Proceedings of IBIC2012, Tsukuba, Japan

MOPA30APPLICATION OF EMMA BPMS TO THE ALICE ENERGY RECOVERY
LINAC

Abstract
The ALICE Energy Recovery Linac Arc1 button
pickups have been recently equipped with EMMA BPM
electronics. These EPICS VME BPMs give bunch-bybunch information about charge and position, allowing
investigation of beam dynamics in ALICE in different
modes of operation. A Mathematica program is designed
to monitor statistically individual bunches (spacing
61.54ns) as well the train as a whole (up to 1625
bunches), allowing the study of jitter and position stabilityof the beam through the Arc1. The Arc1 has been
designed to be isochronous, with the bunch compression
achieved through a separate dedicated bunch compressor
chicane. The Arc1 incorporates two sextupoles forcorrecting non-linear longitudinal matrix terms andexperimental evidence suggests that the off-centred beamin the sextupoles breaks the linear isochronicity. We
present some beam measurement results collected in 2012using these BPMs.

INTRODUCTION
The ALICE (Accelerators and Lasers In CombinedExperiments) facility, shown in Fig.1, is an energy
recovery test accelerator operated at Daresbury
Laboratory since 2006 [2].
The accelerator consists of: a photoinjector with DC
gun (up to 350 keV); buncher and superconducting
booster (typically 6.5 MeV beam energy); an energyrecovery loop (typically 26 MeV beam energy) containing
a superconducting linac module; a bunch compression
chicane; and an FEL undulator.
The main demands on the ALICE beam dynamics andbeam quality come from the IR-FEL and the coherent
THz emission from the compression chicane used for
dedicated experiments. By design the ALICE latticeconsists of an isochronous first arc (Arc1), a bunch
compressor with R56= –0.28m, and a second arc with R56= +0.28m. The arc design is based on triple bend
achromats (TBA) [3], and the R56 is tuneable by thestrengths of quadrupoles within the arc.
The R56 of Arc1 strongly influences the post linac
bunch compression. This has been consistently observedin both THz as well as FEL setups. Due to a previous lackof reliable beam diagnostics in Arc1, it has not beenpossible to investigate beam dynamics in detail,especially through the sextupoles, which are needed toprovide second order correction. It has consistently been
observed that the two sextupoles steer the beam andmodify the transverse optics, making Arc1 non___________________________________________#alexander.kalinin@stfc.ac.uk

isochronous and affecting the beam dynamics in thetransverse as well as longitudinal planes. FEL lasing was
found to be very sensitive to the setting of the first
sextupole, whereas the second sextupole has neverdemonstrated any improvement in either the FEL or THz
setups.
In order to understand the beam dynamics in Arc1 andthe chicane, the pickups 01 to 06 in Arc1 (see inset of Fig.
1), and an additional pickup in the chicane, have been
recently equipped with EMMA BPM electronics. It is
possible to connect any five (from seven) pickups to theelectronics at a time. These BPMs provide information
about misalignments and trajectory errors in Arc1 as wellas providing bunch-by-bunch and train-to-train
information about charge and position.
Additional information from the time-of-flight (ToF)
measurements [4] combined with these observations
should be able to provide a better understanding of beam
dynamics, and help in explaining the current performance
limitations.
We present here the first experimental results obtainedusing these BPMs, and describe the details of BPM
capabilities and the Mathematica processing programused for analysis.

BEAM POSITION MONITORSOne of the ALICE functions is to deliver beam to a NSFFAG EMMA. EMMA’s BPMs [1] are designed for turnby-turn measurements (turn is 55.2ns). Four of them of
the same type are used in the ALICE to EMMA Injection
Line to monitor a single bunch train from ALICE. These
BPMs were modified to work with ALICE many-bunchtrains, which is useful for injection tuning and opens thepossibility to apply these BPMs to ALICE as well. The
ALICE train bunch rate can be set to (1.3GHz/16)/N,
where N=1, 2, ... is an integer. For most of ALICE
experiments, N=5 (bunch spacing T=61.54ns). This rate
has been used for the BPM measurements below. The
train length was up to 1625 bunches (which is typical for
IR-FEL operation). The bunch charge was in the range(30 to 60)pC.
The Arc1 and chicane pickups are rectangle pickups
with two pairs of horizontal buttons symmetrically spacedfrom the x, y planes. In the measurements below wecalculated the beam offset in the simplest way using aformula ((V11-V12‫ט‬V21±V22))/, and the charge simply
as =V11+V12+V21+V22. The pickups have no
fiducials, so the relative positions of the BPM centres tothe quadrupole centres, or the beam pipe, are unknown.
Each two-plane BPM (see [1]) comprises two FrontEnds placed near the pickup. Each of them works with
two opposite button signals. It first converts them into

