Manabu Aoki

Vertically Layered Heat Exchanger for the Solid Nitrogen Heat Capacitor Cooling High-Temperature Superconducting Magnet

This study investigates the heat exchanger structure for the solid nitrogen heat capacitor, which slows the warming rate of the superconducting magnet during a power failure. To improve transient heat transfer of the solid nitrogen, the heat exchanger has a vertically layered structure to limit the thickness of the solid nitrogen. Horizontally wide heat transfer fins maintain thermal contact using the weight of the solid nitrogen. The heat exchanger solidified 2000 cm3 of liquid nitrogen in one hour and cooled the HTS coil to the operating temperature using just the cryocooler. In the temperature rise test, which stopped the cryocooler for one hour, it was estimated that the heat exchanger utilizes 28.3%-35.1% of the heat capacity of the solid nitrogen.

Locating a Mechanical Disturbance Source in a Superconducting Coil Using Pickup Coils on a Diaphragm

In this paper, we report a new method using pickup coils on a diaphragm to locate a mechanical disturbance source in a superconducting coil. Pickup coils with a twisted pair cable have a lower heat penetration compared to acoustic emission (AE) sensors with a coaxial cable. Vibrations of the diaphragm, which are caused by an acoustic pressure wave from a mechanical disturbance source, vibrate the pickup coil in an inhomogeneous magnetic field and generate signals in the pickup coils as inductive voltages. We have applied pickup coils to locate the azimuthal position of mechanical disturbance sources, which induce superconducting magnet quenching. The azimuth position of the mechanical disturbance sources have been located by measuring the time differences of the acoustic wave arrival at each pickup coil. The results using the pickup coils and AE sensors agreed in the angular range below 38°. The measurement error of this method was caused mainly by the detection time lag due to the acoustic wave damping in the coil windings.

An Experimental Study on the Superconducting Coil Behaviors Using Electromagnetic Pulse Signals During Ramping

Measurement and the analysis are done for the pulse signals from magnetic sensors (search coils) near superconducting coils and voltage taps on the coils. The superconducting coils are arranged facing each other, and the magnetic sensors and the voltage taps are expected to detect the coil behavior (motion of the coils, motion of coil windings, and so on) as the pulse signals during the ramping up. One of the causes of the superconducting coil quenching is the occurrence of a small mechanical disturbance around the coil windings. Therefore, information about the coil behavior from these pulses should be useful to get a better understanding of the quenching phenomena. The results of the pulse analysis suggest that the signals can be classified into just two patterns. In addition, only one of them is observed from the signals just before all quenchings; these are supposed as signals of the trigger disturbance of quenching. This fact suggests that the disturbance which causes the quenching has a strong relationship with the specific motion of the coils.

A Study on the Functional Characteristics of Superconducting Loops for External Magnetic Flux Suppression

The functional characteristics of superconducting loops passively driven by an external magnetic flux have been experimentally investigated. The loops can be used for external magnetic flux suppression in superconducting magnets which need to generate highly stable magnetic fields. Two one-turn-loops were installed between two superconducting coils which generate a strong magnetic field, and a Hall sensor and a search coil were set at the center of the system. The effect of external magnetic flux suppression of the one-turn-loops was measured while changing the strength of the magnetic field on the loops. Contrary to expectation, as the magnetic field strength on the loops increased, the suppression rate decreased. The reason for the degradation was identified as a micrometer-order wire motion caused by the electromagnetic force on the loops. Then, the loops were remade and ultra-tightly fixed, before testing again. This time no degradation was observed.