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>>
Stability analysis of the TPX toroidal field coil
The energy stability margin of the TPX/TF conductor has been calculated as a response to heat pulses with short initial quench zones (IQZ=10, 20 cm for 4, 20, 100 ms), and with long initial quench zones (IQZ=2.28, 4.53 m for 20, 50, 100 ms). The short IQZs approximate ramp-rate induced heating, and the long IQZs approximate heating from a plasma disruption. These IQZs are centered in the bore inner leg of the double pancake, where the operating field and temperature are maximum. Energy margin stability curves are plotted as a function of current. The stability of the 10 cm IQZ differs from that for the 20 cm IQZ by less than 20%. Similarly the stability of the 2.28 m IQZ differs from that for the 4.53 m IQZ by less than 20%. However the stability of the short IQZs (10 and 20 cm) is about twice as high as that of the long IQZs (2.28 and 4.53 m). The friction in the long IQZs prevents the conversion of heat to work by helium expansion during the pulse. At the 33.5 kA design current, the minimum calculated stability margin with short IQZs is 390 mJ/cc. The minimum calculated stability margin with long IQZs is 205 mJ/cc. A comparison of the stability margin with the available enthalpy (short IQZs) and with the available internal energy (long IQZs) shows that the conductor utilizes the available helium energy well.<<ETX>>
In-House Industrial Production of the Superconducting Conductor for the 43 T Hybrid Magnet of LNCMI-Grenoble
The hybrid magnet being built at LNCMI-Grenoble is based on the combination of resistive inserts, made up of Bitter and polyhelix coils, with a large bore superconducting outsert. It will produce, in the first step, an overall continuous magnetic field of 43 T in a 34 mm warm bore aperture. The superconducting coil will provide a nominal magnetic field of 8.5 T in a 1.1 m cold bore diameter. It relies on the novel development of an Nb-Ti/Cu Rutherford Cable On Conduit Conductor (RCOCC) cooled down to 1.8 K by a bath of superfluid helium pressurized at 1200 mbar. After a brief reminder of the specificity of hybrid magnets, namely the strong electromechanical coupling between resistive and superconducting coils, the main requirements of the RCOCC for efficient and reliable magnet operation are given. The test and validation phases prior to the industrial production of the RCOCC are reported, including the trial production of the ½ hard-drawn (H02 cold-worked) Cu-Ag 0.05% hollow stabilizer in continuous lengths of 325 m. The novelty of the RCOCC development concerns the assembly and the induction soft-soldering of the Rutherford cable on the Cu-Ag hollow stabilizer allowing a strict control of the interstrand contact resistance. Studies, developments, and the integration of the industrial production line at LNCMI-Grenoble are presented together with the result of the production of all RCOCC unit lengths.