Proceedings of IBIC2012, Tsukuba, Japan

TUPA18DEVELOPMENT OF THE BEAM POSITION MONITORS FOR THE
SPIRAL2 LINAC
M. Ben Abdillah, P. Ausset, J. Lesrel, P. Blache, P. Dambre, G. Belot, E. Marius
Institut de Physique Nucléaire d’Orsay, France
The SPIRAL 2 facility will deliver stable heavy ion
beams and deuteron beams at very high intensity,
producing and accelerating light and heavy rare ion
beams. The driver will accelerate between 0.15mA and 5mA deuteron beam up to 20 MeV/u and also q/A=1/3
heavy ions up to 14.5 MeV/u.
The accurate tuning of the Linac is essential for theoperation of SPIRAL2 and requires from the Beam
Position Monitor (BPM) system the measurements of thebeam transverse position, the phase of the beam withrespect to the radiofrequency voltage and the beamenergy.
This paper addresses all aspects of the design,realization, and calibration of these BPM, whileemphasizing the determination of the beam position andshape. The measurements on the BPM are carried out on atest bench in the laboratory: the position mapping with aresolution of 50 µm is performed and the sensitivity to thebeam displacement is about 1.36dB/mm at the centre ofthe BPM. The characterization of the beam shape is
performed by means of a special test bench configuration.
An overview of the electronics under realization for theBPM of the SPIRAL2 Linac is given.

OVERVIEWSPIRAL2 represents a major advance for research on
exotic nuclei. It will provide better insight into the tableof nuclides, thereby fostering the discovery of new
properties of matter.
SPIRAL 2 Linac is installed in Caen, France. It is
composed of 2 types of cryomodules A and B
respectively for low (β= 0.04) and high (β=0.2) energy
sections. Each of these cryomodules contains respectively
one or two resonators superconducting radiofrequency
cavities.According to beam dynamics calculations, all thecavities operate at 88.0525MHz.
A doublet of magnetic quadrupoles takes place between
the cryomodules for the transverse horizontal and verticalfocusing of the beam.
Measurement of the particle beam position inaccelerators is an essential part of beam diagnostics, andthe Beam Position Monitors (BPMs) provide the basic
diagnostics tool for commissioning and operation ofaccelerators. One of the primary applications of the BPMs
is the stabilization of the particle beam positions throughfeedback.
In order to save room, BPM will be inserted in thevacuum pipe inside the quadrupoles which will be buriedat their turn in the quadrupole magnet.

Different beams are accelerated by SPIRAL2; Table 1
shows the main characteristics of some of these beams at
the start and at the end of the linac.
Table 1: Beam Characteristics
Linac start

Linac End

Beam relative velocity β0.040.2

Beam energy (MeV)

1.4640The specifications of the required BPM resolution forbeam position feedback in SPIRAL2 linac are listed inTable 2.
Table 2: BPM Specifications
Beam position resolution

50 um

Beam position range±20 mmBeam shape resolution

20%

This paper addresses the design, realization andcalibration of these BPMs; it also shows the design
adopted for the realization of the BPMs acquisition cards.

BPM DESIGNSPIRAL2 BPMs have four electrodes mounted directly
within the focusing magnets as shown in Figure 1 (Azoom of the BPM portion is shown on the right) . A BPM
has four electrodes that couple to the beam through theimage charge produced by the beam [1].

Figure 1: BPM position inside the linac.
The influence of the electrode dimensions on the signal
level and harmonic content for different beams wascalculated using the method described by Schulte [2]. The
BPM electrodes are considered as capacitors that areISBN 978-3-95450-119-9
Beam Position Monitor System

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractProceedings of IBIC2012, Tsukuba, Japan

charged by the beam and discharged through a resistor
connected to ground.
BPM diameter and length are respectively equal 48mmand 39mm respectively. Each electrode has an angularcoverage of 63 degrees.

BPM REALIZATIONSPIRAL2 will be equipped with 20 BPM. As
consequence the equipment to be supplied consists of 23
BPMs: one prototype, two pre-series planned for a test
bench to commission the RFQ and will be consideredlater as spares and 20 to be put on operation. Each BPM
has four electrodes and each electrode is composed of afeed-through and capacitance as shown in Figure 2.

Electrode 30
Electrode 38
Electrode 39
Electrode 49

0,06

reflection coefficient

TUPA180,04
0,020-0,02
3,366E-08

3,37E-08

3,374E-08

3,378E-08

3,382E-08

time(s)Figure 3: BPM block TDR responses (zooming at feedthrough location).
Each block is again sent back to CoorsTek for final
integration in the BPM.

Figure 2: Feed-through (left), BPM block(right) and BPM.

