Fiftyone Series - IEEE Xplore
Sensor
Input
Processing
Output
Display, actuators, signals, control
Transformer-based Measurement of Critical Currents in Superconducting Cables: Tutorial 51 Part 51 in a series of tutorials on instrumentation and measurement
Amalia Ballarino, Giuseppe Montenero, and Pasquale Arpaia
S
uperconductivity is a technology with a de- to a detectable voltage at its ends, and then to possiclared interest in several fields of physics and ble damage risks for the superconducting system as engineering. Nowadays, superconducting ca- a whole. bles [1] are largely exploited in fields such as particle Critical current assessment needs accurate measurecolliders [2] and medical machines [3]. The genera- ment of the voltage-current characteristic of the sample tion of electrical energy with tokamaks [4] is which is, in general, a function of temperaa further application of low-temture, current, and magnetic field. perature superconductivity as This is a relatively delicate well. Moreover, the promtask for single wires, nowises arising from High adays carried out through Temperature Superconquasi-industrial standards ductors (Tc>77 K) would [7]. However, for largeextend the range of apsize cables, facilities of 47 plication, for instance, appropriate dimensions 20 to power distribution and functionality are few, [5]–[6]. mainly owing to the diffiA key design parameter culty and cost of providing a for any large-scale applicalarge and complex setup for this tion of superconductivity is the measurement [8]–[9]. maximum current that a superconducFrom an operational point of view (Fig. 1a), tor can carry, also referred to as critical current (Ic). This by keeping constant the external magnetic field Bext and parameter identifies the limit beyond which a super- the working temperature T, the measured electrical beconductor undergoes the phase transition to normal haviour of a cable can be expressed by the following conducting state. Right before I c, the superconduc- voltage-current relationship during the superconducttor’s resistivity starts suddenly to increase, leading ing-resistive transition:
51 9
4 2 Fiftyone 3
in a
56 Series 81
Fig. 1. Conceptual setup of critical current (Ic) assessment. (a) A cable’s segment under test. (b) The typical measured V-I curve during an Ic test.
(1)
where V0 is a reference voltage, I the cable current, and n the resistive transition index. However, at experimental level, Ic has not a univocal definition but depends on the criterion imposed on V0. In standard practice, criteria based either on the electric field or the resistivity are used. According to the electrical field criterion, Ic is defined as the current generating a voltage V0=LEc, where L is the length of the cable between the voltage taps, and Ec is the electric field assumed as criterion. Usually, Ec is chosen equal to 10 μVm-1 or 100 μVm-1. The resistivity criterion assumes a 50
linear relation between the current and the voltage V=cLI/A, where A is the cross section area of the cable and c the resistivity assumed as criterion. Operatively, Ic is defined as the current where the straight line intercepts the curve described by (1), V0= cLIc/A. The resistivity c is usually chosen to be 10-14 Ωm or 10-13 Ωm. In Fig. 1b, a graphical view of the two criteria is shown. Cable critical current tests commonly involve current levels in the order of tens of kA, in principle supplied by large power converters. However, main drawbacks are significant equipment cost and the need for large current leads, resulting in high cryogenic loads and operating costs. For this reason, in this range of current, an interesting alternative is to use superconducting transformers [10]–[15]. A low current is fed to a superconducting primary winding with a large number of turns, inductively coupled to a superconducting secondary with a much smaller number of turns and directly connected to the sample under test. The modest primary current, usually in the range of 100 A, can be generated with relatively inexpensive and standardized power supplies, and the current feed through into the cryogenic environment can be optimized to have lower cryogenic load by orders of magnitude. Such a device provides test capability at moderate capital and operating cost. Beyond the obvious issues of the performance and protection of a superconducting transformer, main concerns during the operation are: monitoring the transformer system as a whole, and measuring the secondary current. These two items determine the successful running of Ic test experiments. The Facility for the Research of Superconducting Cables (FReSCa) [8] at the European Centre for Nuclear Research (CERN) is a reference laboratory for testing superconducting cables [16]. In particular, the FReSCa setup includes: ◗◗ two coaxial cryostats, independently cooled either at 4.3 or at 1.9 K; ◗◗ a background-field magnet, housed in the outer cryostat, with a maximum field of 10.3 T; and ◗◗ a sample insert, either connected to an external roomtemperature 32-kA power supply via copper current leads, self-cooled by the vapour from the 4.3 K-He bath, or including an assembly (Fig. 2a), with a superconducting transformer (Fig. 2b), with maximum currents of 50 A in the primary and 38 kA in the secondary. The secondary current is measured by means of two Rogowski coils (Fig. 2a), displaced on the two branches of the secondary cables and connected in anti-series, consisting of about 2600 turns of 0.2-mm copper wire. The primary winding of the transformer is wound from insulated NbTi wire, with a diameter of 0.542 mm, a Cu/SC (Copper to Superconducting) ratio of 1.35, a residual resistivity ratio of 82, and a filament diameter of 45 μm. The primary has a solenoid shape, with a height of 160 mm, and inner and outer radii of 70 and 88 mm, respectively. The coil has 33 layers, with in total 10850 turns, and an inductance of 11.75 H (Lp). The secondary winding is wound directly over the primary, and consists of 7 turns of a NbTi-Rutherford cable with a width of
IEEE Instrumentation & Measurement Magazine
February 2014
Sensor
Input
Processing
Output
Display, actuators, signals, control
Fig. 3. Architecture of a transformer-based measurement station for superconducting cable test. (© 2012, AIP Rev. Sci. Instrum., used with permission [18].)
Fig. 2. (a) The sample insert, and (b) the superconducting transformer of FReSCa at CERN. (© 2012, AIP Rev. Sci. Instrum., used with permission [18].)
15.1 mm. The secondary is impregnated with epoxy to support the coil mechanically. All along the cable, a copper strip 1 mm thick is welded for mechanical and electro-dynamical stability. The self-inductance of the secondary is 9 μH and the mutual inductance between primary and secondary is 8.77 mH. In this tutorial, the techniques developed at CERN to bring transformer-based test stations to levels of costly reference setups are illustrated. In particular, the issues related to monitoring the superconducting transformer system and measuring the transformer secondary current are addressed. First, the state-ofthe-art system for monitoring a superconducting transformer is illustrated. Results from the characterization and validation of such a system at the FReSCa test station are reported. Then, the design and the preliminary prototype of a new cryogenic DC current transformer are described. Experimental characterization results of the prototype complete the picture.
