There are three basic options for achieving cryogenic temperatures in space: radiative cooling, cryogen cooling, and mechanical coolers. Both the radiative and cryogen options can fail to meet the highly-demanding requirements for future space missions in terms of temperature, long life and low mass. The mechanical coolers that ESA is currently developing can overcome several of these major problems, thereby providing much greater flexibility in overall spacecraft configuration compared with current cryogenic cooling designs.
The European Organization for Nuclear Research (CERN) is preparing for the construction of a new high energy colliding accelerator, the Large Hadron Collider (LHC). High field superconducting dipole magnets with a nominal magnetic field of 10T will be inevitably required to realize the LHC project. This paper reviews the project and progress of development of the 10T dipole magnets for the LHC project.
Temperature is one of the most important parameters for the study of various properties of materials at cryogenic temperatures. In this lecture, the basic knowledge and techniques are described and reviewed, which are necessary for accurate thermometry.
In large superconducting coil applied systems, such as Superconducting Magnetic Energy Storage (SMES), nuclear fusion system and so on, large current leads for charging and discharging coils are needed, so that conduction heat through the leads may cause large heat losses and reduce efficiency of the system. Operating of a power supply (power converter) for a superconducting coil in the same cryostat together with the coil and converting the low voltage and high current power into high voltage and low current power in the cryostat, it will become possible to reduce the size of current leads and power losses of a power supply. Among existing power semiconductor devices such as thyristors and power transistors, power MOSFETs showed lower losses in low temperature region; they showed the minimum on-resistance value at around 70K. Utilizing this characteristic, a power converter using power MOSFETs and diodes was made and operated in a cryostat together with a superconducting coil. As a result, it was confirmed that the stable operation of a power converter in a cryostat is possible and that the size of current leads into a cryostat and power losses of a converter can be reduced.
Performances of pulse-tube refrigerators are discussed using thermoacoustic theory of inviscid fluid. Discussions are based on numerical calculations of distributions of temperature, displacement amplitudes and energy flows in a pulse tube using a method of the second step of thermoacoustic theory by A. Tominaga. Pulse-tube refrigerators are classified to three generations by boundary conditions at the hot end of pulse-tube: the first generation has no displacement at the end, as is the case of basic-type refrigerators. The second generation has finite displacement and phase difference below 90° between displacement and pressure oscillations, as is the case of orifice type refrigerators. The third generation has finite displacement and the phase difference over 90°. The followings are main results of discussions. (1) For the first generation there is heat pumping through the pulse tube due to standing-wave component when the temperature of the cold end is not so low. Large -Fsχ″ is, therefore, favourable to large heat-pumping. The phase loss of the regenerator is expected to be large, since the phase difference at the cold end is small. (2) For the second generation the phase loss of the regenerator is decreased due to progressive-wave component superposed at the hot end. The refrigeration power of the regenerator, however, decreases due to the displacement loss, if the displacement is too large. (3) For the third generation both of the regenerator losses are decreased by reduction of the standing wave component. The pulse tube of the third generation has, therefore, larger refrigeration power and lower limiting temperature than that of the second generation. (4) Small -Fsχ is favourable to reduction of heat input through the pulse tube at the pulse-tube part of the second and the third generation.
A new technique has been developed to remove flux quanta trapped in superconducting film. In 1984, one of the present authors proposed a micro heat flushing (μHF) method for achieving perfect magnetic shielding. In this method, a small area on a superconducting thin film is heated and turned into the normal state. The entire film surface is then swept over by the normal hot spot, which captures the trapped flux and carries it either to an unimportant region or out of the superconductor. It has been shown that the trapped flux movement follows the motion of the Ar laser beam exactly. By scanning the laser beam over the trapped flux quanta, the trapped flux is successfully moved to any desired location.
Discussed are methods of controlling phase difference between oscillations of pressure and displacement at the hot end of a pulse-tube. Electric analogs of fluid mechanical system are discussed in the beginning and fluid mechanical components corresponding to capacitance, inductance and resistance in electric circuits are clarified. In the next the phase difference is discussed in terms of equivalent circuit-network. The moving-plug type by Y. Matsubara and A. Miyake is shown to belong to the third generation of pulse-tube refrigerator. The double-inlet one by S. Zhu et al. belongs possibly to the third generation. A differential pulse-tube refrigerator with a free-piston is proposed.
Resistance vs. temperature relations of 200 carbon resistors (Allen Bradley 100Ω, 1/8W), widely used for cryogenic temperature measurements, were investigated in the range between liquid helium temperature and 20K. Obtained data are statistically evaluated, and it is apparently shown that the difference of the carbon resistor's lot affects the resistance value at the cryogenic temperatures. Four empirical formulas for the resistance vs. temperature relation proposed by other researchers are evaluated. According to this result, the equation, logR+K/logR=A+B/T, proposed by Clement et al. has most precise fitting below 10K. The authors propose a new empirical equation by applying Clement's equation. It is found that the three undecided coefficients, A, B and K of the equation can be calculated from the resistance at 4.2K alone, thus enabling one-point calibration at liquid helium temperature. Resistance changes caused by ten times temperature cyclings from room temperature to liquid helium temperature are also measured. These are within -0.05-+0.02K, though measurments are slightly diverged by temperature cyclings.
Single crystal growth of the Bi2.2Sr2CaCu2Ox superconductor by the traveling solvent floating zone method has been performed with the intension of obtaining large single crystals with high quality and elevating the superconductive transition temperature, Tc. The oxygen partial pressure of growth atmosphere, PO2, and the composition ratio, Sr/Ca, were varied in the crystal growth. The thickness of the crystal in the crystal growth decreased with decreasing oxygen pressure below 22kPa, while no distinct change was observed in the resulting crystals in the range of 22-100kPa. The thickness was the largest at Sr/Ca=1.4. The lattice image of the single crystal by a high resolution TEM with no intergrowth revealed the extremely high quality of the crystal. Mechanism of the crystal growth is discussed.
Geomagnetic field changes are expected prior to volcanic eruptions because temperature and pressure changes associated with magma flows in surrounding rock give rise to temporal magnetic anomalies. It was planned to monitor temporal magnetic changes by a SQUID (Superconducting QUantum Interference Device) sensor in the vicinity of the Sakurajima volcano in an attempt to study the relation between volcanic activity and variations in the geomagnetic field. This paper describes the development of the geomagnetic field sensing system using the SQUID sensor and the measurement of electromagnetic environmental noise generated in Kagoshima city.