Using ac dipoles to localize sources of beam coupling impedance

Abstract

The beam coupling impedance is one of the main sources of beam instabilities and emittance blow up in circular accelerators. A refined machine impedance evaluation is therefore required in order to understand and model intensity dependent effects and instabilities that may limit the machine performance. For this reason, many impedance source localization techniques have been developed. In this work we present the impedance localization technique based on the observation of phase advance versus intensity at the beam position monitors using ac dipoles to force betatron oscillations. We present analytical formulas for the interpretation of measurements together with simulations to benchmark and illustrate the equations. Studies on the method accuracy for different Fourier transform algorithms are presented as well as first exploratory measurements performed in the LHC.

Review of linear optics measurement and correction for charged particle accelerators

“…First order expansions on δk can be found in [25][26][27]. Higher order expressions of the β-beating can be found in [28] using an infinite determinant method and in [27] using the resonance driving terms (RDTs) f 2000 and f 0020 as where q 2000 represents the phase of the RDTat location s. An equivalent relation applies to the vertical plane, after replacing f 2000 with f 0020 .…”

“…The use of destructive single kicks would have required an unaffordable number of cycles. The ac dipole technique also has applications beyond linear optics measurements, extending into the evaluation of collective effects and nonlinear dynamics [26,[64][65][66].…”

“…Refined FTs as NAFF [70] and Sussix [71] are more accurate in the measurement of main frequencies than the regular fast Fourier transform (FFT). However, for the evaluation of the phase advance between 2 BPMs the FFT is preferred [26], due to the fact that the tunes measured at different BPMs fall into the same FFT bin. Alternatively, after accurately measuring the main tune with NAFF or Sussix at all BPMs, the phases can be computed via an FT by looking at the spectral line corresponding to the average tune.…”

First experimental demonstration of forced dynamic aperture measurements with LHC ac dipoles

“…The success and reliability of ac dipoles have allowed them to be used for new applications. The LHC ac dipoles have been used to measure the physical aperture of the collider to assist in collimator alignment [19], to measure amplitude detuning in the presence of head-on beam-beam interactions [20] as well as to measure the machine impedance [21]. More recently ac dipoles have been used in lightsources, such as at ESRF [22], PETRA III [23] and ALBA [24].…”

Local transverse coupling impedance measurements in a synchrotron light source from turn-by-turn acquisitions

“…While the first one has been the choice of reference in light sources like APS or Diamond [8,9], the turn-by-turn method has been mainly applied to larger circumference machines like SPS, RHIC or LHC [10][11][12][13]. This work reports on the use of turn-by-turn method for the impedance measurement in a synchrotron light source whose localized impedances are typically at least one order of magnitude smaller than those in the cited hadron and ion rings.…”