Due to different speeds of the beam particle, expectedsignals at each electrode have different shapes along theLINAC: As shown in Figure 4 , low β beams mainly
induce fundamental tone signals at the electrodes output
and more energetic beams with β up to 0.2 induce signals
with harmonic tones as well. To manage different βvalues, the generator sends signals at two different tones:
the fundamental frequency (88.0525MHz) and its firstharmonic (176.0525MHz). Processing higher tones isabandoned.
0,12

BPM realization is divided into three steps:

Feed-Through Realization
Our industrial partner CoorsTek* is in charge of BPM
fabrication, it delivered 110 feed-through that were tested.
The testing first verifies feed-through isolation thenmeasures its capacitance and TDR* response and finallymatches four feed-through to form a single BPM. Fourfeed-through are matched if the difference between there
capacitances is less than 0.1pF and their TDR responses
are identical.
The 110 feed-through delivered by CoorsTek had
capacitances measuring 1.35pF with a margin of ±0.05pF.
Therefore, feed-through matching only takes account of
their TDR response.

BPM Realization
Once the needed feed-through matched, they were sent
back to CoorsTek for BPM block realization.
Twenty BPM blocks were delivered by CoorsTek, theirisolation was verified, and their capacitance weremeasured and verified to be within ±0.2pF per block. TheTDR responses of each block were verified as well: Anexample is illustrated in Figure 3.

Electrodevoltage(V)c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○BPM CALIBRATION

beta=0.2

0,08

beta=0.04

0,04
0,00
0

12

34

567

8

910

harmonic number
Figure 4: Electrode voltage FFT at different beta beams.
Thanks to BPM calibration, two main parameters aremeasured: BPM sensitivity and the displacement between
mechanical and electrical centres of the BPM.
To achieve these goals, the BPM is mounted inside theautomated test bench shown in Figure 5. The BPM ismounted onto an automated XY table that moves thesensor into a predefined point with coordinates (X, Y).
The signal generator continuously sends a signal along acopper wire inside the BPM. The copper wire could havea circular or elliptical shape and it simulates the beam at
β=1. At each position, the power received by eachelectrode is measured by the probes and the power
meters, the measurements are then saved sequentially into
appropriate files.

___________________________________________*CoorsTek: www.coorstek.com*TDR: Time Domain Reflectometry

ISBN 978-3-95450-119-9
2Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, JapanTUPA18

electrode are recorded. These sweeps are over a range of
40mm with a step of 0.5mm. The goal out of these sweeps
is to measure BPM sensitivity for off centered beams.
The same technique mentioned above is used toannihilate differences in cables attenuation and phasing.
BPM sensitivities at a given point (X, Y) are computedusing Eq. 3 and Eq. 4:

S x ( X ,0)P( Elec3) P( Elec1)
.
X Xe

(3)

S y (0, Y )P( Elec 4) P( Elec 2)
.
Y Ye

(4)

Figure 5 : BPM automated test bench.

This measurement is performed at the fundamental
frequency and its first harmonic.

BPM Mechanical Centre Measurement
The cable inside the BPM has a fixed position. The XY
table moves the BPM on four predefined axes until thecable touches the BPM boundaries.
The four positions measured are then combined to findthe BPM mechanical centre coordinates Xm and Ym with aprecision less than 20μm.

BPM Mapping
BPM mapping aims to measure the difference between
theoretical and measured wire positions. Beam measured
position coordinates Xb and Yb are computed in mm using
Eq. 5 and Eq. 6:

BPM Electrical Centre Measurement
The cable is placed at the BPM mechanical centre
position; the power received by each electrode is thenmeasured. These powers are combined to find BPM
electrical center coordinates Xe and Ye.
Cables between BPM feed-throughs and probes induce
differences in magnitude and phase between measured
voltages at BPM electrodes. To annihilate these additional
differences, measurement is performed at four different
cable configurations: for a given electrode, the received
signal is measured using a different cable at eachconfiguration, therefore, four power measurements areperformed and averaged.
Xe and Ye are computed in mm using Eq. 1 and Eq. 2:XbXeP( Elec3) P( Elec1).
S x ( X ,0)

(5)

YbYe

P( Elec 4) P( Elec 2).
S y (0, Y )(6)

The XY table follows the predefined map shown inblue in Figure 6 (distance between adjacent dots is0.5mm).

21

theoretical position

Measured position

XeXm

Ye

Ym

P( Elec 4) P( Elec 2)
.
Sc

(1)

(2)

P is the power received by each electrode and Sc is theBPM sensitivity to beam displacements at the BPM
electrical centre. After a couple of iterations, Sc is foundto be equal to 1.36dB/mm at fundamental frequency and1.41dB/mm at the first harmonic.

