Superconductivity in high energy particle accelerators

The basics of superconductivity are outlined with special emphasis on the features which are relevant for the application in magnets and radio frequency cavities for high energy particle accelerators. The special properties of superconducting accelerator magnets are described in detail: design principles, magnetic field calculations, magnetic forces, quench performance, persistent magnetization currents and eddy currents. The design principles and basic properties of superconducting cavities are explained as well as the observed performance limitations and the countermeasures. The ongoing research efforts towards maximum accelerating fields are addressed and the coupling of radio frequency power to the particle beam is treated. (orig.)Available from TIB Hannover: RA 2999(02-116) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Surface superconductivity in niobium for superconducting RF cavities

A systematic study is presented on the superconductivity (sc) parameters of the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz cavities for the linear collider project TESLA. Cylindrical Nb samples have been subjected to the same surface treatments that are applied to the TESLA cavities: buffered chemical polishing (BCP), electrolytic polishing (EP), low-temperature bakeout (LTB). The magnetization curves and the complex magnetic susceptibility have been measured over a wide range of temperatures and dc magnetic fields, and also for different frequencies of the applied ac magnetic field. The bulk superconductivity parameters such as the critical temperature T_c=9.26 K and the upper critical field B_c_2(0)=410 mT are found to be in good agreement with previous data. Evidence for surface superconductivity at fields above B_c_2 is found in all samples. The critical surface field exceeds the Ginzburg-Landau field B_c_3=1.695B_c_2 by about 10% in BCP-treated samples and increases even further if EP or LTB are applied. From the field dependence of the susceptibility and a power-law analysis of the complex ac conductivity and resistivity the existence of two different phases of surface superconductivity can be established which resemble the Meissner and Abrikosov phases in the bulk: (1) ''coherent surface superconductivity'', allowing sc shielding currents flowing around the entire cylindrical sample, for external fields B in the range B_c_2 < B < B_c_3"c"o"h, and (2) ''incoherent surface superconductivity'' with disconnected sc domains for B_c_3"c"o"h < B < B_c_3. The ''coherent'' critical surface field separating the two phases is found to be B_c_3"c"o"h=0.81 B_c_3 for all samples. The exponents in the power law analysis are different for BCP and EP samples, pointing to different surface topologies. (orig.)Available from TIB Hannover: RA 2999(04-027) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman