Proceedings of IBIC2012, Tsukuba, Japan

MOPB87

DEVELOPMENT AND FIRST TESTSOF A HIGH SENSITIVITY CHARGE MONITORFOR SwissFEL*

Abstract
The compact X-ray free electron laser SwissFEL,
which is presently under development at the Paul Scherrer
Institut (PSI) in Villigen, Switzerland, will operate atcomparably low charges, allowing the compression of theelectron bunches to a few femto-seconds (nominal 200 pCmode) and even towards the atto-second range (shortbunch 10 pC mode). A high precision charge
measurement turns out to be a challenge, especially in thepresence of dark currents, which may occur from highgradient RF gun and accelerating structure operation. In
response to this challenge, a higher sensitivity chargetransformer and new beam charge monitor electronics
were developed in collaboration between Bergoz
Instrumentation and PSI. The Turbo-ICT captures sub-pC
bunch charge thanks to a new magnetic alloy exhibiting
very low core loss. Transmission over a carrier using
narrow-band cable television technique preserves thesignal integrity from the Turbo-ICT to the BCM-RF.
Electro-magnetic and RF interferences are strongly
attenuated; the dark current signal is suppressed. First
beam test results, which have been performed at the
SwissFEL Test Injector Facility (STIF), are presented inthis contribution.

SwissFEL AND THE SwissFEL TESTINJECTOR FACILITY
SwissFEL is a compact free electron laser user facility
presently under design at the Paul Scherrer Institut in
Villigen, Switzerland [1, 2]. The project comprises two
FEL beam lines, which will be realized in two phases.
The three hard X-ray ARAMIS end stations (phase 1) will
provide highly brilliant SASE radiation from 1 to
7 Ångström while the soft X-ray ATHOS beam lines(phase 2) will range from 7 to 70 Ångström [3].
SwissFEL will be operated at 100 Hz repetition rate withtwo bunches per RF pulse at a bunch distance of 28 ns.
Bunch distribution towards the two FEL beam lines will
be accomplished at electron energies of 2.1 GeV by fastkickers in a switchyard. The nominal operation at low
bunch charges between 10 and 200 pC provides excellent
transverse emittances and allows the utilization ofcompact (in-vacuum) undulators providing full hard Xray photon flux at comparably low electron energies of
5.8 GeV. Full compression of the low charge electron
bunches will lead to ultra-short pulses of < 20 fs (rms at
200 pC) and < 2 fs (rms at 10 pC) in the nominal
SwissFEL operation modes and even to towards the atto____________________________________________* The work was done within a Paul Scherrer Institut / Bergoz
Instrumentation collaboration under a June 2010 MoU.

second range in a specific short bunch mode of operation.
A status of the SwissFEL facility and simulations of its
accelerator and FEL performance have recently beensummarized in [4].
Most of the design aspects for the SwissFEL
accelerator sub-systems as well as the experimental
verification of the initial electron beam parameters arepresently examined at the 250 MeV SwissFEL Test
Injector Facility (STIF) (Fig. 1) [5, 6]. An extensive
experimental program is dedicated towards the generationand measurement of the low charge, low emittanceelectron beam. In this context, prototypes of beam
instrumentation specifically designed for the low charge
operation modes are being designed and tested – such as
cavity beam position monitors [7], transverse profile
monitors [8], bunch compressor diagnostics [9] as well as
the beam charge monitors presented in this paper.
First operational experience at STIF and preliminary
results from (mainly longitudinal) sensitivity studies ofelectron beam parameters [10] indicate that the tightest
tolerances for SwissFEL are driven by the peak current
stability, which is mainly related to the longitudinalstability of the RF system (mainly in the injector) and thestability of the bunch charge. In this respect, a reliable
and high precision bunch charge measurement with an
anticipated resolution of 1% at the low charge (10 pC)
operation mode, especially in the presence of dark current
from the high brightness RF gun and/or the high gradientaccelerating structures, turns out to be an important prerequisite for stable and reproducible SwissFEL user
operation. The design of a prototype bunch charge
monitor, the so called Turbo-ICT, and the related BCMRF electronics as well as first test measurements at the
STIF are presented in the following.

