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内容記述 |
The purose of this work is to study the possibility to apply high-T c superconductors to future accelerator cavities. YBa2 Cu 3 O 7-δ ( YBCO ) film with the c-axis normal to the film surface were deposited by a laser ablation technique on copper subshates of 36mm in diameter. An YSZ/Cr buffer layer was used for the deposition of YBCO film on copper substrates. The surface resistance and the penehation depth of the YBCO film were measured using a cylindrical host cavity operated at 13.6GHz in TE 011 mode.<br /> The deposition on metallic substrates is an important concern from the view point of heat removal in the possible application to accelerator cavities, in which an rf excitation at a high-power level is required. Since the c-axis of the fabricated film is normal to the surface, and, therefore, the surface current flows in the ab-plane of the film, a very low surface resistance was obtained at low temperatures. In order to improve the measurement accuracy at low temperatures, a niobium host-cavity was used to calibrate the data measured by a copper host-cavity. However, since the measurable temperature region in the niobium and copper cavities were reshicted and unoverlapped, some fitting procedure were required and will be described in detail. This accuracy improvement by the two-cavity method enabled us to discuss the temperature dependence of the surface resistance more reliably, and, therefore, the excitation mechanism of the normal charge carriers from the superconducting state.<br /> Since some part of the fabricated film could be remains normal even at very low temperature, the two fluid-mobel was modified to include the residual charge carriers. This modified two-fluid model could well fit the measured surface resistance and penetration depth for a wide temperature range for 4K to 85K, if the residual charge carriers was assumed to be around 20% of the total charge carriers and the normal charge carries increased in a form (T/T c )2.<br /> The effect of the film thickness and substrate properties of the measured surface resistance was calculated by the impedance transformation method. The results show that the measured surface resistance R eff is almost equal to the real surface resistance in the temperature range below 70K. However, above 70K, R eff is lowered to some extent depending on the value of penetration depth, and needs some corrections.<br /> The field dependence of the surface resistance was measured for the c-axis textured YBCO films up to the surface magnetic field of 1000 A/m. One of important results was that the surface resistance increases linearly as the field increases, which is explained better by the critical-state model than by the coupled-grain model.<br /> Comparison of the microwave surface resistance between a-b plane well-textured and weakly-textured samples were made. The surface resistance of the weakly-textured samples was lower than that of the well-textured samples for the circular current on the endplate in the TE 011 excitation mode. However, the increase in the surface resistance with increasing field level was lower for the well-textured samples than for the weakly-textured samples. This suggests the necessity of a-b plane texturing as well as c-axis texturing for superconducting films to be applied to accelerator cavities. |
要旨・審査要旨 / Abstract, Screening Result (227.0 kB)
