The influence of Lorentz force on the ac loss in sub-size cable-in-conduit conductors for ITER

The cable-in-conduit superconductors for the ITER coils have operating current in excess of 40 kA and function under last ramp conditions and fields up to 13 T. The transverse Lorentz force acting on strands may reduce the effective contact resistance between strands in the cable and as a consequence, the coupling loss will increase. This influence is investigated with a sub-size jacketed cable having 81 Cr-coated Nb3Sn strands. The AC loss is measured with a sinusoidal and trapezoidal magnetic field superimposed to a stationary background field of 1 or 2 T while the cable carries a constant transport current up to about 30 kA. The AC loss is determined by a pick-up coil system and partly with a calorimeter for calibration purposes. The nτ at 0 current declines after cyclic loading, from 9 ms in the virgin state to 2 ms after several loads. The increase of the interstrand coupling loss due to Lorentz effects, accompanied by resistance-hysteresis and relaxation effects as observed in the loss are discussed. The total loss increases considerably due to interference of transport current and induced coupling currents with rising transport current and DC field

Electromagnetic and mechanical characterisation of ITER CS-MC conductors affected by transverse cyclic loading, part 1: coupling current loss

The magnetic field generated by a coil acts on the cable which results in a transverse force on the strands. This affects the interstrand contact resistances (Rc), the coupling current loss and current redistribution during field changes. A special cryogenic press has been built to study the mechanical and electrical properties of full-size ITER conductor samples under transverse, mechanical loading. The cryogenic press can transmit a variable (cyclic) force up to 650 kN/m to a conductor section of 400 mm length at 4.2 K. The jacket is partly opened in order to transmit the force directly onto the cable. In addition a superconducting dipole coil provides the magnetic field required to perform magnetisation measurements using pick-up coils. The various Rc's between strands selected from different positions inside the cable have been studied. The coupling loss time constants (nτ) during and after loading are verified for the Nb3Sn, 45 kA, 10 and 13 T, ITER Model Coil conductors. A summary of the results obtained with up to several tens of full loading cycles is presented. A significant decrease of the cable nτ after several cycles is observed. The values of the nτ's are discussed with respect to the Rc measurements and a multiple time constant model (MTC)

Characterization tests of the Nb₃Sn calbe-in-conduit conductors for Se.C.R.E.T.S.

Two Nb3Sn cable-in-conduit conductors have been procured for Se.C.R.E.T.S. (Segregated Copper Ratio Experiment on Transient Stability). The two conductors are identical in the fractional cross sections. The only difference is the location of the copper stabilizer, included either as segregated copper wires or as copper shell in the superconducting strands. A number of characterization tests, on individual strands and cabled conductors, have been carried out to establish a solid data base for the assessment of the results in the main experiment

Study of the Hot-Spot Temperature During Quench in the Nonplanar Coils of W7-X

The quench protection system of the nonplanar coils of the Wendelstein 7-X stellarator was laid out over 15 years ago. At that time, the assessment of the hot-spot temperature by a basic adiabatic model was done using design values for material and operation parameters. After the operating experience in 2016, the hot-spot temperature is reassessed with the thermal-hydraulic program THEA, using the actual values for delay and dump time. The electrical resistivity of the conduit alloy ismeasured over the whole range of temperatures and in magnetic fields on relevant samples of a conductor, exposed to the hardening heat treatment after winding. The results were fed into THEA. Parametric variations are studied in the calculations, e.g., testing the effect of the quench initiation zone, delay time, and dump resistor. The results suggest that the hot-spot temperature of the conduit in the case of quench is safely within the allowable values even when the dump voltage is reduced compared to the initial design value

Design of the HTS Fusion Conductors for TF and CS Coils

The main electrical and mechanical requirements for the LTS fusion conductors of DEMO are retained as a starting point for the development of HTS fusion cables. Based on the HTS coated conductor technology, a flat cable design was proposed by CRPP Swiss Plasma Center (SPC) using the strands made of twisted stack of tapes soldered into copper profiles. Analytical modeling of the cable geometry is developed and presented in this work. The model was used to estimate various properties of cable. Addressing the issue of bending strain and related performance degradation, an optimization model of the cable properties was built, which allows to best fulfill the cable requirements. Design options are developed for both toroidal field (TF) coils operating at 63 kA and central solenoid (CS) coils operating at 50 kA. Paying attention to the DC and pulsed operation of the TF and CS coils, proposals for the design of the forced-flow HTS conductors are reported and discussed for each type of the coils

Design Optimization of Round Strands Made by Twisted Stacks of HTS Tapes

Atwisted stack of high-temperature superconducting (HTS) tapes soldered into copper profiles was used as a strand in the fabrication of the 60-kA/ 12-T HTS cable prototype at the Center for Research in Plasma Physics (CRPP). Considering the strand in general as a modular element for the high-current cables, design parameters of the strand need to be optimized in order to best fulfill requirements of appropriate application field. During the development program of the HTS fusion cable at CRPP, influence of the design parameters on the strand's performance was obtained as input data for the optimization process. In this paper, we summarize the results of the twisting, bending, and transverse pressure tests on strands of various tapes and profile's geometries. Finite-element model for the transverse pressure test was developed, validated with the test results, and used for the design proposals. Effect of the stack aspect ratio, tape properties, and anisotropy, as well as selection of HTS material for the strand, will be discussed

Design and R&D for the DEMO Toroidal Field Coils Based on Nb3Sn React and Wind Method

In 2013, the Swiss Plasma Center proposed a Toroidal Field (TF) layout for the DEMO-EUROfusion tokamak, based on a graded winding made of layers of Nb3Sn (react&wind) and NbTi conductors. The R&D effort led in 2015 to a full size prototype conductor tested up to 82.4 kA at 12.35 T. The test continued in 2016 and new results are presented. In summer 2015 a new reference baseline was issued for the DEMO-EUROfusion tokamak, leading to an update of the TF requirements. The design update is presented in this paper, with the winding pack consisting of 12 single layers of Nb3Sn with "invisible" (no protrusion) inter-layer joints. The high grade Nb3Sn react&wind conductor operates at 63.3 kA, 12.23 T with T-cs > 6.5 K. A new prototype conductor is being manufactured. The main advantages of the graded approach, applied to both the superconductor and the stainless s teel conduit, are a substantial space and cost saving compared to the wind&react approach with pancake winding

Manufacture and performance test result of a 95 kA-class Nb-Ti cable-in-conduit conductor for the low field winding-package of CFETR-TF coil

The engineering design of the CFETR TF prototype coil and conductors has been completed. The wind-package (WP) of the coil is graded into three regions based on the magnetic field distribution for saving cost. High-Jc Nb3Sn strand, ITER-like Nb3Sn strand and Nb-Ti strand are applied for high-field, mid-field and low-field WP respectively. In order to verify the conductor design, full-size short samples have been manufactured for the three types of conductors. The samples are tested in the SULTAN facility at CRPP in Villigen, Switzerland. At present, the test of Nb-Ti conductor for low-field WP is finished, DC and AC tests were performed. In DC test, several current sharing temperature (Tcs) measurements were performed with 500 electromagnetic cycles and one thermal cycle. Additionally, minimum quench energy (MQE) measurement was performed for investigating the stability of the conductor. The test results and analysis are reported in this paper.</p