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CRYOGENIC CURRENT COMPARATORS (CCC) AS LOW INTENSITYDIAGNOSTICS FOR ION BEAMS*
T. Sieber†, D. M. Haider, M. Schwickert, GSI, Darmstadt, Germany
L. Crescimbeni1, F. Schmidl, Friedrich-Schiller-University Jena, Jena, Germany
M. Schmelz, R. Stolz3, V. Zakosarenko4, Leibniz IPHT, Jena, Germany
T. Stoehlker1,2, V. Tympel, Helmholtz Institute Jena, Jena, Germany
N. Marsic
TEMF, Institute for Theory of Electromagnetic Fields, Technical University, Darmstadt, Germany
J. Tan, CERN European Organization for Nuclear Research, Geneva, Switzerland
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 SQUID
based superconducting device for intensity measurement.
It was firstly proposed as a beam diagnostics instrument in
the mid ´90s at GSI. After prove of principle the CCC wasintroduced into other facilities, showing great potential forhigh resolution measurements as well as raisingconsiderable mechanical and cryogenics challenges andcosts.
In the course of planning for FAIR the CCC has beenrevitalized. Systematic investigations started - also
involving now commercially available SQUID systems which led to improvements of detector and cryostat. The
developments resulted in nA spill measurements at GSI(2014) followed by the installation of a CCC in CERN
Antiproton Decelerator (AD), which has become a key
instrument.
Since then optimization of the device is ongoing, withrespect to various operating conditions, system robustness,current resolution and last but not least system costs.
Alternative CCC versions with improved magnetic
shielding have been developed as well as ‚Dual Core‘
versions for background noise reduction. We give an
overview of CCC optimization and development steps,
with focus on applications at GSI and FAIR.

as a matter of principle. The pickup loop is basically a onewinding coil around a high permeability ring core, acting
as a flux concentrator. The latter is used in the ‘classical’
CCC shown in Fig. 1 to ensure efficient coupling of thebeam magnetic field to the SQUID circuit. The
arrangement can be regarded as a transformer with theparticle beam being the primary winding and the pickup
coil the secondary winding. The signal from the pickup coilis fed (via a matching transformer for impedance
matching) to a DC SQUID (Superconducting Quantum
Interference Device) magnetometer, which is operated in acompensation circuit, using a so called Flux Locked Loop
(FLL) electronics [1]. Figure 1 shows the currently usedarrangement, originally developed at the PTB(Physikalisch-Technische Bundesanstalt) [2] and adapted
to the accelerator application at GSI [3].

INTRODUCTION
The Cryogenic Current Comparator measures the beam
intensity via the beam azimuthal magnetic field, which isfor nA currents in the fT range. The device consists of asuperconducting shielding, which provides an attenuationof non-azimuthal external fields in the range -70 dB to 140 dB, depending on the shield geometry (see below). The
shielding guides the superconducting Meissner-Current(compensation current for the beam magnetic field) to theinternal pickup loop, which allows for DC measurements_____________________________________* supported by BMBF under contracts 05P18RDRB1 and 05P18SJRB1†
T.Sieber@gsi.deFigure 1: Classical CCC, shielding geometry with radial
meanders and high permeability ring core.
Recent developments at IPHT Jena have shown that it ispossible to build a CCC without toroidal core, using ashielding with axial meander geometry [4], consequently
the device is called coreless or axial CCC (see Fig. 2). This

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new shielding/pickup design promised a number ofadvantages, like reduced magnetic noise and baseline drift(both mainly caused by the toroidal core), significantly
reduced costs and weight due to inherent better mechanicalstability (which allows for lead as shielding material).
Furthermore, the axial structure offers an easy
manufacturing and most of all a significantly increased
shielding efficiency.
In addition to the two mentioned CCC varieties, a third
CCC type is currently under investigation, which combines
the axial meander geometry with a doubled classicaltoroidal core pickup [5]. This version, called the double
core CCC or DCCC, is an attempt to combine and improve
the positive features, which have been identified at thedifferent CCC systems so far. Figure 2 shows the three
CCC types schematically.

Figure 2: Magnetic shieldings with radial (left) and axial(middle) meanders. The ring-core of the radial CCC isindicated in blue, the detector volume of the axial CCC inturquois. Right: Schematic sectional view of the DCCC
with axial meanders and two independent, fourfold
segmented (due to material width limitations) toroidal
cores.

