12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
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 P 0 3 4

L. Crescimbeni†1,, F. Schmidl, Friedrich-Schiller-University Jena, Jena, Germany
D. M. Haider, A. Reiter, M. Schwickert, T. Sieber
GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany
M. Schmelz, R. Stolz3, V. Zakosarenko4, Leibniz IPHT, Jena, Germany
T. Stoehlker1,2, V. Tympel1, Helmholtz Institute Jena, Jena, Germany
1
also at GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany
2
also at Institute for Optics and Quantum Electronics, Jena, Germany
3also at Technical University Ilmenau, Ilmenau, Germany
4
also at supracon AG, Jena, Germany
Abstract
The Cryogenic Current Comparator (CCC) is a superconducting device based on an ultrasensitive SQUID (superconductive quantum interference device) magnetometer(fT range). Measuring the beam azimuthal magnetic field,
it provides a calibrated non-destructive measurement ofbeam current with a resolution of 10 nA or better, independent from ion species and without tedious calibrations
procedure. The non-interceptive absolute intensity measurement of weak ion beams (< 1 µA) is essential in heavy
ion storage rings and in transfer lines at FAIR. With standard diagnostics, this measurement is challenging forbunched beams and virtually impossible for coasting
beams. To improve the performance of the CCC detector
several upgrades are under study and development: One is
the investigation of a new type of CCC using an alternative
magnetic shield geometry. The so-called “axial” geometry
will allow for much higher magnetic shielding factor, an
increased pick-up area, and a reduced low frequencies
noise component. Further improvements and optimizationsof the detector will be presented. The CCC will be testedon the beamline at the end of 2023 allowing to define thebest possible version for FAIR.

built out of niobium, causing high costs and several mechanical difficulties in the construction process (for example the needing of electron beam wielding). Other than this,the FAIR-Nb-CCC-xD is equipped with a high permeability core, that produces low frequencies noise (due totrapped magnetic field) and introduces the need of a low
pass circuit to avoid resonances, reducing the maximum
bandwidth of the system.
To improve these aspects of the detector a new versionof the CCC, with an axial geometry, has been developed.

NEW AXIAL CORELESS CCC
A new type of CCC, developed together with IPHT
Jena, will be built out of lead with an axial shield geometry
and without a high permeability core (Fig. 1).

INTRODUCTION
After proof of principle with an earlier CCC version[1], the first prototype of CCC for GSI and FAIR is theFAIR-Nb-CCC-xD (Fig. 1) that is part of the family ofCCC-XD that has been developed for the use with thebeamline dimensions at FAIR HEBT (Ø 150 mm).
The CCC is installed in a beam-line cryostat [2]equipped with a helium reliquifier¶ that provides stable operating conditions.
The CCC has been tested in CRYRING@ESR [3] confirming the viability of the detector and its current resolution in the order of nA. The test in CRYRING@ESR hasshown some limitation of the system like the limited slew
rate and the low magnetic shielding. The CCC prototype is____________________________________________*Work supported by AVA – Accelerators Validating Antimatter the EU
H2020 Marie-Curie Action No. 721559 and by the BMBF under contract
No 05P21SJRB1.
†l.crescimbeni@gsi.de¶
Model PT415 from Cryomech Inc, Syracuse, NY, USA

Figure 1: Left: Radial Geometry, like the FAIR-Nb-CCCxD, with high permeability core. Right: Axial geometry
coreless CCC, made of lead, the pickup coil is filled onlywith non magnetic foam for structural support.
The axial CCC is expected to be less sensitive to external perturbations, in particular very low-frequency noise(<1 Hz), caused by magnetic field caught inside the corematerial [4], and less sensitive to mechanical perturbation.
The new coreless lead CCC will have a much higher
screening factor due to the axial magnetic shield composedof 10 layers of lead sheet [5]. The simulation shows a theoretical screening factor higher than 200 dB, reducing theeffect of a magnetic perturbation in the order of 10 µT

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AXIAL CRYOGENIC CURRENT COMPARATOR (CCC) FOR FAIR

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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
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 P 0 3 4(amplitude similar to the dipole ramp perturbation observed with the CCC at CRYRING@ESR) to a level below
detection threshold of the detector.

Figure 2: Simulations of the transfer function for CN4
SQUID and schematic of the two parallel squid system.
Upper: CN4 squid transfer function. Lower: Circuit Schematic, can be seen the two squids and their feedback system, the damping circuit has been added to avoid resonancepeaks in the frequency of interest. The chosen value for theresistance is 0.5 Ohm, to provide the smoothest gain without decreasing the bandwidth.
The improved screening factor, together with the expected lower sensitivity to low-frequency perturbation
should increase the sensitivity of the detector in the accelerator environment. This new CCC will be equipped witha parallel SQUID system (Fig. 2), a sensitive SQUID witha lower bandwidth (type CN4, Leibniz IPHT/supracon AG)
and a higher bandwidth SQUID with lower sensitivity(type CN2, same provider).
The parallel use of two SQUIDs refers to the introduced cascade principle [6]. The two parallel SQUID system, thanks to the higher bandwidth, will allow to sensibly
improve the slew rate of the detector without introducing
additional filtering elements, so without decreasing the resolution due to additional thermal noise, (as it was done for
the FAIR-Nb-CCC-xD in CRYRING [3]).

