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

Abstract

Transverse beam coupling impedance is a source of beam instabilities that limits the machine performance in circular accelerators. Several beam based techniques have been used to measure the transverse impedance of an accelerator, usually based on the optics distortion produced by the impedance source itself. Beam position monitor turn-by-turn analysis for impedance characterization has been usually employed in large circumference machines, while synchrotron light sources have mainly used slow orbit based techniques. Instead, the work presented in this paper uses for the first time turn-by-turn data at ALBA to advance the measurement technique into the range of the typically small impedance values of modern light sources. We have measured local impedance contributions through the observation of phase advance versus bunch charge using the betatron oscillations excited with a fast dipole kicker. The ALBA beam position monitor system and the precision of the turn-by-turn analysis allowed to characterize the main sources of transverse impedance, in good agreement with the model values, including the impedance of an in-vacuum undulator.

Review of linear optics measurement and correction for charged particle accelerators

“…For example, in HEPS the rms horizontal β-beating should be kept below 1.5% [142]. Developments in other disciplines like collective effects such as impedance, space charge and Touschek leading to particle loss also require improving the measurement and control of linear optics [48,[143][144][145][146][147].…”

“…An example is shown in Fig. 3 from [48], where a tune spread of 2 × 10 −6 among the 120 BPMs in one single acquisition is observed at the ALBA storage ring, whereas the spread over 100 acquisitions increases to about 10 −4 . This spread is compatible with the noise in the power supplies, which is specified to 0.01%.…”

“…IV E. Tune jitter measurement at ALBA. In a single acquisition, the rms tune spread over the 120 BPMs (in red) is only 2 × 10 −6 , but this increases to 10 −4 when the measurement is extended over 100 acquisitions due to the power supplies noise [48].…”

“…At ALBA, for example, the vertical normalized chromaticity during normal operation is large (∼4) to fight against beam instabilities. Therefore, linear optics using turn-by-turn methods are performed by changing the machine lattice from the operational one to a lattice with small (∼0) normalized chromaticity [48,55].…”

Analysis of beam position monitor requirements with Bayesian Gaussian regression

“…First, due to radiation damping, chromatic decoherence and nonlinearity, a disturbed bunched-beam trajectory is not a pure linear undamping betatron oscillation [18]. A reduced model (for example, assuming A is a constant), will introduce systematic er-rors [10,[19][20][21]. The second error source is the BPM TbT resolution limit, which results in random noise.…”