Comparison of the calculated and measured stability of a NbTi cable-in-conduit conductor
Calculated curves of cable-in-conduit conductor (CICC) energy stability margins vs. current are compared to experimental curves obtained at Oak Ridge National Laboratory for NbTi single triplex conductors. The conductors ranged in length from 1.8 m to 4.8 m, and had no imposed helium flow. The initiating heat pulse was applied for 16.7 ms over the entire conductor length. The calculated stability curves display the large decrease in energy margin from the low current and high energy margin "well-cooled" stability region, to the high current and low energy margin "ill-cooled" region that was determined experimentally. The calculated "limiting current" of 250 A (boundary between the ill-cooled and well-cooled regions) also agrees with experiment. The multi-valued stability margins measured for lengths of 3.1 and 3.8 m could not be obtained by the computer model. Excluding the multiple stabilities, the calculated margins are generally 30% lower than the experimental values. The decrease in energy margin in the ill-cooled region was found to be due to the low critical to operating temperature difference (2.2 K at 250 A). This prevents an initiating pulse in the ill-cooled region from being able to generate significant Joule heating without quenching, limiting the energy margin.<<ETX>>
Numerical simulation of the stability in a cable-in-conduit conductor developed for fusion magnet applications
The stability margins of the US-Demonstration Poloidal Coil (US-DPC) and the International Thermonuclear Experimental Reactor (ITER) toroidal field (TF) coils have been modeled numerically using the computer program CICC. The computed US-DPC limiting current, I/sub lim/, compares favorably with the values determined experimentally. Using the detailed program CICC output, the authors investigated the DPC quench initiation mechanism in each of the three stability regions. In the ill-cooled region, the imposed heat pulse heats the conductor to the current-sharing temperature. In the transition region, the resistance heating after the pulse must be strong enough to overcome the induced flow reversal. In the well-cooled region, good heat transfer heats the helium during the pulse. After the pulse, these high helium temperatures along with poor heat transfer cause the conductor to quench. Changes in I/sub lim/ agree with Dresner's relationship. I/sub lim/ can be improved by decreasing the copper resistivity, the helium fraction, or the conductor diameter. Preliminary results show the ITER TF coil operating point is in the well-cooled region.<<ETX>>
Stability Measurements of A Large Nb3Sn Force-Cooled Conductor
The Westinghouse coil for the Large Coil Program (LCP) [1] at Oak Ridge National Laboratory will use a cable-in-conduit conductor made of copper-stabilized, multifilamentary (MF) Nb3Sn strands enclosed in a stainless steel jacket, which provides a channel around the conductor to allow forced cooling by supercritical helium. The operating current will be 16 kA with an 8-T maximum field. In this study, the performance of a subsize conductor similar in construction, but with only one-third as many active strands in the cable, was investigated.