Abstract
A superconducting particle accelerator like the LHC (Large Hadron Collider) at CERN, can only be controlled well if the effects of the magnetic field multipoles on the beam are compensated. The demands on a control system solely based on beam feedback may be too high for the requirements to be reached at the specified bandwidth and accuracy. Therefore, we designed a suitable field description for the LHC (FIDEL) as part of the machine control baseline to act as a feed-forward magnetic field prediction system. FIDEL consists of a physical and empirical parametric field model based on magnetic measurements at warm and in cryogenic conditions. The performance of FIDEL is particularly critical at injection when the field decays, and in the initial part of the acceleration when the field snaps back. These dynamic components are both current and time dependent and are not reproducible from cycle to cycle since they also depend on the magnet powering history. In this paper a qualitative and quantitative description of the dynamic field behavior substantiated by a set of scaling laws is presented.
Highlights
The baseline of the LHC control system includes feedforward control intended to reduce the burden on the beam based feedback
Known as the field description for the LHC (FIDEL) [1], this feed-forward system will predict the main field and the harmonics of the superconducting magnets during the whole machine operation cycle. This system is critical during the beam injection and the initial phase of the particle acceleration where the machine magnetic state is dynamic and its reproducibility is, to some extent, unknown
The magnetic field multipoles drift when the magnets are on a constant current plateau
Summary
The baseline of the LHC control system includes feedforward control intended to reduce the burden on the beam based feedback. Known as the field description for the LHC (FIDEL) [1], this feed-forward system will predict the main field and the harmonics of the superconducting magnets during the whole machine operation cycle This system is critical during the beam injection and the initial phase of the particle acceleration where the machine magnetic state is dynamic and its reproducibility is, to some extent, unknown. These dynamic field changes are not reproducible from one powering cycle to another and they are dependent on the powering history of the magnet [9] Extensive research on these phenomena has been done at the hadron electron ring facility (HERA) [10,11] and for the superconducting super collider (SSC) [12]. Results from other magnet productions are quoted, and, in particular, the experience at Fermilab on the Tevatron dipoles, to which CERN has participated actively, and from the HERA reference magnets that have been extensively exploited to steer and optimize operation
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