Results from a Prototype Chicane-Based Energy Spectrometer for a Linear Collider

0.71 MB·English

by  A. Lyapin

arXiv:1011.0337
PreprinttypesetinJINSTstyle-HYPERVERSION
SLAC-PUB-14255
DESY10-146
Results from a prototype chicane-based
1
1 energy spectrometer for a Linear Collider
0
2

n
a
J
7 A. Lyapina,d∗,H.J.Schreiberb, M.Vitib,C. Adolphsenc,R. Arnoldc, S.Boogertd,
2
G. Boormand,M.V. Chistiakovae,F.Gournarisa,V. Duginovf,C. Hastc,

] M. D.Hildrethg, C.Hlainge, F.Jacksonh, O.Khainovskye, Yu.G.Kolomenskye,
h
p S. Kostrominf, K.Kumarc,i,B. Maiheua, D.McCormickc, D.J.Millera, N.Morozovf,
- T.Orimotoe,j, E.Petigurae,M.Sadre-Bazzaze, M.Slaterk, Z.Szalatac,M. Thomsonk,
c
c D. Wardk,M.Wendtl,M. Winga andM. Woodsc
a
s. aUniversityCollegeLondon,London,UK,
c bDeutschesElectronenSynchrotronDESY,HamburgandZeuthen,Germany,
i
s cSLACNationalAcceleratorLaboratory,MenloPark,California,USA,
y
h dRoyalHolloway,UniversityofLondon,Egham,UK,
p eUniversityofCaliforniaandLawrenceBerkeleyNationalLaboratory,Berkeley,California,USA,
[ fJointInstituteforNuclearResearch,Dubna,MoscowRegion,Russia,