A Fully-Digital Monitoring System for Superconducting Transformers In a test station based on a superconducting transformer, the main objective of monitoring the secondary current effectively is to achieve a performance compatible with reference systems based on room-temperature converters. By analyzing the specifications of state-of-the-art systems in the range of 32kA, the following main operational requirements arise for the monitoring system: current measurement with a resolution better than 3 A and stability below +/− 0.5 A per minute (Amin-1), for a corresponding relative ripple due to the control algorithm less than 10-4. The main solution is the use of a high-performance digital integrator [17] with adjustable selfcalibration time inserted in a fully-digital control loop with all of the related benefits of noise, ease of implementation/modification, etc. In this regard, the development of a fully digital monitoring system for superconducting transformers is illustrated [18].
Monitoring System In Fig. 3, the architecture of a measurement station based for superconducting cable test based on a supply transformer and February 2014
Rogowski coils for current measurement is illustrated. In this architecture, the fundamental elements are the measurement system and the control strategy. The former has to provide high-quality measurements of the secondary current (Is), by minimizing undesired uncertainty effects, such as integrator drift and noise, due to the integration of the Rogowski coil signal (VRC). The latter has to follow closely the reference signal (Iref), namely, the desired secondary current, by accurately driving the current into the primary Ip. Moreover, the overall system should be flexible, in order to allow quick modifications of the desired waveforms according to the test needs. In this regard, the only viable solution is to implement a fully digital system, where the signals at the monitoring block can be handled in a digital form and the high-performance digital integration has adjustable calibration time. According to these leading concepts, the measurement and control setup was developed as follows. The output of the control block delivering the Vref signal is based on a waveform generator, implemented through a data acquisition board NI-PXI 6281 of National Instruments. The board drives a four-quadrant power supply Lake Shore Model 622 (Voltage-Controlled current source), supplying the transformer’s primary. The core of the monitoring system is the Fast Digital Integrator (FDI) [17], measuring the transformer’s secondary current via the signal from the Rogowski coils. A timing board NI PXI-6682 of National Instruments is used to generate the trigger signal for the FDI and for the data acquisition board. The boards are housed in a crate PXI U1091AC50 by Agilent. The embedded computer is a Single-Board Computer D9-6U by Mikro Elektronik, hosting the software handling the overall system functions, developed through the Flexible Framework for Magnetic Measurements [20], and implementing the controller algorithms (heart of the control block). In particular, two digital controllers were implemented. The former is a classical Proportional-Integral (PI) algorithm, specifically designed for controlling a superconducting transformer. The latter is the adaptive version of this algorithm based on a Fuzzy Gain Scheduling paradigm [21].
Performance Highlight Some experimental results of critical current assessments, highlighting performance achieved by the above monitoring system at the test station of FReSCa, are reported. The critical current measurements refer only to the PI-based system for brevity. The reader can find details on the implementation
IEEE Instrumentation & Measurement Magazine 51
Fig. 4. Comparisons of V-I curves on an LHC cable of type 2 measured using the reference power supply and the superconducting transformer (© 2012, AIP Rev. Sci. Instrum., used with permission [18].)
of the algorithms, as well as more results, in [21]. Critical currents measured on a cable sample by the superconducting transformer test station are compared with results of the reference setup based on the 32-kA power supply. In particular, an outer layer dipole cable of type 2 for the Large Hadron Collider (LHC) using a criterion of resistivity of 10-14 Ωm was utilized. In Fig. 4, the measured V-I curves on a length of 610 mm of the cable are compared at 3.0 T and 6.0 T, with current ramp rate of 250 As-1. The voltage data overlap satisfactorily and the noise level is well inside the requirements. Indeed, the quality of the V-I curves measured by the developed system is better than the one obtained using the 32-kA power supply. The characterization campaign on the LHC cable showed the compatibility between the results from the new and the reference systems, thus validating the development.
A Cryogenic DC Current Transformer for Measurement Applications Several techniques for sensing current at room temperature are available, referred to as contact and contactless, according to the working principle. At temperature values close to 4.2 K and for high current levels, the only practical solutions are contactless sensors: the superconducting circuit cannot be interrupted. Moreover, Ic measurements require accuracy better than 1%, thus the set of available solutions is further restricted. As shown above, a classical ac solution is the Rogowski coil. Drawbacks of this approach for Ic assessment with a superconducting transformer are the intrinsic ac nature of the device, the difficulty arising from detecting zero-current conditions directly, and the need for quenches of the transformer to reset the device and the measurement process as well. Nevertheless, such a transducer is still a key tool for applications and its importance as backup system is unquestionable. Sensing a dc component with accuracy of 0.1% and below, for currents up to tens of kA, in a cryogenic environment calls for more sophisticated sensors/transducers. Nowadays, 52
devices such as dc measurement current transformers (DCCT) [22], combining ac and dc compensation mechanisms, are available on the market for a wide range of rated currents with bandwidth from dc to a couple of hundreds kHz. Careful calibration of these transducers leads to an accuracy of the order of ppm. The first development of a cryogenic DCCT is due to CEA-Saclay [23] for measuring the secondary current of a superconducting transformer. The basic idea was to modify a well-settled commercial DCCT (MACC+, 600 A by Hitec [24]) for currents up to 50 kA. A cryogenic DCCT would represent one of the most accurate current measurement techniques for testing new generation of superconducting cables up to 100 kA, with superconducting transformers, if the range of operation is extended. Therefore, a substantial improvement of the CEA solution is demanded, but realizing and testing a 100kA transducer prototype would require significant costs and a suitable ad-hoc complex facility not available yet at CERN. For these reasons, a proof demonstrator was realized preliminarily. In particular, the prototype was aimed at validating the main design elements that could affect the performance of a 100-kA DCCT.
