Non-Contact Measuring of Tiniest Currents
Superconducting Cryogenic Current Comparators
Intensities of beam currents in particle accelerators or storage rings need to be measureable. Numerous measurement devices to determine the number of agitated electrically charged particles exist for this purpose. A large problem arises if the following is required:
- beam must not be destroyed;
- very small beam currents have to be measured;
- reduction of measurements to national normal is possible.
Cryogenic Current Comparators (CCC) were developed for electrical metrology to compare amperages in conductive wires with high precision . If a beam of electrically charged particles replaces one of those wires – e.g. ions, protons or antiprotons – it essentially creates a high-resolution, non-destructive measurement device for the particle current intensity. Naturally, the measurement is non-destructive as only the strength of the magnetic field of the moving charges is determined without contact. Particle quantity and velocity remain unchanged. A proof of concept first succeeded in the 90’s in a real accelerator environment at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt with a CCC-sensor built at the institute of solid-state physics (IFK) at the Friedrich-Schiller-University (FSU) Jena [2, 3].
The principle of the measurement resembles an induction-based current clamp as used by electricians. Crucial differences are nine orders of magnitude smaller amperages, and the possibility to measure not only alternating currents (AC), but also direct currents (DC). The key to achieve this is superconductivity with its particular effects . The name of this macroscopic phenomenon of quantum physics originates from superconducting materials, completely losing their DC resistance at very low temperatures, and therefore becoming superb conductors for electric currents.
The heavy metal niobium for example becomes a superconductor at temperatures below 9 K (-264°C). If a ring made of niobium is placed in liquid helium with a temperature of 4.2 K (at normal pressure) and a current is applied once, the current will continue to flow for decades – provided cooling is maintained.
Magnetic field coils in accelerator facilities and also in Magnetic Resonance Tomography (MRT) utilize this effect.
The Meißner-Ochsenfeld-effect – the property of superconductors to completely extrude an external magnetic field from within (diamagnetism) – allows for the most effective existing magnetic shielding. The respective shielding currents on the surfaces of the superconducting material compensate the outer magnetic fields, and create the aforementioned field absence within.
Strong magnetic fields are necessary to keep the charged particles in the particle accelerator in their tracks. The magnetic fields of the beam currents are much smaller. A complex meander structure suppresses interfering magnetic fields. The impact of such filters for magnetic field components can be determined with special software, and facilitates the development of effective designs .
Direct Current Transformer
The law of electromagnetic induction is known since Faraday’s experiments and subject material. A temporally varying magnetic field leads to a temporally changing electric field and vice versa – transformers use this principle. The temporal change of the electric current respectively voltage is necessary for induction. Historically AC voltage prevailed during electrification instead of DC and AC transformers are part of everyday life.
A normally conducting transformer can’t function with direct current – the current flow would only heat up the primary coil. A superconducting transformer in superconducting circuits, however will also transform DC according to the winding number. The cause for are the shielding currents. Superconductivity enables the development of DC meters with transformers.
Cryogenic Ferrite Cores
Fast-freezing iron alloys with a very fine-grained crystalline structure and typical grain sizes of 10 nm, so called ‘nanocrystalline‘ weakly magnetic materials, were optimized with heat treatment while exposed to external magnetic fields for applications with cryogenic conditions. The special ring core GSI328plus was developed in collaboration with researchers and technicians of the IFK, and the company Magnetec for the CCC-XD, which accomplishes an inductivity of 80 µH at a temperature of 4.2 K with an all-over superconducting single-winding take-up reel.
Highly Sensitive Current Sensors
In optics interferometers signify exact and sensitive measuring systems, for instance the length measurement of production machine, the verification of black holes, and medical imaging. The coherence of light or photons is required for optical interference effects. In many superconductors the superconductivity state can be explained with the emergence of electron pairs (Cooper pairs). Cooper pairs share many properties with photons. They can be created, destroyed and interfere in a mutual quantum mechanical state. Such Superconducting Quantum Interference Devices (SQUID) are extremely sensitive current respectively magnetic field sensors, and therefore are used with CCCs. Sensitivity is so high, that the current noise of a 1-Ω- resistance at a temperature of 4.2 K is roughly 50 times larger than the noise limit of modern SQUIDs.
Combined and embedded into a special cryostat that ensures constantly low temperature,s these components form a unique sensor for small current flows. Currently a CCC is continuously running at CERN-AD in Genf, measuring antiproton flows. This sensor was originally developed at the GSI in close cooperation with FSU Jena, but due to its dimensions, cannot be operated there in future. The new Facility for Antiproton and Ion Research (FAIR) project of the GSI requires detectors with significantly larger inner diameters. This new CCC-XD sensor with a free inner diameter of 250 mm is now completed and passed its lab tests. Multiple components made of niobium had to be welded with an electron beam in a vacuum, altogether over 50 kg Nb. A task successfully performed with dedication by the company Josch Strahlschweißtechnik.
Measurements show that the sensor white noise is below 5 pA/√Hz and therefore singular pulses of 1 nA are traceable. A frequency bandwidth of 200 kHz was achieved . Consequently the required parameters are realized for the unique CCC dimensions.
Besides the financial support provided by the BMBF (joined project 05P2015 – R&D accelerator CCC) the cooperation with users, universities and research institutes is crucial for the success of this project. The GSI Darmstadt (M. Schwickert, T. Sieber, et al.) and CER-AD (J. Tan, M. Fernandes, et al.) gave vital impulses. The work cryogenic workgroup of P. Seidel and the IFK has worked on SQUID-sensors for many years and TU Darmstadt (H. De Gersem, et al.) supports with time and cost reducing simulations.
In particular the short paths at the site Jena between university, Leibnitz Institute for Photonic Technologies (IPHT, R. Stolz, et al.), Helmholtz-Institute Jena (HI Jena, T. Stöhlker) and the direct access to a magnetically shielded chamber, as well as the high-performance helium liquefier were and continue to be guarantees for the successful development of cryo-electronic components, such as CCCs for atom and nuclear research.
F. Schmidl1, V. Tympel2
1Universität Jena, Institut für Festkörperphysik, Jena, Germany
2Helmholtz-Institut, Jena, Germany
 I. K. Harvey, “A precise low temperature dc ratio transformer”, Rev. Sci. Instrum. Vol. 43, 1972. doi.org/10.1063/1.1685508
 A. Peters u. a., “Review of the Experimental Results with a Cryogenic Current Comparator”, in Proc. EPAC’96, Barcelona, Spanien, 1996.
 W. Vodel u. a., “20 years of development of SQUID-based Cryogenic Current Comparators for beam diagnostics”, in Proc. IPAC’13, Shanghai, China, 2013.
 P. Seidel, Applied Superconductivity, Handbook on Devices and Applications, Vol. 2, Weinheim, Wiley-VCH, 2015.
 N. Marsic u. a., “Analytical and numerical performance analysis of a Cryogenic Current Comparator”, in Proc. IPAC’17, Kopenhagen, Dänemark, 2017.
 V. Tympel u. a., “Cryogenic Current Comparator for 150 mm Beamline Diameter.” IBIC 2017, Grand Rapids, USA, in Druck.