Research On Transmission Line Characteristics In Low Temperature Environment

The low-temperature transmission lines can be divided into two categories:the low-temperature normal-metal transmission line and the superconducting transmission line.They both have the advantages of low insertion loss,low noise and high quality factor.Therefore,the low-temperature transmission lines are widely used in the special technology fields,such as high energy physics,radio astronomy,quantum communications and quantum computation.The main contents and results of this paper are as follows:In the first part,based on the free-electron-gas model,we studied the main electromagnetic parameters such as electronic conductivity,skin depth and surface impedance of the metallic material at normal state,and analyzed the trend of these electromagnetic parameters changing with frequency and temperature,especially the anomalous skin effect at low temperature.According to the calculation of the conductivity and the surface impedance at low temperature,it is found that the aluminum is the preferred material for future low temperature electronic devices and circuit design because of its best conductivity at the temperature less than 66K,while the silver is suitable for working in a wide temperature range due to its lowest temperature sensitivity.In the second part,using the London equation which based on the two-fluid model,Pippard theory and BCS theory,we studied the electromagnetic parameters of the superconducting metal and the calculation method of these parameters.According to the London equation,it is found that the superconductor has a very high surface quality factor,and its surface resistance is proportional to the square of frequency,while surface reactance is mainly determined by the penetration depth.Compared with the type-? superconductor,it is found that the penetration depth of the type-? superconductor,which introduced by Pippard theory,varies more slowly with the temperature.In the third part,a new test method of the complex propagation constant,which is suitable for cryogenic Dewar environment,is presented.According to the final measured results,at the room temperature,the attenuation of normal metallic coaxial lines is mainly affected by conductor loss and is proportional to the square root of frequency;on the contrary,at the temperature of 4K,the attenuation of normal metallic coaxial lines is proportional to the frequency and is determined by the dielectric loss instead.