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Journal ArticleDOI

Performance improvement of a measurement station for superconducting cable test.

20 Sep 2012-Review of Scientific Instruments (AIP Publishing)-Vol. 83, Iss: 9, pp 095111

TL;DR: A fully digital system, improving measurements flexibility, integrator drift, and current control of superconducting transformers for cable test, is proposed, based on a high-performance integration of Rogowski coil signal and a flexible direct control of the current into the secondary windings.

AbstractA fully digital system, improving measurements flexibility, integrator drift, and current control of superconducting transformers for cable test, is proposed. The system is based on a high-performance integration of Rogowski coil signal and a flexible direct control of the current into the secondary windings. This allows state-of-the-art performance to be overcome by means of out-of-the-shelf components: on a full-scale of 32 kA, current measurement resolution of 1 A, stability below 0.25 A  min−1, and controller ripple less than ±50 ppm. The system effectiveness has been demonstrated experimentally on the superconducting transformer of the Facility for the Research of Superconducting Cables at the European Organization for Nuclear Research (CERN).

Topics: Rogowski coil (61%), Superconducting magnet (55%), Transformer (54%), Electromagnetic coil (54%), Integrator (51%)

Summary (3 min read)

Introduction

  • The system effectiveness has been demonstrated experimentally on the superconducting transformer of the Facility for the Research of Superconducting Cables at the European Organization for Nuclear Research (CERN).
  • On the other hand, for large-size cables, facilities of appropriate dimensions and functionality are few, mainly owing to the difficulty and cost of providing a large and complex set-up for assessing the device properties as a function of the abovementioned parameters 3-8 .
  • The control loop, on the other hand, must account for the physical characteristics of the coupled system formed by the primary winding, and its power supply, the secondary, and the current transducer.
  • In 11 , these issues are addressed both by implementing a custom FPGA-based integrator with higher resolution and by minimizing the residual offset via a dedicated procedure.

II. The Proposed System

  • In Fig. 1, the architecture of a measurement station for superconducting cable test, based on a transformer for supply and Rogowski coils for current measurement, is reported.
  • The secondary current is then obtained by integrating the differential signal VRC from the Rogowski coils.
  • The measured current is finally compared to the reference Iref in order to generate the feedback signal Im compensating for resistive losses.
  • 4/21 In this architecture, the fundamental elements are the measurement system and the control strategy.
  • The above-mentioned requirements are met by exploiting high-performance numerical integration and digital control algorithms, as described below.

A. The measurement and control system

  • In Fig. 2a, the architecture of the measurement and control system is shown.
  • The control reference Vref is generated by a digital waveform generator, with at least 16 bits of resolution in the input range of the voltage-controlled current source (Fig. 2b) in order to accurately control Ip.
  • Then, after digital integration, the measured magnetic flux is: ( ) ( ) ( ) (2) where n stands for a discrete time instant, and offset(n) is the undesired flux contribution arising from the voltage offset on the data acquired from the integrator.
  • Beyond well-known advantages of a fully digital measurement, the proposed architecture allows off-the-shelf boards, advanced digital signal processing, and software flexibility to be exploited.
  • Moreover, a software control algorithm can be implemented if the sampling frequency is less than few hundreds of samples/s.

B. The system under control

  • The first eq. of (3) provides the dependence of the currents on the voltage Vp, and the second eq. of (3) provides the link between primary and secondary currents.
  • 6/21 From (3) the transformer transfer function is derived using the Laplace transform: ( ) ( ) ( ) (4) where GT is the transformer gain, i.e. the current amplification factor, without losses (Rs=0) , and τ the decay time constant of Is, .
  • The (4) justifies the need for a control strategy to counteract the resistive current decay in the secondary circuit 21 .
  • According to 12,13 , the joint resistance and the self-inductance are a function of the current and the field.

C. Digital control algorithm

  • Conversely, in this work, a fully digital measurement system and control algorithm, taking into account only the plant characteristic without further analog signal handling, is proposed.
  • This provides an accurate conversion of the reference voltage V * ref into the primary current Ip.
  • The transformer is modeled according to (4).
  • The transfer function of the Proportional-Integral controller PI(z) can be written for a backward digital integrator as: ( ) (5) where KP and KI are the gains of the proportional and integral actions, respectively.
  • (12) The (12) represents a first-order filter with behavior defined by the pole position inside the unit circle.