Experimental Proof Demonstration of Feasibility for a Cryogenic DCCT The cryogenic DCCT demonstrator was developed according to the following requirements: ◗◗ Maximum primary current 20 kA, to keep cryogenic load not higher than 20 W; ◗◗ Maximum secondary (compensation) current of 10 A; and ◗◗ Maximum sensing element diameter of 70 mm to avoid disturbance field from the powering current leads. The three cores of cryogenic permalloy [26] constituting the DCCT sensing element were designed with a height of 17.3 mm and cores’ radii of r2=29.8 mm and r1=23.3 mm, respectively. Then, a primary current of 20 kA is compensated with 10 A, if a secondary coil of 2,000 turns is implemented, with a current ratio of 2,000:1. In Fig. 5, a core of cryogenic permalloy (a), the three cores with the 200 turns sensing coil (b), and the sensing element assembly (c) of the prototype are shown. The three cores are insulated mutually with Teflon tape. The compensation coils are realized with a copper wire with f=0.315 mm. The demonstrator prototype can be tested with a primary current of 20 kA by exploiting an extra coil wound on top of the sensing element. Using a power supply of 20 A with a primary coil of 1,000 turns gives rise to 20 kAt, corresponding to the desired equivalent maximum current. The excitation coil is wound on the top of the secondary coil using a copper wire of f=0.315 mm. The coils are insulated mutually by a Kapton tape. In Fig. 6, the conceptual architecture of the cryogenic DCCT is illustrated. The voltage VDCCT,out is the output of the conditioning electronics that drives the power amplifier to generate the required compensation/secondary current (I2) of the device. The signals from the sensing element are exploited to
IEEE Instrumentation & Measurement Magazine
February 2014
Sensor
Input
Processing
Output
Display, actuators, signals, control
Figure 7. (a) 1- repeatability of I1,m . (b) The nonlinearity of the DCCT proof demonstrator.
implement the feedback mechanism for the ac and dc components of the primary current (I1) under test.
Prototype Characterization
Fig. 5. The preparation steps of the cryogenic DCCT sensing element. (a) The cryogenic permalloy core. (b) The stack of the three cores insulated with Teflon tape. (c) The final preparation with the 2,000 turns secondary coil.
Fig. 6. Cryogenic DCCT conceptual architecture. February 2014
The cryogenic DCCT proof demonstrator was characterized preliminary by cooling down the sensing element in a cryostat via a liquid helium bath at 4.2 K. The primary current was measured with a reference DCCT MACC+ before going into the cryostat via current feedthroughs. The circuitry of the cryogenic DCCT is kept at room temperature and the secondary current for the compensation of the transducer is measured by means of the voltage drop on a 50-mΩ burden resistor. The static characteristic was then recorded for both the instruments at several plateau levels of I1. In Fig. 7, the 1-σ repeatability (a) of measured primary current (I1,m) by means of the prototype and the nonlinearity (b) arising from the comparison of the data with the reference DCCT are illustrated. The nonlinearity is better than 0.05% for I1, namely, close to the nominal performance of the equivalent room temperature device. For the sake of readability, the primary current axis is expressed without taking into account the factor 1000 from the primary coil. These results highlight the
IEEE Instrumentation & Measurement Magazine 53
effectiveness of the design and a promising improvement of the performance for the final 100-kA transducer prototype.
Instrum., vol. 81, 035107, 2010. [11] E. Barzi, et al., “Superconducting transformer for
Conclusions
superconducting cable tests in a magnetic field,” in 2010 AIP Conf.
This tutorial introduced the reader to available state-of-the-art CERN solutions for the metrological improvement of cable test stations based on a superconducting transformer. The issues related to monitoring a superconducting transformer and to the measuring the transformer’s secondary current have been experienced as having primary relevance. Attention then has been then focused on the design and implementation of a fully digital monitoring system for the superconducting transformer. Results from the system validation (carrying out critical current assessments) at the FReSCa test station highlight the high performance of the system with respect to the reference setup and also the compatibility of the measurement results between the two systems. The issue related to the intrinsic AC nature of Rogowki Coils as current sensing element is addressed. The development steps for a Cryogenic DCCT capable of providing absolute measurement of the secondary current of a superconducting transformer are reported. The experimental proof of the design concepts underlying the proposed current transducer is shown by means of the implementation of a preliminary prototype. Finally, the results of experimental characterization measurements of the prototype prove the effectiveness of the design and foster the investment for a full-scale prototype. Further work will be devoted to the final prototyping of the Cryogenic DCCT and to its integration in the developed monitoring system.
Proc. vol. 1218, p. 421, 2010. [12] C. Berriaud, S. Regnaud, and L. Vieillard, “High current test facility for superconductors at Saclay,” IEEE Trans. Appl. Supercond., vol. 11, no. 1, p. 3190, 2001. [13] P. Bruzzone, et al., “Status report of the SULTAN test facility,” IEEE Trans. Appl. Supercond., vol. 20, no. 3, p. 455, 2010. [14] J. Liu, Y. Wu, Zh. B. Ren, S. T. Wu, Y. Shi, J. Q. Peng, J. L. Chen, F. Long, M. Yu, and L. Qian, “Manufacturing of 50 kA superconducting transformer for ITER correction coil conductor test,” Rev. Sci. Instrum. vol. 81, 044701, 2010. [15] P. Fabbricatore, R. Musenich, and R. Parodi, “Inductive Method for Critical Current Measurement of Superconducting Cables for High Energy Physics Applications,” Nucl. Instrum. Methods, vol. 302, no. 1, p. 27, 1991. [16] A. P. Verweij, C-H. Denarie, S. Geminian, and O. Vincent-Viry, “Superconducting transformer and regulation circuit for the CERN cable test facility,” J. Phys. Conf. Ser. vol. 43, p. 833, 2006. [17] P. Arpaia, L. Bottura, L. Fiscarelli, and L. Walckiers, “Performance of a fast digital integrator in on-field magnetic measurements for particle accelerators,” AIP Rev. Sci. Instrum., vol. 83, no. 2, Feb. 2012. [18] P. Arpaia, L. Bottura, G. Montenero, and S. Le Naour, “Performance improvement of a measurement station for superconducting cable test,” AIP Rev. Sci. Instrum., vol. 83, 095111, 2012. [19] T. Boutboul, C. H. Denarié, Z. Charifoulline, L. R. Oberli, and D. Richter, “Critical current test facilities for LHC superconducting
References
NbTi cable strands,” in Proc. 5th Eur.Conf. on Applied
[1] Y. Iwasa, Case Studies in Superconducting Magnets. New York, NY, USA: Springer Science, 2009.