ISBN 978-3-95450-119-9
Beam Position Monitor System1

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○A. Kalinin#, D. Angal-Kalinin, F. Jackson, J. K. Jones, P. H. Williams,
ASTeC, STFC Daresbury Laboratory, Warrington, U.K

MOPA30

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 1: Layout of the ALICE accelerator (magnified Arc1 layout in a box on the right).
compact 700MHz three-period packets and thenmultiplexes the packets in the time domain into one
channel.
The doublets are transmitted through low loss cables toa remote two-channel VME card. In each channel, thebunch-by-bunch doublets are amplified, detected,
measured, and then stored in memory. After the lastbunch, the card sets the number of detected bunches to beread by EPICS, and then generates a VME interrupt.
Next, each memory is read by EPICS in the time between
successive injections (max. 10Hz rate).
The Arc1 BPM resolution (on the thermal noise plateau     
The BPM output provided by the Mathematica programis given in Fig. 2.

.NET interface (via ActiveX controls) to the underlying
EPICS control system allows the reading of BPM
voltages at several Hz, but not at the 10Hz train repetition
rate. A much faster native .NET EPICS interface is alsoavailable, but does not currently allow passing of therequired array data. This is a planned upgrade, and would
allow BPM readings to be taken at least as fast as the train
repetition rate. Voltage to beam position calculations are
performed within Mathematica, so allowances for BPM
geometry and scaling can be varied external to the control
system. Control of BPM electronics variables, such as
delays and channel attenuations, can be done directlyfrom the Mathematica front-end. The Mathematica
environment allows for rapid analysis of the beam
position data, including FFT analysis, jitter analysis andautomated parameter scans. An interface to several optics
and tracking codes (MAD, Elegant and GPT), can also
allow the simulation of beam dynamics issues, at thesame time as experimental data is being taken andanalysed. Work has been performed to confirm thefeasibility of a feedback system within Mathematica, but
is not implemented due to the limited number of BPMs
currently available.

BPM READINGS ANALYSIS

Figure 2: BPM-01 readings (the photo-injector laser is set
nominally to 30pC bunch charge). The horizontal axis isthe bunch number. The upper plots are transverse bunch
offsets. The left lower plot is the bunch charge (arbitrary
units), the right one is an auxiliary plot of two pairs of
button signals as measured by the ADCs.

MATHEMATICA PROGRAM
Analysis of the BPM signals is performed in the
Mathematica programming environment [5]. A windows

The readings presented in Fig. 2 show typical beam
behaviour. With imperfectly tuned energy recovery, aglobal slope appears on BPMs located in the dispersive
regions.
All plots show an initial ‘jump’, and ‘ripple’ of various
kinds. The first task is to try to identify the sources ofeach component, whether it comes from the beam or it isa feature of the BPM(s). Next, at least, should be tooutline further investigation.
In Fig. 3, DFT spectra of two pairs of pickup voltages
given in Fig. 2 are shown, where full frequency range (0to 8)MHz is divided into low (0 to 350)kHz and high (0.3
to 8)MHz frequency ranges. The DFT resolution is
10kHz. Spectra of other BPMs are similar.
One can see that the ripple of any kind is present ineach signal, independently of the button/channel number.

ISBN 978-3-95450-119-92Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, JapanDFT applied to transverse and charge readings results inspectra that differ from pickup voltage spectra.

MOPA30

into the transverse plane and it can be concluded that theroutine intensity normalisation in the BPM does not
generate errors in the transverse plane.

Figure 3: DFT spectra (arbitrary units) of pickup voltages
given in Fig. 2. The DC components are subtracted.

Low Frequency Ripple
Figure 4: DFT of x, y, and charge BPM-01 readings. A
low frequency range is shown. The frequency axis is in
Hz.

High Frequency Ripple
In the high frequency range (see Fig. 5), the mostprominent peaks in the transverse plane are about 6MHz
(in Fig. 2, this three-bunch-period oscillation is seen as
some ‘hair’ but is well recognizable as a fast ripple in theplots of Fig. 6 below). The plots of three BPMs wereobtained from three measurements done in different
times. Note each peak is accompanied by a smaller side
peak separated by 50 kHz.