III. Experimental Results

  • The proposed system was tested at CERN, on FReSCa, the Facility for Research on Superconducting Cables 4 .
  • 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 secondary is impregnated with epoxy to support mechanically the coil.
  • In the following, (A) the experimental set-up, (B) the controller parameters determination, (C) the measurement system characterization, and (D) the validation results of the proposed system are described.

A. Experimental Set-Up

  • The waveform generator is realized through a data acquisition board NI-PXI 6281 of National Instruments 22 .
  • The board drives a fourquadrants power supply Lake Shore Mod 622 24 , supplying the transformer’s primary (voltage-controlled current source in Fig. 1).
  • The signal-to-noise and distortion ratio is higher than 100 dB.
  • The timing board is a NI PXI-6682 of National Instruments, with 10 MHz of internal clock 26 , used to generate the trigger signal for the FDI and for the data acquisition board.
  • The embedded computer is a Single-Board Computer D9-6U by Mikro Elektronik 28 , hosting the software handling the whole system functions, based on the Flexible Framework for Magnetic Measurements 29 , and implementing the controller algorithm.

B. Controller parameters determination

  • Defining the largest required signal bandwidth for the secondary current enables to specify the sample frequency of the closed-loop operation and thereafter the controller parameters.
  • In practice, this value can be thought much lower because for testing purposes the current has smooth transition to the maximum ramp-rate.
  • The required numeric bandwidth for the control is therefore 0.2 (B/fs).
  • Once the sample rate is defined, the controller parameters K, KI, and KP can be calculated 18 .
  • In Figs. 6, the bounds of the frequency response of the closed-loop transfer function (12), using ideal GT and τ are illustrated for a typical variation of ±30 % of the transfer function parameters (namely, left, the magnitude, and, right, the phase).

C. Measurement System Characterization

  • Main problems in the secondary current measurement arise from the integration equivalent offset and from the repeatability of Rogowski Coils in typical test conditions.
  • For the repeatability tests of the system composed by the Rogowski coils and FDI, the sample was fed directly from the room-temperature 32-kA power supply, through large current leads (Fig. 7a).
  • Stability In Fig. 7b, the experimental set up for the stability tests of the measurement system is illustrated.
  • In Figs. 11b1 and 11b2, the differences between the measured current and the ideal linear reference at ramp up and its average value at the flattop, respectively, are detailed.
  • III, the average values of the measured critical current on the sample under test with the reference 32-kA power supply and the superconducting transformer are compared by reporting also their percentage difference.

IV. Conclusions

  • A fully digital system for the control of transformers for superconducting cable testing is proposed.
  • The digital system is based on a low-drift precision integrator and a simple but robust PI control algorithm, achieving brilliant performance and improving test flexibility.
  • The set-up has also a definite cost advantage for the use of off-the-shelf components.
  • The effectiveness of the architecture was assessed by an experimental implementation aimed at controlling the superconducting transformer available at the Facility for Research on Superconducting Cables of CERN.
  • These results were demonstrated in practical working conditions, measuring the critical current of a NbTi Rutherford cable with well known properties.

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CERN-ATS-2012-057
15/03/2012
EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
CERN - ACCELERATORS AND TECHNOLOGY SECTOR
Performance Improvement of a Measurement Station
for Superconducting Cable Test
P. Arpaia
1,2
, L. Bottura
2
, G. Montenero
1,2
, S. Le Naour
2
1 Engineering Department, University of Sannio, Benevento, Italy
2 CERN, Geneva, Switzerland
A fully digital system, improving measurements flexibility, integrator drift, and current control of superconducting
transformers for cable test, is proposed. The system is based on a high-performance integration of Rogowski coil
signal and a flexible direct control of the current into the secondary windings. This allows state-of-the-art performance
to be overcome by means of out-of-the-shelf components: on a full-scale of 32 kA, current measurement resolution of
1 A, stability below 0.25 Amin
-1
, and controller ripple less than 50 ppm. The system effectiveness has been
demonstrated experimentally on the superconducting transformer of the Facility for the Research of Superconducting
Cables at the European Organization for Nuclear Research (CERN).
Submitted to "Review of Scientific Instruments"
Geneva, Switzerland
CERN-ATS-2012-057
March 2012
Abstract

