Proceedings of IBIC2012, Tsukuba, JapanWECD01OPERATION OF A SINGLE PASS, BUNCH-BY-BUNCH X-RAY BEAMSIZE MONITOR FOR THE CESR TEST ACCELERATOR RESEARCH
PROGRAM*
N.T. Rider, M. G. Billing, M.P. Ehrlichman, D.P. Peterson, D. Rubin, J.P. Shanks, K. G. Sonnad,
CLASSE, Cornell University, Ithaca, NY 14853, U.S.A.
M. A .Palmer, Fermilab, Batavia, IL 60510, U.S.A.
J.W. Flanagan, KEK, Ibaraki 305-0801, Japan
Abstract

INTRODUCTION
The X-ray Beam Size Monitor (xBSM) provides
experimenters in the CESRTA program with the ability tomeasure the vertical beam size of individual particle
bunches on a turn-by-turn, single pass basis. At present
two xBSM instruments have been installed inexperimental areas of the Cornell High Energy
Synchrotron Source (CHESS). One is used for positrons
and the other is used for electrons. Each setup has its own
x-ray source, which is a dipole magnet within the Cornell
Electron Storage Ring (CESR). The critical energy is 0.6
keV during 2 GeV CESRTA operations. A set of invacuum optics focuses the photon flux onto the detector.
The geometry of the beam line provides an opticalmagnification of 2.34 for the positron line and 2.52 forthe electron line. A continuous vacuum vessel, containing
optics elements and filters which are inserted into the xray beam, extends from the x-ray source to the detector.
Images are collected via a custom data acquisition system.
Figure 1 shows the functional layout of the positron line.
The electron line is a near mirror image.
____________________________________________*Work supported by NSF grant PHY-0734867, PHY-1002467 andDOE grant DE-FC02-08ER41538, DE-SC0006505

Figure 1: Layout of the xBSM positron line

VACUUMA multi-stage differential pumping scheme has been
implemented to allow “windowless” transmission pathfrom the x-ray source to the detector. The schemefeatures a series of apertures, turbo pumps and gate valves
along the x-ray beam line. This setup allows us to haveelectronics (printed circuit board, cables, etc) which are
not vacuum compatible within the detector box without
contaminating CESR. This is critical for the proper
operation of the instrument. The vacuum system pressureis automatically controlled and monitored via
programmable logic controllers.

DETECTOR
The detector is a vertical linear array of 32 InGaAs diodes
with a 50 µm pitch and horizontal width of 400 µm. The
InGaAs layer is 3.5 µm thick, and absorbs 73% ofphotons at 2.5 keV. The time response of the detector is
sub-nanosecond.

OPTICSThe xBSM utilizes four different optical elements. For
all beam energies, a vertically limiting slit (referred to asa pinhole) is available. For beam energies less than 2.5GeV, a low energy Fresnel zone plate and a coded
aperture are available. At 4 GeV and greater, a highpower coded aperture can be inserted into the x-ray beam.
The low energy optics are contained on one “chip” which
is made from a 2.5 µm silicon substrate with a 0.7 µm
layer of gold forming the optical features. The highenergy coded aperture is made from a 625 µm silicon
substrate with a 10 µm layer of gold. The coded aperturefeatures of the high energy optics are the same but one
half the scale of the low energy optics. Figure 2 shows
the coded aperture features.

ISBN 978-3-95450-119-9
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1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The CESR Test Accelerator (CESRTA) program targets
the study of beam physics issues relevant to linear collider
damping rings and other low emittance storage rings.
This endeavour requires new instrumentation to study thebeam dynamics along trains of ultra low emittancebunches. A key element of the program has been thedesign, commissioning and operation of an x-ray beam
size monitor capable, on a turn by turn basis, of collecting
single pass measurements of each individual bunch in a
train over many thousands of turns. This new instrument
utilizes custom, high bandwidth amplifiers anddigitization hardware and firmware to collect signals froma linear InGaAs diode array. The instrument has been
optimized to allow measurements with 3x109 to 1x1011
particles per bunch. This paper reports on the operational
capabilities of this instrument, improvements for its
performance, and the methods utilized in data analysis.
Examples of key measurements which illustrate theinstrument’s performance are presented. This device
demonstrates measurement capabilities applicable tofuture high energy physics accelerators and light sources.

WECD01

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 2: Co
oded aperture.
The low ennergy coded aperture
aprovid
des a series ofo
ttransmitting annd opaque featu
ures which ran
nge from 10 µmmtto 40 µm. Analysis
Aof thee coded apertu
ure interference
ppattern yields information about both beambsize andd
pphoton energy
y spectrum. Due
D to the variaation in featuresize, the coded aperture is sensitive
sto a range of x-ray
yeenergies and veryv
small beaam sizes. Thiss, coupled with
htthe relatively highhlevel of trransparency, alllows the coded
daaperture to be useful for low energy and low
w photon counnt
eexperiments.
Analysis off the Fresnel zonezplate has not been fully
yddeveloped.
It has proveen to have a very broaddbbackground coomponent when used with a wide spectrumm
xx-ray beam, andathe introd
duction of a monochromato
morllimits the num
mber of photonss to an unusablee level.
The pinholee consists of a vertically limitinglset ofoaadjustable tunggsten blades whichwallow forr a configurable
slit height. ThisT
allows thee height to bee optimized foor
ddifferent particcle beam energ
gies.

