ITER Conductors Research Articles

Qualification tests and facilities for the ITER superconductors

All the ITER superconductors are tested as short length samples in the SULTAN test facility at CRPP. Twenty-four TF conductor samples with small layout variations were tested since February 2007 with the aim of verifying the design and qualification of the manufacturers. The sample assembly and the measurement techniques at CRPP are discussed. Starting in 2010, another test facility for ITER conductors, named EDIPO, will be operating at CRPP to share with SULTAN the load of the samples for the acceptance tests during the construction of ITER.

Calorimetric method for current sharing temperature measurements in ITER conductor samples in SULTAN

Several Toroidal Field Conductor short samples with slight layout variations have been assembled and tested in the SULTAN facility at CRPP. The measurement campaigns started in 2007 and are still ongoing. The performance of every conductor is expressed in terms of current sharing temperature ( T cs ), i.e. the temperature at which a defined electric field, 10 μV/m, is detected in the cable due to the incipient superconducting-to-normal state transition. The T cs at specific operating conditions is the key design parameter for the ITER conductors and is the main object of the qualification tests. Typically, the average electric field is measured with voltage tap pairs attached on the jacket along the conductor. The inability however to explain observed premature voltage developments opened the discussion about possible alternative measuring methods. The He flow calorimetric method is based on the measurement of the resistive power generation in the conductor. It relies on the detection of very small temperature increases along the conductor in steady state operation. The accuracy and the reliability of the calorimetric method in SULTAN are critically discussed, with particular emphasis on the instrumentation requirements and test procedures. The application of the calorimetric method to the recent SULTAN test campaigns is described with its merits and limits. For future tests of ITER conductors in SULTAN, the calorimetric method for T cs test is proposed as a routine procedure.

A cable twisting model and its application in CSIC multi-stage cabling structure

The ITER conductor is made of a superconductor cable with multi-stage twisted strands. The spatial structure of such a cable is essential for understanding the effect of load and strain on the performance of a superconducting cable. Unfortunately, there have no analytical models so far to describe accurately such twisted spatial structure. This paper proposes a twisting matrix model to accurately describe the spatial structure of a multi-stage twisted superconducting cable. Although the current model only simulates the first step of a cabling process, it still gives approximately neighboring relation between strands and sub-cables. This neighboring relation is essential for any cabling simulation. The 2nd step simulation, i.e., compaction, can be realized by finite element method based on the cable spatial structure constructed by the proposed one. The simulation results by this model agree well with the observations for pitch and orientation.

Preparatory Work to Host the EDIPO Test Facility at CRPP

CRPP has been selected to host the EDIPO test facility for the quality control tests of the ITER conductors. The new facility will be erected next to the existing SULTAN test facility at CRPP (Villigen) in Switzerland and will share the same cryo-plant. Operation of the two facilities is planned in parallel starting 2009. The new facility is designed to test short length samples built to the same specification as for SULTAN. EFDA procures 17 tons superconducting winding under industrial contract that will be delivered to CRPP in late 2008. The preparatory work at CRPP includes the design and procurement of several items as well as the final assembly and commissioning of the facility. An overview and the progress are reported, with focus on the main components: the vacuum vessel, the superconducting transformer and sample holder unit, the cryogenic cooling loop, the power supply and quench protection system, the HTS current leads and the magnetic screen.

Characterization of ITER $\hbox{Nb}_{3}\hbox{Sn}$ Strands Under Strain-Applied Conditions