1/21
Performance improvement of a measurement station for
superconducting cable test
Pasquale Arpaia,
1,2
Luca Bottura,
2
Giuseppe Montenero,
1,2
Sandrine Le Naour
2
1
Engineering Department, University of Sannio, 82100 Benevento, Italy.
2
European Organization for Nuclear Research (CERN), 1217 Geneva, Switzerland
A fully digital system, improving measurements flexibility, integrator drift, and
current control of superconducting transformers for cable test, is proposed. The
system is based on a high-performance integration of Rogowski coil signal and a
flexible direct control of the current into the secondary windings. This allows state-of-
the-art performance to be overcome by means of out-of-the-shelf components: on a
full-scale of 32 kA, current measurement resolution of 1 A, stability below
0.25 Amin
-1
, and controller ripple less than 50 ppm. The system effectiveness has
been demonstrated experimentally on the superconducting transformer of the Facility
for the Research of Superconducting Cables at the European Organization for Nuclear
Research (CERN).
I. Introduction
Superconductivity is a technology with a declared interest in several fields of physics
and engineering
1
. A key design parameter for any large-scale application of
superconductivity is the current carrying capacity, also referred to as “critical current”. Its
assessment needs first and foremost for an accurate measurement of the voltage-current
characteristic of the sample, in general, function of temperature, current, and magnetic field
2
.
This is a relatively delicate task for single wires, nowadays carried out through quasi-
industrial standards. On the other hand, for large-size cables, facilities of appropriate
dimensions and functionality are few, mainly owing to the difficulty and cost of providing a
large and complex set-up for assessing the device properties as a function of the
abovementioned parameters
3-8
.
Cable critical current tests commonly involve current levels in the order of the 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

2/21
to use superconducting transformers
9-17
. 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 on the performance and protection of the primary and
secondary windings, one of main concerns in the operation of a superconducting transformer
for cable test is a suitable control, with a precise and accurate measurement of the secondary
current. Indeed, current measurement is a key component of the control loop. Resistive losses
due to the interconnections between the secondary and the sample lead to an unavoidable
decay of current, unless the primary current is adjusted continuously to compensate and
maintain the sample current at the desired set value. The control loop, on the other hand, must
account for the physical characteristics of the coupled system formed by the primary winding,
and its power supply, the secondary, and the current transducer.
In
10
and
11
, advanced systems for measuring the secondary current and for
compensating its decay are proposed. Both are based on similar concepts for the
improvement of the current measurement quality and the implementation of a suitable
control. The sensing elements for the current are two Rogowski coils connected in anti-series,
providing a good rejection to parasitic couplings. The transducer signal is integrated in time
by using digital integrators, and the integral is converted to current by means of a suitable
calibration coefficient. The signal conditioning and the control are then implemented by
means of customized analog electronics. In
10
, main drawbacks are the resolution and the
offset of the digital integrator. In
11
, these issues are addressed both by implementing a
custom FPGA-based integrator with higher resolution and by minimizing the residual offset
via a dedicated procedure. However, the control loop is still analog (proportional action).
Furthermore, in both the systems, instead of directly monitoring the secondary current, a
voltage produced via a function generator is used as reference signal on the primary power
supply, by giving rise to an indirect control strategy. This makes rather complex both the
common operation of defining an arbitrary cable current cycle and the detection of system
faults: a test cycle defined in terms of secondary current has to be converted in a
corresponding voltage. This requires that the closed-loop transfer function is known with
satisfying accuracy for all the operating conditions.