TURBO-ICT & BCM-RF PRINCIPLEThe Turbo-ICT sensor and the BCM-RF electronics
receiver perform bunch charge measurements with low
noise and high accuracy. The Turbo-ICT combines an
Integrating Current Transformer [11] of a new kind andfront-end electronics in one assembly. The original ICT
developed for LEP in 1989 was redesigned to measurebunch charges as low as 10 pC with 1% resolution.
Several techniques were used to maximize the signal
taken from the beam and minimize the noise from various
sources: beam dark current, electronics noise, RF andother electromagnetic interferences.
To maximize the amplitude of signal taken from thebeam, the ICT integration time is reduced by a factor of
25 compared to the classical ICT making its amplitude 25
ISBN 978-3-95450-119-9

Beam Charge and Current Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○S. Artinian, J. Bergoz, F. Stulle, Bergoz Instrumentation, Saint-Genis-Pouilly, France
V. Schlott, P. Pollet, Paul Scherrer Institut, Villigen, Switzerland

MOPB87

Proceedings of IBIC2012, Tsukuba, Japan

times higher. The shorter signal requires a processing
bandwidth 25 times higher. It causes the bandwidth noise
to increase by 25, while the signal noise increasesc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○by 25 only. The net signal-to-noise ratio is therefore
improved by 25 . The ICT bandwidth increase is madepossible thanks to new alloys whose core losses remain
below 1% up to 350 MHz.
To further increase the signal multiple cores are used[12]. The available signal is multiplied by the number ofcores coupling with the beam. The core windings outputs
are combined to optimize the power transmission fromcores to front-end amplifier.
The first amplifier stage is located in a front-end
electronics box near the sensor. This close proximity isessential to avoid excessive noise pick up. The amplifier
noise performance is mainly defined by input impedance
and transistor transfer gain. A pHEMT transistor withhigh transconductance and low input capacitance is used[16]. The front-end modulates a 180 MHz narrow-band
carrier similar to cable TV signal transmission improving
electromagnetic interference immunity.
In conclusion, the Turbo-ICT and its front-end
transform a beam signal whose frequency spectrum
extends into the THz region into a narrow-band signal
proportional to bunch charge. During this process the darkcurrent signal is rejected by > 40 dB. The setup is shownin Fig. 1.
Beam

Load impedance

Coupling
networkLow noise amplifier

Narrow-bandprocessing&
50 ohm matching

To BCM-RF
through coaxial
cable

Figure 1: Turbo-ICT with two active cores and front-end.
The Turbo-ICT front-end electronics is powered fromthe BCM-RF receiver through its output coaxial cable. In
the BCM-RF, the modulated carrier is processed througha 500 MHz diplexer removing out-of-band signals.
Rejection of 3 GHz dark current signals is about 50 dB.
The receiver implements a logarithmic RF amplifier toachieve 90 dB input dynamic range and a 10 MHz output
bandwidth. Logarithmic amplifiers are however wideband devices which demodulate any wide-band noisepresent on their input. To reduce this noise, a narrow-band
filter is located at the input. The filter’s centre frequency
is adjusted to the RF signal carrier. The log amplifier
output signal is proportional to the logarithm of bunchcharge. A sample and hold circuit holds the signal
magnitude until the next bunch arrives to allow
comfortable readout. Fig. 2 shows the BCM-RF input
circuit.

50ohm
load
From
Turbo-ICTLog
amplifier
S&H

To BCMRF outputs
stagesFigure 2: BCM-RF input circuit.
An AD8310 logarithmic amplifier [13] was chosen for
its wide input dynamic range and its better sensitivity
compared to others models. It is built of six cascaded
cells. Each one has a fixed 14 dB gain. The transition
from one cell to another produces a periodic log
conformance error versus input signal strength of +/0.4 dB [13]. BCM-RF circuit features a Microchip PIC
microcontroller [14] which can be programmed to correct
this systematic error.