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Japan Atomic Energy Agency has developed four types of <formula formulatype="inline"> <tex>$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> strand which can be used in the ITER TF coils. One is a strand made by an internal tin process strand and the others are bronze process strands. The achieved critical current density is more than 790 <formula formulatype="inline"><tex>$\hbox{A/mm}^{2}$</tex> </formula> in the bronze process strands and more than 980 <formula formulatype="inline"> <tex>$\hbox{A/mm}^{2}$</tex></formula> in the internal tin process strand under 4.2 K temperature and 12 T magnetic field and there is hysteresis loss of less than 770 mJ/cc under <formula formulatype="inline"><tex>$\pm$</tex> </formula> 3 T cycle. Since these strands are utilized with an external strain, it is necessary to evaluate strain dependency to confirm the ITER conductor design. An apparatus to measure the strain dependency was newly developed. It has a horseshoe-shaped ring to produce uniform axial compressive or tensile strain along the strand length, a strand being soldered on the outer surface of the ring. The detailed strand characteristics were investigated subjecting the developed strands to a magnetic field from 10 T to 13 T, a strain from about <formula formulatype="inline"><tex>$-$</tex></formula>0.8% to 0.5%, and a temperature from 4.2 K to the critical temperature. When the critical current is normalized to that under the conditions where strain is intrinsically zero, the bronze process strands exhibit better performance than the internal tin process strand. However, the three bronze process strands do not exhibit the same <formula formulatype="inline"><tex>${\rm I}_{c}$</tex></formula>-strain characteristics. Two types of scaling relations are applied to the data, and good expressions of strand performance were obtained by the least square method within 3 A as RMS. </para>

Test Results of Two European ITER TF Conductor Samples in SULTAN

Four conductor lengths were prepared according to the ITER TF conductor design and assembled into two SULTAN samples. The four lengths are not fully identical, with variations of the strand supplier, void fraction and twist pitch. Lower void fractions improve the strand support and increased twist pitches also lower the strand contact pressure but both tend to increase the AC loss and the lower void fraction also increases the pressure drop so that the mass flow rate in the strand bundle area of the cable is reduced. The assembly procedure of the two samples is described including the destructive investigation on a short conductor section to assess a possible perturbation of the cable-to-jacket slippage during the termination preparation. Based on the DC performance and AC loss results from the test in SULTAN, the impact of the void fraction and twist pitch variations is discussed in view of freezing the ITER conductor design and large series manufacture. A comparison with the former generation of conductors, using similar strands but based on the ITER Model Coil layout, is also carried out. The ITER specifications, in terms of current sharing temperature, are fulfilled by both samples, with outstanding results for the conductor with longer twist pitches.

Development and Sultan Test Result of ITER Conductor Samples of Korea

As a way to improve performance of ITER TF conductors, two types of cable-in-conduit conductors were developed in Korea with variations of conduit thickness resulting in the different void fraction of the conductors. The estimated void fractions of the conductors are 31% and 33%. Here we report the details of the TF conductor development and the performance test result of them carried out in SULTAN. Regarding the conductor development, the internal-Sn-processed strand characteristics, strand cabling, twist pitch and characteristics of the conduits for the conductors are presented. For an extended understanding of the conductor design and performance, the SULTAN test results are presented and the effect of the void fraction variations is discussed based on the results.

Signal processing of data from short sample tests for the projection of conductorperformance in ITER magnets

Qualification of the ITER conductor is absolutely necessary. Testing large scale conductorsis expensive and time-consuming. To test 3–4 m long straight samples in a bore of a splitsolenoid is a relatively economical way in comparison with the fabrication of a coil to betested in a bore of a background field solenoid. However, testing short samples maygive ambiguous results due to different constraints in current redistribution inthe cable or other end effects which are not present in the large magnet. Thispaper discusses the processes taking place in the ITER conductor, conditionswhen conductor performance could be distorted and possible signal processing todeduce the behaviour of ITER conductors in ITER magnets from the test data.

A porous medium analogy for the helium flow in CICCs

Cable-in-Conduits are superconducting conductors that consist of a bundle of cabled strands enclosed in a leak-tight pipe, the jacket. Helium flows inside the jacket, in a meandrous flow path defined by the intersticial space among the cabled strands, or in specific passages introduced by design to reduce pressure drop. The main advantage of this design is to bring the coolant in close thermal contact with the superconductor, thus enhancing the heat removal as well as its thermal stability. To date, however, the details of the flow and heat transfer mechanism in this complex geometry are not well understood. We propose to use an analogy between the bundle of strands in the cable and a porous media. The analogy provides simple correlations for pressure drop and heat transfer prediction that contain explicitly permeability, drag factor and thermal dispersion. We use published pressure drop data to show that the range of permeability of a CICC is within the expected values for a porous medium with the same equivalent particle diameter, while the drag factor is consistently lower than what expected from the theory of particle beds, which will require further work to produce a satisfactory explanation. Experimental data for the internal heat transfer obtained from a short ITER conductor sample are in good agreement with the expected contribution of thermal dispersion, which supports or proposal.