Superconductivity, Lyngby, Denmark, p. 8, 2001. [20] P. Arpaia, M. Buzio, L. Fiscarelli, and V. Inglese, “A software
[2] LHC Machine Outreach. [Online] Available: http://lhc-machineoutreach.web.cern.ch/lhc-machine-outreach/. [3] Siemens MR Magnet Technology. [Online] Available: http://
framework for developing measurement applications under variable requirements”, Rev. Sci. Instrum. vol. 83, 115103, 2012. [21] P. Arpaia, A. Ballarino, V. Daponte, D. Maisto, G. Montenero, and
www.siemens.co.uk/en/- about_us/index/manufacturing/
C. Svelto, “A smart current control for superconducting cable
siemens_magnet_technology.htm.
tests,” in Proc. 19th IMEKO TC 4 Symp. and 17th IWADC Workshop
[4] J. Wesson, Tokamaks. New York, NY, USA: Oxford Science Publications, 2012.
Advances in Instrum.and Sensors Interoperability, Barcelona, Spain, 2013.
[5] A. Mann, “Still in suspense,” Nature, vol.475, pp. 280-282, 2011. [6] A. A. Golubov, “High Temperature Superconductivity,” Handbook of Applied Superconductivity, vol. 1, pp. 53-62. Philadelphia, PA, USA: IOP Publishing, 1998.
[22] H. C. Appelo, “The zero-flux DC current transformer, a high precision bipolar wide-band measuring device,” IEEE Trans. Nuc. Scien., vol. 24, no. 3, p. 1810, 1977. [23] C. Berriaud and A. Donati, “A device for measuring high current
[7] International Standard, Critical current measurement- DC critical current of Nb-Ti composite superconductors, CEI-IEC 61788-1, 2006. [8] A. P. Verweij, J. Genest, A. Knezovic, D. F. Leroy, J. P. Marzolf, and L. R. Oberli, “1.9 K test facility for the reception of the superconducting cables for the LHC,” IEEE Trans. Appl. Supercond. vol. 9, no. 2, p.153, 1999.
at cryogenic temperatures,” IEEE Trans. Appl. Supercond., vol. 12, no.1, p. 1264, 2002. [24] MACC-PLUS. [Online] Available: http://www.hitecups. com/?RubriekID=4025. [25] A. Hobson, “The zero-flux current transformer, power apparatus and systems, part III,” Trans. of the Amer. Inst. of Elec. Engineers,
[9] A. G. Prodell and A. Am, “A Facility for Evaluating Superconductors above Atmospheric Pressure at 1.8K,” Adv. Cryog. Eng. 43(A), p. 443,1998.
vol. 72, no, 2, p. 608, 1953. [26] “Nickel Iron and Cobalt Iron Cold Rolled Strips,” Aperam Alloys Imphy. [Online] Available: http:// www.aperam.com/
[10] A. Godeke, D. R. Dietderich, J. M. Joseph, J. Lizarazo, S. O. Prestemon, G. Miller, and H. W. Weijers., “A Superconducting 54
Transformer System for High Current Cable Testing,” Rev. Sci.
alloysandspecialities/fileadmin/pdf/Aperam/Brochure/cold_ rolled_strip_web.pdf.
IEEE Instrumentation & Measurement Magazine
February 2014
Sensor
Amalia Ballarino is a scientist leader of the section of superconducting materials, cables, and devices at CERN. For the High Temperature Superconducting leads for the LHC, she received the award of “Superconductor Industry Person of the Year 2006”. She is Work-Package Leader in the EC FP7 High Luminosity Design Study. In 2012, she became a member of the Applied Superconductivity Conference board of directors for large-scale applications. She is a member of the IEC-TC 90. Giuseppe Montenero ([email protected]) took Bachelor (2003), Master (2008) and PhD (2013) Degrees in Telecommunications Engineering at University of Sannio. Since 2007, he has been carrying out his scientific activity at
Input
Processing
Output
Display, actuators, signals, control
CERN. In 2010, he won the IEEE Award for best young student paper at IEEE I2MTC 2010. Since July 2013, he is Fellow at CERN, at the Superconducting Devices Section, Technology Department. Pasquale Arpaia is professor of Instrumentation and Measurements at University of Sannio and Team Leader at CERN. He is Associate Editor of the Elsevier Journal Computer Standards & Interfaces, and in the past also of IEEE Transactions on Electronics Packaging and Manufacturing. He developed several patents and licenses and funded 4 spin-off companies. His PhD students were awarded in 2006 and 2010 at IEEE I2MTC and in 2012 at IMEKO World Conferences.
continued from page 48
monitor has shown that a photon-counting method can produce high-resolution longitudinal beam profiles in the LHC with a dynamic range of up to 105 within a few minutes. It is now regularly used during operation to optimize and understand beam behaviour and is an essential tool for calibration of the LHC experiments.
005.pdf. [2] D. Cocq, “The wide band normaliser – a new circuit to measure transverse bunch position in accelerators and colliders,” Nuclear Instruments and Methods in Physics Research A, vol. 416, pp.1-8, Oct. 1998. [3] E. B. Holzer, et al., “Beam loss monitoring system for the LHC,”
Summary
IEEE Nuclear Science Symp., Fajardo, Puerto Rico, pp. 1052-1056,
The LHC’s suite of sophisticated beam instrumentation has been essential for fast commissioning, safe and reliable operation, and understanding beam behaviour at these high energies and intensities. Many of the systems, of which only a few have been discussed here, are the fruits of over a decade of R&D, addressing the demand for higher precision, better reliability and faster acquisition. The success of the LHC is therefore, in no small part, due to the beam instrumentation devices that allow the optimization of its performance.
References [1] D. Brandt, “Beam diagnostics for accelerators,” in Proc. CERN Accelerator School CAS, Dourdan, France, 2009, CERN. [Online]. February 2014
Available: http://cds.cern.ch/record/1071486/files/cern-2009-
2005. [4] A. Jeff, et al., “First results of the LHC longitudinal density monitor,” Nuclear Instruments and Methods in Physics Research A, vol. 659, pp. 549-556, Sept. 2011. DOI: 10.1016/ j.nima.2011.08.055
Owain Rhodri Jones ([email protected]) received his B.Sc. in Physics in 1992 and his Ph.D. in 1996 on laser spectroscopy from the University of Wales, Swansea, UK. At CERN, he started as research fellow working on novel beam instrumentation techniques, later taking over responsibility for the beam position and tune measurement systems for the LHC. He is currently head of the Beam Instrumentation Group which provides the beam diagnostic systems for the CERN accelerator complex.
