Measurements of magnetic field pattern in a short LHC dipole model

The magnetic field in superconducting accelerator magnets has a fine struct ure with longitudinal periodicity. This periodic pattern, with period ident ical 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 resul ts obtained powering the magnet with simple current steps and typical opera tion cycles as will be used during accelerator operation. The main result o f the analysis is the time variation of the amplitude of the periodic patte rn, from which we infer the evolution of the current distribution in the ca ble. We discuss the dependence of the pattern amplitude on ramp and pre-cyc le parameters.