eeFACT2016 - Table of Session: TUT3AH (Impedance Issues and Beam Instabilities)

Paper	Title	Page
TUT3AH0	Low SEY Engineered Surface for Electron Cloud Eradication
	R. Valizadeh, O.B. Malyshev, B.S. Sian, S. Wang STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom M.D. Cropper Loughborough University, Loughborough, Leicestershire, United Kingdom B.S. Sian, G.X. Xia UMAN, Manchester, United Kingdom
	Developing a surface with low Secondary Electron Yield (SEY) is one of the principal methods of mitigating the beam-induced electron multipacting and electron cloud in high-energy charged particle accelerators. Since the wall material, surface chemistry, topography and electron energy are the parameters that influence the SEY, common mitigation mechanisms are based on engineering the above parameters. Recently ASTeC has demonstrated that nano- and microstructures engineered on Cu, Al and Stainless steel surface reduces SEY to less than 1. Such structures can be readily produced by nano- and sub-nanosecond pulsed laser. SEY can be further reduced to an even lower value by bake-out and/or photon and/or electron bombardment. A systematic analysis of surface composition and chemistry (using XPS), the surface topography (using SEM), and SEY measurements with primary electron energies ranging from 50 to 1000 eV is reported and correlated to the laser treatment conditions. A test liner was prepared by ASTeC for a first machine test of such new anti-e-cloud technique. The liner has been installed at SPS for testing in the LHC-like beam conditions.
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TUT3AH1	Collective Effects Issues for FCC-ee	100
	M. Migliorati University of Rome La Sapienza, Rome, Italy E. Belli CERN, Geneva, Switzerland G. Castorina, B. Spataro, M. Zobov INFN/LNF, Frascati (Roma), Italy S. Persichelli LBNL, Berkeley, California, USA
	The Future Circular Collider study, hosted by CERN to design post-LHC particle accelerator options in a worldwide context, represents a great challenge under several aspects, which require R&D on beam dynamics and new technologies. One very critical point is represented by collective effects, generated by the interaction of the beam with self-induced electromagnetic fields, called wake fields, which could produce beam instabilities, thus reducing the machines performance and limiting the maximum stored current. It is therefore very important to be able to predict these effects and to study in detail potential solutions to counteract them. In this paper the resistive wall and some other important geometrical sources of impedance for the FCC electron-positron accelerator are identified and evaluated, and their impact on the beam dynamics, which could lead to unwanted instabilities, is discussed.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH1
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TUT3AH2	Instability Issues in CEPC	108
	N. Wang, W. Chou, J. Gao, D.J. Gong, J. He, Q. Qin, Y.S. Sun, D. Wang, Y. Wang, G. Xu, Y. Zhang, H.J. Zheng IHEP, Beijing, People's Republic of China K. Ohmi, D. Zhou KEK, Ibaraki, Japan
	The CEPC is a high energy circular electron-positron collider under design. Large bunch population is required to achieve the design luminosity. Instabilities driven by the coupling impedance are possible limitations for reaching high machine performance. An updated impedance model, including the resistive wall and the main vacuum components, has been obtained for the main ring. Based on the impedance model, the collective instability issues of the beam with the partial-double ring design are discussed.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH2
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TUT3AH3	Beam-based Impedance Measurement Techniques	112
	V.V. Smaluk BNL, Upton, Long Island, New York, USA
	Funding: Work supported by DOE under contract No.DE-AC02- 98CH10886 Characterization of a vacuum chamber impedance is necessary to estimate stability conditions of a particle beam motion, to find a limit of the beam intensity and characteristic times of single-bunch and multi-bunch instabilities. For new accelerator projects, minimization of the vacuum chamber impedance is now the mandatory requirement. For an accelerator in operation, the impedance can be measured experimentally using various beam-based techniques. The beam-impedance interaction manifests itself in measurable beam parameters, such as betatron tunes, closed orbit, growth rates of instabilities, bunch length and synchronous phase. The beam-based techniques developed for measurement of the longitudinal and transverse impedance are discussed, including theoretical basics and experimental results.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH3
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TUT3AH4	Coherent Wave Excitation in a High Current Storage Ring
	A. Novokhatski SLAC, Menlo Park, California, USA
	Funding: Work supported by the U.S. Department of Energy under contract number DE-AC02-76SF00515 We will discuss the fine structure of the forces acting on a particle in a bunch moving in a storage ring. These forces are due to coherent electromagnetic fields, which are excited by a charged bunch, moving along a curved path; or due to diffraction of its own electromagnetic field on irregularities of the metal beam pipe. Transverse forces have much in common, in spite of the different way of excitation. The strength of coherent fields is proportional to the square of the number of particles in the bunch. In case of a large beam current backward reaction of these fields will play an important role in the dynamics of the beam on one side and on the other side it leads to heating of the various elements of the beam pipe. Some part of these fields, electromagnetic waves with frequencies above the cut-out frequency can propagate over long distances in the ring and to be absorbed by some materials with low conductivity. Without proper cooling, the temperature rise can be so great that small pieces of metal (e.g., shielding fingers) can be melted. Additional effects can the spark and increasing the vacuum pressure.
