Myung Su Kim

Experiments on thermal contact conductance between metals below 100 K

The thermal contact conductance is one of the important components in heat transfer mechanism. The accurate estimation of thermal contact conductance is necessary for the development of a conduction-cooled superconducting magnet system because the metallic materials are thermally connected to the refrigerator or heat sink to cool the superconducting magnet without cryogen. The contact resistance occurs at the interface between metals and the amount of conductance can be affected by various factors, such as surface roughness, contact area, and contact pressure. In the superconducting magnet system, there are several metal components in contact with each other for cooling magnet as well as supplying current to the magnet. Therefore, a temperature gradient exists between superconducting magnet and cryocooler. In this study, we have developed the thermal contact conductance measurement system and used the steady state method to measure the thermal conductance between metals. The effects of temperature, contact pressure and interfacial materials on the thermal contact conductance are also discussed.The thermal contact conductance is one of the important components in heat transfer mechanism. The accurate estimation of thermal contact conductance is necessary for the development of a conduction-cooled superconducting magnet system because the metallic materials are thermally connected to the refrigerator or heat sink to cool the superconducting magnet without cryogen. The contact resistance occurs at the interface between metals and the amount of conductance can be affected by various factors, such as surface roughness, contact area, and contact pressure. In the superconducting magnet system, there are several metal components in contact with each other for cooling magnet as well as supplying current to the magnet. Therefore, a temperature gradient exists between superconducting magnet and cryocooler. In this study, we have developed the thermal contact conductance measurement system and used the steady state method to measure the thermal conductance between metals. The effects of temperature, contac...

Design and Fabrication of Vibration-Free Cryostat for Cryocooler-Cooled Superconducting Magnet in CPS

The vibration-free cryostat for a cryocooler directly cooled superconducting magnet is designed, fabricated, and tested for a cryogenic probe station (CPS). A low-temperature superconductor coil has a solenoidal configuration with a copper former which is thermally connected to a cryocooler with 1.5 W refrigeration power at 4.2 K. A pair of current lead, which is composed of a metal element and a high-temperature superconductor element, is employed and cooled by a cryocooler. A sample stage for transient measurement is located in the bore of a superconducting magnet and thermally connected to the cold head of a cryocooler to cool the sample down to liquid helium temperature. No mechanical contact exists between the superconducting magnet and the sample stage in the system. A bellows and mount pads are fabricated to attenuate the transmission of a cryocooler vibration. The structure of the cryostat to support the superconducting magnet and the sample stage has a long thermal path to minimize heat leakage. This paper presents the process of the design, fabrication, and magnet excitation test. The cooling performance of the cryocooler-cooled superconducting magnet is discussed with respect to the thermal resistance along the conduction plate. In addition, the measurement of the vibration in the CPS is presented, and the vibration level is reported.

Thermal Contact Resistance Measurement of Metal Interface at Cryogenic Temperature

Abstract The thermal contact resistance (TCR) is one of the important resistance components in cryogenic systems. Cryogenicmeasurement devices using a cryocooler can be affected by TCR because the device has to consist of several metal componentsthat are in contact with each other for heat transfer to the specimen without a cryogen. Therefore, accurate measurementand understanding of TCR is necessary for the design of cryogenic measurement devices using a cryocooler. The TCR occursat the interface between metals and it can be affected by variable factors, such as the roughness of the metal surface, thecontact area and the contact pressure. In this study, we designed a TCR measurement system at variable temperature using a cryocooler as a heat sink. Copper was selected as a specimen in the experiment because it is widely used as a heat transfermedium in cryogenic measurement devices. We measured the TCR between Cu and Cu for various temperatures and contactpressures. The effect of the interfacial materials on the TCR was also investigated.

Low Temperature Superconducting Magnet Connected to a Cryocooler by Conductive Link

A low-temperature superconducting (LTS) magnet cooled by a cryocooler has been designed, fabricated, and tested for lab-scale material control device. The superconducting magnet incorporates a solenoidal configuration with a copper former which has a 52-mm room-temperature bore. The superconducting coil is installed in a cryostat maintaining high vacuum and is cooled by a two-stage cryocooler. In order to maintain the operating temperature of magnet at the designed level, the cold head temperature of a cryocooler must be lower so that heat can be removed from the superconducting coil. Also, a temperature difference occurs between the magnet and cryocooler and its magnitude is dependent upon the contact resistance at the interface between the metals in the conductive link. The performance of the LTS magnet is investigated with respect to the conductive link between the magnet former and the cold head of a cryocooler. In addition, the effects of the contact pressure and interfacial materials on the temperature distribution along the conductive link are discussed.