CLASSICAL (RADAL) CCC
The classical CCC, as it is shown in Fig. 1, has been
operated successfully in accelerator beamlines at GSI [6],and CERN [7], measuring currents <10 nA at bandwidths
2-10 kHz. For FAIR, significantly larger detector
dimensions are required, therefore a so called CCC-XD(eXtended Dimensions, with inner/outer diameter:
250 mm/350 mm) has been designed and extensively
tested in the laboratory [8] and in CRYRING [9]. Althoughshielding efficiency is anti-proportional to shielding innerdiameter, the CCC-XD reached - due to careful shielding
design, toroid material choice [10] and sophisticated
SQUID electronics and controls [11] - a performance
similar or better than its predecessors. The magnetic
shielding provides (like for the CERN/AD CCC with
inner/outer diameter: 185 mm/280 mm) an attenuation of
external fields of -70 dB. It is made from Niobium, which
is - regarding its mechanical properties - considered thebest choice for CCCs at large dimensions. The muchsmaller GSI CCC prototype [3] (inner/outer diameter:
147 mm/260 mm) was built from Lead.
The CCC-XD, in combination with its speciallydesigned cryostat [12], was originally considered to be thefirst of series for five planned CCC systems at FAIR.
Nonetheless, already during its construction and assembly,

investigations on the coreless axial CCC (see below)
started in parallel, with the goal to eliminate unwanted
effects (low frequency Barkhausen noise, temperaturedependent offset drift, microphony) related to the toroidal
core. Moreover, a more efficient magnetic shielding
seemed desirable because of the disturbing influence of
nearby dipole and quadrupole magnets (see Fig. 3). Despite
these drawbacks, the prototype of the CCC-XD wasoperated successfully in CRYRING@ESR, showing withappropriate filtering of dipole and cryostat effects (seeFig. 3) a current resolution <10 nA at 10 kHz bandwidth.
Slew rate problems at higher currents and fast rise-times
could be solved by damping the SQUID circuit with an
inductive load. Figure 4 shows such a ‘high current’measurement in CRYRING with CCC in comparison to
standard diagnostics, IPM and DCCT (Bergoz PCT).

Figure 3: Effect of dipole ramp in CRYRING on CCC-XD
measurement. Since the dipole ramp is deterministic it canbe subtracted with an appropriate transfer function. The
1.4 Hz pertubation from the cryostat pulse tube cooler,visible on the raw signal, was eliminated by a software
bandblock filter (1-3 Hz) [9].

Figure 4: CCC current measurement compared to IPM and
PCT in CRYRING. Rf = Feedback resistor value,
GBP = Gain Bandwidth Product of the SQUID electronics[9].
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Independent from parallel and future steps of CCC
development, it could be demonstrated in CRYRING thatthe classical CCC system is appropriate for nA current
measurement at slow extracted beams in transfer lines andlow intensity exotic ion beams in the storage rings at FAIR.
Room for optimization was identified in the requirement offiltering external (periodic) disturbances, the necessary
slew rate damping (which reduces current resolution) as
well as in the high costs of the system, mainly because of
the Niobium shielding.

AXIAL CCC DESIGNAs a solution for some of the above mentioned problems
a coreless CCC with axial meander geometry has been
proposed by IPHT Jena. In this design the SQUID is
directly connected to the inner part of the shielding, whichbecomes therefore pickup coil and shielding at the sametime. Figure 5 shows on the left hand side a cross sectionof the shielding torus.

Figure 5: Alternative shielding geometry with axial
meanders (left), the SQUID, housed in the little brown box,
is connected between the two marked dots. Right:
prototype of the Axial CCC made from Lead with attached
SQUID electronics.
Compared to the classical CCC, the production of theaxial version is much simpler, due to the possibility of
wrapping lead sheets around a GFK carrier (compared toelectron beam welding of the Niobium meanders). The
costs for the shielding/pickup are therefore significantly
reduced (to ~1/10). Another big advantage of this design isthe improved magnetic shielding. From the analyticalmodel in Ref. [2] as well as from FE simulations [13] it
was shown that the shielding efficiency depends basicallyon the path length of the meanders. Since each of the axial
meanders has the full length of the detector, acomparatively small number of meanders is sufficient toprovide a much higher damping than with the classicalshielding. Realistic values are -140 dB instead of classical-70 dB, the high attenuation factor could be verified inlaboratory tests and can to large extent solve the problem
of interference from nearby magnetic elements.
Since the high permeability ring core is omitted, thecoupling of the axial pickup to the beam is much weaker
than in the classical setup. The pickup inductance now
depends completely on the detector volume (Fig. 5, left,
volume below the meanders) and is in the order of nH,
compared to µH with toroidal core. This has on the one
hand the advantage of matching the pickup to the SQUID
inductance (~10 nH) without necessity of a matching
TUP036270transformer (which might introduce its own thermal noiseand slew rate problems), on the other hand, the signal fromthe beam magnetic field is much weaker (by a factor ~100)and parasitic inductance (e.g. connection cables) play abigger role. The basic question during axial CCC
development was, if the lower signal strength would becompensated by the improved noise properties and SQUID
coupling, resulting in an improved SNR. First evidence
that this trade off might not work out was the detection ofan unexpected high intrinsic noise in the range between
1 Hz and 1 kHz measured on the axial CCC prototype
during laboratory tests. Figure 6 shows for comparison anoise spectrum from the CCC-XD (blue) compared to theaxial CCC (black). The expected positive effect from themissing core is not evident, even at very low frequencies<1 Hz. Also in the critical region for microphonic effects(~10-100 Hz) the noise of the axial CCC is an order ofmagnitude higher.