TEST SETUP AT STIF
The purpose of the test is to compare measurementsfrom a classical ICT with BCM-IHR electronics tomeasurements from a Turbo-ICT with BCM-RF
electronics. ICT and Turbo-ICT are installed at STIF
close to each other. Long cables bring their output signals
to their respective electronics. The bunch charge is alsomeasured by a BPM which is calibrated vs. a wall current
monitor. Output values from both BCMs are digitalized
and processed by LabVIEW.
During the charge scan the bunch charge is decreasedby the control room in small steps from a starting value of
200 pC/bunch. For every bunch charge LabVIEW takes
20 measurements, averages them and calculates standard
deviation. The standard deviation expresses themeasurement noise, since the bunch charge variations are
assumed to be minimal. The average value is thenprocessed by a reverse function to get the bunch charge: alinear for the ICT/BCM-IHR and an exponential for theTurbo-ICT/BCM-RF. Figure 3 shows the ICTs/BCMs test
setup.
Long coaxial
Sucofeed 7/8
cable

BCM-RF

Turbo-ICTNI-USB9162

PC
LabVIEW

Data file

BCM-IHRICT
Trigger

Figure 3: Test setup at STIF.

ISBN 978-3-95450-119-92Beam Charge and Current Monitors

Proceedings of IBIC2012, Tsukuba, Japan

250

1000.0

100.0

100.0

10.0

10.0

1.0

1.0

0.1
0.5

1.5

100

100

50

50

0

0
0

10

20

30

40

50

Output View (mV)

Figure 4: ICT/BCM-IHR response (squares, left axis),
signal over noise ratio (triangles, right axis) and reference
charge line (left axis).
Figure 5 shows the BPM relative deviation compared to
ICT/BCM-IHR from 200 pC to 30 pC. Below this value
the ICT/BCM-IHR signal was too noisy to be properly
analysed. The relative deviation stays within +/-1%,
which confirms the assumption that BPM and ICT/BCMIHR have a linear response. As a consequence, theICT/BCM-IHR response was extrapolated to lower
charge values using BPM measurements. This
extrapolation is the calibration reference for the TurboICT/BCM-RF below 30 pC. At higher charges the BCMIHR measurements are accurate enough to be directly
used as the reference.

2.5

3.0

3.5

0.1
4.0

Figure 6: Turbo-ICT/BCM-RF response (squares, leftaxis), signal over noise ratio (triangles, right axis) andreference charge line (left axis).
The Turbo-ICT/BCM-RF relative deviation is within+/-5%, which is slightly higher than expected (Fig. 7). We
have identified three different sources: the log amplifier,the BCM-RF electronics and the BPM. Although thelinearity of ICT/BCM-IHR and BPM has been confirmed
above 30 pC, linearity of our reference could not bechecked for lower charges. It is planned to address this
topic in future measurements. Having established betterreference will possibly allow recognizing the log
amplifier conformance error signature. More points will
be taken in order to identify it and try to correct it byusing mathematical functions or a predefined correction
table.
5%
4%
3%

Relative deviation (%)

150

Input S/N

150

2.0

Output View (V)200

Bunch charge (pC)

200

1.0

2%
1%
0%
-1%
-2%
-3%

5%

-4%

Relative deviation (%)

4%
3%

-5%
0.1

2%
1%

1.0

10.0

Bunch charge (pC)

100.01000.00%

Figure 7: Turbo-ICT/BCM-RF relative
compared to BPM charge measurement.

-1%
-2%
-3%

deviation

-4%
-5%
0

50

100

150

200

250

Bunch charge (pC)

Figure 5: Relative deviation of BPM measurements
compared to ICT/BCM-IHR measurements.
ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○2501000.0

Input S/N

The BCM-IHR was calibrated on site using a 1 nC
calibrated source and calibrated attenuators. The linearity
error measured during the calibration is less than 1%. The
BPM had been calibrated previously by the STIF teamusing a wall current monitor.
A charge scan from 200 pC to 0.13 pC is performed bycontrol room operators in fourteen steps. Figure 4 shows
the ICT/BCM-IHR response and its signal over noise
ratio. The signal over noise ratio decreases linearly withbeam charge. The ICT/BCM-IHR measures 120 pC with
1% resolution (S/N = 100). In laboratory conditions theBCM-IHR measures 70 pC with 1% resolution, which isalmost twice better. One reason may be dark current
making the ICT/BCM-IHR signal noisier.

Figure 6 shows the Turbo-ICT/BCM-RF response.
Even at 5 pC, the signal over noise ratio remains close to90, i.e. 1.1% resolution, which is slightly better than
expected.

