Multipoles Induced by Inter-Strand Coupling Currents in LARP <formula formulatype="inline"> <tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> Quadrupoles

Abstract

The U.S. LHC Accelerator Research Program has been developing Nb 3 Sn quadrupole magnets of progressively increasing performance and complexity for the High-Luminosity LHC project. The magnets are wound with Rutherford cables following the wind-and-react process. The resulting inter-strand coupling can generate strong field distortions during current ramp. The latest series of 120 mm aperture magnets (HQ) are designed and built for high field quality, offering an opportunity for detailed studies of these effects. Magnetic measurements of firstgeneration HQ magnets showed strong ramp-rate dependence. A stainless-steel core was introduced for the second generation of magnet coils to control the inter-strand coupling currents and the resulting dynamic multipoles. We report the observed dynamic effects and compare with calculations taking into account the coil geometry and cross-contact resistance in the Rutherford cable. In particular, the dependence of field quality on width and position of the stainless steel core is discussed.

Test Results of the LARP Nb3Sn Quadrupole HQ03a

“…Detailed analysis [11] led to critical revisions of the coil parameters and components which were validated through dedicated tests in a mirror structure [12][13] and gradually implemented in the HQ02 series [14]. The HQ02a model confirmed that the previous performance issues had been resolved, but its gradient was limited to 182 T/m due to the choice of a conservative pre-load level [15][16][17]. The next assembly, HQ02b, featured an increased pre-load and achieved a maximum gradient of 194 T/m at 1.9K, corresponding to 94% of the short sample limit [18][19].…”

Magnetic Analysis of the Nb3Sn Low-Beta Quadrupole for the High-Luminosity LHC

“…However, some non-allowed components (a3, a4, a7 and a8) show larger ramp rate dependence. They are a factor 10 times smaller than seen in HQ01 [14] (un-cored cable), a factor 5 times smaller than in HQ02 [15] (core coverage 60 %) and a factor 2 times larger than in the CERN 11 T [16] (core coverage 90 %). When ramping stops during the stair step measurements, multipole decay of the non-allowed harmonics with a stronger dependence on the ramp rate is observed at each measurement level.…”

Dipole Magnets above 20 Tesla: Research Needs for a Path via High-Temperature Superconducting REBCO Conductors

“…If a low electrical contact resistance is required (Section 3.4), would it lead to pronounced dynamic field errors that depend on the current ramp rates as observed in LTS magnets [181][182][183][184][185][186]? Measurements from recent Roebel and CORC R dipole magnets show no pronounced dynamic field errors [174,187].…”