IEEE Instrumentation & Measurement Magazine 55
Input
Processing
Output
Display, actuators, signals, control
Transformer-based Measurement of Critical Currents in Superconducting Cables: Tutorial 51 Part 51 in a series of tutorials on instrumentation and measurement
Amalia Ballarino, Giuseppe Montenero, and Pasquale Arpaia
S
uperconductivity is a technology with a de- to a detectable voltage at its ends, and then to possiclared interest in several fields of physics and ble damage risks for the superconducting system as engineering. Nowadays, superconducting ca- a whole. bles [1] are largely exploited in fields such as particle Critical current assessment needs accurate measurecolliders [2] and medical machines [3]. The genera- ment of the voltage-current characteristic of the sample tion of electrical energy with tokamaks [4] is which is, in general, a function of temperaa further application of low-temture, current, and magnetic field. perature superconductivity as This is a relatively delicate well. Moreover, the promtask for single wires, nowises arising from High adays carried out through Temperature Superconquasi-industrial standards ductors (Tc>77 K) would [7]. However, for largeextend the range of apsize cables, facilities of 47 plication, for instance, appropriate dimensions 20 to power distribution and functionality are few, [5]–[6]. mainly owing to the diffiA key design parameter culty and cost of providing a for any large-scale applicalarge and complex setup for this tion of superconductivity is the measurement [8]–[9]. maximum current that a superconducFrom an operational point of view (Fig. 1a), tor can carry, also referred to as critical current (Ic). This by keeping constant the external magnetic field Bext and parameter identifies the limit beyond which a super- the working temperature T, the measured electrical beconductor undergoes the phase transition to normal haviour of a cable can be expressed by the following conducting state. Right before I c, the superconduc- voltage-current relationship during the superconducttor’s resistivity starts suddenly to increase, leading ing-resistive transition:
51 9
4 2 Fiftyone 3
in a
56 Series 81
Fig. 1. Conceptual setup of critical current (Ic) assessment. (a) A cable’s segment under test. (b) The typical measured V-I curve during an Ic test.
(1)
where V0 is a reference voltage, I the cable current, and n the resistive transition index. However, at experimental level, Ic has not a univocal definition but depends on the criterion imposed on V0. In standard practice, criteria based either on the electric field or the resistivity are used. According to the electrical field criterion, Ic is defined as the current generating a voltage V0=LEc, where L is the length of the cable between the voltage taps, and Ec is the electric field assumed as criterion. Usually, Ec is chosen equal to 10 μVm-1 or 100 μVm-1. The resistivity criterion assumes a 50
linear relation between the current and the voltage V=cLI/A, where A is the cross section area of the cable and c the resistivity assumed as criterion. Operatively, Ic is defined as the current where the straight line intercepts the curve described by (1), V0= cLIc/A. The resistivity c is usually chosen to be 10-14 Ωm or 10-13 Ωm. In Fig. 1b, a graphical view of the two criteria is shown. Cable critical current tests commonly involve current levels in the order of tens of kA, in principle supplied by large power converters. However, main drawbacks are significant equipment cost and the need for large current leads, resulting in high cryogenic loads and operating costs. For this reason, in this range of current, an interesting alternative is to use superconducting transformers [10]–[15]. A low current is fed to a superconducting primary winding with a large number of turns, inductively coupled to a superconducting secondary with a much smaller number of turns and directly connected to the sample under test. The modest primary current, usually in the range of 100 A, can be generated with relatively inexpensive and standardized power supplies, and the current feed through into the cryogenic environment can be optimized to have lower cryogenic load by orders of magnitude. Such a device provides test capability at moderate capital and operating cost. Beyond the obvious issues of the performance and protection of a superconducting transformer, main concerns during the operation are: monitoring the transformer system as a whole, and measuring the secondary current. These two items determine the successful running of Ic test experiments. The Facility for the Research of Superconducting Cables (FReSCa) [8] at the European Centre for Nuclear Research (CERN) is a reference laboratory for testing superconducting cables [16]. In particular, the FReSCa setup includes: ◗◗ two coaxial cryostats, independently cooled either at 4.3 or at 1.9 K; ◗◗ a background-field magnet, housed in the outer cryostat, with a maximum field of 10.3 T; and ◗◗ a sample insert, either connected to an external roomtemperature 32-kA power supply via copper current leads, self-cooled by the vapour from the 4.3 K-He bath, or including an assembly (Fig. 2a), with a superconducting transformer (Fig. 2b), with maximum currents of 50 A in the primary and 38 kA in the secondary. The secondary current is measured by means of two Rogowski coils (Fig. 2a), displaced on the two branches of the secondary cables and connected in anti-series, consisting of about 2600 turns of 0.2-mm copper wire. The primary winding of the transformer is wound from insulated NbTi wire, with a diameter of 0.542 mm, a Cu/SC (Copper to Superconducting) ratio of 1.35, a residual resistivity ratio of 82, and a filament diameter of 45 μm. The primary has a solenoid shape, with a height of 160 mm, and inner and outer radii of 70 and 88 mm, respectively. The coil has 33 layers, with in total 10850 turns, and an inductance of 11.75 H (Lp). The secondary winding is wound directly over the primary, and consists of 7 turns of a NbTi-Rutherford cable with a width of
IEEE Instrumentation & Measurement Magazine
February 2014
Sensor
Input
Processing
Output
Display, actuators, signals, control
Fig. 3. Architecture of a transformer-based measurement station for superconducting cable test. (© 2012, AIP Rev. Sci. Instrum., used with permission [18].)
Fig. 2. (a) The sample insert, and (b) the superconducting transformer of FReSCa at CERN. (© 2012, AIP Rev. Sci. Instrum., used with permission [18].)
15.1 mm. The secondary is impregnated with epoxy to support the coil mechanically. All along the cable, a copper strip 1 mm thick is welded for mechanical and electro-dynamical stability. The self-inductance of the secondary is 9 μH and the mutual inductance between primary and secondary is 8.77 mH. In this tutorial, the techniques developed at CERN to bring transformer-based test stations to levels of costly reference setups are illustrated. In particular, the issues related to monitoring the superconducting transformer system and measuring the transformer secondary current are addressed. First, the state-ofthe-art system for monitoring a superconducting transformer is illustrated. Results from the characterization and validation of such a system at the FReSCa test station are reported. Then, the design and the preliminary prototype of a new cryogenic DC current transformer are described. Experimental characterization results of the prototype complete the picture.
