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COMPARISON OF DIFFERENT BUNCH CHARGE MONITORSUSED AT THE ARES ACCELERATOR AT DESY
T. Lensch∗ , D. Lipka, R. Neumann, M.Werner
Deutsches Elektronen-Synchrotron DESY, Germany
CHARGE MONITORS AT ARES

Abstract
The SINBAD (Short and INnovative Bunches and Accelerators at DESY) facility, also called ARES (Accelerator Research Experiment at SINBAD), is a conventional
S-band linear RF accelerator allowing the production of lowcharge ultra-short electron bunches within a range of currently 0.01 pC to 250 pC. The R&D accelerator also hosts
various experiments. Especially for the medical eFLASH experiment an absolute, non-destructive charge measurement
is needed. Therefore different types of monitors are installed
along the 45 m long machine: A new Faraday Cup design had
been simulated and realized. Further two resonant cavities(Dark Current monitors) and two Beam Charge Transfomers(Toroids) are installed. Both, Dark Current Monitors andToroids are calibrated independently with laboratory setups.
At the end of the accelerator a Bergoz Turbo-ICT is installed.
This paper will give an overview of the current installationsof charge monitors at ARES and compare their measured
linearity and resolution.

Faraday Cup
Simulations of at DESY existing Faraday Cup geometries
showed 4 % to 22 % of primary and secondary charged particles escaping from the cup. This does not fulfill the ARES
demand for an absolute charge measurement at the Gun. Dueto this, a new Faraday Cup geometry had been simulated foran energy of 5 MeV. This geometry of a copper cup withan inner diameter of 15 mm, inner depth of 24 mm and an
Aluminum plate inlay shows only 0.6 % of charged particles escaping from the Faraday Cup in the simulation. Also
different beam configurations (pencil beam, out of center,angled beam) had been taken into account. None of themshowed more than 1 % loss of charged particles [4]. Figure 1 shows the final design of the simulated Faraday Cupfor ARES prior vacuum installation.

INTRODUCTION
The SINBAD facility (Short and INnovative Bunches
and Accelerators at DESY) hosts various experiments inthe field of production of ultra-short electron bunches andnovel high gradient acceleration techniques. ARES (Accelerator Research Experiment at SINBAD), is a conventional,
45 m long S-band linear RF accelerator allowing the production ultra-short electron bunches (fs to sub-fs) with highstability. The machine is operated with a repetition rate of
10 Hz with a charge range of 0.01 pC up to 250 pC. ARES
is used for applications related to accelerator R&D in thefield of advanced and compact longitudinal diagnostics andaccelerating structures development, test of new accelerator
components, machine learning and others. In the field of
medical application studies an experiment has been set up toperform studies on cancer irradiation techniques with electron beams. For this experiment an absolute, non-destructive
charge measurement is essential [1] .
As ARES is also related to Accelerator R&D several dedicated charge monitors had been installed: one Faraday Cup
and a Dark Current Monitor (DaMon) at the gun section, one
DaMon and two Beam Charge Transformers (Toroid) along
the machine and one Bergoz Turbo-ICT [2] in-vacuum at theend of the beam line. An additional Bergoz in-air ICT [3] isinstalled 30 cm next to the T-ICT outside the beamline.
∗ timmy.lensch@desy.deFigure 1: New Faraday Cup mounted with ceramic spacer
on a mover rod.
The signal from the Faraday Cup is connected to an MicroTCA system [5] via a 15 MHz low pass and a pre amplifier.
A Struck SIS8300-L2D [6] ADC board samples the pulse
with 125 MS/s. The charge is reconstructed using a pulse
form fit with three samples as described in Ref. [7]. For
calibration of the electronics and cables an electrical pulse
which corresponds to 1 nC is fed into the front end box input.
An appropriate scaling factor is set in the software.