Figure 5: High frequency peaks in three BPMs (BPM01
blue, BPM03 magenta, BPM04 brown).
The origin of this high frequency oscillation, which ismostly prominent in the transverse plane and variable inISBN 978-3-95450-119-9
Beam Position Monitor System3

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Consider the low frequency range in Fig. 4. It looks
that the minor peaks near 300kHz in all three plots areproduced by DC-DC converters of BPM VME cards, thatfeed analogue electronics. According to the datasheet, theswitching frequency of the converters lays near 300kHz.
In the charge plot in Fig. 2, this ripple is seen as a faster
oscillation (period is about 55 bunches).
In the charge spectrum, the peak (20 to 30) kHz is
probably produced by a ripple of the ALICE DC gun highvoltage, which is obtained from a switch-mode PS. Its
switching frequency is just about 20kHz. [6] This ripple(period is about 800 bunches) is practically notrecognizable in Fig. 2. Gun voltage variation causes
electron energy variation, and this can cause loss ofelectrons due to changes in longitudinal (and transverse)
dynamics, which in the gun region is highly sensitive toelectron velocity, space charge, etc. In addition, theenergy modulation should directly manifest itself in thetransverse plane in dispersive regions.
The peaks (60 to 80) kHz in the charge spectrum
correspond to a slower oscillation in Fig. 2 (period isabout 300 bunches). This oscillation that can also be seenin the Faraday Cup signal, and is assumed to be due to thephoto-injector laser pulse-to-pulse power variation.
Peaks in the transverse spectra in the range (30 to 200)
kHz appear to be produced by a magnetic field ripple of
the ALICE dipole and quadrupole magnets (in thehorizontal plot of Fig. 2, an oscillation of 170 bunches
period corresponds to a most prominent peak at~100kHz). We have observed that this ripple depends onmagnet current settings. The magnets are fed fromvarious types of switch-mode PSs, with switchingfrequency ranges from 20kHz to 80kHz [6]. One can
suppose that stray magnetic field from a magnet in thisfrequency range can affect the beam through someadjacent thin-wall bellows. One can see that a low
frequency ripple in the charge reading does not penetrate

MOPA30

Proceedings of IBIC2012, Tsukuba, Japan

strength for each pickup, is not clear. We noticed that in
the beginning of our BPM run on ALICE, this fast
oscillation amplitude was considerably lower than that in
the data taken presented here. One can carefully suggestthat the oscillations might be caused by a dipole TM
mode that is excited by off-centre bunches in thebuncher/booster/linac RF cavities. In the BPM, thisoscillation is under-sampled by the ADC which brings thefrequency down. Note such purely transverse oscillation
can manifest itself in the charge as well, due to, forinstance, imbalance of propagation times in cables, etc.
The ripple of various kinds above exceeds several times
the BPM thermal noise position resolution. For somebunch-by-bunch measurements on ALICE this accuracy
deterioration is still not decisive. Further investigation
using both beam-based models of the accelerator, as wellas engineering analysis of the BPMs, is required to obtaina fuller picture of the noise seen.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Charge Transient: Transverse Plane Transients.
All three transients seen in Fig. 2 are shown in detail in
Fig. 6. A charge increase over first forty bunches seen in
Fig. 6, is a feature of the photo-injector DC gun and has
been observed in the Faraday Cup signal as well.
Attributing the transients in the transverse plane to BPM
intensity normalisation error only does not look correct
because, as shown above, the leakage of charge ripple
into the transverse plane is low. This conclusion is
supported by the fact that for a selected charge transient,
the polarity and height of the transverse transients can besomewhat varied depending on the specific ALICE
tuning. The transients observed require further
investigation.

dispersion is large. Measurements of jitter were takenover 100 trains. To remove the effect of initial transients,
the first 100 bunches in each train were discarded in theanalysis.
The bunch-to-bunch jitter within a train was found as
simply as bunch-to-bunch position rms of all bunches inall 100 trains (for each train the centre of mass position issubtracted). The results are shown in Fig.7.

Figure 7: Distribution of bunch-to-bunch position
variation in 100 trains of 1500 bunches for BPM-01(blue) and BPM-05 (magenta).
The bunch-to-bunch jitter rms on BPM-01 (BPM-05)
was found to be 60 Pm (220Pm). At present it is notpossible to conclude quantitatively the relative
contributions of different beam-sizes and dispersions at
the two BPMs to this jitter, since the lattice optics are notwell known. In addition, on BPM-05 a significant slope of
position was observed over the train (probably due toimperfect energy recovery conditions) which further
complicates the analysis.
Thus, jitter measurements need to be carefully
examined alongside additional information on lattice
optics and beam loading to provide meaningful
information. The fast BPMs provide new scope to pursue
these studies on ALICE, and which were previously
infeasible. In addition, it is envisaged to use thesemeasurements in combination with fast diagnostics on the
ALICE IR-FEL radiation to further explore the effects ofthis beam jitter on the beam dynamics of the FEL.