BPM Sensitivity Measurement
Once the BPM electrical centre is measured, twosweeps of the XY table (one is horizontal and the other isvertical) are performed and the powers received by eachYposition(mm)14P( Elec3) P( Elec1).
Sc

70-7-14-21-21-14-70

71421

Xposition(mm)Figure 6 : Example of BPM mapping at 88.0525MHz.

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

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The following steps are performed:

TUPA18

Proceedings of IBIC2012, Tsukuba, Japan

Figure 6 shows good agreement between theoreticaland measured position around BPM centre. Thisagreement is distorted at BPM boundaries because of theapproximation in the sensitivities Sx and Sy

BPM Quadrupole Moment Measurement
Estimation of beam shape is based on the computationof σx²-σy² that uses the following formula given byR.H.Miller [2]2y1 V1 V2
K V1 V2

V3 V4
V3 V4Xb2Yb2

Where σx and σy are the beam rms half widths in the xand y directions, Xb and Yb are beam position coordinates
regarding its electrical centre and V1, V2, V3 and V4 are
the sensing electrodes voltages.
The parameter K depends on beam velocity β, BPM
radius and the electrode angular coverage. For β=1,K=0.0028mm-2.
Two different copper wires were used to check Miller’s
formula: A cylindrical wire with diameter of 5mm and an
elliptically shaped wire with major axe length of 5mm
and minor axe length of 3mm. The two wires were
mounted at the BPM electrical centre. 2mm sweeps(horizontal and vertical) were performed around the BPMelectrical centre, the ratio at the left of Miller’s formula
was measured and the difference σx²-σy² was deduced.
As shown in Figure 7, xσ ²-σy ² is close to 0 for circularshaped cable and around 2 for elliptical shaped cable.
2,52

σx²-σy² (mm2)c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○2x1,5

circular shaped cable horizontal sweep
elliptical shaped cable horizontal sweep
circular shaped cable vetical sweep
elliptical shaped cable vertical sweep

10,50-0,5-1,5-1-0,500,5Xb or Yb(mm)11,5Figure 7 : σx²-σy² for different beam positions.
The factor K is expected to be stronger for low beta beams,
exhaustive studies shows K=0.0051mm-2 at β=0.04.

BPM ACQUISITION CARD
Each BPM is supplied with an electronic acquisition
module. The module processes fundamental frequency or
the first harmonic tones contained in the signals delivered
by BPM electrodes in order to deliver beam position,
shape, energy and phase. It also warns the operator if thebeam is 5mm close to BPM boundaries.
The design of the BPM is done in collaboration withour partner BARC*. The BPM module, shown in Figure

8, consists of an analogue board and a digital board. The
design of the BPM module is based on the scheme ofauto-gain equalization using offset tone. In this scheme
the gain of different channels is equalized with respect tothe injected offset tone. The scheme consists of digitally
generating an amplitude and phase stable calibration tone
having frequency slightly offset from the RF Reference.
This calibration tone is added to each of the 4 incoming
sensor signals. Each combined signal is then passed
through respective analogue channel, which essentiallyincorporates the functionality of programmableamplification and selectable band-pass filtering. The
amplification/attenuation is identically set for all the four
analogue channels.
X1

Digital board

Analog board
ADC

Xb
fs

X2

Yb

ADC
fs

Y1

ADC

Y2

FPGA

Energy

fs

Phase

fs

Interlock

ADC

10 MB
Memory

ADC

Shape

fs
DAC
fs

Attenuator and other control signals
fs

Fundamental frequency

Clock
Synthesizer
Communication bus

Figure 8: BPM acquisition module.
These conditioned signals (four electrodes, one selectedreference and one calibration tone) are digitized byrespective ADCs on the digital board. The digitized datafrom the ADCs is sent to FPGA for calculation of output
parameters. The results are also stored in a local memory
for analysis and diagnostic purposes. Two DACs are kept
for diagnostic purposes for viewing the variables in thesignal path. A clock synthesizer generates the samplingclock for ADCs, DACs and FPGA using the 88.0525
MHz reference signal. The system has provision for
digital input and output signals on the front panel forinterfacing to the external world or to be used during
testing and debugging.

CONCLUSIONSPIRAL2 BPM measurements show a good agreement
to expected sensitivity and quadrupole moment. The next
step is to realize and test the BPM acquisition card.

REFERENCES[1] P. Ausset and al “Beam diagnostics for the SPIRAL2
accelerator, SPIRAL2 week, Jan 2011, Caen, France.
[2] J.Bosser, “Beam instrumentation”, CERN Editions, CERNPE-ED-001-92.
[3] R.H. Miller and al., “Nonintercepting emittance monitor”
12th Int. Conf. on high-energy accelerators. Fermi Nat
Accelerator Lab, 11-16 Aug 1983.

___________________________________________*BARC: Bhabha Atomic Research Centre www.barc.ernet.in

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