3 gUniversityofNotreDame,NotreDame,Indiana,USA,
v hDaresburyLaboratory,Daresbury,UK,
7
iUniversityofMassachusetts,Amherst,Massachusetts,USA
3
3 jCaliforniaInstituteofTechnology,Pasadena,California,USA,
0 kUniversityofCambridge,Cambridge,UK,
.
1 lFermiNationalAcceleratorLaboratory,Batavia,Illinois,USA,
1
E-mail: [email protected]
0
1
:
v ABSTRACT: The International Linear Collider (ILC) and other proposed high energy e+e− ma-
i
X chines aim to measure with unprecedented precision Standard Model quantities and new, not yet
r discovered phenomena. Oneofthemainrequirements forachieving thisgoalisameasurement of
a
the incident beam energy with an uncertainty close to 10−4. This article presents the analysis of
datafrom aprototype energy spectrometer commissioned in2006–2007 inSLAC’sEndStation A
beamline. Theprototypewasa4-magnetchicaneequippedwithbeampositionmonitorsmeasuring
small changes of the beam orbit through the chicane at different beam energies. A single bunch
energy resolution close to 5·10−4 was measured, which is satisfactory for most scenarios. We
alsoreportontheoperationalexperience withthechicane-based spectrometerandsuggestwaysof
improving itsperformance.
KEYWORDS: Beam-lineinstrumentation, Instrumentation forparticleaccelerators andstorage
rings-highenergy, Hardwareandaccelerator control systems.
Contents
1. Introduction 1
2. Testbeamsetupandspectrometer hardwareconfiguration 2
3. Performanceoftheprototypespectrometer 6
3.1 Reconstruction ofthebeamorbitinthemiddleofthechicane 6
3.2 Estimateofthebeamenergyandscalecorrection 8
3.3 Resolution oftheenergy BPMs 10
3.4 Dipolemagnets 11
3.5 Energyresolution ofthespectrometer 12
3.6 XtoYcoupling 15
4. Suggestionsforfutureexperiments 15
5. Summary 17
1. Introduction
The physics potential of the next e+e− Linear Collider depends greatly on precision energy mea-
surements of the electron and positron beams at the interaction point (IP). Beam energy measure-
mentsaremandatoryfortheprecisiondeterminationofthefundamentalpropertiesofparticlescre-
atedintheprocessesofinterest. Forexample,measuringthetopmasstoorderof100−200MeVor
measuringthemassoftheStandardModelHiggsbosontoabout50MeVusingtheHiggs-strahlung
process requires the luminosity-weighted collision energy to be known to a level of (1−2)·10−4
toavoidthisbeingthedominantuncertainty [1].
The strategy proposed in the International Linear Collider (ILC) design report [2] is to have
redundant beam-based measurements capable of achieving a 10−4 relative precision on a single
beam, which would be available in real time as a diagnostic tool to the operators. Also, physics
reference channels, such as e+e− → m +m −g , where the muons are resonant with the known Z-
mass, are expected to provide valuable cross-checks of the collision energy scale, but only long
afterthedatahadbeenrecorded.
The primary method planned to perform the beam energy (E ) measurements at the ILC is a
b
non-invasive energy spectrometer using beam position monitors (BPMs). The proposed setup is
similar to that used for calibrating the energy scale for the W-mass measurement at LEP-II [3].
AttheILC,however, theparameters ofthespectrometer aretightly constrained toprovide limited
emittance dilutionatthehighest ILCenergyE =500GeV.
b
∗
Correspondingauthor
–1–
Initially, a 3-magnet chicane located upstream of the interaction point just after the energy
collimators of the beam delivery system (BDS) was proposed [4]. However, the baseline ILC
spectrometer design uses two dipole magnets to produce a beam displacement x, while two more
magnets return the beam to the nominal beam orbit. Forsuch a chicane, the beam energy (to first
order)isthengivenby
c·e·L
E = Bdl , (1.1)
b x Z
magnet
where L is the distance between the first two magnets, Bdl the integral of the magnetic field in
eachmagnet,cthespeedoflightandetheelectric chargReoftheelectron.