3/21
In this paper, a flexible system improving the state-of-the-art of superconducting cable
testing based on transformers is proposed to overcome state-of-the-art solution by meeting
the overall target of a secondary current control for a 32 kA full range, with resolution better
than 3 A, relative ripple less than 10
-4
, and stability below 0.5 Amin
-1
. The secondary current
measurement is improved by compensating the residual offset of a high-performance digital
integrator by adjusting the self-calibration time. The measurement system is integrated in a
fully-digital control loop, with all the related benefits
18
, i.e. good noise margins, easiness in
the implementation/modification, and so on. In particular, in Section 2, the proposed
measurement system with the associated digital control algorithm is illustrated. In Section 3,
experimental results from the on-field characterization and validation of the proposed system
at CERN, in the Facility for Research on Superconducting Cables (FReSCa)
4
are illustrated.
II. The Proposed System
In Fig. 1, the architecture of a measurement station for superconducting cable test,
based on a transformer for supply and Rogowski coils for current measurement, is reported.
The objective is to generate a test current in the sample I
s
, i.e. in the transformer secondary,
ideally equal to the set point I
ref
. At this aim, the main issue is to provide a direct control of
the secondary current I
s
. The control block, according to the feedback signal I
m
from the
measurement system, acts to provide a reference voltage V
ref
to the power supply of the
primary, a voltage-controlled current source. The source drives the current I
p
into the
primary, by inducing a secondary current I
s
. In turn, I
s
is sensed by means of two Rogowski
coils, suitably positioned in phase opposition and connected in anti-series in order to reject
parasitic electromagnetic couplings to stray fields. The coils voltage signal V
RC
is
proportional to the secondary current rate dI
s
/dt. The secondary current is then obtained by
integrating the differential signal V
RC
from the Rogowski coils. The measured current is
finally compared to the reference I
ref
in order to generate the feedback signal I
m
compensating
for resistive losses.
FIG. 1. Architecture of a transformer-based measurement station for superconducting cable test.

4/21
In this architecture, the fundamental elements are the measurement system and the
control strategy. The former has to provide high-quality measurements, by minimizing
undesired uncertainty effects, such as integrator drift and noise. The latter has to follow
closely the reference signal, i.e. the desired secondary current I
s
, by accurately driving the
current into the primary I
p
. Moreover, the overall system should be flexible, in order to allow
quick modifications of the desired waveforms according to the test needs.
The above-mentioned requirements are met by exploiting high-performance numerical
integration and digital control algorithms, as described below.
A. The measurement and control system
In Fig. 2a, the architecture of the measurement and control system is shown. The
control reference V
ref
is generated by a digital waveform generator, with at least 16 bits of
resolution in the input range of the voltage-controlled current source (Fig. 2b) in order to
accurately control I
p
. The Rogowski coils signal V
RC
, acquired by means of a digital
integrator, is related to the secondary current in the ideal case as:




(1)
where K
RC
(VsA
-1
) is the sensitivity coefficient of the Rogowski coils in anti-series
configuration. Then, after digital integration, the measured magnetic flux is:

󰇛
󰇜

󰇛
󰇜

󰇛
󰇜
(2)
where n stands for a discrete time instant, and
offset
(n) is the undesired flux contribution
arising from the voltage offset on the data acquired from the integrator. This integration
equivalent offset signal arises from both all the circuit before the integrator input as well as
from the integrator itself. In practice, design objectives are achieved by imposing the
following requirements on the integrator:
Signal-to-Noise Ratio (SNR) better than 90 dB, for resolution in current readings
better than 1 A on a full-scale of 32 kA;

