Measurements of Magnetic Field Pattern in a Short LHC Dipole Model

The magnetic field in superconducting accelerator magnets has a fine structure with longitudinal periodicity. This periodic pattern, with period identical to the cable twist pitch, is originated by uneven current distribution within the cable. Here we present results of measurements of the periodic pattern performed in an LHC dipole model. We report in particular the results obtained powering the magnet with simple current steps and typical operation cycles as will be used during accelerator operation. The main result of the analysis is the time variation of the amplitude of the periodic pattern, from which we infer the evolution of the current distribution in the cable. We discuss the dependence of the pattern amplitude on ramp and pre-cycle parameters.