Stephen March

Commissioning of the Main Coil of the EDIPO Test Facility

The EDIPO test facility is erected at CRPP Villigen with the aim of providing a flexible, high field test bed for high current force flow superconductors. The EDIPO main coil is a tilted-head race-track pair wound by a graded Nb3Sn cable-in-conduit conductor. The whole project, partly funded by the European Commission, started in 2004 and entered the commissioning phase in 2013. The final steps of instrumentation and installation of the main coil, delivered by industry in May 2011, lasted about 18 months. The first cool-down of the facility started in November 2012. The commissioning of the main coil, including the precise measurement of the generated magnetic field, was carried out in March 2013. At an operating current of 17.2 kA, a ± 1% homogenous field of 12.35 T was generated over a length of 900 mm in the center of the test well, 140 mm × 91 mm in cross section. Details about cool-down, flux jumps, forces and displacements, field map, and charging rate are presented in this paper.

Results of the TFEU6 Sample Tested in SULTAN

This paper presents the results of the latest European Conductor Performance Qualification Sample for the ITER toroidal field coils. The standard qualification test was performed in the SULTAN facility at CRPP. The sample consisted of two 3.5-m lengths of cable in conduit conductor (CICC), which utilized bronze route Nb3Sn strands from Bruker-EAS. The sample was prepared at CRPP with the conductor terminations and the bottom joint fabricated by the solder filling method. In order to prevent slippage between the jacket and cable, SULTAN samples systematically have crimping rings added to the extremity of each leg. In this case, one leg also had additional crimping rings on both sides of the high field region, which experiences a greater electromagnetic load during the test. As per a standard SULTAN sample, temperature sensors and voltage taps were installed along each conductor. In order to monitor the strain effects of the electromagnetic load cycles on the sample, strain gauges were bonded to each leg both in the high and low field regions. Pickup coils were wound on the high field region of each leg for critical temperature measurements via the magnetization method. The results of the SULTAN test are described in this paper and the effect of the additional crimping rings in the high field region discussed. Additional investigative work into residual jacket strain of CICCs is also presented.

The EDIPO Project: Status and Outlook

The aim of this paper is to present an up to date review of the status of the European DIPOle (EDIPO) project - whose objective is to build a new facility to perform both DC and AC tests of large force flow superconductor samples in high magnetic background field (up to ~12.5 T) - as well as an outlook on the final commissioning of the magnet in CRPP-PSI foreseen for 2012. Concerning the actual status, the results presented focus on the recent steps carried out in the manufacturing of the magnet assembly, in particular the cold mass assembly, its impregnation and the final acceptance tests. Moreover, results are reported regarding the identification and repair of an inter-turn short carried out on EDIPO Pole 1 in the second half of 2010. As far as the commissioning of EDIPO is concerned, an up-to-date report is provided on the preparation of the facility at CRPP (e.g. mechanical structures, high temperature current leads, quench detection, control system, etc.) as well as its acceptance and final commissioning (e.g. magnet reception tests, commissioning program, etc.).

Operation and Test Results From the SULTAN Test Facility

In the period between August 2010 and August 2011 the SULTAN test facility has been continuously operated, with over one dozen test campaigns. Among the ITER conductors, five Toroidal Field (TF) samples, two Central Solenoid (CS) samples and four NbTi samples for Poloidal Field (PF), Correction Coil and Main Busbar coming from Japan, Russia, Korea and China have been tested with duration of test campaign ranging from two to nine weeks each. Two non-ITER conductor samples are also tested in the same period, a high current NbTi cable-in-conduit (CICC) for the secondary winding of the EDIPO flux pump and a prototype high field conductor for the hybrid magnet of the Radboud University (NL). The possibility to apply “off-SULTAN” thermal cycles from room temperature to liquid nitrogen has been realized for two samples. A novel method for in-situ measurement of the critical temperature in CICC has been successfully applied in SULTAN, opening the possibility to experimentally assess the thermal strain of the filaments at various stages of the test campaign, i.e. as a function of the load cycles and degradation. Residual strain measurements on the steel jacket of the TF conductors have been carried out at RT and 77 K.

Effect of Thermal Loading on

The ITER CICC will undergo a number of cool-down and warm-up cycles over the lifetime of the plant. The standard SULTAN based ITER conductor qualification test normally includes one thermal cycle in the test sequence. In many samples a performance degradation was observed following this thermal loading. In order to investigate the effect of multiple thermal cycles on the TF conductor short sample, additional repeated thermal cycles to liquid nitrogen temperature were carried out on the left leg of the CNTF3 sample and the JATF5 sample. Thermal cycles using SULTAN are very time consuming, about four days, with a corresponding cost of around 32 kEuro. Ten thermal cycles will give an estimation of the degradation upon repeated thermal loading, but would require a prohibitive amount of time in SULTAN, and therefore cause a significant delay in the testing of other time critical samples. As a large fraction of the change in thermal contraction occurs between room temperature and liquid nitrogen temperature, a purpose made facility and program was developed. This ad-hoc facility allowed faster, more cost effective thermal cycles that crucially did not interfere with SULTANs ongoing test program. Cooling and heating was provided by means of forced flow nitrogen. The sample was contained within a vacuum to pre- vent the formation of moisture or ice. During warm-up, a heater distributed around the CNTF3A was also used. These cycles were performed both before and after electromagnetic loading. The results of these tests indicated that thermal loading before the first electromagnetic load cycle did not result in a worsening of the conductor performance. The tests following repeated thermal cycling after electromagnetic loading show a thermal cycle causes a performance degradation but a large number of consecutive thermal cycles do not appear to have a significant effect.

{\rm Nb}_{3}{\rm Sn}

For full cryogenic test of CERN 600 A high temperature superconducting (HTS) current leads prior to integration into the Large Hardron collider (LHC), a dedicated facility has been designed, constructed and operated at the University of Southampton. The facility consists of purpose-built test cryostats, 20 K helium gas supply, helium gas flow and temperature control systems and quench protection system. Over 400 such leads have already been successfully tested and qualified for installation at CERN. This paper describes various design and operation aspects of the test facility and presents the detailed cryogenic test results of the CERN 600 A current leads, including steady state 20 K flow rates.

CICC Performance

The Iter TF joints are of a twin-box design and the critical parameters of the overall resistance are 1) the contact between cable and termination, and 2) the resistance between two terminations. This paper describes applicability of non-destructive examination (NDE) to these joints. The TFEU joint was adapted to make the joint demountable and the contact area was artificially degraded. The TFEU Joint was measured in the range 30-70 kA, 0-6 T. With no artificial degradation, the resistance of the TFEU Joint was measured to be better than the inter-pancake criterion of 3 nΩ at 2 T, 68 kA. At high fields (6 T) the voltage-current (V I) characteristic of the joint is nonlinear and the resistance is higher than expected. The nonlinearity is worse when the joint is artificially degraded. An FEA model was used to demonstrate that the magneto-resistant coppers contribution to the overall joint resistance is low (<; ~1 nΩ) and does not explain the high field behavior. The nonlinear V I behavior is due to poor current redistribution within the joint, which is related to the resistance of the strand-bundle to copper interface. CRPP is developing a room temperature NDE technique based on resistance profiles to investigate this interface. Resistance measurements at low current and field, or high current and low field, do not guarantee performance at high field; joint tests under the operating conditions are required. Tests on the upper terminations of the TFEU Joint showed that large defects in the contact area between two terminations could be tolerated, when the joint has a good strand-bundle to copper contact resistance and effective current redistribution.

Full Cryogenic Test of 600 A HTS Hybrid Current Leads for the LHC

In the framework of a collaboration, CRPP (Centre de Recherches en Physique des Plasmas) and WEKA AG have developed high-temperature superconductor (HTS) current leads for currents in the range of 3 to 30 kA, which are suitable for industrial fabrication. In the development project, two 10 kA HTS current leads, mainly distinguished by the design of the copper heat exchanger and the transition zone between the HTS module and the heat exchanger, have been manufactured by WEKA AG and tested at CRPP. The test of the current leads covered their behavior under normal operating conditions as well as in the case of a loss of flow. Furthermore, a quench of the current leads was initiated by increasing of the helium temperature by means of heaters immediately before the inlet. The measured quench temperatures provide an estimate of the operational limits of the 10 kA HTS current leads.

Applicability of Non-Destructive Examination to Iter TF Joints

Conventional superconducting switches for power applications, which operate at liquid helium temperature, generally utilize Nb-Ti superconductor in a cupro-nickel matrix. For superconducting circuits based on High Temperature Superconductors (HTS) that work at higher temperatures, the associated superconducting switches must also be based on HTS. This paper addresses the issues concerning the requirements and the appropriate design of HTS switches, including approaches to fast triggering.

Results of the Test of Industrially Manufactured HTS Current Leads With Novel Design Features

EDIPO is the new 12.5 T superconducting test facility that is being installed at CRPP in Villigen, Switzerland. It will be placed beside the existing test facility SULTAN. The main purpose of both facilities is testing samples for the ITER project. EDIPO will provide a uniform high field region of 1 m, i.e. longer than SULTANs ( 45 cm). EDIPO includes the main magnet and a superconducting transformer to power it with a current up to 100 kA in the test sample. The energy stored in the dipole is 16 MJ and in the undesired event of a quench this energy must be safely extracted. A Quench Detection System (QDS) is required to trigger the quench protection system for both the dipole and the transformer. This paper describes the overall QDS from the conceptual design, components, manufacture and commissioning as well as interactions with related systems.