Investigation of the periodic magnetic field modulation in LHC superconducting dipoles

Abstract

Abstract-The windings of high-field accelerator magnets are usually made of Rutherford-type superconducting 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. This effect, resulting from quasipersistent currents, was investigated with a Hall probes array inserted inside the aperture of the LHC superconducting dipoles, both in short models and full-scale prototypes. The amplitude and the time dependence of this periodic field oscillation have been studied as a function of the magnet current history. The origin and the impact on the LHC dipoles stability of the nonuniform current redistribution producing such a field modulation are discussed.Index Terms-LHC, periodic field pattern, persistent eddy-current, quench performance, Rutherford type cable, superconducting magnets.

Performance of the LHC final prototype and first pre-series superconducting dipole magnets

“…Results show significant improvements in this respect as compared to the second generation of prototype magnets, thanks to the strand coating (SnAg ) and oxidation process developed at CERN for the third generation prototypes, pre-series, and series magnets [12]. The control of the contact resistances between strands of the cable is also confirmed by loss measurements, ramp rate effect on the field quality [13] and by the amplitude of the periodic field modulation measured inside apertures of the third generation prototypes and first pre-series dipoles [14].…”

Study of time dependent magnetic field variation due to current redistribution in Rutherford cable

“…In various magnets for accelerators, periodic patterns and time variations were reported [3]- [7]. Time-varying periodic patterns suggest that imbalance currents redistribute, and induce local magnetic field variations, which cause magnetization change.…”

Conductor Limited Quenches of LHC Superconducting Main Dipoles

“…To interpret results and to determine the amplitude of nonuniform current distribution with long time constants in some reference cases, this experimental investigation has been associated to local field measurements using Hall plates [7], [8]. Effects of Boundary Induced Coupling Currents (BICC) were extensively studied at CERN [9] and offer an explanation for the difference between DRQ and VRQ results.…”

“…4 shows a typical example of the effect of the powering history on the quench currents. VRQ gives a larger mainly because the V-precycle reduced the BICC whereas the heating due to uniform current distribution is nearly the same in both cases [7]- [9]. One of the main parameters needed to interpret these results is the cross-contact resistance between strands of the Rutherford cable.…”