24 Works

CERN Yellow Reports: Conference Proceedings, Vol 2 (2018): Proceedings of "Tracking Collimation" ICFA Workshop

CERN
Beam collimation and machine protection have become essential aspects for modern high stored energy accelerators. The understanding of operating facilities and the performance extrapolations for future machines demands unprecedented accuracy in simulations of beam cleaning systems. This is critical for the Large Hadron Collider (LHC) and for its High-Luminosity (HL) upgrade project and for the Future Circular Collider (FCC) presently under study, but also for a number of other lower energy accelerators that operate with...

CERN Yellow Reports: Conference Proceedings, Vol 1 (2018): Proceedings of the ICFA Mini-Workshop "Impedances and Beam Instabilities in Particle Accelerators"

CERN
The Impedance of Flat Metallic Plates with Small Corrugations...5 Feedback Systems for Multibunch Beam Diagnostics and Instabilities Suppression...39 A rectangular Waveguide with Dielectric Coatings as a dedicated Wakefield Dechirper at ELBE...87 Analytical Impedance Models for Very Short Bunches...133 The Circulant Matrix Model and the Role of Beam-beam Effects in Coherent Instabilities...25 Studies for the SPS Travelling Wave Cavities Upgrade...57 Design of Low-Impact Impedance Devices: the new Proton Synchrotron Booster Absorber Scraper (PSBAS)...111 Observation and Damping...

Studies for the SPS Travelling Wave Cavities Upgrade

P. Kramer & C. Vollinger
The Super Proton Synchrotron\,(SPS) 200\,MHz accelerating system poses an intensity limitation for the planned High Luminosity\,(HL)-LHC upgrade due to excessive beam loading as well as higher-order modes\,(HOMs) contributing to a longitudinal multi-bunch instability. The mitigation of these HOMs together with a shortening of the cavities is therefore an essential part of the LHC Injectors Upgrade\,(LIU) project. A brief introduction to the accelerating structure as well as its present and future cavity configurations is given. First...

New Longitudinal Beam Impedance Formula with 8 Terms

O. E. Berrig & F. Paciolla
A previous formula for longitudinal beam impedance was developed by H. Tsutsui and consists of 13 terms. The formula by H. Tsutsui is developed up to terms of second order in the transverse offsets. In this report we develop a new formula, which use symmetries from the Lorentz reciprocity principle and the multipolar decomposition of transverse fields to reduce the number of terms from 13 to 8. This new formula is also developed up to...

The future of the E-P Instability in the SNS Accumulator Ring

N. Evans & M. Plum
Concerns about the e-p instability drove many decisions during the design of the accumulator ring for Oak Ridge’s Spallation Neutron Source. To date, these decisions seem justified since e-p activity has been observed at the mm scale as a broadband transverse excitation, but it has not been a limiting factor (or even a contributor to normal losses) during operation up to the design power of 1.4 MW. However, the proton power upgrade (PPU) [1] will...

Design of Low-Impact Impedance Devices: the new Proton Synchrotron Booster Absorber Scraper (PSBAS)

L. Teofili, M. Migliorati, I. Lamas Garcia & T. L. Rijoff
At CERN the HL-LHC (High Luminosity Large Hadron Collider) and the LIU (LHC Injection Upgrade) projects call for an increase in beam parameters such as energy, intensity and brightness. To achieve this goal the whole accelerator complex will be upgraded. Systems, equipment and devices need to be redesigned and rebuilt accounting for the demanding new beam features. In this framework device impedance is a key parameter. It is essential to evaluate and to minimize the...

A rectangular Waveguide with Dielectric Coatings as a dedicated Wakefield Dechirper at ELBE

F. Reimann & U. Van Rienen
Based on the Rayleigh-Ritz method and an eigenmode expansion of the wake function, we devised a semi-analytical description of the wakefield inside a dielectrically lined rectangular waveguide. We implemented this method and employed it to determine the geometrical parameters of a dedicated dechirper for the use at the light source ELBE in Dresden-Rossendorf. In 2016, a construction team at ELBE built a dielectrically lined rectangular waveguide with the given geometry. Subsequently, we carried out experiments...