Figure 6: Noise spectra measured at the FAIR CCC-XD(blue), the GSI prototype from [3] (red) and one of the first
prototypes of the axial CCC, shown in Fig. 5.
This result is still analyzed and under discussion,
damping properties of the matching transformer and thetoroidal core might play a bigger role than expected. Also,
the exact reproduction of this measurement is quitedifficult. Nonetheless, from recent laboratory tests it also
seems that the axial pickup is extremely sensitive to rfdisturbances, a kind of influence that can (partially) beshielded in the laboratory, but not in the accelerator
environment, since it propagates along the beamtube,
through the detector.
A detailed report from development work and latest test
measurements can be found in Ref. [14]. The applicationof the axial detector at FAIR seems at the moment
questionable and has to be further investigated. In any casewe consider the cost reduction due to axial meander
production from Lead and the superior magnetic shielding
properties as extremely valuable results from axial CCC
development.

DUAL CORE CCC (DCCC)
The DCCC was originally designed to eliminate the low
frequency disturbances from magnetization jumps, whichhave been observed in the Nanoperm© toroidal cores. Since
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these jumps occur randomly in each core, they can easily
be subtracted from the beam current signal (seen by both
pickups/cores simultaneously), which leads to an improved
SNR.
Since the CCC-XD for FAIR had so far the best noise
behaviour and current resolution, the components chosen
for the pickup and SQUID system of the DCCC wereidentical to the CCC-XD. To combine all advantages our
CCC systems have shown so far and to facilitate theproduction (and reduce costs) the magnetic shielding was
designed as an axial type, built from lead, which resulted
in a setup like shown in Fig. 2, right. Figure 7 shows acompleted prototype of the DCCC with 300 mm outer
diameter and closed axial shielding.

is operated at superfluid Helium temperature (2.1 K), toavoid the disturbances from He boil off.

Figure 8: Comparison of current noise from an advanced
axial CCC prototype (black) and a dual core CCC (red) at
XD-dimensions (outer diameter: 350 mm).

SUMMARY AND OUTLOOK

Figure 7: DCCC prototype, built from lead with diameter
300 mm and length 600 mm. Left: Pickup coils #1 and #2
mounted to the inner cylinder of the shielding (covered
with tape). Right: DCCC completed and closed, with theouter meander shielding and the SQUID housing.
The DCCC has been extensively tested to investigate theinfluence of different geometries and magnetic corematerials on the current noise density and in particular on
resonant behaviour and eigenmodes of the system, whichhave strong influence on the current resolution as well as
on operation stability [15]. Additional use of mu-metalshielding was tested as well as different ways of connecting
the pickup coils (e.g. in series and parallel).
During this process it could be shown that inversely
connected SQUIDS will suppress the influence of highfrequency noise by adding the SQUID signals either
analogue by differential amplifier or digitized via softwaredifferentiation. In total the optimization of the CCC LC
circuit (L: pickup inductance, C: meander capacitance)
together with the elimination of Barkhausen and rf noiselead to a strongly reduced noise floor. Figure 8 shows thenoise density spectra of the DCCC compared to anadvanced axial CCC prototype. Obviously the DCCC noise
density is much lower due to the possibility of noise
subtraction by dual core.
Independent from properties of a special CCC type, it
could be shown during the same measurement campaignthat further noise reduction can be achieved if the system

Two CCC detectors, based on the classical PTB design,
made from Niobium, are in standard operation at CERN
AD or have shown solid performance in CRYRING at
GSI/FAIR. We conclude that our CCC system, thecombination of cryostat and classical CCC detector, isappropriate to serve as a basis for nA current diagnostics in
FAIR and other machines. In parallel an alternative axial
shielding/pickup design has been developed to avoid
unfavorable properties of the classical CCC. This axial
CCC has recently been tested in the laboratories at Jena andin the beamline cryostat at GSI, it was found to provide asignificant improvement concerning magnetic shielding
efficiency as well as production effort, materials and costs.
However, regarding the current measurement respectively
the CCC functionality in general the axial CCC is still
suffering from too high sensitivity to background noise andhas to be further investigated. As a third variety a dual CCC
has been designed to combine the advantages of the twoearlier versions. In addition to a high pickup efficiency andsuperior magnetic shielding properties the usage of two
pickups and SQUIDs in parallel provides excellent noise
spectra. Beam experiments with the DCCC are planned for
2024.

REFERENCES[1] F. Kurian, “Cryogenic Current Comparators for precise Ion
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ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

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ISSN: 2 6 7 3 - 5 3 5 0
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