The 4-magnet chicane avoids spurious beam displacement signals in the BPMs due to the
inclination ofthebeamtrajectory, andthussystematic errors inE measurements. Forthisreason,
b
a 4-magnet spectrometer, which maintains the beam axially with respect to the axis of the cavity
BPMs,seemspreferabletoamoreconventional3-magnetchicane. Inbothcasesthemagneticfield
in the spectrometer chicane can be recorded and reversed for studying systematic effects without
changing thebeamdirection downstream ofthespectrometer [5].
Adispersionof5mmatthecentreofthechicanecanbeintroducedroutinelywithoutasignif-
icantdegradationofthebeamemittanceduetosynchrotronradiation. Whenoperatingwithafixed
dispersion of 5 mm over the whole energy range, a micrometre-level BPM resolution is needed.
This resolution can be achieved with cavity BPMs [6]. Since the spectrometer bending magnets
need tooperate at lowfields when running the ILCatthe Z-pole, the magnetic fieldmeasurement
may not be accurate enough to provide the required level of precision. A significantly improved
BPMresolution would, however, allowthemagnetstoberunatthesamefieldforboththeZ-pole
andhighest energyoperation.
Some original energy resolution studies of the SLAC prototype 4-magnet chicane were pre-
sented inreference [7]. Theanalysis used calibrated beam position readings but revealed that due
to small differences between the magnets in the chicane the beam inclination also needs to be
considered. Theanalysis hasherebeenextended byusingcomplexBPMreadings thatcontain the
informationonboththebeamoffsetandinclination. Thisapproacheliminatestheneedforposition
calibration oftheBPMs,whilethewholesystemcanbecalibrated bymeansofanenergy scan.
Inthispublication weestimatetheresolution ofthespectrometer tocompareitwiththeresult
of 8.5·10−4 measured in [7]. We also consider the impact of different systematics on the energy
measurement inordertoimprovetheresolution tothe10−4 levelinfutureexperiments.
2. Testbeam setup andspectrometer hardware configuration
A prototype test setup for a 4-magnet chicane was commissioned in 2006 (the T-474 experiment)
and extended in 2007 (the T-491 experiment) in the End Station A (ESA) beamline at the SLAC
NationalAccelerator Laboratory [8].
Inourexperiments theelectron beamgenerated bythemainLinearAccelerator atSLACwas
transported to the ESAexperimental area through the 300 mlong A-line, which includes bending
and focusing magnets, diagnostic instruments, such as stripline and Radio Frequency (RF) cavity
–2–
Figure1. Betafunctions(b ,b )andhorisontaldispersion(D )intheA-lineandESAbeamline.
x y x
BPMs, charge sensitive toroids, a synchrotron light monitor, profile screens and waveguide pick-
ups. TheSLAClinacprovidedsinglebunchesat10Hzandanominalenergyof28.5GeV,abunch
charge of 1.6·1010 electrons, a bunch length of 500 m m and an energy spread of 0.15%, i.e. a
beam with properties similar to the ILC expectations at the highest energy currently available for
electrons.
Thesebeam parameters allowedustotestthecapabilities oftheproposed spectrometer under
realistic conditions. Two feedback systems were in place for the ESA beam: one for its position
and one for theenergy. Theposition feedback stabilised the beam position and angle using cavity
BPMsandcorrectormagnetsupstreamoftheESAarea. Theenergyfeedbackstabilisedtheenergy
by controlling the phase of the klystrons, and thus the accelerating gradient, in one of the linac
sections. The energy feedback was also used for offsetting the energy from the nominal value in
approximately 50MeVstepswithina±100MeVrange,thusprovidingaroughenergycalibration
forthespectrometer.
Remaining beam energy drifts change the beam orbit through the transfer line, resulting in
increased beam losses as the trajectory wanders off the optimal one. Monitoring these losses and
correcting for the drifts manually, the linac operators kept the beam energy within a ±1% range