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Abstract: This book is not suitable for the majority of readers of Physics Education. It is a highly specialized textbook which is intended to give a `concise introduction to the theory and practice of Fourier transforms ... and avoiding unnecessary mathematics'. The author states that no previous knowledge of the subject is needed. However, I believe that the subject matter will only be appreciated by readers who already have a fairly detailed knowledge of optics and electrical circuit theory, probably at third-year university level. The `new' student of physics, electrical and electronic engineering and computer science will find the book heavy going because insufficient background information is supplied; the physicist will probably benefit from reading a textbook such as Hecht's Optics. In order to support the authors' contention that Fourier methods are not a theoretical device, various important applications are employed as illustration, such as extracting information from noisy signals, designing electrical filters, treating experimental data and `cleaning' TV pictures, and so on. The first chapter introduces the Fourier transform, and is relatively easy reading, but the second chapter is too highly condensed. It touches on the Dirichlet conditions, introduces some of the theorems used to manipulate Fourier pairs and then devotes much of the remainder of the chapter to the concept of the convolution. There is a tremendous amount of detail which the unfamiliar reader may find quite daunting. It is a pity that the convolution of two top-hat functions is not generated diagramatically: the author assumes that the result (a triangle) is obvious from inspection. The remaining chapters deal with the applications mentioned previously. The book has an attractive cover, and is well produced using good quality paper. I would expect the book to be bought by libraries rather than private individuals.

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Abstract: ASC'98 Palm September 14- 18, 1998 D~~!i11JG L8/11- Jt: fj.:2/hF be:;}'! Critical Current of Superconducting Rutherford Cable in High Magnetic Fields with Transverse Pressure Daniel R. Dietderich and Ronald M. Scanlan, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Robert P. Walsh and John R. Miller National High Magneti c Field Laboratory, Tallahassee, FL 32706 Abstract-For high energy physics appli cations supereon· dueting cables are subjected to large stresses and high magnetic fields during service. It is essential to know how these cables perform in these operating conditions. A loading fixture capable of applying loads of up to 700kN has been developed by NHMFL for LBNL. This fixture permits uniform loading of straight cables over a 122 nun length in a split-pair solenoid in field s up to 12 T at 4.2 K. The first results from this system for Rutherford cables of internal-tin and modified jelly roll strand of Nb,Sn produced by IGC and TWC showed that lillie permanent degradation occurs up to 210 MPa. However, the cable made from internal· tin s't rand showed a 40 % reduction in at lIT and 210 MPa while a cable made from modified jelly roll material showed onJy a 15 % reduction in Ie at liT and 185 MP•. I. INTRODUCTION Large forces are produced when superconducting magnets are ene rgized . Therefore, it is necessary to support the conductor to prevent large displacements that can strain the conductor beyond its elastic limit. These same forces also reduce the critical current of the conductor before the elastic limit is reached. To produce an optimum magnet design which best utilizes the superconductor material and maximizes the field the critical current variation with pressure is required. For a cosine 0 magnet design large pressures are produced at the mid-plane of the magnet on the large face of the Rutherford cable. The magnets (i.e. common coil) presently being designed and built in the Superconducting Magnet Group (SMG) of Lawrence Berkeley National Laboratory such that the cable will be bi-axially loaded on both the face and the edge with the edge loading being about twice that applied to the face. Therefore information for loading in both orientations is required. The results presented here are the first measurements performed at the National High Magnetic Field Laboratory (NHMFL) in a system designed for the SMG. These tests were performed on cable designed for and used in the inner coi ls ofthc world record dipole magnet D20 [I]. Manu sc ript received September 14, 1998. Thi s work was supported by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, High Energy Physics Division, U. S. Department of Energy, under Contrac t No. DE-AC03-6SFOOO98 . These cables were fabricated in our Group from modified jelly roll wire produced by Teledyne Wah Chang (TWC) and from internal tin wire manufactured by Intermagnetics General Corporation (IGC). The results for both cables showed that very little if any permanent degradation occurred for transverse loads up to 185 to 210 MPa. However, the critical current of the cable with IGC strand was reduced by 40 % with a transverse pressure of 210 MPa at II T. ·The I, of the cable with TWC strand was only reduced by 15 % at II T and 185 MPa. The behavior of both cables with loading was also different. The TWC strand showed a quadratic behavior with increasing press ure while the IGC strand showed a linear behavior. These results have important ramifications for high field magnet designs beyond the 13.5 T of D20. Not only must the strain level be controlled to prevent irreversible damage at these hi gher field s but the reduction in critical current due to the lower T, and B with high pressure must be controlled. II. MATERIALS AN D TEST SYSTEM A. Strand and Cable Characteristics Two strands made by two manufacturers were used jn the Rutherford cables of this study. Cable 523 was made from IGC strand while cable 522 was made from TWC strand. Each cable was rectangular and contained 37 strands. The cabling parameters are listed in Table I. Strand specifications are given in Table II. The two strands had very different internal geometries. The TWC strand had more Cu stabili zer and each of its 120 sub·e1ements had its own diffusion barrier. (Fig. I (a» The IGC strand had less Cu stabilizer and a diffusion barrier around all of its 19 sub· elements (Fig. I (b». Both cables were hea t treated at the same time and received the following schedule: 210 °C for 121 h, 340 °C for 60 h, and 660 °C for 259 h. For heat treatment the samples were sealed in a stainless steel retort that was continuously purged with fl owing argon. The data has not been corrected for self· field but due to the bifilar nature of the test arrangement the correction s h o ~ld be small.

