A review of the radiation damage problems in insulators expected to be used in a superconducting (SC) magnet of fusion reactors has been given. Organic insulators, when compared with inorganic ones, show great advantage in fabricability, but they suffer from larger sensitivity to irradiation. In organic insulating materials electrical properties such as dielectric strength or dielectric loss and thermal properties such as thermal conductivity are less problematical than mechanical properties. In view of the possible pulse operation in the fusion reactor SC magnets, it should be stressed that the accumulation of the data on mechanical properties against time varying load at cryogenic temperatures, such as strain rate dependent fracture stress (and/or strain), impact strength or fatigue in necessary. Radiation effects on organic insulator are again less problematical to electrical and thermal properties than to mechanical properties. Maximun allowable neutron fluence for the organic insulator has been considered at most 1018n·cm-2. The design of the SC magnet for fusion reactors should take these results into consideration.
Effects of deformation rate on the mechanical properties of epoxy resin used in superconducting magnets have been studied at cryogenic temperatures. Compressive and flexural tests were made to reveal the mechanical behaviors. In the case of compressive tests, the increase of deformation rate caused the increase of elastic modulus and the decrease of breaking stress and strain. In the case of flexural tests, different results in breaking stress was obtained. The results indicate that following two problems must be elucidated for practical use of polymers, i.e. (i) impact strength of the polymers, (ii) the stress condition applied to the polymers in the magnet.
Heat transfer into saturated He II was investigated in an experiment which simulates the use of He II as coolant for the stabilization of superconducting magnets. One must distinguish two critical values of the recovery heat flux (i.e. recovery from film boiling to non-boiling), namely for recovery from silent film boiling and recovery from noisy film boiling. There are also two threshold values of the power of short-time heat pulse which trigger a transition from the non-boiling heat transfer into film-boiling. The increased cooling capacity of He II at about 1.9K seems to compensate for the reduction of the thermal parameters of the stabilizing material at the low temperature.
Thermally driven acoustic oscillation is one of nuisances for cryogenic researches and applications, because it disturbs measurements and accompanies a considerable amount of heat flux into low temperature devices. This paper reports a rough picture of the thermally driven acoustic oscillation and some information on the stability limit of the oscillation. Corresponding two types of experiments were carried out using helium gas. The first type of experiments shows that 1. Periods of the oscillation and temperature distribution in the dewar suggest the oscillation is one of gas-column oscillations. 2. The oscillation amplitude is very sensitive to inner radius of the tube. 3. Heat flux into the dewar due to the oscillation is roughly proportional to the square of the oscillation amplitude. The second type of experiments with U-shaped tube shows that 4. The stability limit of the oscillation is expressed by dimensionless quantities, TH/TC and ατ/r2, where TH and TC are temperatures of hot and cold parts and α, τ and r are the thermal diffusivity of the gas at the hot part, the oscillation period and the inner radius of tube, respectively. 5. The calculations of the stability limit by N. Rott are confirmed experimentally. 6. The stability limit is expressed by a simple relationship between TH/TC and ατ/r2.
A transportable sealed cell was designed for a thermometric fixed point device at low temperatures, which is made of copper and filled with a pressurized sample gas. Two types of cryostats, an adiabatic cryostat for precise measurements and a commercially available refregerator for practical use, were employed for realization of the triple points; e-H2, n-H2, Ne, O2, N2, Ar, Kr, Xe, CO2 and CH4. The precise realizations indicated that the triple points examined were reproducible to ±0.2mK on several runs for different cells. A practical method for realization with an accuracy of ±10mK were also examined for argon and neon. All results obtained showed that the sealed cell is a useful device as a reference standard of temperature both for precise measurements and for practical use.