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Proceedings of IBIC2016, Barcelona, Spain
OPERATION OF THE BEAM POSITION MONITOR FOR THE SPIRAL 2
LINAC ON THE TEST BENCH OF THE RFQ
P. Ausset†, M.B. Abdillah, F. Fournier, IPN, Orsay, France
S.K. Bharade, G. Joshi, P.D. Motiwala, BARC, Mumbai, India
R. Ferdinand, D. Touchard, GANIL, Caen, France
Abstract
SPIRAL2 project is based on a multi-beam superconducting LINAC designed to accelerate 5 mA deuteron
beams up to 40 MeV, proton beams up to 33 MeV and 1
mA light and heavy ions (Q/A = 1/3) up to 14.5 MeV/A.
The accurate tuning of the LINAC is essential for the
operation of SPIRAL2 and requires measurement of the
beam transverse position, the phase of the beam with
respect to the radiofrequency voltage, the ellipticity of the
beam and the beam energy with the help of Beam Position Monitor (BPM) system. The commissioning of the
RFQ gave us the opportunity to install a BPM sensor,
associated with its electronics, mounted on a test bench.
The test bench is a D-plate fully equipped with a complete set of beam diagnostic equipment in order to characterize as completely as possible the beam delivered by the
RFQ and to gain experience with the behavior of these
diagnostics under beam operation. This paper addresses
the first measurements carried with the BPM on the Dplate: intensity, phase, transverse position and ellipticity
under 750 KeV proton beam operation
Copyright © 2016 CC-BY-3.0 and by the respective authors
GENERAL DESCRIPTION OF SPIRAL2
SPIRAL2 facility is being installed in Caen, France. It
includes a multi-beam driver accelerator (5mA/40Mev
deuterons, 5mA/14.5MeV/A heavy ions). The injector is
constituted by an ECR ion source (Q/A= 1/3), an ECR
deuteron/proton source, a low energy beam transfer line
(LEBT) followed by a room temperature RFQ which
accelerates beam up to an energy of 0,75MeV/u. A medium energy transfer line (MEBT) transfers the beam to the
superconducting Linac.
The Linac is composed of 19 cryomodules: 12 contain
one ȕ = 0.07 cavity and 7 contain two ȕ = 0.12 cavities.
All cavities in the cryomodules operate at F =
88.0525MHz.
The superconducting Linac is designed to accelerate
deuterons, protons, heavy ions Q/A= 1/3 and Q/A = 1/6
for a future injector. (Table 1)
Table 1: SPIRAL 2 Main Beam Parameters
Particle
Energy
Current Max
(mA)
(MeV/u)
Proton
5
2 - 33
Deuteron
5
2 - 20
Q/A = 1/3
1
2 - 14.5
Q/A= 1/6
1
2-8
SPIRAL2 nominal mode of operation is planned to be
C.W. mode. The considerations on commissioning and
tuning periods of the LINAC lead to consider also pulsed
mode operation in order to minimize the mean power of
the beam. The shortest duration of a macro-pulse will be
100 ȝs. The repetition rate may be as low as 1Hz and as
high as 1 kHz. The intermediate configurations have to be
taken in account in order to reach the C.W. operation. The
step to increase or decrease either the macro pulse duration or the repetition rate will be 1ȝs.
SPIRAL2 BEAM POSITION MONITORS
General Description
A doublet of magnetic quadrupoles is placed between the
cryomodules for the horizontal and vertical transverse
focusing of the beam. Beam Position Monitors (BPM), of
the electrostatic type, is inserted in the vacuum pipe located inside the quadrupoles of the LINAC.
Each BPM sensor contains four probes on which beam
image currents induce bunched-beam electrical signals.
The electronics board associated with each BPM sensor
processes the electrical signals and enables the measurement of beam transverse position, phase, energy and
transverse beam ellipticityߪ௫ଶ െ ߪ௬ଶ , where ıx and ıy are
the standard deviations of the transverse size of the beam.
BPM Acquisition Modes
The BPMs data and measures are acquired under CW
or pulsed mode operation in three modes: Normal, post
mortem and electrode signal reconstruction.
A synchronizing signal “SF” is distributed simultaneously to the SPIRAL2 diagnostics, including the BPM to
indicate that the beam is present during its high level.
Normal mode: the electronics module acquires the data
on the SF rising edge after a delay and during a given
integration time. The integration time must be less than
the time where the SF signal is high. Both the delay and
the integration times are selected by the operator.
Data: beam centroid transverse position and beam ellipticity, electrode received signal amplitude and vector sum
in phase and magnitude of the four electrodes are transferred to the VME local memory on the fall of “SF” signal. EPICS driver reads the data every 200ms from the
local VME memory.
BPM Specifications
The main specifications for the BPM system are summarized in Table 2.