A General Overview on the 2017 ICFA Mini-Workshop on Impedances and Beam Instabilities in Particle Accelerators

G. Rumolo, M. R. Masullo & S. Petracca
The ICFA mini-Workshop on impedances and beam instabilities in particle accelerators, which took place in Benevento (Italy) from 18 to 22 September 2017, was intended to continue the tradition of dedicated conferences on the subject of beam coupling Impedances in particle accelerators, initiated with the 2014 ICFA mini-Workshop on “Electromagnetic Wake Fields and Impedances in Particle Accelerators” held in Erice (organized by Vittorio Vaccaro, Maria Rosaria Masullo and Elias Métral). The aim of the event...

Vlasov Solvers and Macroparticle Simulations

N. Mounet
We review here two essential methods to evaluate growth rates of transverse coherent instabilities arising from beam-coupling impedance in a synchrotron, namely Vlasov equation solvers and tracking simulation of macroparticles. We will discuss the basics of the two methods, reviewing in particular the theoretical grounding of Vlasov solvers -- giving all the necessary formulas in the case of the DELPHI solver. We will then assess the advantages and limitations of the two methods, by showing...

Impedance and Instabilities in Hadron Machines

B. Salvant
Coherent beam instabilities related to impedance effects represent one of the main limitations to increasing beam intensity in circular accelerators. This contribution was presented at the ICFA mini-workshop on impedance and beam instabilities in particle accelerators on 18-22 September 2017 in Benevento, and aimed at providing a brief overview of the situation in circular hadron machines at the moment of the workshop.

Electron Cloud Effects

G. Iadarola & G. Rumolo
Secondary electron emission in combination with the elecromagnetic fields generated by a bunched beam can lead to the formation of electron clouds in the beam chambers of particle accelerators. The interaction of the circulating beam with the e-cloud can lead to transverse instabilities, beam losses, transverse emittance degradation. Moreover, the electrons impacting on the chamber's walls induce other unwanted effects like energy deposition (particularly critical for superconducting machines) and vacuum degradation. This contribution summarizes the...

Needs and Solutions for Machine Impedance Reduction

C. Vollinger, A. Farricker, T. R. Kaltenbacher, P. Kramer & B. K. Popovic
Particle beams with highest possible beam intensities are requested nowadays, hence in modern circular accelerators, the consideration of beam coupling impedance issues is of increasing relevance. Classical sources of beam coupling impedance are RF-systems, injection and extraction kickers, but also beam diagnostic elements such as wire scanners where the object itself forms an undesired cavity, and beam pipe transitions, namely tapers or steps. Optimally, for any machine, impedance mitigation shall take place already during the...

2D and 3D Collimator Impedance Modeling and Experimental Measurements

N. Biancacci, D. Amorim, S. Antipov, G. Mazzacano, B. Salvant, E. Metral, O. Frusciello, M. Zobov & A. Mostacci
In order to safely operate high energy particle accelerators, an advanced collimation system in terms of absorber material choice, alignment precision and low impedance impact is often required. In this work we will summarize the recent advancement on collimator impedance modeling starting from simple 2D structures, involving resistive jaws and transition tapers, to complex 3D structures, involving multilayer coatings, embedded beam position monitors, contact fingers, and jaw segmentation. Measurements techniques based on the stretched and...

The Circulant Matrix Model and the Role of Beam-beam Effects in Coherent Instabilities

X. Buffat
The role of beam-beam interactions in coherent instabilities in high energy colliders is discussed with a particular emphasis on the circulant matrix model. This model, based on the development of a one turn matrix including all linearised coherent forces, is particularly suited for the study of the stability of complex configurations involving different forces. Thus it allows for the study of interplays, e.g. between the effect of the beam-beam interactions and the beam coupling impedance....

Beam Transfer Function Measurements and Transverse Stability in presence of Beam-Beam

C. Tambasco, J. Barranco, T. Pieloni, L. Rivkin, X. Buffat & E. Metral
Instability thresholds are explored at the Large Hadron Collider (LHC) by means of the computation of the Landau Stability Diagram (SD). In the presence of diffusive mechanisms, caused by resonance excitations or noise, the SD can be reduced due to the modification of the particle distribution inside the beam. This effect can lead to a possible lack of Landau damping of the coherent modes previously damped by lying within the unperturbed SD area. The limitations...

