Marco Calvi

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.

Methods, Accuracy and Reliability of ITER Conductor Tests in SULTAN

In the last decade, a large number of high current, force flow superconductors have been tested as short length samples in the SULTAN facility. The object of the test ranged over transient stability, thermal-hydraulic behavior, AC losses, joint resistance and proof-of-principle for innovative conductor design. Recently, with the ITER cable-in-conduit conductors (CICC), the basic DC transport properties have been the focus of the SULTAN test. The critical steps of the sample assembly and instrumentation are described, with emphasis on the application of the temperature sensors, verification of the signal treatment chain and calibration. The post-processing and the data reduction are focused on the assessment of the current sharing temperature, T cs: the conventional method of electrical field threshold detection by voltage taps is compared with the current sharing power detection by steady state gas-flow calorimetry. The longitudinal strain state of the conductors is discussed through the results of strain gauges applied on the jacket. Eventually, the value of a certified conductor test is highlighted in the frame of the quality control for the ITER magnets.

Status Report of the SULTAN Test Facility

One year of operation and test activity of the SULTAN test facility at CRPP-Villigen, from October 2008 to October 2009 is reviewed. The main improvements of the facility include a new control system for the cryo-plant and a new electric motor for the helium compressor. The range of operation for the SULTAN samples has been improved in terms of cyclic loading rate. The test campaigns from October 2008 to October 2009 include eight ITER TF conductor samples, two JT60SA samples and a number of other developmental samples. The highlights of the test campaign and the statistical data about cool-downs, warm-ups and test duration are reported. For the eight ITER TF samples, more detail is given about the joint development, the standard test program and the data reduction for the assessment of the results. Eventually, an outlook in the next operation period is also discussed.

Stability Analysis of the LHC Cables for Transient Heat Depositions

The commissioning and the exploitation of the LHC require a good knowledge of the stability margins of the superconducting magnets with respect to beam induced heat depositions. Previous studies showed that simple numerical models are suitable to carry out stability calculations of multi-strands cables, and highlighted the relevance of the heat transfer model with the surrounding helium. In this paper we present a systematic scan of the stability margin of all types of LHC cables working at 1.9 K against transient heat depositions. We specifically discuss the dependence of the stability margin on the parameters of the model, which provide an estimate of the uncertainty of the values quoted. The stability margin calculations have been performed using a zero-dimensional (0-D) numerical model, and a cooling model taking into account the relevant helium phases which may appear during a stability experiment: it includes Kapitza thermal resistance in superfluid He, boundary layer formation and heat transfer in He I, and considers the transition from nucleating boiling to film boiling during He gas formation.

Quench Analysis of the European High Field Superconducting Dipole Magnet EDIPO

The European high-field superconducting dipole magnet, currently under development, will create magnetic fields of up to 12.5 T for performance tests of high-current superconducting cables. To study the behavior of the dipole during a quench (the energy stored is 16 MJ at 16.5 kA) a complex simulation model was developed and integrated in the 1-D thermal hydraulic code THEA. The detailed quench analysis has shown that the dipole can be adequately protected by proper selection of the discharge time constant and the quench detection voltage for all disturbances except if the initial normal zone is 50 m, an unlikely event. In this case the peak helium pressure exceeds the limit for plastic deformation.

Vibrating Wire Technique and Phase Lock Loop for Finding the Magnetic Axis of Quadrupoles

The tolerance in the alignment of the quadrupoles in the linacs and in the undulator lines of the Swiss Free Electron Laser (SwissFEL), the next project at the Paul Scherrer Institute (PSI), will be about 1 μm. This accuracy will be reached using beam alignment techniques. To minimize the commissioning time, it is also requested to install the quadrupoles with a precision of ±50 μm. To achieve this goal, the vibrating wire technique will be used in association with other systems to determine the position of the magnetic axis of the quadrupoles with respect to external fiducials. After a general introduction to the project and to the instrumentation principle, the novel approach developed at PSI is presented. The main innovation consists in the use of a phase lock loop (PLL) for maintaining the wire in its resonance condition. This approach simplifies the operation of the system in case the wire is moved or replaced and increases substantially the reliability of the measurement outcomes in the frame of a series test campaign.

Inter-Strand Resistance in the Termination of ITER Conductors

In large, cabled superconductors, a non-negligible level of current unbalance among the strands is unavoidable in dc operation due to non-homogeneous contact resistance distribution at the electrical connections. When the overloaded strands saturate their current carrying capability and start to develop current sharing voltage at the high field section, the excess current must be transferred to less loaded strands. The inter-strand current re-distribution takes likely place at the electrical connections, where the inter-strand resistance is lower. Two samples of electrical termination, made of large cable-in-conduit conductors, were formerly prepared and tested for contact resistance distribution at CRPP. One of these samples is now re-arranged to measure the inter-strand resistance in different combination. A discussion of the results allows ruling out speculations about the impact of current re-distribution effects on the voltage-current and voltage-temperature characteristics in short length conductor test.

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.

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.

Interstrand Resistance and Contact Resistance Distribution on Terminations of ITER Short Samples

The current uniformity among the strands in large cabled conductors is affected mostly by the contact resistance distribution and the interstrand resistance in the electrical terminations. While the contact resistance distribution between the strands and the copper of the termination affects the current distribution among the strands, the resistance between the strands in the termination (inter-strand resistance) has an effect on the re-distribution of the current from the most to the less loaded strands. The contact resistance distribution of an ITER conductor termination is measured in the JORDI (JOint Resistance Distribution) facility at CRPP. The sample is prepared starting from a section of Nb3Sn Cable in Conduit Conductor (CICC) and the joint is assembled following the procedure (solder dipped) used for most of the ITER qualification samples tested in the SULTAN facility. The cable is opened into 150 groups of strands (elements) and each element is connected to low temperature shunt resistors which split the current from a 10 kA power supply. The voltage drop is sensed between each element and the contact surface of the joint made equi-potential by a superconducting soldering alloy. The resistance of each channel is deduced from the measured voltage drop. The resistance distribution measured under imposed current uniformity allows deducing the current distribution in normal operation, namely under imposed voltage. The same sample is then adjusted to measure the inter-strand resistance. Two strands at the time are fed with a current of the order of 100 A, the voltage between the selected strands is sensed and the equivalent resistant deduced. The results of both measurements are compared with those obtained in previous experiments, carried out on different conductor layouts.