Yoon-Myung Kang

A CRYOCOOLED 61-CHANNEL MEG SYSTEM

Abstract An LTS-SQUID-based MEG system cooled by a GM/JT cryocooler was manufactured, and its clinical performance was estimated. The system consists of 61 axial gradiometer channels arranged on a spherical surface. The white noise level of the system was below 20 fT/Hz1/2. The periodic noise induced by the cryocooler could be eliminated by using a digital noise subtraction process. We measured the somatosensory evoked magnetic field of the human brain.

A cryocooled helmet-shaped MEG measurement system

We developed a cryocooled helmet-shaped magnetoencephalographic measurement system. We determined the sources of white noise that contribute to the total white noise of this system and determined the source of cyclic noise generated by the Gifford-McMahon-Joule-Thomson cryocooler. Design modifications based on these results will reduce the white noise of our future cryocooled systems to 5 fT Hz-1/2 for cyclic noise to 1 pT.

Preparation of YBCO Films by CP-Process for HTS Microwave Filters

Double-sided YBCO films were prepared on LaAlO3(00l) substrates (20×20×0.5 mm3) by a coating-pyrolysis (CP) process through single annealing in a 80-mm-diameter tube furnace or by means of IR lamp heating. Superconducting transition temperatures about 90 K were obtained for the epitaxial films by dc-resistance measurement. The surface resistance at around 10 GHz was measured by the dielectric resonator method to be 0.6 mΩ at 60 K, which is comparable to those made by other high vacuum deposition techniques. Moreover, we fabricated a 5-pole microwave filter on the epitaxial c-axis YBCO films, for which an insertion loss less than 0.1 dB was obtained at 70 K.

A Cryocooled SQUID System with Minimal Clearance Between the Pick-up Coil and the Specimen

We have developed a SQUID system that have a clearance of about 3 mm between the pick-up coil and the specimen and is cooled by a Gifford-McMahon (GM)/Joule-Thomson (JT) cryocooler. Previous work showed that cryocooled SQUID systems, which do not require liquid helium, can successfully measure biomagnetic fields [1][2][3]. To eliminate the noise caused by the cryocooler, such systems either utilize digital filters or interrupt the cooler expansion operation during the measurements. In our new system, however, we reduced this noise by replacing the ferromagnetic materials of the cryocooler, and we measured magnetcardiogram (MCG) signals in real-time without the need for interrupting the cooler expansion operation.

DEVELOPMENT OF A 4 K GM/JT CRYOCOOLER

4 K Gifford-McMahon/Joule-Thomson cryocoolers, which have refrigeration capacity rating of 3.5 W at 4.3 K and efficiency relative to Carnot 4%, were developed. One prototype has operated for 20,000 hours without JT service and several other units for 10,000 hours each in the field. No JT blockage problems have been observed. Several types of interface were also developed for application, e.g., recondensing helium in the cryostat for MRI or single crystal growth, and cooling sensors or mixers for far-infrared wave measurements.

Experimental Study on Thermal Contact Conductance at Liquid Helium Temperature

Publisher Summary This chapter employs two methods to estimate this thermal contact conductance at liquid helium temperature–an empirical equation, and the Wiedemann-Franz law. When cooling an object in a cryogen-free, cryocooled system at liquid helium temperature, it is important to quantitatively estimate the thermal resistance that occurs at the contact surface between the cooling stage and the object. It compares the estimated conductance with that measured at liquid helium temperature, both methods are proved valid. Additionally, it measures the thermal conductivity and the electric resistance of phosphor bronze screen stacks at room temperature and at low temperature. For electrically conductive contacts consisting of electrically conductive blocks and interposers such as indium foil, the thermal conductance at liquid helium temperature can be estimated with Wiedemann-Franz law when the temperature dependency of the properties is considered. In contrast, the thermal conductance of phosphor bronze stacks whose surface was oxidized could not be estimated with Wiedemann-Franz law between 4K and 300K.