Analytical Impedance Models for Very Short Bunches

I. Zagorodnov
We discuss several analytical models for impedances of very short bunches. The approximate analytical models are compared with direct solution of Maxwell's equations.

Feedback Systems for Multibunch Beam Diagnostics and Instabilities Suppression

A. Drago
Transverse instabilities in storage rings can limit both beam and single bunch currents. Vacuum chamber impedance can be source of instabilities for positive and negative charge stored beams and also parasitic e-clouds can produce other undesirable effects for positive charge stored beams. Transverse bunch-by-bunch feedback systems are implemented in storage rings as active devices for instability suppression and for state of art beam diagnostics. In this paper the following topics are discussed: basics on bunch-by...

Multiphysics Simulations of Impedance Effects in Accelerators

C. Zannini
Multiphysics is a computational discipline that involves multiple physical models. Nowadays commercial 3D simulation codes allow to couple electromagnetic and thermo-mechanical effects. Considering all the induced effects of the electromagnetic fields is fundamental in the development of electromagnetic devices. In this paper, the importance of Multiphysics simulations applied to beam coupling impedance is discussed.

The Impedance of Flat Metallic Plates with Small Corrugations

K. Bane
This report summarizes recent work on using a device composed of two flat, metallic plates with small corrugations (sometimes called a dechirper) at the end of linac-based, X-ray free electron lasers (FELs) for energy chirp control and as a passive fast kicker. We present an analytical theory of the wakefields/impedances of such dechirpers, followed by comparisons with numerical simulations, and comparisons with measurements at the Linac Coherent Light Source (LCLS). We show that agreement in...

Observation and Damping of Longitudinal Coupled-bunch Oscillations in the CERN PS

H. Damerau, A. Lasheen & M. Migliorati
Longitudinal coupled-bunch instabilities pose a major limitation to the intensity and quality of LHC-type beams in the CERN Proton Synchrotron (PS). The oscillations are excited above transition energy and mainly driven by the impedance of the main accelerating cavities at around 10 MHz. When approaching the flat-top these cavities are partly short-circuited. However, due to the sweeping revolution frequency, the spectral components of the beam move towards the resonances of the high frequency cavities at...

Modeling of fast Beam-Ion Instabilities

L. Mether, A. Oeftiger & G. Rumolo
Beam-induced ionisation of residual gas in the vacuum chamber generates ions, which in an electron machine can accumulate around a passing bunch train. If the density of trapped ions becomes sufficiently high, a fast beam-ion in- stability will be excited. The development of the instability can be prevented by keeping the pressure of the residual gas below a certain value. This contribution describes the mod- eling of fast beam-ion instabilities and presents simulation studies of...

Impedance Theory and Modeling

E. Metral
The “impedance” limits the performance of all the particle accelerators where the beam intensity (or beam brightness) is pushed, leading to beam instabilities and subsequent increased beam size and beam losses, and/or excessive (beam-induced RF) heating, which can deform or melt components or generate beam dumps. Each equipment of each accelerator has an impedance, which needs to be characterised and optimised. This impedance is usually estimated through theoretical analyses and/or numerical simulations before being measured...

The Birth and Childhood of a couple of Twin Brothers

V. Vaccaro
The context in which the concepts of Coupling Impedance and Universal Stability Charts were born is described in this paper. The conclusion is that the simultaneous appearance of these two concepts was unavoidable.

Effect of HOM Frequency Shift on Bunch Train Stability

J. Repond & E. Shaposhnikova
Future operation of the Large Hadron Collider (LHC) at CERN foresees an increase of the nominal luminosity by a factor of ten. The High-Luminosity LHC (HL-LHC) project necessitates increased intensities, good quality beam delivered by the injector chain. An important quality of the beam is its stability. In the LHC injector, the Super Proton Synchrotron (SPS), the longitudinal Coupled-Bunch Instability (CBI) during the acceleration ramp is a limiting factor in the production of high intensity...

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