KEKB is one of the most challenging accelerators in the world aiming to achieve the peak luminosity of 1×1034 cm-2s-1 for the study of B-meson physics. To achieve such a high luminosity, ampere-class beams of electron and positron have to be accumulated in the 8 GeV-ring (HER) and the 3.5 GeV-ring (LER) separately, and be controlled precisely to collide each other in the BELLE detector located at the collision point. These intense beams need the accelerating RF cavities that have sufficiently damped higher modes (HOM) not to excite uncontrollable beam instabilities caused by the field induced by the beam. Therefore new accelerating cavities have been developed in KEKB for both superconducting (SC) and normal conducting (NC) cavities. Since the beginning of machine commissioning in 1998, the peak luminosity has been improved gradually as increasing the beam intensity of both rings, and achieved the peak luminosity of 1.4×1034 cm-2s-1 with the beams of 1.2 A in HER and 1.6 A in LER so far. In HER, eight SC damped cavities have been installed together with twelve NC cavities, and share the RF voltage of 11 MV and the beam power of 2.4 MW. This superior accelerating performance has opened up a new application of high intensity acceleration for the SC cavity.
The fundamental two-phase flow characteristics of slush nitrogen in a pipe are numerically investigated to develop effective cooling performance for long-distance superconducting cables. First, the governing equations of two-phase slush nitrogen flow based on the unsteady thermal non-equilibrium two-fluid model are constructed and several flow characteristics are numerically calculated taking into account the effects of slush volume fraction, thermodynamic behavior of slush, and duct shape. Furthermore, the numerical results are compared with previous experimental results on pressure loss measurement and visualization measurement in two-phase slush nitrogen flow along the longitudinal direction of the pipe. According to this research, it is found that reducing the pressure loss using a two-phase slush flow is possible under the high Reynolds number condition and by applying the appropriate volume fraction of slush particles. The optimized thermal flow conditions for cryogenic two-phase slush nitrogen with practical use of latent heat for slush melting are predicted for the development of a new type of superconducting cooling system.
Slush nitrogen is a mixture of liquid and solid nitrogen. Fine solid particles are included and dispersed in the liquid phase. It enables the utilization of latent heat and make it possible to increase the quantity of refrigeration supply. Therefore, it is expected that it will be applied as a refrigerant for high-temperature super conductivity devices. This study deals with the pipe flow characteristics of slush nitrogen. The relationship between pressure loss and velocity is investigated experimentally. Flow velocity and mass solid fraction are made to be control parameters. As a result, on the condition that the solid proportion is less than 0.2, pressure loss in the pipe flow increases as the solid proportion increases. However, this increasing rate decreases with increasing flow velocity, and it is found that the pressure loss does not increase even if the solid proportion is increased under the condition of very high flow velocity. Furthermore, observation and classification of the flow pattern by visualization was carried out. The result of the visualization indicates that the flow pattern of the slush nitrogen can be classified into four patterns: uniform flow, layer flow, slide flow and separation flow. In addition, the relationship between the Reynolds number and tube friction coefficient was deduced and an empirical formula was derived.
We have investigated the superconducting properties and surface morphologies of c-axis-oriented SmBCO thick films on MgO substrate deposited by pulsed laser deposition. The critical current values in the SmBCO films deposited using the x=0.04 and x=0.08 (Sm-rich) targets increased with increasing film thickness, whereas their values in the films using x=0 targets were held constant. From the AFM images, the terrace widths of the Sm-rich thick films are wider than the widths of the stoichiometric films. Therefore the surface roughness arising from the increasing film thickness is improved using the Sm-rich film.
We have developed a “traveling” superconducting quantum interference device (SQUID) magnetic imaging system for practical nondestructive testing (NDT). For years it has been widely acknowledged that DC-SQUID sensors are adversely affected by ambient noise (e.g., earth's magnetic field), therefore they are not effective when applied to sense large areas through self-propelling motion. We recently succeeded in developing an advanced SQUID magnetic imaging system in which the SQUID sensor continuously travels back and forth by itself over the surface of an object being tested without using magnetic shielding. Technically, the self-propelled scheme is useful to enlarge the available scanning area of SQUID-NDT without limitation. In order to demonstrate the capability, we represent data relating to the detection of ferromagnetism due to plastic deformation by tensile testing using type-304 austenitic stainless steel. We also found that the traveling SQUID system developed is useful for evaluating artificial internal cracks embedded in austenitic welds.