around 28.5GeVduringtherun.
Figure 1 shows the horisontal and vertical beta functions as well as the horisontal dispersion
throughout the A-line and ESA. The maximum dispersion and horisontal waist are at the loca-
tion of the high power momentum slits. The dispersion is then minimised throughout the ESA
experimental area. Detailedinformation ontheopticsstudiesintheESAcanbefoundin[9,10]
Thesetup,asschematicallyshowninfigure2,includesfourbendingmagnetsdenotedas3B1,
–3–
Figure2. SchematicrepresentationoftheprototypespectrometerinESA.
3B2, 3B3 and 3B4, forming a chicane in the horizontal plane and high-precision cavity BPMs
upstream, downstream and in between the dipole magnets. Two of them (BPMs 4 and 7) in the
middle of the chicane were instrumented with precision movers. When the magnets were turned
on, these BPMswere mechanically movedtoensure the beam offset fitsthe dynamic range ofthe
BPM electronics. These movers were also used for position calibrations. Horizontal positions of
threeBPMs(3,4and7)weremonitored withaZygointerferometer [11].
The 10D37 magnets from the old SPEAR injection beamline, refurbished for the use in the
chicane, are 37" long, 10" wide on the pole faces and have a 3" gap (approximately 94, 25 and
8cmrespectively), thefirstmagnetofthechicane canbeseen infigure3. Theywereruninseries
from a single power supply to minimise relative drifts. The magnets were studied during a set of
measurements intheSLACMagnet Measurement Laboratory. Magnetic fieldmapsofthevertical
fieldcomponentB weretakenusingNMRandHallprobes,whileeach Bdl wasmeasuredusing
y
a flipcoil, which wascalibrated against amoving wire system. StabilityRand reproducibility were
atthefocusofthesemeasurements. Detailsofthefieldmeasurements canbefoundin[7,12,13].
Figure3. OneoftheSLACILCtypeBPMs(BPM5)followedbythefirstmagnetofthechicane(3B1)in
theESAbeamline(left),oneoftheSLACtypeBPMsusingrectangularcavities(BPM9)(right).
In situ at ESA,two NMR probes with different, but overlapping working ranges and initially
also one Hall probe were installed in the first magnet 3B1, while one NMR probe was positioned
in each of the other three magnets, so that field integral values could be monitored. In the test
–4–
dataruns,thenominalmagneticfieldintegralwassetat0.117T·m,whichcorresponds toacurrent
of 150 A. The stray field outside the magnets in the middle of the chicane was monitored using
two low-field fluxgate magnetometers. One was placed on the girder to obtain the horizontal (x)
andvertical(y)fieldcomponents andtheotheronthebeampipemeasuring they-component only.
Properties oftheprobesandthefluxgatemonitorsaresummarisedinfigure4.
X
NMR0, working range 0.043−0.13 T
ReadingB
y
BPM4, BPM 7 Fluxgate on girder
ReadingB /B
y x
Hall probe (only in March) Z Beam Direction
3B1 (cid:0)(cid:1) 3B2 3B3 3B4
(cid:0)(cid:1)
NMR1, working range 0.09−0.26 T Fluxgate on beam pipe Wire scanner NMR4, working range 0.09−0.26 T
ReadingBy ReadingBy ReadingBy
NMR2, working range 0.09−0.26 T NMR3, working range 0.09−0.26 T
ReadingB ReadingB
y y
Figure4. Magneticfielddiagnosticsinthespectrometerchicane.
The readout unit for the NMR probes provided one internally-averaged reading every 2.5 s.
The probes were multiplexed, sharing the same readout. Typically 9 readings were obtained for
eachprobebeforeswitchingtothenextprobe,totallinganobservation timeofabout20s. Thegap
between observations, while other probes were read out, was about one minute, while an energy
scantookabout3minutesat10Hzbeamrepetition frequency. Therefore, onlyslow(compared to
thedatarate)variations ofthemagneticfieldcouldbetrackedreliably.
In order to measure the beam orbit, 8 cavity BPMs, all operating in the RF S-band, were
installed. Three ofthem wereSLACprototype ILCBPMs(3, 4, 5)using cylindrical cavities with
x-andy-waveguidesforthedipolemodecouplingandmonopolemodesuppression(figure3,left).
