Current distribution inside Rutherford-type superconducting cables and impact on performance of LHC dipoles

The windings of high--field superconducting accelerator magnets are usually made of Rutherford--type cables. The magnetic field distribution along the axis of such magnets exhibits a periodic modulation with a wavelength equal to the twist pitch length of the cable used in the winding. Such a Periodic Field Pattern (PFP) has already been observed in number of superconducting accelerator magnets. Additional unbalanced currents in individual strands of the cable appear to be causing this effect. The present thesis describes the investigation of the PFPs performed with a Hall probes array inserted inside the aperture of the LHC superconducting dipoles, both in the small--scale model magnets with a length of one meter and in full--scale prototypes and pre--series magnets with fifteen meters of length. The amplitude and the time dependence of this periodic field oscillation have been studied as a function of the magnet current history. One of the main parameters influencing the properties of the PFP is the cross--contact resistance between the strands of the cable. An estimation for these values is achieved by the so--called Field Advance (FA) measurements performed again with a Hall probes set--up. Due to eddy currents a difference in the field values for the ramp--up and the ramp--down of a current cycle is generated, which is a linear function of the applied ramp rate. The resulting slope is furthermore indirectly proportional to the corresponding cross--contact resistance. Two types of so--called interstrand coupling currents, uniform and non--uniform, are induced by a changing magnetic field and flow not only within the individual strands but also between the strands of the cable. Therefore some parts of the strands can carry a total current which is larger than the transport current. This phenomenon locally reduces the difference between the total strand current and the critical current of the superconductor and can provoke a premature quench of the superconducting magnet, i.e. a transition to the normal state. Considering theoretical models and experimental results the impact of the current distribution on the quench performance of the LHC dipoles is discussed. Finally an estimation for the influence of these currents on the magnet stability with respect to quench during operation conditions is given.