Fast-slow mode coupling instability for coasting beams in the presence of detuning impedance

Biancacci, Nicolò; Métral, E.; Migliorati, M.

doi:10.1103/physrevaccelbeams.23.124402

Cited by 9 publications

(16 citation statements)

References 10 publications

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“…For this, the sign of chromaticity should be carefully chosen, to add to the contribution through the slippage factor and not to subtract, and this requires a sign of the chromaticity equal to the sign of the slippage factor, such that the chromatic frequency is positive: this is why the sign of the chromaticity is changed when transition energy is crossed in some machines. The case when modes couple is currently under investigations [10][11][12][13].…”

Section: Chromaticitymentioning

confidence: 99%

“…This led to several new instability mechanisms such as the destabilising effect of resistive transverse dampers [6] and the losses of transverse Landau damping due to linear coupling [7], noise [8], and beam-beam [9]. However, even some more basic mechanisms such as the transverse mode coupling instability (TMCI) for coasting beams were never discussed or predicted in the past [10][11][12][13], and therefore, there is still a lot to be done, despite the huge amount of work and results of the last decades.…”

Section: Introductionmentioning

confidence: 99%

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General mitigation techniques for coherent beam instabilities in particle accelerators

Métral

2021

Eur. Phys. J. Plus

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An important number of coherent beam instability mechanisms can be observed in a particle accelerator, depending if the latter is linear or circular, operated at low, medium or high energy, with a small or a huge amount of turns (for circular machines), close to transition energy or not (below or above), with only one bunch or many bunches, with counter-rotating beams (such as in colliders) or not, if the beam is positively or negatively charged, if one is interested in the longitudinal plane or in the transverse plane, in the presence of linear coupling between the transverse planes or not, in the presence of nonlinearities or not, in the presence of noise or not, etc. Building a realistic impedance model of a machine is a necessary step to be able to evaluate the machine performance limitations, identify the main contributors in case an impedance reduction is required, and study the interaction with other mechanisms such as optics (linear and nonlinear), RF gymnastics, transverse damper, noise, space charge, electron cloud, and beam–beam (in a collider). Better characterising an instability is the first step before trying to find appropriate mitigation measures and push the performance of a particle accelerator, as some mitigation methods are beneficial for some effects and detrimental for some others. For this, an excellent instrumentation is of paramount importance to be able to diagnose if the instability is longitudinal or transverse, single bunch, or coupled bunch, involving only one mode of oscillation or several, and the evolution of the intrabunch motion with intensity is a fundamental observable with high-intensity high-brightness beams. Finally, among the possible mitigation methods of coherent beam instabilities, the ones perturbing the least the single-particle motion (leading to the largest necessary dynamic aperture and beam lifetime) and easiest to implement for day-to-day operation in the machine control room should be preferred.

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Section: Chromaticitymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

General mitigation techniques for coherent beam instabilities in particle accelerators

Métral

2021

Eur. Phys. J. Plus

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“…The mode coupling instability for coasting beams was first discussed in a recent paper [1], with a four-step approach. First, we extended the pyHeadTail simulation code to simulate transverse (and longitudinal) coherent instabilities for coasting beams.…”

Section: Introductionmentioning

confidence: 99%

“…Second, to gain confidence in what had been done, we first benchmarked the new simulations with the classical transverse coasting beam approach from Laclare [2] and an excellent agreement was reached, as can be seen in Figs. 3 of [1], for both the real and imaginary parts of the complex tune shift. Third, we introduced the detuning impedance in pyHeadTail simulations with coasting beams and found that the results did not agree anymore with the classical approach.…”

Section: Introductionmentioning

confidence: 99%

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Self-consistent derivation of the transverse mode coupling instability for coasting beams using the linearized Vlasov equation

Biancacci,

Métral,

Migliorati

2021

Preprint

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The mode coupling instability for coasting beams has been discussed in a previous paper using macroparticle tracking simulations from the pyHeadTail code and a simple analytical formula which was proposed as an extension of the ansatz used for the single-particle formalism. In this paper, we propose a self-consistent derivation of this formula based on the linearized Vlasov equation. The proposed mode coupling instability for coasting beams was never predicted or discussed in the past and we believe that the reason is twofold. First, to derive it analytically from the linearized Vlasov equation, one should not make the usual approximation sin(φ) (e jφ )/(2j), where φ is the transverse betatron phase, but really consider the two terms of sin(φ) = (e jφ − e −jφ )/(2j) as the second term is the one responsible for the mode coupling in coasting beams. It should be stressed here that mode coupling is found already with driving impedance only. Note that the previous approximation is also usually made for bunched beams and this case should therefore also be carefully reviewed in the future. Second, by including the detuning impedance, the coupling is much stronger and this is what we found also in pyHeadTail simulations.

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Wake field, impedance and collective instability

Métral¹

2021

J. Inst.

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The first mention of the impedance concept appeared on November 1966 in the CERN internal report Longitudinal instability of a coasting beam above transition, due to the action of lumped discontinuities by V.G. Vaccaro. Then, a more general treatment of it appeared in February 1967 in the CERN yellow report Longitudinal instabilities of azimuthally uniform beams in circular vacuum chambers of arbitrary electrical properties by A.M. Sessler and V.G. Vaccaro. The concept of wake field came two years later, in 1969, in the paper The wake field of an oscillating particle in the presence of conducting plates with resistive terminations at both ends by A.G. Ruggiero and V.G. Vaccaro. This was the beginning of many studies, which took place over the last five decades, and today, impedances and wake fields continue to be an important field of activity, as concerns theory, simulation, bench and beam-based measurements. Building a reliable impedance or wake field model of a machine is the first necessary step to be able to evaluate the machine performance limitations, identify the main contributors in case an impedance reduction is required, and study the interaction with other mechanisms such as optics nonlinearities, transverse damper, noise, space charge, electron cloud, beam-beam (in a collider), etc. Beam collective instabilities, and their mitigation, cover a wide range of effects in particle accelerators and they have been the subjects of intense research. As the machines performance was pushed new mechanisms were revealed and nowadays the challenge consists in studying the interplays between all these intricate phenomena, as it is very often not possible to treat the different effects separately. With the increasing power of our computers this becomes easier but the need to continue and develop theories remains, to have a better understanding of the interplays between all these effects: the subject of impedance and beam instabilities in particle accelerators is far from being exhausted, as testified by the many new instability and stabilizing mechanisms which have been recently explained or discovered. Furthermore, in the context of the studies for possible future accelerators, some uncharted territories remain such as, for instance, the collective instabilities during the necessary ionization cooling for a muon collider.

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Fast-slow mode coupling instability for coasting beams in the presence of detuning impedance

Cited by 9 publications

References 10 publications

General mitigation techniques for coherent beam instabilities in particle accelerators

General mitigation techniques for coherent beam instabilities in particle accelerators

Self-consistent derivation of the transverse mode coupling instability for coasting beams using the linearized Vlasov equation

Wake field, impedance and collective instability

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