Development of large current superconductors using high performance Nb3Sn strand for ITER

In the ITER model coil tests, degradations of the critical current were observed in Nb3Sn cable-in-conduit conductors when a large transverse electromagnetic force was applied to them. Both mass production technique of strand having high critical current enough to recover the degradation and also trial fabrication of the ITER-TF cable and conduit section were successfully performed. These results demonstrate feasibility of fabrication of the TF conductor using the high performance Nb3Sn strands. In addition, critical current degradation by spatially periodic transverse load was both experimentally and numerically studied. The simulation results show that the transverse resistance among the filaments is significantly small compared to that calculated from bronze resistivity. It is explained by the effect of superconducting current across the filament cross-section. This current however disappears when a large bending strain is applied to the strand, resulting in deterioration of the transverse resistance. It suggests that small degradation of the TF model coil (TFMC) conductor is not attributed to sufficiently low resistance among filaments as is expected in the optimized design of the ITER conductor but high resistance of the TFMC strand critical current against the transverse load. On the other hand, the developed strand has almost the same resistance against the transverse load as the TFMC strand. Therefore, the small degradation of the TF conductor, i.e., stable operation of the TF conductor is preliminarily anticipated.

Development of Strain-Applying Apparatus for Evaluation of ITER ${\rm Nb}_{3}{\rm Sn}$ Strand

Japan Atomic Energy Agency has successfully developed Nb3Sn strand which fulfills ITER requirements. Because Nb3Sn is very susceptible to external strain which reduces critical current, critical temperature, and critical field, it is necessary to evaluate strain dependency of these Nb3Sn strands to confirm an ITER conductor design. An apparatus to measure the strain dependency was newly developed. It has a horseshoe-shaped ring and a strand is soldered on its outer surface. This shape produces uniform axial compressive or tensile strain along strand length by expanding or closing the opening of the ring. Critical current can be measured by the apparatus under a magnetic field up to 15 T, a temperature range of 4.2 K to 15 K, and strains. The maximum allowable current is about 300 A. The details of the apparatus and results of strand characterization are presented.

Results of Contact Resistance Distribution in NbTi and Nb$_{3}$Sn ITER Conductor Termination

The contact resistance distribution among the current-carrying elements of three ITER conductor terminations is measured in the JORDI (joint resistance distribution) test facility at CRPP. One sample is based on Nb3Sn strands, the other two are NbTi cable-in-conduit with different cable pattern and termination layout. The current from a 10 kA power supply is split by precision, low temperature shunt resistors into up to 96 parallel channels, each connected to the first or second cable stage of the conductor sample. The voltage drop is sensed between each current carrying element and the equi-potential surface of the joint connecting the conductor termination to the facility current leads. The current distribution in normal operation of a superconducting coil (imposed voltage) is straightforward deduced from the resistance distribution measured in JORDI under imposed current balance. For the same termination layout, the resistance distribution of the Nb3Sn sample is much flatter than in the NbTi samples.

Review of Nb 3Sn conductors for ITER

The ITER magnets experiencing high magnetic field (>11 T), the central solenoid (CS) and the toroidal field (TF) system, will make use of complex conductors using Nb 3Sn as superconducting material. This superconductor offers the required capability in temperature and magnetic field but shows sensitivity to strain. The two ITER model coils, the CS model coil (CSMC) and the TF model coil (TFMC) tested in 2000–2002 have given confidence in the industrial feasibility and the operating capability of the ITER magnets. However, careful analyses have revealed a degradation in terms of temperature margin. Taking advantage of the progress made by the superconductor industry since the model coil fabrications, the ITER Nb 3Sn conductors have been redesigned using higher performance (advanced) strands to recover the original designed temperature margin without changing the conductor envelopes. An R&D program was launched among the ITER participant teams (PTs) to understand the source of degradation which was found to be likely the bending of the strands inside the cable under the electromagnetic load. Meanwhile, new strands were ordered to industrial companies, to manufacture and to test full-size conductors. The European Union PT tested by mid-2006 two full-size conductor samples with advanced Nb 3Sn strands but using the TFMC conductor design, they showed performance much below expectation with regard to strand performance and to ITER TF conductor design. Overall performance is not much above the TFMC conductor sample performance. This poor performance has been attributed to more sensitivity to deformation of advanced strands. Nevertheless, tests of real ITER TF conductor samples are needed to draw final conclusions. The first samples should be tested at the end of 2006 and in 2007. However, one must already be prepared to redesign the ITER conductors as well as to modify and adapt the strand specifications, using the new tools (test facilities, mechanical models) developed since the model coil tests. In addition, the way to extrapolate strand and full-size short sample test results to ITER coils is not straightforward and has to be agreed among the ITER team and the ITER PTs.