Each of the five SLAC BPMs (A-line-type BPMs 1 and 2, and linac-type BPMs 9, 10, and 11)
consistsofthreecavities: tworectangularcavitiesforxandyseparatelytoavoidx-ycouplings,and
one cylindrical cavity to provide charge and phase information [14] (figure 3, right). BPM 7 was
a dedicated ILC prototype designed and manufactured in the UK for the use in the spectrometer.
Unfortunately, thismonitor could notbeused intheanalysis due tomanufacturing problems [15].
Micrometre level resolution was measured for BPMs 1 and 2, while BPMs 3, 4, 5, 9, 10 and 11
demonstrated a resolution below 1 m m. Details on the performance of the BPM system and the
A-lineconfiguration canbefoundin[6].
BPMs12and24areplacedinthebendingarcregionoftheA-line,wherehorizontaldispersion
reaches about 0.5 m. For our experiment they were instrumented with the same high-sensitivity
electronics asallother BPMsintheESAbeamline, sothattheenergy measurements intheA-line
andinthechicanecouldbeperformed simultaneously andcross-checked againsteachother.
–5–
3. Performance oftheprototype spectrometer
3.1 Reconstruction ofthebeamorbitinthemiddleofthechicane
Asthechicanemagnetsbendthebeaminthex-direction,wearemainlyinterestedinthehorizontal
beam position and angle, and, unless specified otherwise, we refer to the x-coordinate throughout
thissection.
In our system, signals generated by the BPMswere digitised and stored in data files for each
event, i.e. for each beam trigger. They are digitally demodulated in the analysis [6]. A complex
digital local oscillator signal allows decoding of both the amplitude and the phase of the signal’s
phasoralongthewaveform. Sampledatapointclosetothepeakandnormalisedbythephasorfrom
the reference cavity, the converted waveforms give the real, in-phase (I), value and the imaginary,
quadrature (Q),value,whichcontaintheinformation onthebeamoffsetaswellastheinclination.
Theoffsetofthebeamtrajectory inthemiddleofthechicanehastobemeasuredwithrespect
to the nominal orbit position reconstructed using BPMsoutside ofthe chicane. In order toform a
predictionofthebeampositionattheBPM4locationwetookdatawithzerocurrentinthemagnets
and selected a “quiet period”, when neither the beam nor the hardware settings were altered. We
then correlated the I and Q readings of BPM 4 with the data from other BPMs. Forming the
prediction canbevisualised ascontinuing thebeamtrajectory lineconnecting thepointsmeasured
byotherBPMsuptoBPM4location. Thebestsetoflinear correlation coefficients minimisesthe
offsetbetweenthatlineandthemeasuredpointsforthemajorityofthebeampasses.
Data from a run with magnets on could also be used for relative measurements and would
resultinabetterprediction,however,duetotheresidualdispersioninthebeamline,beampositions
before and in the middle of the chicane are correlated. Hence, only data from a run with magnets
offwereused.
BPMs9,10and11werenotusedforthepredictionbecause,whenmagnetsareon,theimpact
of the chicane on the beam orbit is not fully compensated, and the beam offset in these BPMs is
energy-correlated.
Due to alignment errors, there is also a correlation between the vertical beam position and
anglebeforethechicaneandthehorizontalbeampositionandangleinthemid-chicane. Therefore,
both x and y readings from the BPMsupstream of the chicane (x1, x2, x3, x5, y1, y2, y3 and y5)
wereusedintheanalysis.
Inordertoreconstruct thebeam orbit inthemid-chicane, the IandQvalues from BPM4are
correlated tothe I and Qvalues from the upstream BPMs. This means solving anoverdetermined
setoflinearequations:
Ax=b, (3.1)
where A is a matrix containing the readings of the n selected BPMs for m beam pulses (we used
severalthousands), andanadditional unitycolumnfortheoffset:
–6–
I I ··· I Q Q ··· Q 1
1,1 2,1 n,1 1,1 2,1 n,1
I I ··· I Q Q ··· Q 1
1,2 2,2 n,2 1,2 2,2 n,2
A= ... ... ... ... ... ... ... ... ...,
 