Beam Charge Transformers (Toroids)
Different types of in-house developed charge monitors
based on transformers (Toroids) had been used at DESY machines since the 1980s. Beginning in 2010 an optimizationof existing Toroids for the E-XFEL and FLASH accelerator
had been started. These new monitors are based on one or
two ferrite cores with four pickup coils, each consisting of asingle winding. These four coils are combined outside theToroid housing. The read out procedure is the same as withTU3I04
06 Beam Charges and Current Monitors

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the Faraday Cup. Details of the optimization details canbe found in Ref. [8]. An additional calibration coil is usedto fed an electrical pulse which corresponds to 1 nC to theToroid core(s). An appropriate scaling factor is configured
in software to match the calibration pulse. Figure 2 shows aToroid Beam Charge Monitor installed in ARES.

Figure 2: Toroid installed in the ARES accelerator. The
diameter of the beam pipe is 34 mm, the length of the Toroid(flange to flange) is 150 mm.

Dark Current Monitors
The Dark Current Monitor (DaMon) was invented to measure dark current of accelerators with 1.3 GHz acceleration
freqeuncy. The monitor consists of a stainless steel resonatorwith TM010 mode at 1.3 GHz with a loaded quality factor
about 200. This results in a bandwidth of about 6 MHz anda decay time of 50 ns. The cavity is connected to a RF front
end electronic (RFFE) which implements two channels. Oneis a dark current channel DC with a local oscillator, down
conversion and a logarithmic detector. The second channel
is for measuring charge, Q with a logarithmic detector only.
For a 3 GHz accelerator like ARES, both channels, DC andQ, are used for charge measurement to increase the dynamicrange. For calibration an Arbitrary Waveform Generator(AWG), which feeds a signal to the DaMon RFFE, is used.
Details of the system and the calibration can be found inRef. [9].

Bergoz Turbo ICT
At the very end of the beam line a Bergoz Turbo-ICT (TICT) is installed in vacuum. Together with the BCM-RF-E
electronic the T-ICT is operated with a 10 Hz timing trigger
in the Sample&Hold mode. After the T-ICT conversion has
finished a trigger is given to a Keithley DMM7510 High
Resolution Digital Volt Meter (DVM) which is read out over
Ethernet [10]. A look up table, implemented in softwarewith data from Bergoz’ calibration to further improve theresult. The measurement range given by Bergoz is 50 fC to300 pC, the noise is 10 fCrms or 1% of bunch charge.

Bergoz ICT
For test purposes a Bergoz in-air ICT is installed direct after the end of the beamline, 30 cm after the T-ICT. A detailed
description of the ICT principle can be found in Ref. [11].

The in-air ICT is read with an 12 bit oscilloscope, which isalso doing averaging, integrating and scaling the 70 ns longpulse to get a value in pC. Figure 3 shows both, T-ICT invacuum and the neighboured in-air ICT.

Figure 3: T-ICT (left) installed in vacuum, output window of
the beam line (black cover in the middle) and the in-air ICT(copper colored toroidial shape on the right). The electrons
are used for in-air experiments direct downstream the in-airICT.
The ICT installation is improvised yet. Later the setup
might be moved easily to other DESY machines which allows
the installation in-air (take-away-charge-reference).

MEASUREMENTS
For the linearity and resolution measurements the ARES
accelerator was setup with 100 % beam transport down tothe T-ICT and the in-air ICT at (45 m). In total 14 different
charges in the range from 0.1 pC to 153 pC at a bunch repetition rate of 10 Hz had been carried out. For each charge
500 values of each monitor had been taken synchronously.
As the Faraday Cup measures destructive, its data had beentaken independently (move FC in, take 500 values, move
FC out, take 500 values for the other monitors, move FC in,change charge, ...). The DaMon DC channels are limited
to less than 20 pC. Therefore a sweep from 0.1 pC to 13 pC
had been carried out. The DaMon Q channels, the Toroids
and the T-ICT had been measured with a sweep of 1 pC to153 pC.