PROTOTYPE CONSTRUCTIONThe first prototype of the axial coreless CCC has been
built in collaboration with IPHT Jena (Fig. 3).
The first part that has been built were the 10 layers
meander structure, composing the magnetic shielding. The
external meander is built out of a 1 mm pb sheet, placed on
the inside of a 250 mm radius fiberglass cylinder to provide

structural integrity. The internal structure of 9 meanders are
instead built out of 0.25 mm thick Pb-Sl sheet, overlappedwith thin electric-insulation layers. The meander structureis then connected to the upper and lower surface of theCCC, built out of 1.5 mm Pb sheets.
At the end of the meander structure the pickup-coil has
been built up, in this process two coils of 0.25 mm thickness Pb-Sl sheet are wrapped around a structure composedof XPS Polystyrene foam to provide the required stabilization.
The meanders and the pickup coil are then enclosed ina fiberglass cylinder of 320 mm to provide rigidity to thesystem and protect it from deformation due to handlingduring test and installation. The structure is then completedwith a last layer of 1 mm Pb sheet that connects the upperand the lower extremity of the CCC.

Figure 3: Construction steps of axial CCC. Up left: Schemeof axial CCC. Up right: First meander of the meander structure, the fiberglass cylinder can be seen on the upper part.
Down Left: XPS support structure for the pickup coils during assembly. Down Right: Axial lead CCC completed, on
the upper corner it’s also possible to see the squid box.
The Pickup coil is finally connected through two Pb
strips to the squid box installed on top of the CCC. The Pbbox in which the SQUIDs are enclosed is necessary toshield them from external magnetic fields.
All the soldering has been performed with Pb-Sn material, to make sure that also the soldering connection aresuperconductive, and the insulant is kept in place by cryoglue.

FIRST TEST OF AXIAL CORELESS CCC
The Axial CCC has first been tested in the laboratory
at FSU Jena (Fig. 4). This test has allowed us to measure
the performance of the detector in a controlled environment. The following measurement steps have been performed to stepwise characterize the properties of the axial
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Figure 4: Left: FSU Jena Laboratory with wide-neck cryostat. Right: Axial coreless CCC during installation, thesquid box and the cabling is also visible on the top of theCCC. The white windings on the cryostat are the Helmholtz coils for the magnetic shield test. The CCC is at restin the bottom part of the cryostat.
Current resolution: Using the calibration line of thecryostat it was possible to estimate the current resolutionof the system, around 10 nA (Fig. 5).

times higher than the noise floor measured in thelab with radial core CCCs, because of low-passfiltering, which is not present in the axial CCC.
The low-pass filter is required to avoid resonances, due to the high magnetic permeability
core inductance, that together with the capacity ofthe magnetic shield meander structure behave as
an LC circuit. In the coreless CCC the inductanceof the pickup coil is much lower, so the low-passfilter is not needed. The resonance frequency ofthe system is much higher than the maximum
bandwidth of the used squids.
Slew Rate: A first estimation of the slew rate has been
performed in the cryostat in Jena, estimating the value ataround 4 μA/μS, a strong improvement from the0.16 μA/μS of the FAIR-nB-CCC-xD [3]. It will be possible to reach higher slew rates with further optimization of
the hardware (like squid cabling) or of the squid circuit.
This will allow to use the CCC for monitoring a higher
spectrum of signals without decreasing the current resolution.
Magnetic Shielding: The FSU cryostat is equippedwith a pair of Helmholtz coils (Fig. 4) to provide a magnetic field up to 1 mT. The previous test of the FAIR-NbCCC-xD has shown a quite high sensitivity to external
magnetic field. In CRYRING@ESR the dipole field(~100 μT) produced a signal from the SQUID in the orderof 10 nA. For the axial CCC we were not able to detect any
squid response to the external magnetic field of 1 mT. Only
using a waveform generator to drive the Helmholtz coil wewere able to distinguish a small response in the frequency
spectrum of the noise, estimating a SQUID signal lower
than 0.1 nA. This result doesn’t allow to calculate the exact
screening factor of the axial CCC or even to verify a theoretical value of ~200 dB (which is anyway not feasible),but it is good enough to confirm that the new shield geometry strongly improves the magnetic screening factor, effectively removing external magnetic fields from the CCC
signal.