A Fully-Digital Monitoring System for Superconducting Transformers In a test station based on a superconducting transformer, the main objective of monitoring the secondary current effectively is to achieve a performance compatible with reference systems based on room-temperature converters. By analyzing the specifications of state-of-the-art systems in the range of 32kA, the following main operational requirements arise for the monitoring system: current measurement with a resolution better than 3 A and stability below +/− 0.5 A per minute (Amin-1), for a corresponding relative ripple due to the control algorithm less than 10-4. The main solution is the use of a high-performance digital integrator [17] with adjustable selfcalibration time inserted in a fully-digital control loop with all of the related benefits of noise, ease of implementation/modification, etc. In this regard, the development of a fully digital monitoring system for superconducting transformers is illustrated [18].
Monitoring System In Fig. 3, the architecture of a measurement station based for superconducting cable test based on a supply transformer and February 2014
Rogowski coils for current measurement is illustrated. In this architecture, the fundamental elements are the measurement system and the control strategy. The former has to provide high-quality measurements of the secondary current (Is), by minimizing undesired uncertainty effects, such as integrator drift and noise, due to the integration of the Rogowski coil signal (VRC). The latter has to follow closely the reference signal (Iref), namely, the desired secondary current, by accurately driving the current into the primary Ip. Moreover, the overall system should be flexible, in order to allow quick modifications of the desired waveforms according to the test needs. In this regard, the only viable solution is to implement a fully digital system, where the signals at the monitoring block can be handled in a digital form and the high-performance digital integration has adjustable calibration time. According to these leading concepts, the measurement and control setup was developed as follows. The output of the control block delivering the Vref signal is based on a waveform generator, implemented through a data acquisition board NI-PXI 6281 of National Instruments. The board drives a four-quadrant power supply Lake Shore Model 622 (Voltage-Controlled current source), supplying the transformer’s primary. The core of the monitoring system is the Fast Digital Integrator (FDI) [17], measuring the transformer’s secondary current via the signal from the Rogowski coils. A timing board NI PXI-6682 of National Instruments is used to generate the trigger signal for the FDI and for the data acquisition board. The boards are housed in a crate PXI U1091AC50 by Agilent. The embedded computer is a Single-Board Computer D9-6U by Mikro Elektronik, hosting the software handling the overall system functions, developed through the Flexible Framework for Magnetic Measurements [20], and implementing the controller algorithms (heart of the control block). In particular, two digital controllers were implemented. The former is a classical Proportional-Integral (PI) algorithm, specifically designed for controlling a superconducting transformer. The latter is the adaptive version of this algorithm based on a Fuzzy Gain Scheduling paradigm [21].
Performance Highlight Some experimental results of critical current assessments, highlighting performance achieved by the above monitoring system at the test station of FReSCa, are reported. The critical current measurements refer only to the PI-based system for brevity. The reader can find details on the implementation
IEEE Instrumentation & Measurement Magazine 51
Fig. 4. Comparisons of V-I curves on an LHC cable of type 2 measured using the reference power supply and the superconducting transformer (© 2012, AIP Rev. Sci. Instrum., used with permission [18].)
of the algorithms, as well as more results, in [21]. Critical currents measured on a cable sample by the superconducting transformer test station are compared with results of the reference setup based on the 32-kA power supply. In particular, an outer layer dipole cable of type 2 for the Large Hadron Collider (LHC) using a criterion of resistivity of 10-14 Ωm was utilized. In Fig. 4, the measured V-I curves on a length of 610 mm of the cable are compared at 3.0 T and 6.0 T, with current ramp rate of 250 As-1. The voltage data overlap satisfactorily and the noise level is well inside the requirements. Indeed, the quality of the V-I curves measured by the developed system is better than the one obtained using the 32-kA power supply. The characterization campaign on the LHC cable showed the compatibility between the results from the new and the reference systems, thus validating the development.
A Cryogenic DC Current Transformer for Measurement Applications Several techniques for sensing current at room temperature are available, referred to as contact and contactless, according to the working principle. At temperature values close to 4.2 K and for high current levels, the only practical solutions are contactless sensors: the superconducting circuit cannot be interrupted. Moreover, Ic measurements require accuracy better than 1%, thus the set of available solutions is further restricted. As shown above, a classical ac solution is the Rogowski coil. Drawbacks of this approach for Ic assessment with a superconducting transformer are the intrinsic ac nature of the device, the difficulty arising from detecting zero-current conditions directly, and the need for quenches of the transformer to reset the device and the measurement process as well. Nevertheless, such a transducer is still a key tool for applications and its importance as backup system is unquestionable. Sensing a dc component with accuracy of 0.1% and below, for currents up to tens of kA, in a cryogenic environment calls for more sophisticated sensors/transducers. Nowadays, 52
devices such as dc measurement current transformers (DCCT) [22], combining ac and dc compensation mechanisms, are available on the market for a wide range of rated currents with bandwidth from dc to a couple of hundreds kHz. Careful calibration of these transducers leads to an accuracy of the order of ppm. The first development of a cryogenic DCCT is due to CEA-Saclay [23] for measuring the secondary current of a superconducting transformer. The basic idea was to modify a well-settled commercial DCCT (MACC+, 600 A by Hitec [24]) for currents up to 50 kA. A cryogenic DCCT would represent one of the most accurate current measurement techniques for testing new generation of superconducting cables up to 100 kA, with superconducting transformers, if the range of operation is extended. Therefore, a substantial improvement of the CEA solution is demanded, but realizing and testing a 100kA transducer prototype would require significant costs and a suitable ad-hoc complex facility not available yet at CERN. For these reasons, a proof demonstrator was realized preliminarily. In particular, the prototype was aimed at validating the main design elements that could affect the performance of a 100-kA DCCT.
