Fatigue properties of a commercial epoxy have been studied at RT, LNT and LHeT. The low temperature reactor irradiation effects have also been studied at LHeT. Repeated flexural stress with frequency of 2Hz was applied to the samples cured into cylindrical shapes of 4.0mm diameter and 30mm length. The samples were in contact with the coolant of liquid helium or liquid nitrogen during the fatigue experiments. Reactor irradiation was performed in Low Temperature Loop of Kyoto University Research Reactor with the fast neutron fluence of about 2.5×1016n·cm-2 and γ doses of 2.8×108rad. Normally the design parameters at cryogenic temperatures are apt to be based on the data at RT because the temperature decrease brings higher mechanical strength. However the fatigue strength of this epoxy at LHeT was about a half of that at LNT and was as low as that at RT data. For this reason the fatigue data at LHeT where superconducting magnets are used should be accumulated exactly. On this experimental result, a similar tendency of the fatigue properties between at LNT and at LHeT was observed in the stress-number of cycles curves represented by normalized stress on ordinate against fatigue life on abscissa with logarismic scale. It suggests the possibility of the substitution with fatigue data at LNT for that at LHeT. Low temperature irradiation brought on the epoxy used in this study the decrease of fatigue strength and the increase of scattering of fatigue strength. These results indicate that careful design should be performed in practical use of these organic materials to fusion reactor.
The dynamics of a superconducting inductor and a thyristor converter (I-C) unit have been studied. For the optimal control of I-C unit, the dynamics of the unit must be described. But difficulties arise in the deduction of dynamics of a superconducting magnet (SM) from the properties of conductors and the coil configurations. To avoid these difficulties, the identification of dynamics through input/output relations has been studied. For this study, the input/output data are acquired through the following procedures. The experimental system is the I-C unit composed of a 12 phase thyristor power converter and a 100kJ superconducting magnet. The input gate-signal of random waveform is given to the thyristor coverter and the response waveforms of currents and voltages of the I-C unit are sampled and memorized to form an input/output (I/O) sequence. The I/O sequence is identified by a multi-input/multi-output discrete time system (MI/MO DTS), and the dynamics of the I-C unit is described. The discrete time system which relates the input and output waveforms of the I-C unit is derived by being based on least square method such that the euclidean norm between the measured and simulated outputs is minimized. The simulated outputs of the derived MI/MO DTS have closely fitted to the measured waveforms. The derived systems revealed that one of the equivalent circuits of SM was a simple connection of R-L and the parameters were estimated so that the simulated waveforms of the equivalent circuit were closely fitted to the measured ones.