TEST OF AXIAL CCC IN THE BEAMLINE CRYOSTAT AT GSI

Figure 5: Current resolution of CN4 squid: The black
waveform is the rectangular waveform used as input, thered waveform is the squid signal. Maximum current resolution is around 10 nA, lower signal can’t be distinguishedover the noise floor.
There are two main reason for this low current resolution compared to classical CCC [1 nA]:
• Less coupling of the pickup to the magnetic fieldof the beam due to the lack of the high permeability core• Higher noise component at higher frequencies,
the noise floor measured with the axial CCC(around 5-10 nA) is unexpectedly more than ten

We proceeded to install the axial CCC in the beam-line
cryostat in GSI (Fig. 6). Even if the test in the well shieldedlaboratory Jena was successful, the high noise component
we found there has arisen some worries, which unfortunately found confirmation in the test.
The sensitive squid has shown a very unstable V-ɸcharacteristic (Fig. 7), obtained by applying a well defined
signal to a coil wound around the SQUID or to the calibration line. This produces a ‘calibration’ field that is used toobtain the modulation of the squid (in Volt) as a functionof the applied field (ɸ).
The working point of the squid in the Flux Looked
Loop (FLL) mode is set where the characteristic curve
crosses the zero voltage line. To have a stable working
point that allows to use the squid in the FLL mode, whichis essential for the data acquisition, the characteristic curve
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should cross the zero line in a definite point. If the characteristic curve is not stable due to excessive noise, like at
our measurement shown in Fig. 6, it is impossible to definea stable working point for the squid.

Figure 6: GSI setup, left: cryostat scheme, upper right: cryostat vacuum vessel, lower right: picture of the CCC during
installation. The CCC is installed over the He-container
tube, the thermal shield tube and finally the UHV tube ofthe beamline.

CONCLUSION AND NEXT STEPSThe test of the axial coreless CCC has highlighted theissues of the prototype, in particular the low coupling withthe beam field due to the lack of the core that together withthe high noise floor level, especially at higher frequencies,
make the prototype not able to work in the accelerator environment. The test at FSU Jena laboratory has, however,confirmed the validity of the axial magnetic shielding andthe effectiveness of the squid cascade system to improve
the slew-rate of the detector.
Even if it is confirmed that the axial coreless CCC will
not be the solution for FAIR, we are able to use the information achieved on the effectiveness of the axial geometry
and the squid cascade system to develop a new prototypeof CCC of axial geometry, the Double Core axial CCC, inspired on the design of the CCC Sm (Smart & small) [7].
At the end of 2023 the FAIR-nB-CCC-xD will betested on the beam line at GSI, allowing us to perform afinal test of the cryostat and of the detector on the transferlines, as they will be used in FAIR. In the first half of 2024
we will test the first prototype of the double core axial CCC
in the laboratory, followed by a beamline test in summer
2024.

REFERENCES[1] A. Peters et al., ”A Cryogenic Current Comparator for theabsolute Measurement of nA Beams”, AIP Conf. Proc. 451,pp.163-180, 1998.
[2] D. M. Haider et al., “Versatile Beamline Cryostat for the
Cryogenic Current Comparator (CCC) for FAIR”, in Proc.
IBIC'19, Malmö, Sweden, Sep. 2019, pp. 78-81.
doi:10.18429/JACoW-IBIC2019-MOPP007[3] L. Crescimbeni et al., “The Cryogenic Current Comparator at
CRYRING@ESR”, in Proc. IBIC'22, Kraków, Poland, Sep.
2022, pp. 300-303.

Figure 7: Upper: Squid characteristics in the laboratory inJena. Lower: Squid characteristic at GSI cryostat. It’s possible to see how the characteristic has a deformed shape inrespect to the optimal one measured in Jena, moreover thenoise has in some points almost half of the full amplitudeof the characteristic curve. With a noise component so highthe characteristic is very unstable and it’s not possible tofind a stable working point
The test at GSI has shown that the axial coreless CCC
is currently not a valid detector for installation in the beamline cryostat. The absence of a stable working point makes
it impossible to perform beam measurement with the CCC.
It was only possible to use the low sensitivity SQUID,
that can work independently of the FLL mode, to collectinformation on the noise (estimated to be higher than 1 µA,
around 50 times higher than the noise floor of the classicalradial CCC with core). Further investigations will mainly
use the low sensitivity SQUID to get a better understanding
how the noise couples that strongly to axial coreless CCC
and also why the noise is much higher than expected.

doi:10.18429/JACoW-IBIC2022-TUP31[4] V. Zakosarenko et al., “Coreless SQUID-based cryogenic
current comparator for non-destructive intensity diagnostics
of charged particle beams”, Supercond. Sci. Technol., vol. 32,
p. 014002, 2018. doi:10.1088/1361-6668/aaf206[5] H. De Gersem, N. Marsic et al., “Finite-element simulationof the performance of a superconducting meander structure
shielding for a cryogenic current comparator”, Nucl. Instrum.
Methods Phys. Res. Sect. A, vol. 840, pp. 77–86, 2016.
doi:10.1016/j.nima.2016.10.003[6] T. Schönau and M. Schmelz, et al., “SQUID-based setup forthe absolute measurement of the Earth’s magnetic field”,Supercond. Sci. Technol., vol. 26, number 3, p. 035013, 2013.
doi:10.1088/0953-2048/26/3/035013[7] V. Tympel et al., “Creation of the First High-InductanceSensor of the New CCC-Sm Series”, in Proc. IBIC'22,
Kraków,
Poland,
Sep.
2022,
pp.
469-472.
doi:10.18429/JACoW-IBIC2022-WEP30

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