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Cites background from "Performance improvement of a measur..."

  • ...Widely used current sensors are Rogowski and pick up coils [6], [7], [9], [11]....

    [...]

  • ...in addition to the difficulty of operating a superconducting transformer [11], a major metrological issue is the measurement...

    [...]

  • ...This classical solution has drawbacks such as intrinsic AC nature and limited measurement time due to the need of low drift integration (to keep accuracy of the measured current within the range of hundreds of ppm [11])....

    [...]


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TL;DR: The promises arising from High Temperature Superconductors (Tc>77 K) would extend the range of application, for instance, to power distribution and the generation of electrical energy with tokamaks.
Abstract: Superconductivity is a technology with a declared interest in several fields of physics and engineering. Nowadays, superconducting cables [1] are largely exploited in fields such as particle colliders [2] and medical machines [3]. The generation of electrical energy with tokamaks [4] is a further application of low-temperature superconductivity as well. Moreover, the promises arising from High Temperature Superconductors (Tc>77 K) would extend the range of application, for instance, to power distribution [5]-[6].

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TL;DR: A smart monitoring system forsuperconducting cable test is proposed with an adaptive current control of a superconducting transformer secondary, based on Fuzzy Gain Scheduling, which allows the controller parameters to adapt continuously, and finely, to the working variations arising from transformer nonlinear dynamics.
Abstract: A smart monitoring system for superconducting cable test is proposed with an adaptive current control of a superconducting transformer secondary. The design, based on Fuzzy Gain Scheduling, allows the controller parameters to adapt continuously, and finely, to the working variations arising from transformer nonlinear dynamics. The control system is integrated in a fully digital control loop, with all the related benefits, i.e., high noise rejection, ease of implementation/modification, and so on. In particular, an accurate model of the system, controlled by a Fuzzy Gain Scheduler of the superconducting transformer, was achieved by an experimental campaign through the working domain at several current ramp rates. The model performance was characterized by simulation, under all the main operating conditions, in order to guide the controller design. Finally, the proposed monitoring system was experimentally validated at European Organization for Nuclear Research (CERN) in comparison to the state-of-the-art control system [P. Arpaia, L. Bottura, G. Montenero, and S. Le Naour, "Performance improvement of a measurement station for superconducting cable test," Rev. Sci. Instrum. 83, 095111 (2012)] of the Facility for the Research on Superconducting Cables, achieving a significant performance improvement: a reduction in the system overshoot by 50%, with a related attenuation of the corresponding dynamic residual error (both absolute and RMS) up to 52%.

