Search

Plasma synthesized boron powder has been prepared under a variety of RF plasma conditions to examine the suitability of these powders for the preparation of powder‐in‐tube MgB2 wire. Particle size emerging from the RF torch typically ranges from 5 nm to 200 nm and lattice imaging studies in a transmission electron microscope show large portions of both amorphous and beta rhombohedral crystalline material. In situ powder‐in‐tube wire that is made with a continuous tube filling and forming process consistently gives critical current densities ranging from 20,000 to 100,000 A/cm2 at 5 K and 5 Tesla for a powder containing about 4% carbon. As the temperature rises, the critical current density of 100,000 A/cm2 occurs at 4.3 T at 10 K, 3.5 T at 15 K, and 2.5 T at 20 K. In preparation for studies of an ex‐situ powder‐in‐tube process, we have studied the size of reacted MgB2 powder formed in a magnesium vapor. Reaction rates are much slower than for solid state diffusion in the in‐situ process and care is needed to hold particle size under the micrometer range.

Five rotor coils (four plus a spare) intended for a prototype 2 MW generator were fabricated and tested. For each coil, multifilamentary MgB2 strand was wound around a stepped former in a wind and react mode using S‐glass insulation in combination with vacuum epoxy impregnation. The stepped, ellipsoidal coils had maximum in‐plane dimensions of 26.7 cm×13.1 cm and a total thickness of 5.4 cm, and were wound with approximately 580 m of MgB2 strand per coil. Each of the coils were measured separately for Ic and magnetic field in the bore at 4.2 K and for one coil Ic and B were also measured as a function of temperature. The bore field as a function of position along the z‐axis was also determined near the critical current at 4.2 K. The coils typically reached 186 A at 4.2 K generating a 1.7 T field, while at 20 K the Ic was 117 A with a bore field of 1.1 T field.

AC transport current and applied magnetic field loss measurements have been made on MgB2 superconducting strands. We tested six‐filament strands made via an in‐situ route with Nb chemical barriers, a Cu‐inter‐filamentary matrix, and ODS Cu (glidcop) outer sheaths. Strands were untwisted and measured in self magnetic field at temperatures from 4.2 K to Tc. Transport ac losses agreed reasonably well with theoretical predictions for a wire of round cross‐section. AC losses in applied magnetic field perpendicular to the wire axis have been measured in fields up to 140 mT (peak) in frequency range 50–200 Hz in liquid helium bath at 4.2 K. There is a reasonable agreement of the losses with the critical state model at 50 Hz. At 200 Hz the losses are dominated by eddy currents in Cu matrix. Eddy current losses in the glidcop sheath are negligibly small.