Electrical Contact Resistance of Multi-Contact Connector in Semi-Retractable Current Lead

Heat leakage through current leads is the dominant contributor to cryogenic heat load for various high field magnets. The semi-retractable current lead is a good option because the conductive heat leak can be eliminated after the magnet is charged. It is composed of a normal metal element and an HTS element. The normal metal element is disengaged from the HTS element through the multi-contact connector without disturbance to the insulating vacuum space and without requiring complete removal of the normal metal element. In this paper, the electrical contact resistance of multi-contact connector is measured to confirm the feasibility of our application. The effects of current level, operating temperature and size on the electrical contact resistance in a lockable set are also discussed.

Development of Temperature Sensor Calibration System Using Cryocooler

The selection of the temperature sensor in a cryogenic system depends on the temperature range, shape, and accuracy. An accurate temperature sensor is essential for improving the reliability of an experiment. We have developed a calibration system for cryogenic temperature sensors using a two-stage cryocooler. To reduce the heat load, a thermal shield is installed at the first stage with multiple layer insulation (MLI). We have also developed a sensor holder for calibrating more than 20 sensors simultaneously in order to save time and reduce costs. This system can calibrate sensors at variable temperatures via temperature control using a heater. In this paper, we present the design and fabrication of the temperature sensor calibration system and a representative experimental result.

Intermediate Joint of Current Lead in Conduction-Cooled Superconducting Magnet System

The semi-retractable current lead is composed of a normal metal element, conducting the current from room temperature to intermediate temperature, and a high-temperature superconducting element, conducting the current down to liquid helium temperature. The normal metal element is disengaged from the high-temperature superconducting element through the multicontact connector without disturbance to the insulating vacuum space and without requiring complete removal of the normal metal element. The current lead is applied into the conduction-cooled superconducting magnet system developed by ourselves. The intermediate joint with a lockable set point is thermally connected to the first-stage of a cryocooler and carries current through a strip of louvered material. The performance of current lead is investigated with respect to the electrical contact resistance during the magnet charging process. The effects of current level and operating temperature on the heat generation in the intermediate joint are also discussed.

Characteristics of Conduction-Cooled Binary Current Leads Used in Cryogen-Free Probe Station

The binary current lead cooled by a two-stage cryocooler for an NbTi superconducting magnet used in cryogenic probe station was designed, fabricated, and tested. The current lead was composed of metal and a high-temperature superconductor (HTS) element. The temperatures at the joints between metal and the HTS element as well as between the HTS element and the NbTi coil were measured during cool down and the magnet charging process. The cryogenic loads at each stage were derived from the measured temperature and cooling capacity curve of a cryocooler. An HTS current lead was bolt jointed to the metal element and NbTi coil at both ends, so contact resistance was unavoidable. When the superconducting magnet was charged, the temperature at each joint increased with supplied current depending on the amount of heat generation resulting from the contact resistance. The cryogenic load was corrected because additional loads such as support conduction and thermal radiation were affecting the temperatures at each stage. The correlation of heat generation through the binary current lead was investigated in terms of supplying current, contact resistance, and cryogenic load.

Development of Conduction-Cooled Cryostat for Superconducting Probe Station

A conduction-cooled cryostat has been developed for NbTi superconducting magnet used in a cryogenic probe station. In a cryogenic probe station, a sample stage for transient measurement is located in the bore of a low-temperature superconducting (LTS) magnet and thermally connected to the second-stage cold head of a cryocooler to cool the sample down to the liquid helium temperature. Two separate structures exist in order to maintain isolation between the LTS magnet and sample stage. The system is cooled by a two-stage cryocooler, and the LTS magnet is charged and generates the designed magnetic field. When the magnet is charged and maintains a certain magnetic field, the temperature of sample stage is varied depending on the amount of heating. The thermal linkage between the LTS magnet and second-stage cold head as well as the sample stage and second-stage cold head is investigated in terms of the stability of LTS magnet, magnetic field, and temperature of sample stage. The mechanical module including the thermal link to control the sample stage is integrated into the system and the results of the performance test are reported.

Continuous Cooling System for Superconducting Magnets Using a Cryocooler

A continuous cooling system has been designed, fabricated, and tested for superconducting magnet applications. The objective of this paper is the development of a cooling module through a natural circulation loop using a cryogenic liquid for indirect cooling of high-temperature superconductor (HTS) magnets. Since HTS magnets are located in narrow room-temperature bores, several ancillary requirements such as space restriction and cooling passage structure require technical attention. In this paper, the design and fabrication of a continuous cooling system for HTS magnets are presented while taking into account cryogenic loads and system integration. A preliminary test has been carried out using a cryocooler, and the results are reported in detail. The effect of the total heat load to the magnet on the temperature distribution in the system is presented. In addition, long-term operation of the continuous cooling module for superconducting magnet applications is discussed.