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Effects of Boundary-Induced Coupling Currents in LHC Dipoles During Ramping

This study examines the dynamic field effects in main LHC dipoles, focusing on the impact of boundary-induced coupling currents (BICCs) during ramping procedures. The generation of a field ripple, characterized by a period equivalent to the twist pitch length, is analyzed. We explore the decay of harmonics and the snapback phenomenon as current ramps resume, influenced by pre-cycle parameters such as flat-top current and duration. Results from a comprehensive parametric study of a model LHC magnet reveal the dependencies of decay and snapback associated with varying injection currents, showing significant harmonic effects with increasing current levels.

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Effects of Boundary-Induced Coupling Currents in LHC Dipoles During Ramping

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  1. Dynamic field effects in main LHC dipoles • During ramps, boundary-induced coupling currents (BICCs) are induced in the LHC cable creating a field ripple with period  twist pitch length • BICCs diffuse along the cable during stationary excitation with time constants  1000 s, leading to demagnetization of allowed harmonics of the order of a few units (decay) • As soon as current ramping is resumed, the field snaps back to (almost) the previous hysteresis curve with an exponential law vs. magnet current • Decay and snapback amplitudes depend to some extent upon all cycle parameters, mostly: - increase (~linearly) with pre-cycle flat-top current- increase with pre-cycle flat-top duration- decrease with pre-injection duration

  2. Decay and snapback in model LHC dipoles vs. injection current Parametric study carried out on 1 m long model magnet shows no clear evidence of the dependence of absolute harmonic effects vs Iinj

  3. Decay and snapback in main LHC dipoles vs. injection current Absolute B3 decay: reduction of 15% from 0.45 TeV to 1 TeV injection(influence of longer ramp to injection not taken into account)

  4. Decay and snapback in main LHC dipoles vs. injection current Normalized B3 decay: reduction of a factor 2.6 from 0.45 TeV to 1 TeV injection

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