Figure 3: Data acquisitionn channel
Each off these data acquisition channnels is synchroonized
to the stoorage ring via a 24 MHz insstrumentation clock.
This clockk is derived froom the CESR mmaster oscillatoor andcontains encoded trigggers and turns markers. A local
timing booard receives tthis 24 MHz cclock and utilizes avoltage coontrolled oscilllator, phase ddetection circuuit andprogramm
mable delays too generate the ssample clocks for all
32 channnels. These individual saample clocks haveadjustablee delays in 100 ps incrementts to allow foor finephase adjuustments.
This timming control ccapability is uused to peak sample
the signaal generated w
when x-rays arre absorbed bby thediode deteector. Figure 4 shows the ssignals generatted bytwo 4 ns spaced bunchees and their coorresponding samplepoints.

FILTERRS/SLITSIn order too empirically investigate annd control theddetector respon
nse, we have created a movab
ble stage whichhccontains a variiety of horizon
ntally limiting slits
s and filterss.
T
These slits an
nd filters are insertediinto thet x-ray beammbbetween the optics
oand thee detector. Th
he horizontally
yllimiting slits area useful in reducingr
the amountaof flux
xdduring high ennergy and highh current operaation. Slit sizees
ffrom 35 µm too 171 µm havee been tested. The filters areuused to changge the spectrall content of thhe x-ray beammw
which reachess the detector, and thus derrive the energy
yspectrum baseed on the response. We havve tested 6 µmmddiamond and 2 µm molybdenum filters and have plans toottest a 6 µm alu
uminium filter.

DATA ACQQUISITION
NThe xBSM
M instrument has an ind
dependent dataaacquisition chhannel for eachh of the 32 detectorddiodess.
EEach channel consists of a transimpedance amplifier, avvariable gain amplifier (-4 dB to 20 dB)), a 12 bit 3000M
MSPS analog to digital connverter, a field programmableggate array (FPPGA), a local 1 Msample SRRAM buffer andda programmab
ble sample clo
ock delay. Fig
gure 3 shows asingle channel.

mpling of 4 nS spaced bunchees.
Figurre 4: Peak sam
A singlle channel sam
mples each of tthese bunches whenclocked aat 250 MHz with the sam
mpling clock edgesaligned wwith the peak oof the signal. N
Note that the image
shown is a composite mmade by shiftinng the sample pointin 10 ps increments. AAll 32 channeels are synchroonizedwith the CESR turn mmarker and ggated with a bbunchpattern w
which matches the CESR filll pattern. Thee peakvalue deteected on all 322 channels is collected and uused toform a dettector image.

ISBN 978-3-95450-119-92

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Proceedings of IBIC2012, Tsukuba, JapanWECD01

SOFTW
WARE

PINH
HOLE IMAAGE ANALYYSISThe pinholee is a simple optical device which has asingle slit andd an opaque maskingmmateriaal. In order tooggenerate a moodel for the immage we apply the derived xx
rray spectrum to a numerical calculation of Fraunhofeer
ddiffraction. The broad energ
gy spectrum sm
mooths out theoouter diffractio
on features andd allows us to approximate
atheiimage as a summ of two gausssians plus a fllat backgroundd.
F
Figure 5 show
ws the model (lline with pointts) and the twooggaussian function (solid red line).
lF
Figure 6: Pinhoole detector im
mage with fit.

CODEED APERTUURE IMAGGE ANALYSSISThe codded aperture iss a multi-slit ooptical device w
which
generates a diffractionn pattern at tthe detector. Thisdiffractionn pattern is caalculable usingg the derived x-ray
spectrum and the detailled geometry aand materials of thecoded apeerture. We paarameterize thee diffraction ppattern
as the sum
um of 12 gausssians. Figure 7 shows the mmodelsmoothedd to a source sizze of 7 µm (linne with dots), tthe 12
gaussians (various coloors, solid liness), and the ressulting
function ((single solid red line).

Figgure 7: Code aaperture modell and function.
ure 5: Pinhole model and funnction.
Figu
This functio
on is used to fit the detector imageidata anddccalculate a beeam size and imageioffset ono the detectorr.
FFigure 6 show
ws a single buncch, single turn detector imagew
with a fit appliied.

This fuunction is used to fit the deteector image datta andcalculate a beam size aand image offfset on the dettector.
Figure 8 sshows a single bunch, single turn, detector image
with a fitt applied whichh corresponds to a source ssize of
9.85 µm.

