It passed more than 30 years since the ultra-high vacuum technique was developed. The ultimate pressure available in an ultra-high vacuum system gradually decreased with the advent of sputter ion pumps (SIP), turbomolecular pumps (TMP) and cryopumps (CP), but still remains in the range of 10-9Pa now. Recently there are increasing number of reports for the generation of extremely high vacuum (XHV), which corresponds to the pressure less than 1×10-10Pa. The vacuum pumps used for the generation of XHV are liquid-nitrogen cooled sublimation pumps, liquid-helium and liquid-nitrogen cooled cryopumps, and helium-refrigerator cooled cryopumps. They are closely related with cryogenic engineering. We summarize recent successful works for the generation of XHV. The production technique of XHV is advancing steadily, but total and partial pressure gauges which can measure such low pressure without disturbing vacuum condition have not been developed yet, and they are strongly demanded.
The recent development of SQUID system for biomagnetic study is remarkable. The high sensitive SQUID system makes possible to study the brain function and the research for the application of biomagnetic measurement to the diagnosis is in progress. In this review, the techniques for measuring the weak magnetic fields emanating from active positions in a human body are described and the estimation of source location of the biomagnetic field is described, The SQUID sensor, magnetically shielded room, reduction of magnetic noise and multi-channel SQUID systems are presented.
CAT and MRI dramatically changed modern medicine by its excellent anatomical resolution. For functional analysis of central nervous system, however, good old EEG (electro-encephalo-graphy) is still the main weapon. Although computer topography brought some improvement in this field, poor spatial resolution has been the main shortcoming for EEG. This is because voltage distribution of neuron activity over the skull is easily influenced by the complicated human head structure. It is known that magnetic field is emitted from the brain together with electrical neuron activity. Distribution of magnetic field over the skull is less sensitive to the complicated head structure. Thus it is theoretically possible to calculate the accurate location of neuron activity by recording magnetic field over the skull. The magnetic field, however, is too weak for conventional magnetic sensors to detect. Recently SQUID (superconducting quantum interference device) has been introduced as an extremely sensitive magnetic sensor. With SQUID system, it is possible to record weak magnetic field from the brain. Multi-channel SQUID system is the main concern since accuracy of neuron activity localization and time needed for recording are dramatically improved. Study of tonotopic and amplitopic organization of human auditory cortex is presented as an example of SQUID application. Multi-channel DC-SQUID system, installed at the Center for Neuromagnetism of New York University Medical Center, was used. Auditory stimulus of various frequency and intensity was applied to the right ear in randomized order with randomized interstimulus interval. The magnetic response was recorded from left hemisphere with one probe fixed at maxima and the other probe at minima of the magnetic field. Signals were then averaged and dipole location was calculated using spherical model method. The location of auditory response was in primary auditory cortex (Area 41). The location of neuron activity shifted medially as auditory stimulus frequency increased. On the other hand, location of neuron activity tended to shift anterioly in the main as auditory stimulus intensity increased. Thus, SQUID system made it possible to analyze minute neuron network function and its location without any invasion and anesthesia to human. SQUID system has been mainly used to study auditory, visual and somatosensory response of the human brain. Magnetic signal during motor activity is also analyzed to elucidate the mechanism of motor initiation, The main current interest of SQUID application is the diagnosis of epilepsy. Magnetic signal emitted during epilepsy is large and distinctive. With SQUID system, location of epilepsy focus and its modality of spreading can be detected. Once the focus is found, it may be possible to treat epilepsy with minor surgery such as electrical cauterization without opening the skull. From the technical point of view, however, the current SQUID system is still under development. The number of channel and complicated structure may be renovated by utilizing recent IC production technology.
From the required characteristics of structural material for high response excitation superconducting generators, precipitation hardened Ni base alloys such as Inconel 718, were considered as candidate materials. To improve weldability and make a large size ingot, the chemical composition range of 4.0≤Nb+Ti≤4.5% was determined. Based on the above chemical composition, a thinwalled forged ring, welded at the center, was manufactured. Material strength and physical property tests were conducted systematically from 4K to room temperature for base metal and weld metal of this ring. From the material strength tests, as expected, the material strength rose with falling temperature, but ductility and toughness were almost constant for both of these materials. Also, it was shown that if the sample was re-annealed and aged after welding, original characteristics of the base metal were recovered in this precipitation-hardened material. The physical properties of thermal expansion, electrical resistivity, and magnetization, were also measured. From the comparison of mechanical and physical properties of this material with those of Inconel 718, it was clarified that the differences of characteristics of these materials were closely related with chemical compositions of Nb+Ti and Ni, respectively.