Experimental Proof Demonstration of Feasibility for a Cryogenic DCCT The cryogenic DCCT demonstrator was developed according to the following requirements: ◗◗ Maximum primary current 20 kA, to keep cryogenic load not higher than 20 W; ◗◗ Maximum secondary (compensation) current of 10 A; and ◗◗ Maximum sensing element diameter of 70 mm to avoid disturbance field from the powering current leads. The three cores of cryogenic permalloy [26] constituting the DCCT sensing element were designed with a height of 17.3 mm and cores’ radii of r2=29.8 mm and r1=23.3 mm, respectively. Then, a primary current of 20 kA is compensated with 10 A, if a secondary coil of 2,000 turns is implemented, with a current ratio of 2,000:1. In Fig. 5, a core of cryogenic permalloy (a), the three cores with the 200 turns sensing coil (b), and the sensing element assembly (c) of the prototype are shown. The three cores are insulated mutually with Teflon tape. The compensation coils are realized with a copper wire with f=0.315 mm. The demonstrator prototype can be tested with a primary current of 20 kA by exploiting an extra coil wound on top of the sensing element. Using a power supply of 20 A with a primary coil of 1,000 turns gives rise to 20 kAt, corresponding to the desired equivalent maximum current. The excitation coil is wound on the top of the secondary coil using a copper wire of f=0.315 mm. The coils are insulated mutually by a Kapton tape. In Fig. 6, the conceptual architecture of the cryogenic DCCT is illustrated. The voltage VDCCT,out is the output of the conditioning electronics that drives the power amplifier to generate the required compensation/secondary current (I2) of the device. The signals from the sensing element are exploited to
IEEE Instrumentation & Measurement Magazine
February 2014
Sensor
Input
Processing
Output
Display, actuators, signals, control
Figure 7. (a) 1- repeatability of I1,m . (b) The nonlinearity of the DCCT proof demonstrator.
implement the feedback mechanism for the ac and dc components of the primary current (I1) under test.
Prototype Characterization
Fig. 5. The preparation steps of the cryogenic DCCT sensing element. (a) The cryogenic permalloy core. (b) The stack of the three cores insulated with Teflon tape. (c) The final preparation with the 2,000 turns secondary coil.
Fig. 6. Cryogenic DCCT conceptual architecture. February 2014
The cryogenic DCCT proof demonstrator was characterized preliminary by cooling down the sensing element in a cryostat via a liquid helium bath at 4.2 K. The primary current was measured with a reference DCCT MACC+ before going into the cryostat via current feedthroughs. The circuitry of the cryogenic DCCT is kept at room temperature and the secondary current for the compensation of the transducer is measured by means of the voltage drop on a 50-mΩ burden resistor. The static characteristic was then recorded for both the instruments at several plateau levels of I1. In Fig. 7, the 1-σ repeatability (a) of measured primary current (I1,m) by means of the prototype and the nonlinearity (b) arising from the comparison of the data with the reference DCCT are illustrated. The nonlinearity is better than 0.05% for I1, namely, close to the nominal performance of the equivalent room temperature device. For the sake of readability, the primary current axis is expressed without taking into account the factor 1000 from the primary coil. These results highlight the
IEEE Instrumentation & Measurement Magazine 53
effectiveness of the design and a promising improvement of the performance for the final 100-kA transducer prototype.
Instrum., vol. 81, 035107, 2010. [11] E. Barzi, et al., “Superconducting transformer for
Conclusions
superconducting cable tests in a magnetic field,” in 2010 AIP Conf.
This tutorial introduced the reader to available state-of-the-art CERN solutions for the metrological improvement of cable test stations based on a superconducting transformer. The issues related to monitoring a superconducting transformer and to the measuring the transformer’s secondary current have been experienced as having primary relevance. Attention then has been then focused on the design and implementation of a fully digital monitoring system for the superconducting transformer. Results from the system validation (carrying out critical current assessments) at the FReSCa test station highlight the high performance of the system with respect to the reference setup and also the compatibility of the measurement results between the two systems. The issue related to the intrinsic AC nature of Rogowki Coils as current sensing element is addressed. The development steps for a Cryogenic DCCT capable of providing absolute measurement of the secondary current of a superconducting transformer are reported. The experimental proof of the design concepts underlying the proposed current transducer is shown by means of the implementation of a preliminary prototype. Finally, the results of experimental characterization measurements of the prototype prove the effectiveness of the design and foster the investment for a full-scale prototype. Further work will be devoted to the final prototyping of the Cryogenic DCCT and to its integration in the developed monitoring system.
Proc. vol. 1218, p. 421, 2010. [12] C. Berriaud, S. Regnaud, and L. Vieillard, “High current test facility for superconductors at Saclay,” IEEE Trans. Appl. Supercond., vol. 11, no. 1, p. 3190, 2001. [13] P. Bruzzone, et al., “Status report of the SULTAN test facility,” IEEE Trans. Appl. Supercond., vol. 20, no. 3, p. 455, 2010. [14] J. Liu, Y. Wu, Zh. B. Ren, S. T. Wu, Y. Shi, J. Q. Peng, J. L. Chen, F. Long, M. Yu, and L. Qian, “Manufacturing of 50 kA superconducting transformer for ITER correction coil conductor test,” Rev. Sci. Instrum. vol. 81, 044701, 2010. [15] P. Fabbricatore, R. Musenich, and R. Parodi, “Inductive Method for Critical Current Measurement of Superconducting Cables for High Energy Physics Applications,” Nucl. Instrum. Methods, vol. 302, no. 1, p. 27, 1991. [16] A. P. Verweij, C-H. Denarie, S. Geminian, and O. Vincent-Viry, “Superconducting transformer and regulation circuit for the CERN cable test facility,” J. Phys. Conf. Ser. vol. 43, p. 833, 2006. [17] P. Arpaia, L. Bottura, L. Fiscarelli, and L. Walckiers, “Performance of a fast digital integrator in on-field magnetic measurements for particle accelerators,” AIP Rev. Sci. Instrum., vol. 83, no. 2, Feb. 2012. [18] P. Arpaia, L. Bottura, G. Montenero, and S. Le Naour, “Performance improvement of a measurement station for superconducting cable test,” AIP Rev. Sci. Instrum., vol. 83, 095111, 2012. [19] T. Boutboul, C. H. Denarié, Z. Charifoulline, L. R. Oberli, and D. Richter, “Critical current test facilities for LHC superconducting
References
NbTi cable strands,” in Proc. 5th Eur.Conf. on Applied
[1] Y. Iwasa, Case Studies in Superconducting Magnets. New York, NY, USA: Springer Science, 2009.