ISBN 978-3-95450-119-9
Miscellaneous and others

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○A Matlab based
bgraphicaal user interfaace is used tooccontrol the datta acquisition anda motorized features of theiinstrument. AllA data is writtten to disk andd is available toobboth real time and offline annalysis program
ms. The controol
pprogram has limited
lanalysiis functionality
y that providees
ffitting and display of a sam
mple of the turnn by turn beam
msize measurem
ments in reall time and allows
afor theppossibility of realr time particcle beam tuningg.
using
theOffline
analysis
aisperformed
xxBSMRootFittter. This loccally developeed, C++ based
daanalysis softwaare uses the CE
ERN ROOT paackage to bookk,store, fit and display histog
grams. It leveerages MINUIT
T
ffor fitting and FFTW for freq
quency analysis. It provides avvariety of plotts and analysiss. xBSMRoottFitter providees
systematic com
mpensations suchsas pedesttal subtractionn,cchannel by chaannel calibratio
ons, gain rangee scaling, bunch
htto bunch croosstalk compensation and bad channeelsuppression.

WECD01

Proceedings of IBIC2012, Tsukuba, JapanFFigure 11: Imaage Offset Overr 1024 turns.
Fig 8: CodedCaperture detector image with fit.

SIZE ANND OFFSETT MEASURE
EMENTSThe xBSM is capable of bunch
bby bunch, turn by turnn,
single shot meeasurements off beam size annd offset of theiimage on thee detector. Figures 9 thhrough 12 are
rrepresentative measuremen
nts of a singgle bunch ofoppositrons in CESR. The size of a single bunchbis plotteddoover 1024 turnns along with thet frequency spectrum
sof thesize variations.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 12: Spectrum of image offseet over 1024 tuurns

PPHYSICS M
MEASUREEMENTS

9 Single buncch size over 10024 turns.
Figure 9:

The xxBSM is in regular usee during CEESRTA
experimenntal runs. Thhe following arre two exampples ofexperimennts which havve collected aand analysed xxBSM
data. Thee physics phennomena which are evident inn theseplots are outside the sccope of this ppaper. Work is ongoing to ddecouple system
matic instrumeentation effectss frombeam phyysics.
Intra beeam scattering (IBS) is a pheenomenon of ccurrent
dependent
nt beam size inn small beams. The xBSM iss used
to measurre the beam sizze and detectorr offset of indivvidual
bunches vversus beam cuurrent over maany turns. Figuure 13
representss an IBS experiment using a single bunnch of
positrons undergoing ddecay from 10011 particles tto 109
particles. Data sets were collectted under ideenticalconditionss using the pinnhole and codedd aperture.

FFigure 10: Freequency spectrrum of bunch size of a singlebbunch.
The offset ofo the image on the detecto
or can also bepplotted, giviing the ex
xperimenter beambmotion
niinformation. The peak in the spectrum
m of offsets inn
F
Figure 12 appeears at the vertiical betatron tuune.
Figure 133: Positron IB
BS experimentt beam sizes vversus
current.
ISBN 978-3-95450-119-9
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Proceedings of IBIC2012, Tsukuba, JapanWECD01

During the IBS experimeents, the imagee offset is alsooaanalysed. Figu
ure 14 is an immage offset specctrum plot for asimilar currentt decay as abovve. A current dependent tune
shift can be observed moving
mfrom approximately
y2210 kHz to 235
5 kHz.

ure 14: IBS im
mage offset specctrum.
Figu
Experimentss in the beamm dynamics program
pstudyyeelectron induced multi-bunchh phenomenonn in CESR. The
xxBSM is used
d to measure the beam sizze and detecto
orooffset for indiv
vidual bunchess along trains ofo bunches oveerm
many turns. TheT spacing beetween bunches can be varieddiin 4 nS increments while the bunch current
cis helddcconstant at ap
pproximately 1 mA. Figurre 15 shows a
ccomparison off different bun
nch spacing annd the effect ofo
tthe electron clo
oud on the bun
nch size along thet train.

measurement will utilize high speed diodeThis mdetectors with rise and fall times on the order of 335 ps.
This respponse time couupled with a ffour channel pphased
digitizer w
will allow forr four longituddinal “slices” oof thebunch to bbe measured.

SUUMMARY
The xxBSM is rouutinely used in the CEESRTA
experimenntal program for precisionn measuremeent ofbeams witth vertical sizee as small as 9 µm. There is aan ongoing efffort to underrstand systemaatic effects annd to
optimize the analysis of images. Thee device has pprovenessential tto the CESRTA
A program to unnderstand intraabeamscatteringg and the emittaance diluting eeffect of the eleectron
cloud.

REF
FERENCESS
Alexander et all, TH5RFP0266, PAC09[1] J.P. A[2] J.P. A
Alexander et all, TH5RFP027, PAC09[3] J.W. Flanagan et all, TH5RFP048,, PAC09
PAC10[4] D. P. Peterson et all, MOPE090, P
PAC10[5] J.W. Flanagan et all, MOPE007, P[6] N.T. Rider et al, MO
OP304, PAC11aa train off bunches.
Figure 15: Beam size along

FUTTURE MEAASUREMENNTSOne future extension of the xBSM co
oncept involvees
mmeasuring the instabilities anda head-tail oscillations
oof asingle bunch. Figure 16 deemonstrates a possible bunchhooscillation andd four slices of the bunch sizee.

ISBN 978-3-95450-119-9Miscellaneous and others

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 16: Bunch sliccing.