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References
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Journal ArticleDOI
Arjan Verweij1, J. Genest1, A. Knezovic1, D. Leroy1, J.-P. Marzolf1, L.R. Oberli1 
Abstract: A new test facility (FRESCA-Facility, reception of superconducting cables) is under construction at CERN to measure the electrical properties of the LHC superconducting cables. Its main features are: independently cooled background magnet, test currents up to 32 kA, temperature between 1.8 and 4.5 K, long measurement length of 60 cm, field perpendicular or parallel to the cable face, measurement of the current distribution between the strands. The facility consists of an outer cryostat containing a superconducting NbTi dipole magnet with a bore of 56 mm and a maximum operating field of 9.5 T. The magnet current is supplied by an external 16 kA power supply and fed into the cryostat using self-cooled leads. The lower bath of the cryostat, separated by means of a so called lambda-plate from the upper bath, can be cooled down to 1.9 K using a subcooled superfluid refrigeration system. Within the outer cryostat, an inner cryostat is installed containing the sample insert. This approach makes it possible to change samples while keeping the background magnet cold, and thus decreasing the helium consumption and cool-down time of the samples. The lower bath of the inner cryostat, containing the sample holder with two superconducting cable samples, can as well be cooled down to 1.9 K. The samples can be rotated while remaining at liquid helium temperature, enabling measurements with the background field perpendicular or parallel to the broad face of the cable. Several arrays of Hall probes are installed next to the samples in order to estimate possible current imbalances between the strands of the cables.

57 citations


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TL;DR: The subject matter will only be appreciated by readers who already have a fairly detailed knowledge of optics and electrical circuit theory, probably at third-year university level, and the author states that no previous knowledge of the subject is needed.
Abstract: This book is not suitable for the majority of readers of Physics Education. It is a highly specialized textbook which is intended to give a `concise introduction to the theory and practice of Fourier transforms ... and avoiding unnecessary mathematics'. The author states that no previous knowledge of the subject is needed. However, I believe that the subject matter will only be appreciated by readers who already have a fairly detailed knowledge of optics and electrical circuit theory, probably at third-year university level. The `new' student of physics, electrical and electronic engineering and computer science will find the book heavy going because insufficient background information is supplied; the physicist will probably benefit from reading a textbook such as Hecht's Optics. In order to support the authors' contention that Fourier methods are not a theoretical device, various important applications are employed as illustration, such as extracting information from noisy signals, designing electrical filters, treating experimental data and `cleaning' TV pictures, and so on. The first chapter introduces the Fourier transform, and is relatively easy reading, but the second chapter is too highly condensed. It touches on the Dirichlet conditions, introduces some of the theorems used to manipulate Fourier pairs and then devotes much of the remainder of the chapter to the concept of the convolution. There is a tremendous amount of detail which the unfamiliar reader may find quite daunting. It is a pity that the convolution of two top-hat functions is not generated diagramatically: the author assumes that the result (a triangle) is obvious from inspection. The remaining chapters deal with the applications mentioned previously. The book has an attractive cover, and is well produced using good quality paper. I would expect the book to be bought by libraries rather than private individuals.

40 citations


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Abstract: Superconducting transformers are an important innovation for future power transmission and transportation systems. Powerful, lightweight, energy-saving and environmentally friendly they offer enormous benefits compared to their conventional counterparts. Siemens is developing a 1-MVA demonstrator transformer for laboratory testing, exhibiting innovative features like horizontal design, cabled-conductor windings and a closed cooling cycle with sub-cooled nitrogen. Being one of the most promising applications Siemens has started a programme towards the development of on-board transformers for electrical rail vehicles. This paper summarises world-wide efforts in the development of superconducting transformers and reports on the progress achieved at Siemens.

39 citations


Journal ArticleDOI
Abstract: The critical current degradation of a few sample Rutherford-type Nb/sub 3/Sn cables is investigated as a function of transverse pressure. A comparison is made between Nb/sub 3/Sn strands produced by the powder-in-tube, bronze, and modified jelly roll processes. The (keystoned) Rutherford cables are charged at 11 T under transverse pressures up to 250 MPa. Large differences in critical current reduction are observed, ranging from 6 to about 60% at 200 MPa, depending on the type of Nb/sub 3/Sn. It appears that about 40% of the total reduction is irreversible. Moreover, the irreversible part shows relaxation, and a partial recovery is possible by thermal cycling. >