Assessment of conductor degradation in the ITER CS insert coil and implications for theITER conductors

Nb3Sn cable in conduit-type conductors were expected to provide an efficient way of achievinglarge conductor currents at high field (up to 13 T) combined with good stability toelectromagnetic disturbances due to the extensive helium contact area with the strands.Although ITER model coils successfully reached their design performance (Kato et al 2001Fusion Eng. Des. 56/57 59–70), initial indications (Mitchell 2003 Fusion Eng. Des. 66–68971–94) that there were unexplained performance shortfalls have been confirmed. Recentconductor tests (Pasztor et al 2004 IEEE Trans. Appl. Supercond. 14 1527–30) andmodelling work (Mitchell 2005 Supercond. Sci. Technol. 18 396–404) suggest that theshortfalls are due to a combination of strand bending and filament fracture under thetransverse magnetic loads. Using the new model, the extensive database from the ITER CSinsert coil has been reassessed. A parametric fit based on a loss of filament area andn (the exponent of the power-law fit to the electric field) combined with a more rigorousconsideration of the conductor field gradient has enabled the coil behaviour to beexplained much more consistently than in earlier assessments, now fitting theNb3Sn strain scaling laws when used with measurements of the conductor operating strain,including conditions when the insert coil current (and hence operating strain) werereversed. The coil superconducting performance also shows a fatigue-type behaviourconsistent with recent measurements on conductor samples (Martovetsky et al 2005 IEEETrans. Appl. Supercond. 15 1367–70). The ITER conductor design has already beenmodified compared to the CS insert, to increase the margin and provide increasedresistance to the degradation, by using a steel jacket to provide thermal pre-compressionto reduce tensile strain levels, reducing the void fraction from 36% to 33% andincreasing the non-copper material by 25%. Test results are not yet available for thenew design and performance predictions at present rely on models with limitedverification.

Potential and desire for HTS application in thermonuclear fusion

Superconducting magnets are stringent for fusion reactors with magnetic confinement to provide an economic energy balance. Large-scale development programmes have been executed worldwide to achieve in time the needed technology. The ultimate result of this effort, so far, is the ITER magnet system with the most sophisticated LTS. However, if one thinks about the design of a fusion DEMO and later reactors, the option of HTS must be considered seriously in view of the potential advantages of these conductors concerning higher operation temperature, temperature margin, high field properties and cryogenic power saving. Extrapolating from the long period needed for the ITER conductor development, it is time now to start with HTS development for fusion reactors to be able to decide their applicability. It is still a long way for the HTS to become comparable with LTS, however, continuous progress can be seen. Beside the large confinement magnets, HTS will also be of advantage for current leads, bus bars and gyrotron magnets. The state of the art of HTS is such, that already now such components can be constructed with HTS.

Test Results of a Small Size CICC With Advanced&lt;tex&gt;$rm Nb_3rm Sn$&lt;/tex&gt;Strands

In the scope of the design activities for a 12.5 T dipole magnet, a short length of cable-in-conduit conductor (CICC) with steel jacket was prepared in spring 2005 as a pre-prototype of the high field dipole conductor. The small, rectangular conductor uses for the first time Nb3Sn strands, with Jc=1100 A/mm2 at 12 T, 4.2 K, also planned in the qualification samples for the ITER conductors. At CRPP, a hairpin sample for SULTAN has been prepared from the short conductor length, heat treated and instrumented. The test program included pressure drop, AC losses and full DC characterization before and after cyclic loading. At large, the test results indicate that the high performance of the advanced strands is preserved in the small size, steel jacketed CICC

