M.M. Steeves

Conductor fabrication for the ITER model coils

The Nb/sub 3/Sn cable-in-conduit 40 kA, 13 T conductors proposed for the next generation ITER tokamak are being tested by the construction of a 600 MJ solenoid and a 80 MJ race track coil. The conductors are being manufactured in a world wide collaboration, with task sharing among the four partners of the ITER project. To date, the qualification and /spl sim/20% of the procurement of the Nb/sub 3/Sn strands is complete. Several hundred meters of dummy copper cable and short lengths of superconducting cable are available. The production of jacket sections is advanced and the two jacketing lines are set up for production. The manufacture of 98 tonnes of cable-in-conduit conductors will be completed in autumn 96.

Measurements of ramp-rate limitation of cable-in-conduit conductors

The ramp-rate limitation found in the US-demonstration poloidal coil (US-DPC) test was studied in a laboratory scale experiment. The ramp-rate sensitivity has been identified on a 27-strand cable-in-conduit conductor at a background ramped field to 9.5 T with various ramp rates of 0.5 T/s to 2 T/s, simulating the US-DPC test conditions. A model assuming the existence of periodic disturbances is proposed in which the disturbance frequency is directly proportional to the ramp rate of the square of field. A semi-empirical formula was developed which fits the ramp-rate limitation data of both the US-DPC large coil and the 27-strand cable. The ramp-rate limitation does not occur for currents below the conventional limiting current.<<ETX>>

The US demonstration poloidal coil

The United States Demonstration Poloidal Coil (US-DPC) has been built and will be tested at the Japan Atomic Energy Research Institute (JAERI). The 2-m-diameter, 30-kA, Nb/sub 3/Sn solenoid is designed for 10-T operation with charging and discharging from zero to full field at ramp rates of up to 10 T/s. The fabrication is described. A summary of problems encountered in the manufacture of the wind-react-insulate coil made from cable-in-conduit conductor (CICC) is given.

Production of tin core modified jelly roll cable for the MIT Multipurpose Coil

A report is given on the effort to produce 600 kg of tin core MJR (modified jellyroll) wire with a critical magnetization current density of greater than 800 A/mm/sup 2/ for 10 T, 4.2 K, 10- mu V/m operation while keeping the non-Cu cross section and at non-Cu loss rate at less than 460 kJ/m/sup 3/ (+or-3 T cycle). The wire will be used in MPC (Multipurpose Coil) conductor modules as part of the US Demonstration Poloidal coil (US-DPC). The 18 subelement design provides three-micron-thick filaments to achieve a compromise between non-Cu loss and critical current density requirements. Vanadium diffusion barriers and titanium additions to the filaments are used to reduce loss and provide reaction flexibility. >

Experimental parameter study of subsize Nb 3 Sn cable-in-conduit conductors

An experimental study of the influence of design parameters on the short sample critical current of Nb 3 Sn cable-in-conduit conductors at 4.2 K is presented. These internally cooled and cabled superconductors (ICCS) are subsized versions of the conductor used in both the Westinghouse Large Coil and 12 Tesla Programs. Parameters investigated are cable compaction, conduit material, twist pitch and activation time and temperature. The results indicate that mechanical coupling between the conduit and cable has a strong effect on the residual strain of the Nb 3 Sn and hence its superconducting properties.

Effects of Incoloy 903 and Tantalum Conduits on Critical Current in Nb3Sn Cable-in-Conduit Conductors

Strain sensitivity of the critical properties of Nb3Sn is well documented.1–3 In particular, the dependence of critical current on longitudinal strain becomes dramatic with increasing magnetic field.3 Therefore, any optimization of high field critical current in Nb3Sn composites must consider the longitudinal strain state as well as the microstructure of the superconductor. Microstructure is determined at the formation temperature, and the longitudinal strain state by cool-down from the formation temperature to the operating temperature, typically from 1000 K to 4.2 K. The residual longitudinal strain depends upon relative thermal contractions, volume fractions, elastic moduli and yield stresses of the various component materials that make up the Nb3Sn composite.4,5

The US-DPC, a poloidal coil test insert for the Japanese Demonstration Poloidal Coil Test Facility

A superconducting pulsed poloidal coil being built by MIT will consist of three double pancakes of Nb/sub 3/Sn cable-in-conduit conductor operating at a maximum coil-envelope current density of 50 A/mm/sup 2/ at 4.2 K in a peak field of approximately 10 T. Peak current and peak ramped field will be 30 kA and approximately 10 T/s, respectively. The coil, conductor, proposed experiment, and supporting research and development efforts are discussed. >

Experimental determination of stability margin in a 27 strand bronze matrix, Nb 3 Sn cable-in-conduit conductor

A small coil of internally cooled cabled superconductor was fabricated for experimental determination of the stability margin. The conductor is 27 strands of Nb 3 Sn in a bronze matrix. The sheath material is JBK-75 superalloy. The bifilar coil was potted in epoxy in the annulus of a pulse coll set. Experimental results are presented defining critical pulse energy as a function of normalized operating current and background field. The relationships between dB/dt, pulse energy and stability margin are derived by calculation and confirmed by experimental calibration.

Manufacture and Evaluation of Tin Core Modified Jelly Roll Cables for the US-DPC Coil

The US-Demonstration Poloidal Coil (US-DPC) is being built by the Massachusetts Institute of Technology as a first step test in developing an advanced cable in conduit conductor for the central solenoid (ohmic heating coil) of a Tokamak style Fusion Reactor. Three full-scale prototype pancake modules are being manufactured using Tin Core Modified Jelly Roll Nb3Sn superconductor wire and will be operated in early 1990 at the Japan Atomic Energy Research Institute (JAERI). Teledyne Vah Chang Albany produced the wire (0.78 mm strand), supervised chrome plating and cabling operations and delivered three 225 strand, 168 meter long cables in 1988. Short sample tests show the wire to be uniform in properties from lot to lot. Wire extracted from the cables has also been tested; though the wire is significantly deformed in the cabling process at crossover points, the performance in these areas is not degraded by more than 5% relative to undeformed wire. We report on the chrome plating, cabling, short sample critical current and hysteresis loss data of strand and critical current data for cable strand.

Critical currents of Nb/sub 3/Sn wires for the US-DPC coil

The critical current of titanium-alloyed internal-tin, jelly-roll Nb/sub 3/Sn wire for use in the US Demonstration Poloidal Coil (US-DPC) was evaluated. It was confirmed from 14 randomly-selected samples that the critical-current values were uniform and consistent: the noncopper critical-current density was approximately 700 A/mm/sup 2/ at 10 T and 4.2 K, in agreement with expectations. A 27-strand cable-in-conduit conductor (CICC) using the low-thermal-coefficient-of-expansion superalloy Incoloy 905 yielded a critical current 5-7% below the average value of the single-strand data.