38 citations


"Performance improvement of a measur..." refers background in this paper

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15 Apr 2011
Abstract: ASC'98 Palm September 14- 18, 1998 D~~!i11JG L8/11- Jt: fj.:2/hF be:;}'! Critical Current of Superconducting Rutherford Cable in High Magnetic Fields with Transverse Pressure Daniel R. Dietderich and Ronald M. Scanlan, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Robert P. Walsh and John R. Miller National High Magneti c Field Laboratory, Tallahassee, FL 32706 Abstract-For high energy physics appli cations supereon· dueting cables are subjected to large stresses and high magnetic fields during service. It is essential to know how these cables perform in these operating conditions. A loading fixture capable of applying loads of up to 700kN has been developed by NHMFL for LBNL. This fixture permits uniform loading of straight cables over a 122 nun length in a split-pair solenoid in field s up to 12 T at 4.2 K. The first results from this system for Rutherford cables of internal-tin and modified jelly roll strand of Nb,Sn produced by IGC and TWC showed that lillie permanent degradation occurs up to 210 MPa. However, the cable made from internal· tin s't rand showed a 40 % reduction in at lIT and 210 MPa while a cable made from modified jelly roll material showed onJy a 15 % reduction in Ie at liT and 185 MP•. I. INTRODUCTION Large forces are produced when superconducting magnets are ene rgized . Therefore, it is necessary to support the conductor to prevent large displacements that can strain the conductor beyond its elastic limit. These same forces also reduce the critical current of the conductor before the elastic limit is reached. To produce an optimum magnet design which best utilizes the superconductor material and maximizes the field the critical current variation with pressure is required. For a cosine 0 magnet design large pressures are produced at the mid-plane of the magnet on the large face of the Rutherford cable. The magnets (i.e. common coil) presently being designed and built in the Superconducting Magnet Group (SMG) of Lawrence Berkeley National Laboratory such that the cable will be bi-axially loaded on both the face and the edge with the edge loading being about twice that applied to the face. Therefore information for loading in both orientations is required. The results presented here are the first measurements performed at the National High Magnetic Field Laboratory (NHMFL) in a system designed for the SMG. These tests were performed on cable designed for and used in the inner coi ls ofthc world record dipole magnet D20 [I]. Manu sc ript received September 14, 1998. Thi s work was supported by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, High Energy Physics Division, U. S. Department of Energy, under Contrac t No. DE-AC03-6SFOOO98 . These cables were fabricated in our Group from modified jelly roll wire produced by Teledyne Wah Chang (TWC) and from internal tin wire manufactured by Intermagnetics General Corporation (IGC). The results for both cables showed that very little if any permanent degradation occurred for transverse loads up to 185 to 210 MPa. However, the critical current of the cable with IGC strand was reduced by 40 % with a transverse pressure of 210 MPa at II T. ·The I, of the cable with TWC strand was only reduced by 15 % at II T and 185 MPa. The behavior of both cables with loading was also different. The TWC strand showed a quadratic behavior with increasing press ure while the IGC strand showed a linear behavior. These results have important ramifications for high field magnet designs beyond the 13.5 T of D20. Not only must the strain level be controlled to prevent irreversible damage at these hi gher field s but the reduction in critical current due to the lower T, and B with high pressure must be controlled. II. MATERIALS AN D TEST SYSTEM A. Strand and Cable Characteristics Two strands made by two manufacturers were used jn the Rutherford cables of this study. Cable 523 was made from IGC strand while cable 522 was made from TWC strand. Each cable was rectangular and contained 37 strands. The cabling parameters are listed in Table I. Strand specifications are given in Table II. The two strands had very different internal geometries. The TWC strand had more Cu stabili zer and each of its 120 sub·e1ements had its own diffusion barrier. (Fig. I (a» The IGC strand had less Cu stabilizer and a diffusion barrier around all of its 19 sub· elements (Fig. I (b». Both cables were hea t treated at the same time and received the following schedule: 210 °C for 121 h, 340 °C for 60 h, and 660 °C for 259 h. For heat treatment the samples were sealed in a stainless steel retort that was continuously purged with fl owing argon. The data has not been corrected for self· field but due to the bifilar nature of the test arrangement the correction s h o ~ld be small.

36 citations


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Q1. What contributions have the authors mentioned in the paper "Performance improvement of a measurement station for superconducting cable test" ?

In this paper, a fully digital system, improving measurements flexibility, integrator drift and current control of superconducting transformers for cable test, is proposed.