Superconducting wiggler magnets for beam-emittance damping rings

Ultra-low emittance beams with a high bunch charge are necessary for the luminosity performance of linear electron-positron colliders, such as the Compact Linear Collider (CLIC). An effective way to create ultra-low emittance beams with a high bunch charge is to use damping rings, or storage rings equipped with strong damping wiggler magnets. The remanent field of the permanent magnet materials and the ohmic losses in normal conductors limit the economically achievable pole field in accelerator magnets operated at around room temperature to below the magnetic saturation induction, which is 2.15 T for iron. In wiggler magnets, the pole field in the center of the gap is reduced further like the hyperbolic cosine of the ratio of the gap size and the period length multiplied by pi. Moreover, damping wiggler magnets require relatively large gaps because they have to accept the un-damped beam and to generate, at a small period length, a large magnetic flux density amplitude to effectively damp the beam emittance. These requirements and the described physical properties and laws make designing, building and operating damping wigglers difficult. Only low-temperature superconducting wiggler magnets wound with Nb-Ti or Nb$_{3}$Sn strands can economically provide the required magnetic flux density. So far, all superconducting wiggler magnets have been wound by using Nb-Ti strands, and the wiggler magnets have been immersed in liquid helium boiling at an atmospheric pressure at 4.2 K. In damping rings, superconducting wiggler magnets are operated in series of up to 26 magnets. This method results in a larger heat load compared to that of the standard application in light sources. Therefore, in this thesis, a full heat load study is presented, and an advanced indirect cooling scheme for wiggler magnets is proposed in order to reduce the gap size and the complexity of the cryogenic system. The essential elements of the design are that the beam-pipe and coils are in a vacuum, they have minimum thermal contact, and they are separately cooled. To reach larger magnetic flux densities and to increase the reliability of the damping wiggler magnets, the use of Nb$_{3}$Sn wiggler magnets is proposed. In the proposed Nb$_{3}$Sn wiggler magnets, the minimum quench energy at the operating current is more than one magnitude larger than that of the Nb-Ti counterpart. The deposition of a minimum quench energy establishes a propagating normal-conducting zone in the superconducting filaments, in which the temperature is above the critical temperature. Superconductors exposed to a steady field, current, and strain pass over to their normal conducting state if their operating temperature is above their critical temperature. In this thesis, a method for the construction and operation of Nb-Ti and Nb$_3$Sn superconducting damping wiggler magnets for ultra-low beam emittance rings is presented. A design optimization is performed to find the optimal parameters of the superconducting wiggler magnets. Manufacturing methods for two different design concepts for advanced superconducting wiggler magnets wound by using Nb-Ti and Nb$_{3}$Sn strands are developed. Trial coils and short-model magnets were built and tested. It was found that Nb-Ti and Nb$_{3}$Sn superconducting damping wiggler magnets could be manufactured and operated in ultra-low emittance rings.