Bunch charge (pC)

MEASUREMENTS & RESULTS

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Proceedings of IBIC2012, Tsukuba, Japan

MOST RECENT RESULTS
Recently measurements were performed by PSI. The
results show (Fig. 8) that after setup improvements theBCM-IHR signal is much less noisy. Its measurement isstill useful down to 10 pC. This is advantageous for future
calibration. Moreover, BCM-RF response on fig. 9
confirms our previous results. Its measurement looks
accurate even down to 1 pC. These are preliminary results
which required careful analysis.
1000

Charge (pC)

100

10

1

0.1
0

5000

10000

15000

20000

25000

30000

Measurement Number

1000
3.7

3.2

2.7
10

2.2

Charge (pC)100

Output Voltage (V)c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 8: charge measured by ICT/BCM-IHR (green) andBPM (blue).

1.7
1
1.2

0.7
0

5000

10000

15000

20000

Measurement Number

25000

0.1
30000

Figure 9: charge measured by Turbo-ICT/BCM-RF (pink)
and BPM (blue).

We plan to repeat the charge scan with more points inorder to establish a better charge reference, to confirm thefirst measurements, to identify the log conformance error
signature and to explore even lower beam charge. The
influence of dark current will be studied in depth. Further,charge measurements using the nominal two-bunch beam
will be performed.

REFERENCES[1] R. Ganter (Ed.), SwissFEL Conceptual Design Report, PSI
report 10-04 (2009)[2] for more information see: http://www.psi.ch/swissfel/[3] B.D. Patterson (Ed.), Ultrafast Phenomena at the
Nanoscale: Science Opportunities at the SwissFEL X-ray
Laser, PSI report 90-10 (2009)[4] S. Reiche, “Status of the SwissFEL Facility at the Paul
Scherrer Institute”, Proc. FEL 2011, Shanghai, China,
August 2011, p. 223.
[5] M. Pedrozzi (Ed.), SwissFEL Injector Conceptual Design
Report – Accelerator Test Facility for SwissFEL, PSI
report 10-05 (2010)[6] M. Pedrozzi et al., SwissFEL Injector Test Facility – Tests
and Plans, Proc. FEL 2011, Shanghai, China, August 2011,
p. 625.
[7] F. Marcellini et al., Design of Cavity BPMs for SwissFEL,
this conference, TUPA24.
[8] R. Ischebeck et al., Transverse Profile Monitor for
SwissFEL, this conference, TUPB80.
[9] G.L. Orlandi et al., Bunch-Compressor Transverse Profile
Monitors of the SwissFEL Injector Test Facility, thisconference, MOPB82.
[10] B. Beutner et al., Operation Modes and Longitudinal
Layout for the SwissFEL Hard X-ray Facility, Proc. FEL
2011, Shanghai, China, August 2011, p. 235.
[11] K. B. Unser, “Design and Preliminary Tests of a Beam
Intensity Monitor for LEP,” in Proceedings of the 1989
Particle Accelerator Conference, IEEE, Piscataway, NJ,
1989, vol. 1, p. 71.
[12] Turbo-CT
European
Patent12356014,
BergozInstrumentation, pending.
[13] Analog Devices AD8310 logarithmic amplifier datasheet.
[14] Microchip PIC 18F2458 microcontroller datasheet.
[15] B. Keil et al., Commissioning of the Low-Charge Resonant
Stripline BPM System for the SwissFEL Test Injector,
Proc. FEL 2010, Malmö, Sweden, August 2010, p. 429.
[16] Avago Technologies ATF-58143 datasheet.

ACKNOWLEDGMENTS
Bergoz Instrumentation would like to thank PSI for
giving us the opportunity to test the new Turbo-ICT and
BCM-RF.

CONCLUSION
We have described first measurements at STIF using a
new non-destructive high sensitivity beam charge
monitor. The new instrument was developed by Bergoz
Instrumentation with guidance from Paul Scherrer
Institut. Measurements were compared and calibrated
with a classical ICT associated with BCM-IHR
electronics and a BPM. Results show that the new beam
charge monitor can measure 10pC single bunch charge
with 1% resolution.
ISBN 978-3-95450-119-9
4Beam Charge and Current Monitors