In order to obtain a predictive equation of Charpy impact absorbed energy of austenitic weld metals at 4.2K, a quantitative relation between absorbed energy at 4.2K and influencing factors, such as ferrite number, carbon content and oxygen content was studied experimentally. These factors were selected based on the fact that the cryogenic toughness of austenitic weld metal is influenced by δ ferrite, chromium carbide and silicon-mangannese type oxide. The absorbed energy at 4.2K was obtained by a method of converting the lateral expansion at 4.2K into the absorbed energy by using the lateral expansion vs. absorbed energy diagram of each weld metal at 77K. The lateral expansion at 4.2K was obtained by testing the impact specimen in the glass vessel filled with liquid helium. In weld metals containing δ ferrite, the ferrite number was measured by a ferrite indicator. On the other hand, in weld metals containing no δ ferrite, the ferrite number was obtained with the help of the modified Szumachowski's constitution diagram. Following experimental equation relating the absorbed energy at 4.2K (vE) to the ferrite number (FN), the carbon content (%C) and the oxygen content (%O) was developed by regression analysis of the experimental data: vE[J]=90.6-4.56(FN)-44.2(%C)-824(%O) It is expected that the above equation is applicable to predict the absorbed energy of the austenitic weld metal in the as-welded condition at 4.2K.
BENKEI, which was a large window frame conventional magnet at KEK, has been converted to a superconducting magnet. By the conversion, the pole gap has been doubled from 0.5m to 1.0m retaining an analyzing power of 2Tm. Several new techniques were adapted to coil windings and cryostat fabrication. The superconducting BENKEI has shown satisfactory performances for long term operation.
This paper describes the results of an experimental study on d.c. breakdown voltage characteristics on supercritical helium under the non-uniform electric field to assist the development of superconducting magnets cooled by supercritical helium. At a certain value of the density, ρc, a maximum breakdown voltage is observed in the breakdown voltage versus density characteristics. In the case of negative needle, the breakdown voltage is proportional to the density ρ below ρc. When ρ>ρc, the breakdown voltage becomes almost independent of the density. The ρc v.s. temperature curve, obtained from the measurements at different temperatures, agrees well with a pseudo-critical line, which is the phase transition line between pseudo-gas and pseudo-liquid phase of supercritical helium. It is pointed out that the breakdown mechanism supercritical helium in pseudo-liquid phase is defferent from the mechanism in pseudo-gas phase.
The neutron irradiation effects on cryostability of composite superconductors for fusion reactor are studied based on Maddock's condition. In particular, to estimate the effects of 14MeV neutrons we assumed that the irradiation-induced degradation of critical temperature, critical current density and conductivity of stabilizer are determined by the damage energy depending on the neutron energy spectrum. The cryostability is found to decrease sensitively with increasing the fraction α of fusion neutrons with energy of 10-14MeV to the total neutrons; (1) The Cu/superconductor ratio Rns, to stabilize the conductor, must be increased remarkably with increasing α as well as the total dose of the neutron fluence. The optimized Rns has a maximum value of about 80 in case of Nb3Sn with transition temperature Tc=15K, critical current density Jc