I I ··· I Q Q ··· Q 1
1,m 2,m n,m 1,m 2,m n,m
 
andbisthevectorofeitherIorQreadings ofBPM4:
I Q
BPM4,1 BPM4,1
   
I Q
BPM4,2 BPM4,2
bI = .. , bQ= .. .
 .   . 
   
I  Q 
BPM4,m BPM4,m
   
We applied the Singular Value Decomposition (SVD) method [16] to solve these equations.
FromtheSVD
w
1
 
w
2
A=U· ... ·VT, (3.2)
 
 
 w 
2n+1
 
andthesolutions ofeq.3.1canbefoundas:
x =V·diag(1/w )·UT ·b , (3.3a)
I j I
x =V·diag(1/w )·UT ·b . (3.3b)
Q j Q
x and x are vectors of coefficients, which relate the Is and Qs of all selected BPMs to those of
I Q
BPM4sothataprediction canbemade:
I = I I ... I Q Q ... Q 1 ·x , (3.4a)
BPM4,pred 1 2 n 1 2 n I
(cid:16) (cid:17)
Q = I I ... I Q Q ... Q 1 ·x . (3.4b)
BPM4,pred 1 2 n 1 2 n Q
(cid:16) (cid:17)
The difference between the predicted and the measured values is the residual. In our case,
the RMS residual is the precision of the orbit prediction and the resolution of BPM 4 added in
quadrature. Itsetsthelimitonthespectrometer resolution. Themeasuredandpredicted valuesfor
IandQareplottedagainsteachotherinfigure5. Thepointsintheseplotsliearoundthey=xsolid
lines, which means the prediction works correctly. The histograms in the bottom part of figure 5
showtheresiduals, forboththeIandQvalues.
It is clear that the I and Q residuals for BPM 4 are small compared to the average I and Q
values, but the results in figure 5 are still hard to interpret quantitatively. In order to set the scale
we used the mover scan data. During the mover scan BPM 4 was moved in 0.25 mm steps from
−0.5to+0.5mmwithrespecttothenominalposition. Theprecisionofthemoversystemisabout
10m m,butthemovescanalsobeobservedbytheinterferometer withasub-micrometreprecision.
–7–
Figure5. BPM4readingspredictedfromotherBPMsinthebeamline:IpredictedvsImeasured(topleft),
QpredictedvsQmeasured(topright),Iresidual(bottomleft),Qresidual(bottomright).
Figure 6 shows the scan data as well as the position residual, which was calculated for the data
used in the SVD computations above. A position residual of 2.73 m m was determined, which is
closetotheestimatein[7](2.3 m m).
The residual is larger than our earlier published value [6], which was close to 1 m m. This is
due tothe movementofBPM4from itsoriginal location between BPMs3and 5tothemiddle of
thechicane andexclusion ofBPMs9,10and11fromthisanalysis. Therefore, BPM4,whichwas
previously inthe“centreofgravity”, hereisattheedgeoftheBPMsystem. Clearly,theprecision
oftheorbitreconstruction atBPM4wasaffected.
Together with the 5 mm nominal beam offset in the middle of the chicane for magnets on,
the 2.73 m m precision of the BPM system sets an energy resolution limit of 5.5·10−4 for our
spectrometer prototype.
3.2 Estimateofthebeamenergyandscalecorrection
TheIandQreadingspredictedforBPM4byallotherBPMscanbesubtracted fromthemeasured
valuesand,whenthemagnetsareon,provideinformationonhowthebeamtrajectorychangeswith
theenergy.
When turning the magnets on, we also moved BPM 4 by 5 mm in order to keep the beam
centred. ThismovementwasobservedbytheZygointerferometer. Accordingtotheinterferometer,
BPM4movedby5.0034 mmbetween our selected runs withmagnets onand magnets off. Using
–8–
Beam offset in BPM4 during a mover scan BPM4 residual
m 0.6 EEnnttrriieess 88660099
m
et, 0.4 500 MMeeaann 99..552255ee--0077
s RRMMSS 00..000022772299
Off 400
0.2
0 300
-0.2 200
-0.4 100
-0.6
0
0 200 400 600 800 1000 1200 1400 -0.02 -0.015 -0.01 -0.005 0 0.005 0.01 0.015 0.02
Event number Residual, mm
Figure6. BPM4positionforahorizontalmoverscan(left),BPM4residualduringaquietperiod(right).
theIQrotation andscalefromthemoverscan,wecanpredictthechanges oftheIandQvaluesof
BPM 4. This results in offsets of I =−8784 and Q =−4605, which were added to the I and Q
0 0
valuesfromtheenergy scanafterthepredictions hadbeensubtracted (figure7,topleft).
Prediction subtracted Q vs I for BPM4 Calibration plot for the energy spectrometer
u. V
Q, a.-4690 g, Me100
n
-4700 etti 50
s
k
-4710 ac
db 0
-4720 ee
y f
g
-4730 er -50
n
E
-4740
-100
-4750
-9040 -9020 -9000 -8980 -8960 -8940 -10170 -10160 -10150 -10140 -10130 -10120 -10110
I, a.u. BPM amplitude change, a.u.
Beam energy measured by the spectrometer Spectrometer noise
GeV28.65 90 EEnnttrriieess 11881133
y, 80 MMeeaann --00..0022115522
nerg 28.6 70 RRMMSS 1100..2255
E
28.55 60
50
28.5
40
28.45 30
20
28.4
10
28.35
0
0 200 400 600 800 10001200140016001800 -50 -40 -30 -20 -10 0 10 20 30 40 50
Event number Measured residual, MeV
Figure7. Beamenergymeasurements:predictionsubtractedQvsIforBPM4(offsetbyQ andI totake
0 0
into account the 5.0034 mm move), with a fit to the data shown (top left), energy calibration plot for the
spectrometer(topright),beamenergymeasuredduringthescan(bottomleft),spectrometernoisemeasured
offtheenergyline(bottomright).
Althoughasmallinclinationofthebeamorbitisintroducedalongwiththeoffsetinthemiddle
–9–