Results of Contact Resistance Distribution in ITER Size Conductor Termination

JORDI (JOint Resistance DIstribution), a dedicated test facility for the measurement of contact resistance distribution in full-size, ITER type conductor terminations, has been successfully commissioned at CRPP. In contrast to earlier attempts, where the contact resistance in terminations was measured in current unbalanced condition, in JORDI the contact resistance is measured under imposed balanced current distribution among all current-carrying elements of the termination. The current from a 10 kA power supply is split by precision, low temperature shunt resistors into 98 parallel channels, each connected to the first cable stage of a full size ITER conductor. The voltage drop, in the range of few tens of nanovolt, is simultaneously sensed between each current carrying element (first cable stage) and the equipotential surface of the joint between the conductor termination and the facility current lead. We report first results of the contact resistance distribution in a prototype of typical ITER terminations, made of NbTi based cable-in-conduit conductor. The results of the measurements are analyzed based on a simple statistical model

Transverse heat transfer coefficients on a full size dual channel CICC ITER conductor

Dual channel Cable-In-Conduit Conductors (CICC) provide low hydraulic resistance and faster central channel circulation, limiting superconductors temperature rise. The Poloidal Field Insert Sample (PFIS) was tested in the SULTAN facility to evaluate the thermal coupling between the CICC channels upon an experimental heat transfer coefficient assessment. Simple assumptions on the flow – homogeneous central and annular temperatures, no jacket conduction, no steel inertia and diffusivity – lead to a one-dimensional thermal model fully solved in its transient response to a Heavyside temperature evolution at the inlet, using a Laplace transformation. Transient temperature step data fitted with the analytical resolution provide heat transfer coefficients as a function of mass flow rate, compared to crude predictions. The transient measurements provided consistent measurements on the full range of mass flow rate in both vertical flow directions, whereas steady state homogenization characteristic length measures pursuing the same goal suffer from annular isothermal assumption. Recommendations are made for the thermohydraulic instrumentation of future conductor samples.

The European Nb 3Sn advanced strand development programme

Strands relevant for fusion with high critical current densities and moderate hysteresis losses were developed and already produced on industrial scale. Based on these achievements EFDA-CSU Garching has launched a Nb 3Sn strand development and procurement action inside Europe in order to assess the current status of the Nb 3Sn strand production capability. All six addressed companies have replied positively to the strand R&D programme which includes the three major Nb 3Sn production techniques namely the bronze, internal-tin and powder-in-tube (PIT) route. According to the strand requirements for the ITER TF conductor a critical current density of 800 A/mm 2 (at 12 T, 4.2 K and 10 μV/m) and overall strand hysteresis losses below 500 kJ/m 3 have been specified as the minimum guaranteed strand performance. The second major objective of this programme is to motivate the strand manufacturers to develop and design high performance Nb 3Sn strands optimised for the ITER conductor. For this purpose, a target critical current density of 1100 A/mm 2 has been added to the specification. This paper describes the strategy behind the strand development programme, the actual status of the strand production as well as first preliminary results obtained from the strand suppliers.

The CEA JOSEFA Test Facility for Subsize Conductors and Joints

The JOSEFA (joint subsize experiment facility) experimental test facility, installed at CEA/Cadarache is devoted to perform tests at cryogenic temperature on subsize superconducting conductor and joint samples under parallel or transverse magnetic field. This facility was built in 1993 to investigate the performances of joints of cable-in-conduit conductors at subsize level and further upgraded in the framework of European tasks. The samples of hairpin type using subsize ITER conductors are cooled by a circulation of supercritical helium in a temperature range from 5 to 15 K and tested at a maximum current up to 10 kA. Two different helium bath cooled magnets allow to apply DC or AC transverse magnetic field up to 3.5 T or longitudinal magnetic field up to 7.5 T. A sliding system with a 240 mm stroke on the sample cryostat allows to test separately in the same sample either the conductor or the joint performances. The paper reports on how, through the conductor and joint development tasks, the facility performances were successfully increased and tested. The ITER TFMC joints using Nb/sub 3/Sn conductors were first developed on this facility. The last developments, performed on the ITER PF NbTi conductors and joints proved this facility to be a versatile and useful tool for superconducting magnet developments and showed the interest of possible upgrading to finalize conductor design.