BEAM ORBIT AND DISPERSIONMEASUREMENTS

Figure 6: Transients in BPM-01.

JITTER MEASUREMENTS
The BPMs were used for preliminary measurements ofthe transverse horizontal beam jitter at different locationsin the lattice.
Two locations were used at first, BPM-01, at theentrance to Arc1 where the dispersion is small, and BPM05, at the entrance to the final dipole in Arc1, where theBeam orbit through the arc was measured by switching
on/off the arc quadrupoles. The horizontal beam orbit in
the quadrupoles varies from fractions of a mm up to 8
mm. The next stage will be to carry out beam basedalignments using these BPMs. This will then allow us tocorrect for second order terms using the sextupoles, as
was originally envisaged.
Dispersion was measured in Arc1 by changing the beamenergy using the gradient of the first linac cavity (LC1).
For a central beam kinetic energy of 26 MeV, a unitchange of LC1 grad set gives a 1.07 % change in beam
momentum. LC1 gradient was changed from 33 to 36(~3% energy variation), with a nominal gradient set at35.3 for a beam kinetic energy of 26 MeV. The measured
dispersion at entry to the arc (BPM01) was found to be~25mm instead of zero. This is likely to be due to leaking

ISBN 978-3-95450-119-9
4Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japanof some dispersion from the injection line. The dispersion
was measured on BPMs 02-05 with and withoutswitching on the sextupoles. The dispersion measurement
results are shown in Fig.8. The setting of SEXT01 to 3A
corresponds to an actual FEL setup (where linearisationand compression of the electron bunch is important).
SEXT02 does not normally show any improvement in the
FEL setup and is also set up at 3A for these measurements
to understand its effects.
The linear dispersions are compared against simulationsin Table 1. Due to low dispersion at BPM03 and 04
locations, the second order dispersion contribution showsa slight curvature on the data. Since the beam passes offaxis in the sextupoles, as alignment studies in the arc
demonstrate, switching on the sextupoles change thelinear dispersion (through quadrupole kick) as well as
increases the second order dispersion.

MOPA30

CONCLUSIONS
Several EMMA BPMs have been modified to workwith ALICE bunch trains to give bunch-by-bunch andtrain-to-train information about charge and position.
Analysis of BPM signals is done through Mathematica
program. In a bunch-by-bunch picture, the BPM readings
show an initial jump and ripple of various kinds. Analysisof these peculiar features point us to investigate several
ALICE technical systems as well as the BPMs
themselves, which we plan to do in future ALICE
runs. The BPMs provide us with useful information aboutbunch-to-bunch and train-to-train jitter, and correlating
this information with ALICE IR FEL detector will
enhance our understanding of transverse/longitudinal
dynamics in ALICE. The experimental data confirms thatthe beam trajectory in Arc1 is significantly off-centred,
explaining the observations of difference in ToF
measurements when sextupoles are switched on. We also
see significant differences to linear and second orderdispersion due to off-centred beam in the sextupoles.

The authors would like to acknowledge: Graham Cox
for creating and supporting the .NET interface to EPICS,
as well as general EPICS support; Ian Kirkman has
provided much assistance in simulating the electromagnetic properties of the ALICE BPMS; and Rob Smithfor providing technical support for the BPM hardware.

REFERENCES[1] A. Kalinin et al, “Computing Bunch Charge,

Figure 8: Dispersion Measurements at different BPM
locations. S1(2)_3A is for SEXT01(02) at 3A. Fitted
equations are shown next to the legend for each case.
Table 1: Linear Dispersion in Arc1
Location

Simulated
Dispersion (m)Experimental
Dispersion (m)

BPM01

0.0

0.025

BPM02

0.33

0.352

BPM03

-0.0968-0.068+0.068 with S1=3A

BPM04-0.08521-0.080+0.025 with S1=3A

BPM05

0.34240.356
0.214 with S1=3A
0.323 with S2=3A

Position, and BPM Resolution in Turn-by-Turn
EMMA BPMs”, IPAC12.
[2] Y. M. Saveliev, “ALICE: Status, Developments andScientific Programme”, IPAC12.
[3] H. L. Owen et al, “Choice of Arc Design for the ERL
Prototype at Daresbury Laboratory”, EPAC04.
[4] F. Jackson et al, “Electron Beam Dynamics in the
ALICE IR-FEL Facility”, FEL12, Kyoto, Japan.
[5] Wolfram Research, Inc., Mathematica, Version 8.0,
Champaign, IL (2011).
[6] C. White, R. J. Smith, private communications.

ISBN 978-3-95450-119-9
Beam Position Monitor System5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○ACKNOWLEDGEMENTS