses same current arc bend magnet• sy: Coded Aperture Mask

– Single-shot (single-bunch, single turn) resolution expected to besufficient.
– Single-shot mode probably be needed for low-emittance tuning, basedon experience at CesrTA.

• sx: Possible, if single-shot measurements are needed– SRM should work for this in slow integration mode

• Being developed in collaboration with Cornell U. and U. Hawaii.
– Testing different components at CesrTA and ATF2

• Development work continuing on detector and readout

LABM: Large Angle Beamstrahlung
Monitor• The radiation of the particles of one beam due to the bending force of theEM field of the other beam
• Beamstrahlung POLARIZATION at specific azimuthal points provides
unique information about the beam-beam geometry.

G. Bonvicini

LABM: Layout• 4 viewports and associated 4mm2 mirrors on Beam Pipe (KEK),
at 7-8 mrad w.r.t beam axis
• 4 narrow acceptance telescopes, which count photons in twopolarizations and 4 bands (32 channels total, WSU)
• Build flexible electronics for a variety of beam measurements(Hawaii, Tabuk, WSU)
• Interface with EPICS (Luther)• Discussions begun with company to make Be extraction
mirrors (2 mm x 2.8 mm). (KEK)
Optical channel

Tunnel floor
Optics box

LABM: Optical box locations

Tsukuba B4 (below accelerator floor level)

LABM: Optical channel• Designed opticalchannel. Built firstelbow and two test
pipes. (Wane)LABM: Optics box• Optics Box

Light collector

PMT conveyor
Belt for relative
calibrationSummary• Optical Channel prototype designed and built• Optics Box designed. Production has started• Work on electronics has started.
• Consulting with company on extraction mirror
design.

Photon Monitors Schedule• 2012:– SRM• Mirror + holder heat distortion test
• Extraction chamber design– XRM• Continue heat and imaging tests with high-power optics at CesrTA• Continue Electronics and detector development• Design beamline, determine magnet apertures required. Start fabrication ofdownstream beamline components.

– LABM• Construction of optics box and PMT calibration conveyor• Start electronics readout development• Preparation for possible Frascati beam test in 2013

• At SuperKEKB turn-on:– SRM & XRM commissioning• Sometime soon after (real, non-bakeout) turn-on:– LABM comissioningSparesEffect of amount of contact surface
between mirror and holder
6 mm of contact surface(contact equivalent to brazing)

0.5 mm of contact surface

Reducing effective contact surface to 1/10 of ideal raises
maximum temperature by about a factor of 2.
(Distortion ~ linear with temperature.)Examples of no-good holder designs