Overview of verification tests on AC loss, contact resistance and mechanical properties of ITER conductors with transverse loading up to 30 000 cycles

Abstract

The ITER magnet system uses cable-in-conduit conductor (CICC) technology with individual strands twisted in several stages resulting in a rope-type cable, which is inserted into a stainless steel conduit. The combination of high current (up to 68 kA) and background magnetic field (up to 13 T) results in large transverse Lorentz forces exerted on the conductors during magnet system operation. The high transverse forces, accompanied with the cyclic nature of the load, have a strong influence on the conductor properties. The Twente Cryogenic Cable Press is used to simulate the effect of the Lorentz forces on a conductor comparable to the ITER magnet operating conditions. An overview is presented of the AC coupling and hysteresis loss, mechanical deformation characteristics and inter-strand contact resistance measurement results obtained on full-size ITER CICCs measured in the Twente Cryogenic Cable Press. The aim of this work is to characterize conductors' electromagnetic and mechanical properties during cycling of the load up to 30 000 cycles. The evolution of the magnetization (AC coupling loss time constant nτ), mechanical properties and inter-strand resistance R c between selected strands is presented along with loading history. The R c between first triplet strands is also measured as a function of applied load. It is shown that transverse load cycling has a strong influence on the CICC properties. An overview of the results for eight toroidal field conductors, two central solenoid conductors, three poloidal field conductors of different types (PF1&6, PF4, PF5), one main bus-bar and one correction coil conductor is presented.

AC loss, interstrand resistance and mechanical properties of prototype EU DEMO TF conductors up to 30 000 load cycles

“…The total loss of the RW sample decreases with cycling of the load and the AC loss behaviour is similar to what is observed earlier for ITER Nb 3 Sn CICCs [17]. The total loss of WR sample decreases at frequencies below the AC loss peak while at frequencies above the peak, the total loss rises with load cycle number in figure 7.…”

“…When cyclic loading is started, coupling loss time constants of both samples decreases showing behaviour qualitatively similar to measured previously on ITER Nb 3 Sn CICC conductors [17]. The coupling loss is higher in the fully loaded state as the interstrand coupling is increased due to compaction.…”

“…Both samples have high coupling loss in the virgin state; 5.8 s for RW and 34.5 s for WR with perpendicular field orientation and offset field. That is an order of magnitude higher than ITER Nb 3 Sn conductors with nτ values in the range of 0.5-1.0 s in virgin condition [17].…”

“…This is supported by the contact resistance measurement presented in section 3.3. There it is shown that the average inter-petal resistance of the WR sample is an order of magnitude less than the lowest R c observed for ITER CS conductors while the average intra-petal resistance is in the same range as ITER CS conductors [17].…”

“…The design of the cryogenic press is described in detail in [11]. The press was previously used to test various types of NbTi and Nb 3 Sn cable-in-conduit conductors (CICC) [12][13][14][15][16][17]. The sample position in the press gap is such that the press applies force perpendicular to the wide side of the sample, i.e.…”