The magnetic bearings with an oxide superconductor have been studied and a possibility for industrial-scale application is discussed. This paper describes the performances of a circular superconducting bearing at temperatures of 4.2K and 77K. The characteristics of the bearings composed of YBCO superconductor fabricated by MPMG (Melt-Powdering-Melt-Growth) process and conventional NbTi metal are compared with each other. It is clarified that the magnetic bearing with the YBCO disk, which virtually behaves as a perfect diamagnetic substance at 4.2K has a load capacity on the same order of that of conventional magnetic one, a radial restoring force of this bearing results from magnetic flux trapped in the superconductor, and high bearing stiffness is obtained at a certain region of a load-displacement hysteresis loop.
Two types of copper-stabilized Nb-Ti superconducting wires No. 1 and 2, were tensile-tested at various temperatures from 7 to 293K. Samples No. 1 and 2 have copper ratios of 3.92 and 7.72, cross sections of 0.86mm×1.7mm and 0.5mm diameter, 740 and 24 filaments of which diameter are 22 and 38μm, and were twisted and none-twisted, respectively, Temperature and strain rate dependences of yield stress, 0.2% proof stress, the stress corresponding to yielding in superconducting filaments, tensile-strength, elongation to fracture and serrated deformation were investigated in detail. The main results obtained are as follows: (1) Yield stress and proof stress increase slightly with decreasing temperature and the stress corresponding to yielding in the filaments and tensile strength increase with decreasing temperatures to about 15K and then decrease at temperatures below 15K in both the samples. (2) Elongation increases with decreasing temperature to about 15K, and then decrease in both the samples. Values of elongation at 7K of sample No. 1 show considerable scatter (0.7-18%) and values of elongation at temperatures above 15K show no scatter. Very low values of elongation seems to be due to remarkable discontinuous slip in thin regions of the filaments. (3) Yield stress, proof stress, the stress corresponding to the yielding in the filaments and tensile strength increase with increasing strain rate at 293K and show independence of strain rates at 7K in both the samples. (4) Elongation of sample No. 2 at 7 and 293K increase with increasing strain rate, On the other hand, elongation of sample No. 1 is independent of strain rates at 293K and values of elongation show considerable scatter at 7K regardless of strain rates.
Critical current density and its history effect in Bi-based high-Tc superconducting tapes were measured by ac inductive method in various magnetic fields and temperatures. A rapid decrease in the critical current density with elevating temperature and the history effect were observed in the same low temperature region between 4.2K and 30K. The coincidence suggests that the two phenomena are caused by “low temperature weak links” in specimens. Weak link region is considered to lose superconductivity at temperature higher than 30K. Above 30K, strongly bonded area retains superconductivity and Jc changes slowly with increasing temperature showing no history effect.
The electrical breakdown phenomena in liquid nitrogen under atmospheric pressure for point-to-plane electrode configuration have been investigated using a photo optical current measuring technique and a 790 nano-second rectangular high-voltage pulse generator. The waveforms of the prebreakdown current in liquid nitrogen were essentially the same as those in hydrocarbon liquids and in polyethylene. Discharge from positive point was more intense and faster than from negative point. A linear relation to formative time lag and gap spacing was observed for gap length more than some critical values, which indicates an average velocity of the discharge propagation in the most part of gap spacing. The velocities are deduced to be the range 13-18km/s for positive point and 3-12km/s for negative point under the applied voltage range 20-50kV. These values are an order higher than those in hydrocarbon liquids and are same order as those in chlorinated liquids for both point polarities.
The magneto-resistance of a thin film platinum resistance thermometer, PTF-7, was measured between 20 and 200K under magnetic fields of up to 8T. It showed a high stability during the thermal cycles between room temperature and liquid nitrogen temperature. The variation among the resistance values at 20, 50, 100 cycles was <1mK in terms of temperature for 8 of 10 sensors. Its magneto-resistance, ε, was well fit by an empirical formula ε=D(T)B2/(1+E(T)B) with two fitting parameters, D(T) and E(T). The variation of the magneto-resistance among the sensors was found to be 5%, which is 30 times as large as that of new-JIS fine wire platinum resistance thermometers, GR 0705. The anisotropy of the magneto-resistance was also measured and found to be as large as 50% depending on the direction of the field applied to the sensors.
A heat-capacity measurement system has been developed to establish a reliable calorimetric study under magnetic fields. The system is based on the thermal relaxation method by use of a temperature-difference detection system, which incorporates a pair of small thin film resistance thermometers, PTF-7 and is little affected by magnetic fields. Its performance under magnetic fields has been studied by use of a reference sample, whose heat capacity is independent of magnetic fields. SRM 720 provided by NIST, which is made of synthesized sapphire, was selected for this purpose. As this material is non-magnetic insulator, its heat capacity is expected to have small field dependence. The performance of the total system has been studied between 40 and 90K under magnetic fields of up to 7T. The resolution of the system was estimated to be about 2% and independent of the magnitude of magnetic fields. The measured heat capacities under magnetic fields of 0T, 5T and 7T agreed with each other within the resolution of the measurement.