Observation of a periodic pattern in the persistent-current fields of the superconducting HERA magnets

During an experimental study of the time dependence of a HERA dipole, it was found that the sextupole field exhibits a sinusoidal structure along the axis of the magnet. A similar periodic structure was found for the main dipole field with the help of a nuclear magnetic resonance probe. The wavelength of the periodic pattern is compatible with the transposition pitch of the Rutherford-type cable in the magnet coils. The effect is related to superconducting currents in the cable because it vanishes when the magnet is warmed up above its critical temperature. The structure was found to exist in all HERA dipoles measured afterwards ( approximately 10) and also in a superconducting coil without iron yoke. With a specially developed 2-cm-long pickup coil, it was found that all accessible multipole components in dipole and quadrupole magnets are modulated along their axis.<>