Linearity
The upper plots of the Figs. 4 to 8 show the monitor charge
as a function of the Faraday Cup, the lower plots show theratio of the monitor to Faraday Cup as a function of theFaraday Cup charge. The error bars shown are the standard
deviation caused by beam charge variation.
Figure 8 shows the linearity fit slope, fit offset and thein-air ICT charges relative to the Faraday Cup. As this setup
is improvised yet, this measurement should not be taken asa face value yet.

Resolution using Standard Deviation (rms)
A reference charge was defined by using all existing charge
information (except the device under test), including also theTU3I04
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d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U 3 I 0 4Figure 4: Linearity of the DaMon DC channel.

Figure 6: Linearity of the Toroids.

Figure 5: Linearity of the DaMon Q channel.

Figure 7: Linearity of the T-ICT.

existing Cavity BPMs. This reduced the statistical variationof the charge determination and it was used to determine
the resolution of the monitor under test. This procedure ismachine optics independent. A detailed description of thiscalculation can be found in Ref. [12]. To get better results,

also charges measured by seven Cavity BPMs had been
included. These devices can measure charge as well, but are
not calibrated independent [13]. The resolution had been
calculated for the DaMon channels DC and Q, the Toroids
and the T-ICT. Figures 9 and 10 show the resolutions.

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Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3IBIC2023, Saskatoon, Canada
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ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U 3 I 0 4Figure 9: Absolute and relative resolution of the DaMon DCchannels in the range of 0.1 pC to 13 pC.

Figure 8: Top: measured charge of the in air ICT as a function of Faraday Cup charge; below the ratio of the in-air ICT
to Faraday Cup charge as a function of Faraday Cup charge.
Both in the range of 30 pC to 153 pC.
As the in-air ICT could not be measured synchronous tothe other monitors, the resolution could not be determined.

Results
Table 1 shows the linearity and resolution measured atARES.

Figure 10: Absolute and relative resolution of DaMon Qchannels, Toroids and T-ICT, each in the range of 1 pC to153 pC.
The DaMon DC channels are limited to a maximum
charge of about 20 pC. For a range from 0.1 pC to 13 pC
the DaMon DC channels show reasonable linearity results.
The resolution measured up to about 6 pC is the best of all
charge monitors compared during this measurements.
For the charge range from 1 pC to 153 pC both, the DaMon Q channels and the T-ICT show reasonable results oflinearity with a linear fit slope of almost 1 and an offset ofclose to 0 pC.

Table 1: Resolution results for all monitors. Upper values are the upstream monitor.
DaMon DCDaMon Q

ToroidT-ICT

Charge Range

0.1 pC - 13 pC

1 pC - 153 pC

0.1 pC - 13 pC

0.1 pC - 13 pC

Linear fit Slope
Monitor 1
Monitor 2

1.04
1.06

0.96
1.04

1.18
1.15

1.01
—

Linear fit Offset
Monitor 1
Monitor 2-0.002 pC-0.006 pC

0.26 pC
0.31 pC-0.02 pC
-0.02 pC

0.11 pC—(@0.1 pC - @13 pC)
4 fC - 111 fC
3.4% - 0.83%
3.7 fC - 90 fC
3.36% - 0.59%(@1 pC - @153 pC)
34 fC - 366 fC
3.4% - 0.2%
37 fC -427 fC
3.7% - 0.23%(@1 pC - @153 pC)
108 fC - 119 fC
10.8% - 0.1%
151 fC - 154 fC
15.1% - 0.1%(@1 pC - @153 pC)
31 fC - 1621 fC
3.1% - 1.1%——Resolution<6 pC

Resolution>6 pC – 60 pC

Resolution>60 pC

Absolutecharge

Resolution
Monitor 1
Monitor 2
Use forTU3I04172

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The Toroids show a linear fit slope of 18 respectively
15% higher compared to the Faraday Cup but also an offset of nearly 0 pC. Nevertheless, the relative plot in Fig. 2shows good linear accordance to the Faraday Cup, so that acorrection can easily be applied. Depending on the charge
range, different areas of resolution could be determined inour setup. The DaMon Q channels show the best resolution between 6 pC – 60 pC, the Toroids for charges greaterthan 60 pC. As the upstream Toroid in ARES has a dual
core installation the resolution is better (10.8%) than thedownstream Toroid (15.8%), which only has a single core
installed. For higher charges both Toroids converge to aresolution of 100 fC. The T-ICT’s resolution might sufferfrom the installation in ARES: the cable between the T-ICT
in the beamline and the Bergoz electronics outside had tobe extended by about 30 m. The cable damping had been
compensated, but environmental noise is captured anyway.
However, the monitor is suitable for absolute measurements
out of the box.

