Direct amplitude detuning measurement with ac dipole

Phys. Rev. ST Accel. Beams

Schmidt

et al. 2014

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The nonlinear dynamics of a circular accelerator such as the Large Hadron Collider (LHC) may significantly impact its performance. As the LHC progresses to more challenging regimes of operation it is to be expected that the nonlinear single particle dynamics in the transverse planes will play an increasing role in limiting the reach of the accelerator. As such it is vital that the nonlinear sources are well understood. The nonlinear fields of a circular accelerator may be probed through measurement of the amplitude detuning: the variation of tune with single particle emittance. This quantity may be assessed experimentally by exciting the beam to large amplitudes with kicks, and obtaining the tunes and actions from turn-by-turn data at Beam Position Monitors. The large amplitude excitations inherent to such a measurement also facilitate measurement of the dynamic aperture from an analysis of beam losses following the kicks. In 2012 these measurements were performed on the LHC Beam 2 at injection energy (450 GeV) with the nominal magnetic configuration. Nonlinear coupling was also observed. A second set of measurements were performed following the application of corrections for b 4 and b 5 errors. Analysis of the experimental results, and a comparison to simulation are presented herein.

Section: Ordermentioning

confidence: 99%

“…It has been observed in simulations of detuning with amplitude at top energy in the LHC, that the linear coupling could significantly influence the nonlinear phenomenology of the machine [23]. The quality of the linear coupling model is therefore a particular concern for study of the nonlinear dynamics.…”

Section: Modeling the Lhc Nonlinearitymentioning

confidence: 99%

Measurement of nonlinear observables in the Large Hadron Collider using kicked beams

Phys. Rev. ST Accel. Beams

Schmidt

et al. 2014

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“…It is well known that the particle motion in the presence of forced oscillations differs from the free motion following a single kick. This has led to methods which account for this difference in the motion when reconstructing the optics parameters [9][10][11][12]. However, these studies stay at linear order in the strength of perturbing components and can therefore not be used to predict the ADeCTA.…”

Section: Introductionmentioning

confidence: 99%

Suppression of amplitude dependent closest tune approach and its behavior under forced oscillations

Persson

Phys. Rev. Accel. Beams

2019

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An amplitude dependent closest tune approach (ADeCTA) was first observed in the LHC in 2012. This potentially harmful effect can be generated by linear coupling in conjunction with octupoles. In this article we demonstrate experimentally and in simulations a configuration of the octupoles to suppress this recently observed effect. Furthermore, ADeCTA is investigated for the first time with driven oscillations. We show analytically that skew quadrupoles and normal octupoles do not generate ADeCTA for forced oscillations in the same way as for free oscillations. This adds an additional constraint to measure it since the forced oscillations, generated by an ac dipole, are the preferred option for optics measurements in the LHC. The analytical result is confirmed by both measurements and simulations.

“…However, upon reduction of the LHC β Ã from 1 to 0.6 m in 2012, a significant degradation of the beam lifetime was observed [12]. Optics studies at injection [13] and top energy [14] also revealed larger than expected discrepancies between the measured and predicted first and second order amplitude detuning. Nonlinear errors in the triplets and separation dipoles are one of several candidates for the source of these reductions in performance.…”

Section: Introductionmentioning

confidence: 99%

First measurement and correction of nonlinear errors in the experimental insertions of the CERN Large Hadron Collider

Phys. Rev. ST Accel. Beams

Giovannozzi

et al. 2015

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Nonlinear magnetic errors in low-β insertions can contribute significantly to detuning with amplitude, linear and nonlinear chromaticity, and lead to degradation of dynamic aperture and beam lifetime. As such, the correction of nonlinear errors in the experimental insertions of colliders can be of critical significance for successful operation. This is expected to be of particular relevance to the LHC's second run and its high luminosity upgrade, as well as to future colliders such as the Future Circular Collider. Current correction strategies envisioned for these colliders assume it will be possible to calculate optimized local corrections through the insertions, using a magnetic model of the errors. This paper shows however, that reliance purely upon magnetic measurements of the nonlinear errors of insertion elements is insufficient to guarantee a good correction quality in the relevant low-β Ã regime. It is possible to perform beam-based examination of nonlinear magnetic errors via the feed-down to readily observed beam properties upon application of closed orbit bumps, and methods based upon feed-down to tune have been utilized at RHIC, SIS18, and SPS. This paper demonstrates the extension of such methodology to include direct observation of feed-down to linear coupling in the LHC. It is further shown that such beam-based studies can be used to complement magnetic measurements performed during LHC construction, in order to validate and refine the magnetic model of the collider. Results from first attempts of the measurement and correction of nonlinear errors in the LHC experimental insertions are presented. Several discrepancies of beam-based studies with respect to the LHC magnetic model are reported.