Author_Institution :
Dept. of Mater. Sci. & Eng. & Welding Eng., Ohio State Univ., Columbus, OH, USA
Abstract :
As a starting point we begin with an introduction to the general parallel-resonant circuit as a model for the RF resonant cavity with its surface resistance, Rs, and voltage-amplification parameter, Q. Early RF cavities were of polished high purity Cu. Factors controlling the Rs of a Cu cavity are: temperature, T, operating frequency, f, and electrical resistivity, ρ, parameters associated with which are electron mean-free-path (l) and skin depth (δ). Surface resistance decreases with decreasing T and ρ in the normal skin-effect regime until a limit is reached at δ = l. Beyond this point the cavity wall enters the “anomalous-skin-effect regime” within which Rs is temperature independent and of order mΩ. To further reduce Rs and increase Q the cavity needs to be lined with or constructed from superconducting material. Currently the superconductor of choice is Nb whose BCS-based Rs (i.e. RsBCS) is some 3 to 4 orders of magnitude smaller than that of Cu. The RsBCS of the Nb cavity decreases rapidly with temperature, hence the advantage of cooling by superfluid helium. In general RsBCS also decreases rapidly with increase in Tc, the superconductor´s transition temperature. Thus replacing Nb with an ideal uniform coating of Nb3Sn, for example, would in principal reduce RsBCS by 105. Many factors can perturb the Rs of superconducting cavities and act to reduce the Q-factor, among them are: (i) Trapped magnetic flux from some external magnetic field which should expose the surface to no more than a few tens of μT, (ii) enhanced surface RF magnetic field (associated with surface defects) which limits the attainable accelerating field, (iii) effects associated with cavity shape and surface imperfections such as field emission - nd thermal breakdown. Finally the operating parameters of the LHC and the ILC are presented in terms of the accelerating properties of their SRF cavities.
Keywords :
magnetic fields; particle accelerators; skin effect; superconducting transition temperature; surface resistance; Nb3Sn; Q-factor; RF cavities; RF resonant cavity; accelerating field; anomalous-skin-effect regime; cavity shape; cavity wall; electrical resistivity; electron mean-free-path; enhanced surface RF magnetic field; external magnetic field; field emission; high energy particle accelerators; normal skin-effect regime; operating frequency; operating parameters; parallel-resonant circuit; skin depth; superconducting cavities; superconducting material; superconducting radio-frequency cavities; superconductor transition temperature; superfluid helium; surface defects; surface imperfections; surface resistance; thermal breakdown; trapped magnetic flux; voltage-amplification parameter; Acceleration; Cavity resonators; Linear particle accelerator; Niobium; Radio frequency; Surface resistance; ILC; LHC; normal metal cavity; radiofrequency cavity; skin effect; superconducting cavity;