___________________________________________
†ausset@ipno.in2p3.fr
ISBN 978-3-95450-177-9
642
BPMs and Beam Stability
Proceedings of IBIC2016, Barcelona, Spain
Table 2:Main SPIRAL2 BPM Specifications
Parameter
Position
Phase
Ellipticity
Beam current
Measurement
resolution
± 50m
± 150m
± 0.5deg
± 20%
Measurement
range
± 10 mm
± 20 mm
±180 deg
0.15 – 5 mA
BPM Sensor Mechanical Design
Capacitive sensors have been selected (Electrodes aperture diameter: 48 mm, length in the direction of the beam:
39 mm, subtended lobe-angle: 62°) (Figure 1).
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ITB is positioned after the focusing quadrupole following
the first re-buncher of the M.E.B.T. Two other focusing
quadrupoles are placed between the re-buncher, and the
RFQ. A beam stopper able to withstand nearly the full
power of the beam terminates the ITB which includes 18
beam diagnostics identical to the SPIRAL2 driver ones.
The aim of the ITB is to fully characterize the properties
of the beam accelerated by the RFQ and also to study the
behaviour of these diagnostics. All kinds of measurements
may be carried: beam intensity, transverse beam position,
profiles and emittance, phase and longitudinal emittance
with a beam energy equal to 750KeV/A.
Control command operation gathers the measurements
performed by all these diagnostics almost on real time
(every 200ms). Figure 3 shows the ITB.
BPM Current Dynamic Range
Figure 1: Left: SPIRAL2 BPM central block with the
capacitive electrodes. Right: BPM with its flanges.
The first tests of the BPM and its associated electronics
was to check the beam current dynamic range over which
the BPM electronics are working properly. The beam
current is given by the Faraday cup (CF) of the ITB on
one side and by the magnitude of the four vector sum of
the four electrodes given by the BPM electronics at F =
88.0525 MHz and 2.F on the other side. Horizontal and
vertical slits located in the LEBT vary the beam intensity.
A dedicated test bench based on a coaxial transmission
line has been designed and built in order to characterize
each BPM: electrical center coordinates, position and
ellipticity sensitivities at ȕ=1[1].
Each BPM sensor feeds an electronic module through
eighty meter long coaxial cables. The 20 BPM electronics
modules are located in three VME 64x crates. Each module contains an analog and a digital board. The design of
the analog module of the card is based on the scheme of
auto-gain equalization using offset tone having frequency
slightly offset from the RF reference [2]. The electronic
module is able to work either at 88.0525 MHz or at
176.1050 MHz to deliver the required information.
The Accelerator Control Division of Bhabha Atomic
Research Centre realized the BPM Electronics modules.
Two prototypes of the BPM readout electronics module
were qualified in IPN leading to several upgrades in order
to meet specifications. The prototypes results are in a
good agreement with results obtained with the measurement setup used to characterize the BPMs. [3].
BPM TESTS ON THE SPIRAL2 INTERMEDIATE TEST BENCH (ITB)
General Description of the ITB
An “Intermediate Tests Bench” (ITB) has been assembled as part of the injector commissioning plan [4]. The
Figure 2: BPM beam current dynamic range.
Measurements were performed on fall January 2016.
They showed an upper limitation of the measurement
range (CF current = 3.5mA) at F due to the saturation of
the front end of the BPM electronics (see Figure 2) where
as it is fully operational at 2.F. This saturation is assigned
to a difference in signal magnitude obtained during the
beam operation compared to that obtained by the beam
dynamics simulation.
Further tests were performed on this issue on June 2016
where 10dB attenuations were added at the analogue
inputs of the readout electronics analogue inputs, this
leaded to the following dynamic range: 75 A - 5.5 mA at
F and 60 A – 5.5 mA at 2.F.
ISBN 978-3-95450-177-9
BPMs and Beam Stability
643
Copyright © 2016 CC-BY-3.0 and by the respective authors
BPM Sensor Readout Electronics Module
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Proceedings of IBIC2016, Barcelona, Spain
Figure 3: View of the Intermediate Test Bunch equipped with a full set of beam diagnostics.
BPM Phase Measurement
within 1° for medium and high current where as it is less
precise for the TOF at low beam currents.
Copyright © 2016 CC-BY-3.0 and by the respective authors
The phase relative to the accelerating RF signal has
been measured simultaneously by the BPM and by one of
the three electrostatic P.U. electrodes of the time of flight
(TOF) energy measurement system mounted on the ITB.
The BPM is measuring the phase at F and 2.F whereas the
TOF is only measuring it at F. The RFQ phase was swept
over 360° with a 10° step over different beam currents.
The results of the BPM and TOF measurements were
gathered in order to show the mean value of the measured
phase and the range over which the phase is fluctuating.
Figure 5: Beam phase measurement by a BPM at
176.105MHz and a TOF PU electrode at 88.0525MHz.
BPM phase measurements were also simultaneously
performed at 2.F. with the TOF electrode (same manner
as previously) however the beam conditions were different. TOF measurements at F were performed. The results
are sketched in fig. 5. The same comments reported at F
apply for 2.F.
Beam Position Measurements
Figure 4: Beam phase measurement with BPM and a PU
–TOF electrode at 88.0525MHz.
The results (Fig. 4) show a proper behaviour and a good
agreement with the electrode of the TOF system over the
measured beam current dynamic range: the fluctuation is
The beam position measurements were simultaneously
performed by the BPM and the secondary emission monitor (SEM) profiler (pulsed mode operation) located after a
drift space downstream in the ITB. The results obtained
with the BPM measurements at F and 2.F have been com-
ISBN 978-3-95450-177-9
644
BPMs and Beam Stability
Proceedings of IBIC2016, Barcelona, Spain
Figure 6 : Beam position measurement at 88.0525MHz.
The expected linear response of the BPM is confirmed.
The results are gathered in the table 3 for F and 2.F. The
BPM position sensitivity remains roughly constant over
the current dynamic range. Considering the low value of
the velocity (ȕ ~ 0.04) of the beam accelerated by the
RFQ, we took in account Shafer study [5] which states
that the beam position sensitivity at a frequency f for a
beam travelling in a cylindrical chamber of radius r
should be multiplied by (1+G) where G is:
ʹǤ ߨǤ ݂Ǥ ݎଶ
ʹǤ ߨǤ ݂Ǥ ݎଷ
ܩൌ ͲǤͳ͵ͻǤ ൬
൰ െ ͲǤͲͳͶͷ ൬
൰
ߚǤ ߛǤ ܿ
ߚǤ ߛǤ ܿ
With the help of the measured positions of the SEM
profiler, we were able to calculate easily the expected
position on the BPM. Therefore we applied the adequate
correction to the sensitivities at 88.0525MHz and
176.105MHz drawn from our measurements on our laboratory test bench (S= 25.7 mm at ȕ ~ 1). It came out in
good agreement with the estimations of Shafer.
Table 3: Beam Position Sensitivity
Beam
Current
4mA
1.5mA
0.25mA
Measured
Sensitivity
F
2*F
21.9mm 16.7mm
21.6mm 16.3mm
21.6mm 16mm
Theoretical
Sensitivity
F
2*F
22.3mm 15.9mm
22.3mm 15.9mm
22.3mm 15.9mm
BPM Ellipticity Measurement
The quantity ߪ௫ଶ െ ߪ௬ଶ is delivered by the BPM elec-
tronics and as well drawn from the SEM profiler measurements. The BPM ellipticity sensitivity was set to 354
mm² for all measurements. The beam 2D shape was mod-
ified by changing the current in the quadrupole Q13 located before the BPM (beam current equal to 1.2mA).
The current of Q13 was swept over a range [2.5A - 30A]
with a 2.5A step leading to significant changes of the
ellipticity.
The linear behaviour of the BPM ellipticity towards
Q13 current is confirmed by the measurements (fig. 7).
The same behaviour is confirmed by the BPM measurements at 176.105MHz. Further investigations should be
run to explain the discrepancies between beam dynamics
simulations and measurements.
Figure 7: Beam ellipticity measurement at 88.0525MHz.
CONCLUSION
The BPM sensors realized with its associated electronics for the Linac of SPIRAL 2 have been put on operation
on the ITB of SPIRAL2. We will be able to measure the
transverse position of the beam and the phase of the beam
with respect to the RF signal. However further investigations have to be made concerning the ellipticity measurement. A second BPM sensor with its electronics module
will be mounted very soon on the ITB in order to check
the energy measurement by means of the BPM.
ACKNOWLEDGMENTS
It is a pleasure to acknowledge the constant support of
the SPIRAL 2 team during these experiments.
REFERENCES
[1] M. Ben Abdillah and P.Ausset, “Development of Beam
Position Monitors for the SPIRAL2 LINAC”, in Proc. 1st
Int. Beam Instrumentation Conf. (IBIC’12), Tsukuba, Japan,
October 2012, paper TUPA18, pp. 374-377.
[2] G. Joshi et al., “An offset tone based gain stabilization
technique for mixed-signal RF measurement systems”, Nucl.
Instr. Meth., vol. A 795, 2015, pp. 399-408.
[3] P. Ausset and M. Ben Abdillah, “Development of the Beam
Position Monitors system for the LINAC of SPIRAL2”, in
Proc. 2nd Int. Beam Instrumentation Conf. (IBIC’13), Oxford, UK, September 2013, paper WEPC15, pp. 702-705.
[4] P. Ausset et al, “SPIRAL2 injector diagnostics”, in Proc.
DIPAC 09, Basel, Switzerland, pp. 110-112.
[5] R.E. Shafer, “Beam Position Monitor Sensitivity for Low-ȕ
Beams”, in Proc. 17th Linac Conf. (LINAC’94), Tsukuba,
Japan, August 1994, paper THPA84, pp. 905-907.
ISBN 978-3-95450-177-9
BPMs and Beam Stability
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Copyright © 2016 CC-BY-3.0 and by the respective authors
pared to the profiler measurements (frequency independent). The BPM position sensitivity was set to 25.7 mm for
all the measurements. The beam position was changed by
varying the current of a steerer DC13 located in the quadrupole Q13 located just before the BPM. It was swept
over the range [-5A; 4A] with a 1.5A step. An example is
given in Figure 6
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