Fe-sheathed MgB2 superconducting tapes were prepared through an ex-situ process and powder-in-tube (PIT) technique using powders treated in organic acid solutions. The treatment realizes the substitution of carbon, derived from the remnants of the organic solvents being adsorbed onto the surface of MgB2 grains, into boron in MgB2 and pulverization of MgB2grains. The amount of carbon substitution is x = 0.02−0.03 in MgB2-xCx. The carbon substitution is effective for increasing Bc2,and grain boundaries act as pinning centers. Therefore, this improves the Jc property of tapes in the high-magnetic-field region.Depending on the solution, the Jc values at 4.2 K and 10 T increase up to ten times compared to tapes using powders withouttreatment. These values are comparable to those obtained for the ex-situ processed tapes with the best-ever reported performance.Tapes using MgB2-xCx (x ≥ 0.05) powders clearly show weak grain coupling, and hence the enhancement of transport Jc is limited.
In the JT-60SA project, the JT-60U tokamak is replaced with a full superconducting tokamak. The heliumrefrigerator cools the superconducting coils by circulating supercritical helium of 4.4 K and 0.6 MPa in the circulation loop atcertain mass flow rate. Since the cooling power of the helium refrigerator is determined by the heat load of the superconductingcoils, estimating the heat generation and required mass flow rate to acquire sufficient temperature margin is of crucial importance.In this paper, we attempted to optimize the mass flow rate in the superconducting coils to satisfy a temperature margin of 1 K. Wethen show that the consequent maximum pressure drop in the circulation loop that results in minimizing the heat load of thesupercritical helium circulation pump is 81 kPa.