Impact of mechanical and thermal cycles at different operating conditions on the ITER toroidal field coil conductor performance

Abstract

The impact of electromagnetic (EM) and thermal cyclic loading (referred as warm-up-cool-down—WUCD) on the Nb3Sn cable in conduit conductors for the toroidal field magnets of the ITER tokamak should be carefully assessed for the proper operation of the machine. The experience gained during the production phases of the TF conductors revealed that a degradation occurs when thermal and EM cyclic loading is applied at the nominal operating conditions of 10.78 T and 68 kA. Since at the beginning of the tokamak operation the nominal conditions will not be achieved, it is worth investigating to what extent lower EM loads affect the conductor performance, and the threshold of EM load that triggers the WUCD degradation. The aim of this investigation is to identify critical levels which determine the onset of this degradation, both in terms of thermal and EM cycles, and to study the performance of strands from different suppliers. For this assessment, a set of conductor samples were manufactured including all types of strands to be used in the TF coils of the ITER machine. These samples were subjected to a set of EM and thermal cycles representing in a realistic way the actual operation of the conductors in the ITER machine. The tests were performed in the SULTAN facility of the Swiss Plasma Center at working conditions ranging from 5.4 T–34 kA to 10.78 T–68 kA. This work presents the results obtained in these tests in terms of current sharing temperature (T cs), effective strain, rate of degradation at different working conditions and their implications for the prospected conductor performance in the machine.

Lessons learned from European and Japanese production of ITER toroidal field coils

“…The ITER TF coil procurement is divided into procurements of TF conductor, TF structure and TF coil as shown in figure 3. The TF conductor employing Nb 3 Sn superconductor is manufactured by China, Europe, Japan, Korea, Russia, and America after qualifying each TF conductor in SULTAN test facility at Swiss Plasma Center and central solenoid model coil (CSMC) test facility at QST [3][4][5][6][7]. Then, they are shipped to Europe and Japan where the TF coils are manufactured.…”

Time-extended inductive tokamak discharges with differentially-tilted toroidal field coils

“…where ε(t) = a(t)/R 0 is the inverse aspect-ratio, κ is the plasma ellipticity, and we have introduced the two functions 4 − 6ε(t) 5 + 3.7ε(t) 6 ]. The mutual inductance between plasma and the various set of coils are calculated from their defining relation, M = Φ B /I c where Φ B is the flux linked with the plasma and I c the current in the coils creating the flux.…”

“…The adoption of superconducting magnets, which has overcome the problem of coil heating during the discharge, has retained that of EM stresses [4], while introducing a new limit on the critical magnetic field that superconductors (in particular, low temperature ones) can sustain: the magnetic field on the axis in superconducting tokamaks is limited by its value at the inner toroidal field coil interface with the plasma. Even with superconducting coils, however, the time duration of the discharge is restricted by the amount of time-changing magnetic flux linked to the plasma, in short the 'volt-second (V-s)', that the central solenoid (CS) can supply to induce the plasma current.…”

Performance review of the joints for the ITER poloidal field coils

“…In the PFJRF2 sample, the impact on the joint performance of thermal cyclic loading-referred to as warm-up-cool-down (WUCD) applied after the electro-mechanical cyclic loading [33]-was also investigated. To this purpose, the AC losses in this sample were also measured after the WUCDs.…”

“…The impact of EM cycles [33] on the joint resistances is shown in figure 6. The results presented are obtained by considering the voltage signals acquired at the far sections, with an operating current of 55 kA and a background field of 3 T.…”