The lasing of the first free-electron laser (FEL) in the world was successfully carried out in 1977, so the history of FELs as a light source is not so long. But FELs are now utilized for research in many scientific and engineering fields owing to such characteristics as tunability of the wavelength, and short pulse and high peak power, which is difficult utilizing a common light source. Research for industrial applications has also been carried out in some fields, such as life sciences, semiconductors, nano-scale measurement, and others. The task for the industrial use of FEL is the realization of high energy efficiency and high optical power. As a means of promoting realization, the combining of an FEL and superconducting linac is now under development in order to overcome the thermal limitations of normal-conducting linacs. Further, since tuning the wavelength is carried out by changing the magnetic density of the undulator, which is now induced by moving part of the stack of permanent magnets, there is uneasiness in the moving part and partial magnetic depression by irradiation. If the superconducting coil is applied for the undulator, it is believed that wavelength tunability will improve in terms of reliance and usability. As a result, the collaboration between superconducting/cryogenic engineers and FEL engineers is very fruitful because superconducting FEL is essential for industrial use.
Numerical simulation has been performed to analyze the heat and fluid flow in pulse tubes and to clarify the working principle of refrigeration in basic pulse-tube refrigerators. Transient axisymmetric two-dimensional equations of continuity, momentum and energy were solved utilizing the TVD method. A physical model combining the pulse tube with the wall and regenerator is used for numerical simulation. The pulse tube for this study is a stainless-steel pipe of 150mm in length, 5mm in inner diameter and 1mm in thickness, and contains a heat exchanger of 30mm in length. The dimensions of the regenerator are 60mm in length and 12.5mm in inner diameter. Woven 200 mesh wire screens of copper are used as the regenerator material. Air is selected as the working gas. Heat exchange between the pulse-tube wall and the working gas in the pulse tube is assumed to be convective heat transfer. In this paper, we analyze the transient behaviors of pressure and gas temperature, traces and temperature changes of the gas elements in the pulse tube and heat transfer between the working gas and tube wall.
AC losses in multi-strand superconducting cables, utilized in large-scale applications such as fusion machines, are governed by the contact resistance between strands. Especially, in cable twisted at multiple-stages, a variety of magnetic field diffusion time constants exist and these correspond to the quantity of inter-strand coupling loss in each cabling stage. The rate of magnetic field change is less than several T/s in an average fusion machine. Under this condition, the magnetic field penetrates the cable well and the coupling current circuit with the larger time constant causes larger AC loss. Here, the time constant is equal to the leakage inductance divided by the resistance along the coupling current loop. Therefore, by evaluating the coupling current in the larger loop, which consists of a higher twisting stage (e.g., usually the final cabling stage), the loss in the entire cable can be determined. The leakage inductance between sub-cables can be estimated by considering the electrical centers. On the other hand, inter-sub-cable contact resistance was not previously evaluated due to its complexity. In this study, we established an inter-sub-cable contact resistance model that allows the AC loss in cable with multiple twisting stages to be evaluated numerically. The modeling of contact resistance between sub-cables is discussed in detail.
In this study, the cryogenic performance of a regenerator that utilizes fibrous media is estimated. The decrease in the specific heat of the matrix causes a loss of thermal efficiency in cryogenic regenerators. Water vapor condenses in regenerators as frost, which is one of the serious problems in cryogenic air separation plants. A single-blow experiment and numerical analysis considering the effect of temperature dependence on thermal properties as well as the effect of phase change of water vapor were performed. Good agreement of the temperature time variation in the regenerator was obtained for experimental and numerical results. The decrease in specific heat of the matrix causes a loss in efficiency, especially when the switching period is long. Furthermore, the evaporation and condensation of water vapor in a regenerator is simulated under the condition of continuous operation. It was calculated that frost deposition occurs where the temperature gradient is large, and the amount of frost increases linearly with operating time. The drying process is needed once a day for the present condition. The interval increases for lower inlet temperature and higher gas pressure.
The first and the second laws of thermodynamics are reformulated taking current view point into consideration. Extensive state quantity, flow and local production rate are one set of concepts. The first law states that the local production rate related to energy is zero, and the second law states that the local production rate related to entropy is positive or zero. Relations between entropy flow and heat flow are discussed. For reversible process and steady-state, heat flow is proportional to entropy flow, and the proportional constant is the temperature. For periodical steady-state, heat flow is also proportional to entropy flow, but the proportional constant is time average of instantaeous temperature. The law of minimum entropy production rate is also a fundamental law of thermodynamics, and it is essential for discussions on thermodynamical stability.