Superconductivity, Lyngby, Denmark, p. 8, 2001. [20] P. Arpaia, M. Buzio, L. Fiscarelli, and V. Inglese, “A software
[2] LHC Machine Outreach. [Online] Available: http://lhc-machineoutreach.web.cern.ch/lhc-machine-outreach/. [3] Siemens MR Magnet Technology. [Online] Available: http://
framework for developing measurement applications under variable requirements”, Rev. Sci. Instrum. vol. 83, 115103, 2012. [21] P. Arpaia, A. Ballarino, V. Daponte, D. Maisto, G. Montenero, and
www.siemens.co.uk/en/- about_us/index/manufacturing/
C. Svelto, “A smart current control for superconducting cable
siemens_magnet_technology.htm.
tests,” in Proc. 19th IMEKO TC 4 Symp. and 17th IWADC Workshop
[4] J. Wesson, Tokamaks. New York, NY, USA: Oxford Science Publications, 2012.
Advances in Instrum.and Sensors Interoperability, Barcelona, Spain, 2013.
[5] A. Mann, “Still in suspense,” Nature, vol.475, pp. 280-282, 2011. [6] A. A. Golubov, “High Temperature Superconductivity,” Handbook of Applied Superconductivity, vol. 1, pp. 53-62. Philadelphia, PA, USA: IOP Publishing, 1998.
[22] H. C. Appelo, “The zero-flux DC current transformer, a high precision bipolar wide-band measuring device,” IEEE Trans. Nuc. Scien., vol. 24, no. 3, p. 1810, 1977. [23] C. Berriaud and A. Donati, “A device for measuring high current
[7] International Standard, Critical current measurement- DC critical current of Nb-Ti composite superconductors, CEI-IEC 61788-1, 2006. [8] A. P. Verweij, J. Genest, A. Knezovic, D. F. Leroy, J. P. Marzolf, and L. R. Oberli, “1.9 K test facility for the reception of the superconducting cables for the LHC,” IEEE Trans. Appl. Supercond. vol. 9, no. 2, p.153, 1999.
at cryogenic temperatures,” IEEE Trans. Appl. Supercond., vol. 12, no.1, p. 1264, 2002. [24] MACC-PLUS. [Online] Available: http://www.hitecups. com/?RubriekID=4025. [25] A. Hobson, “The zero-flux current transformer, power apparatus and systems, part III,” Trans. of the Amer. Inst. of Elec. Engineers,
[9] A. G. Prodell and A. Am, “A Facility for Evaluating Superconductors above Atmospheric Pressure at 1.8K,” Adv. Cryog. Eng. 43(A), p. 443,1998.
vol. 72, no, 2, p. 608, 1953. [26] “Nickel Iron and Cobalt Iron Cold Rolled Strips,” Aperam Alloys Imphy. [Online] Available: http:// www.aperam.com/
[10] A. Godeke, D. R. Dietderich, J. M. Joseph, J. Lizarazo, S. O. Prestemon, G. Miller, and H. W. Weijers., “A Superconducting 54
Transformer System for High Current Cable Testing,” Rev. Sci.
alloysandspecialities/fileadmin/pdf/Aperam/Brochure/cold_ rolled_strip_web.pdf.
IEEE Instrumentation & Measurement Magazine
February 2014
Sensor
Amalia Ballarino is a scientist leader of the section of superconducting materials, cables, and devices at CERN. For the High Temperature Superconducting leads for the LHC, she received the award of “Superconductor Industry Person of the Year 2006”. She is Work-Package Leader in the EC FP7 High Luminosity Design Study. In 2012, she became a member of the Applied Superconductivity Conference board of directors for large-scale applications. She is a member of the IEC-TC 90. Giuseppe Montenero ([email protected]) took Bachelor (2003), Master (2008) and PhD (2013) Degrees in Telecommunications Engineering at University of Sannio. Since 2007, he has been carrying out his scientific activity at
Input
Processing
Output
Display, actuators, signals, control
CERN. In 2010, he won the IEEE Award for best young student paper at IEEE I2MTC 2010. Since July 2013, he is Fellow at CERN, at the Superconducting Devices Section, Technology Department. Pasquale Arpaia is professor of Instrumentation and Measurements at University of Sannio and Team Leader at CERN. He is Associate Editor of the Elsevier Journal Computer Standards & Interfaces, and in the past also of IEEE Transactions on Electronics Packaging and Manufacturing. He developed several patents and licenses and funded 4 spin-off companies. His PhD students were awarded in 2006 and 2010 at IEEE I2MTC and in 2012 at IMEKO World Conferences.
continued from page 48
monitor has shown that a photon-counting method can produce high-resolution longitudinal beam profiles in the LHC with a dynamic range of up to 105 within a few minutes. It is now regularly used during operation to optimize and understand beam behaviour and is an essential tool for calibration of the LHC experiments.
005.pdf. [2] D. Cocq, “The wide band normaliser – a new circuit to measure transverse bunch position in accelerators and colliders,” Nuclear Instruments and Methods in Physics Research A, vol. 416, pp.1-8, Oct. 1998. [3] E. B. Holzer, et al., “Beam loss monitoring system for the LHC,”
Summary
IEEE Nuclear Science Symp., Fajardo, Puerto Rico, pp. 1052-1056,
The LHC’s suite of sophisticated beam instrumentation has been essential for fast commissioning, safe and reliable operation, and understanding beam behaviour at these high energies and intensities. Many of the systems, of which only a few have been discussed here, are the fruits of over a decade of R&D, addressing the demand for higher precision, better reliability and faster acquisition. The success of the LHC is therefore, in no small part, due to the beam instrumentation devices that allow the optimization of its performance.
References [1] D. Brandt, “Beam diagnostics for accelerators,” in Proc. CERN Accelerator School CAS, Dourdan, France, 2009, CERN. [Online]. February 2014
Available: http://cds.cern.ch/record/1071486/files/cern-2009-
2005. [4] A. Jeff, et al., “First results of the LHC longitudinal density monitor,” Nuclear Instruments and Methods in Physics Research A, vol. 659, pp. 549-556, Sept. 2011. DOI: 10.1016/ j.nima.2011.08.055
Owain Rhodri Jones ([email protected]) received his B.Sc. in Physics in 1992 and his Ph.D. in 1996 on laser spectroscopy from the University of Wales, Swansea, UK. At CERN, he started as research fellow working on novel beam instrumentation techniques, later taking over responsibility for the beam position and tune measurement systems for the LHC. He is currently head of the Beam Instrumentation Group which provides the beam diagnostic systems for the CERN accelerator complex.
IEEE Instrumentation & Measurement Magazine 55