CONCLUSION AND OUTLOOK
Three DESY in-house developed monitors had been presented briefly. These monitors had been compared to eachother and to two commercial available monitors (ICT andT-ICT) from Bergoz. Taking the new Faraday Cup as a (destructive) reference at the Gun Section it could be shownthat the in-house developed (non destructive) Toroids andthe DaMons are very linear compared to the Faraday Cup.
Additionally the Bergoz T-ICT at the end of the beam line
shows a very good linearity as well. Also the resolutionof all (non destructive) monitors could be calculated. All
this monitors can be read out with the required repetition
rate of 10 Hz. In the future we would like to improve thecalibration scheme of the Toroids. The in-air ICT can bemoved and be used as a kind of take-away-charge-reference
to easy cross-check charge measurements in other DESY
accelerators.

Wentowski, Zlatan Pisarov, Klaus Knaack, Artem Novokshonov, Jörg Neugebauer, Sergey Strokov, Bastian Lorbeer,Hans-Thomas Duhme, Igor Krouptchenkov and Jorgen LundNielsen† from the Diagnostics group. For their patienceduring our time consuming measurements and iterations
we would thank also the ARES operation team: Florian
Burkhart, Willi Kuropka, Hannes Dinter, Frank Mayet, Max
Kellermeier and Sonja Jaster-Merz.

REFERENCES[1] F. Burkart, R.W. Aßmann, H. Dinter, S. Jaster-Merz, W. Kuropka, F. Mayet, and et al., “The ARES Linac at DESY”, in
Proc. LINAC’22, Liverpool, UK, Aug.-Sep. 2022, pp. 691–694. doi:10.18429/JACoW-LINAC2022-THPOJO01[2] https://www.bergoz.com/products/turbo-ict/[3] https://www.bergoz.com/products/ict/[4] Sergey Strokov, private communication[5] https://www.picmg.org/openstandards/microtca[6] https://www.struck.de/sis8300-l2.html[7] M. Werner et al., "A Toroid Based Bunch Charge Monitor
System with Machine Protection Features for FLASH andXFEL", in Proc. IBIC’14, Monterey, CA, USA, 2014, pp. 521–524.
[8] M. Werner et al., "Sensitivity Optimization of the Standard
Beam Current Monitors for XFEL and FLASH II", in Proc.
DIPAC’11, Hamburg, Germany, 2011, pp. 197–199.
[9] D. Lipka et al., "Charge Measurement with Resonators atARES", presented at IBIC’23, Saskatoon, CA, September
2023, paper TUP037, this conference.
[10] https://www.tek.com/de/products/keithley/
digital-multimeter/dmm7510#[11] K. B. Unser, "Measuring bunch intensity, beam loss andbunch lifetime in LEP", Report CERN-SL-90-27-BI", 1990,
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ACKNOWLEDGEMENTS[12] N. Baboi et al., “Resolution Studies at Beam Position Monitors at the FLASH Facility at DESY”, in Proc. BIW’06,
Batavia, IL, USA, May 2006, AIP Conf. Proc., vol. 868,
pp. 227–237. doi:10.1063/1.2401409

We would like to thank for development, installation andcommissioning of the charge monitors and all related work:
Maike Pelzer, Jürgen Kruse, Christian Wiebers, Norbert[13] B. Lorbeer et al., "Cavity BPM Electronics for SINBAD
at DESY", in Proc. IBIC’22, Kraków, Poland, Sep. 2022,
pp. 413–415. doi:10.18429/JACoW-IBIC2022-WEP14

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