A high-density and high-speed flowing helium-plasma is produced quasi-steadily (1 ms) by use of a magneto-plasma-dynamic arcjet (MPDA) in various external magnetic field configurations. In a uniform magnetic field configuration, an ion acoustic Mach number M i of the plasma flow is limited to be nearly unity. In a divergent magnetic nozzle configuration, on the other hand, the Mach number increases up to almost 3. The Mach number increases in proportion to the gradient of the magnetic field. Spatial variations of M i are well predicted by an isentropic model of compressible flow. The Mach number decreases in the downstream region due to charge-exchange collisional processes that are caused by a limited pumping capability of surface-recombined neutral gases.

Recently, we proposed a method of forming superconducting joints between high-temperature superconducting wires based on the incongruent melting of an intermedium, called crystalline joint by melted bulk. Using this technique with GdBa2Cu3O7−δ-coated conductors, a superconducting joint with a critical current of 7 A at 77 K was obtained. In the present study, a joint between multi-filamentary Bi2Sr2Ca2Cu3O10+δ (Bi2223) tapes was prepared without an intermedium, employing incongruent melting of Bi2223 filaments in the tapes. A superconducting joint was successfully fabricated, with critical currents of 12.2 A and 177 A at 77 K and 4.2 K, respectively. A Bi2223 coil terminated with the present joint method was operated in persistent current mode and showed an ultra-low field decay rate with a characteristic resistance of 10−12 Ω at 77 and 4.2 K.

This paper presents experimental and simulation results of a screening current induced magnetic field (SCF) in a high temperature superconductor (HTS) insert that constitutes a low-/high-temperature superconductor (LTS/HTS) NMR magnet. In this experiment, the HTS insert, a stack of 50 double-pancake coils, each wound with Bi2223 tape, was operated at 77 K. A screening current was induced in the HTS insert by three magnetic field sources: 1) a self field from the HTS insert; 2) an external field from a 5-T background magnet; and 3) combinations of 1) and 2). For each field excitation, which induced an SCF, its axial field distribution and temporal variations were measured and compared with simulation results based on the critical state model. Agreement on field profile between experiment and simulation is satisfactory but more work is needed to make the simulation useful for designing shim coils that will cancel the SCF.

No abstract

No abstract