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TUT3AH5	Electron Cloud and Ion Effects and Their Mitigation in FCC-ee	120
	K. Ohmi, H. Fukuma, M. Tobiyama KEK, Ibaraki, Japan
	Electron cloud and fast-ion instabilities are serious issues for colliders operated with high current and many bunches. We discuss the instabilities in FCC-ee/hh based on simulations and experiences of KEKB/SuperKEKB.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH5
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TUT3AH6	Electron Cloud at SuperKEKB	125
	H. Fukuma, K. Ohmi, Y. Suetsugu, M. Tobiyama KEK, Ibaraki, Japan
	Several countermeasures such as ante-chambers, TiN coating, clearing electrodes, grooved structure and so on, have been taken to mitigate the electron cloud effects in the SuperKEKB positron ring. During phase 1 operation of SuperKEKB, which continued from this February to June, the electron cloud effects such as beam size blow-up, non-linear pressure increase, betatron tune shift along a bunch train and transverse coupled bunch instability were observed. Permanent magnets attached at bellows that generate longitudinal magnetic field in vacuum chambers were effective to reduce the electron cloud. This talk will cover following subjects about the electron cloud at SuperKEKB, 1) mitigation methods against the electron cloud, 2) observation of the electron cloud effects in phase 1 operation and 3) plan of further reduction of the electron cloud toward phase 2 operation which will start in the late FY2017.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH6
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TUT3AH7	Electron Cloud and Collective Effects in the Interaction Region of FCC-ee	130
	E. Belli, G. Rumolo CERN, Geneva, Switzerland E. Belli University of Rome "La Sapienza", Rome, Italy E. Belli INFN-Roma, Roma, Italy M. Migliorati University of Rome La Sapienza, Rome, Italy M. Migliorati INFN-Roma1, Rome, Italy
	The FCC-ee is an e⁺e⁻ circular collider designed to accommodate four different experiments in a beam energy range from 91 to 350 GeV and is a part of the Future Circular Collider (FCC) project at CERN. One of the most critical aspects of this new very challenging machine regards the collective effects which can produce instabilities, thus limiting the accelerator operation and reducing its performance. The following studies are focused on the Interaction Region of the machine. This talk will present preliminary simulation results of the power loss due to the wake fields generated by the electromagnetic interaction of the beam with the vacuum chamber. A preliminary estimation of the electron cloud build-up is also reported, whose effects have been recognized as one of the main limitations for the Large Hadron Collider at CERN.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH7
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TUT3AH8	An Overview of Active Coupled-bunch Instability Control
	D. Teytelman Dimtel, San Jose, USA
	Feedback control of coupled bunch instabilities has become an ubiquitous feature of modern electron and positron storage rings. Instabilities due to impedances (resistive wall, RF cavity fundamental and higher order modes, resonant modes elsewhere in the ring) as well as effects from ions and electron cloud have all been successfully suppressed by such systems. But feedback is not a panacea, like any physical system it has internal dynamics and noise which limit its effectiveness. With the new aggressive machine designs, it is especially important to understand these limitations. In this work I will describe the topology of the modern instability control system, emphasizing the limiting factors. Whenever possible, simplified scaling relations will be introduced in order to help in the machine design process. Presentation of the current methods will be followed by a discussion of promising future research directions that should help extend the reach of such feedback control.
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TUT3AH9	Feedback Systems for FCC-ee	136
	A. Drago INFN/LNF, Frascati (Roma), Italy
	In this paper, some preliminary considerations on the feedback systems for FCC-ee are developed. Bunch-by-bunch feedback systems have been de-signed in the last years for other e⁺/e⁻ colliders like PEP-II, KEKB, DAΦNE, SuperB and SuperKEKB. In all these cases, similar approaches have been implement-ed, even if some design variations have been suitable or necessary for different reasons. Bunch-by-bunch feedback systems are based on the concept that the barycenter of each bunch moves with harmonic motion around the equilibrium point in three planes (L, H, V). The feedback copes with the forcing excitation by producing damping correction for each individual bunch. This is possible managing every single bunch by a dedicated processing channel in real time. For FCC-ee the very high number of stored bunches re-quires much more power in terms of processing capa-bility for the feedback systems. Ring length (100 Km) and very low fractional tunes must be also considered requiring for a more effective strategy in the feedback system design.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH9
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Paper

Title

Page

TUT3AH0

Low SEY Engineered Surface for Electron Cloud Eradication

R. Valizadeh, O.B. Malyshev, B.S. Sian, S. Wang
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
M.D. Cropper
Loughborough University, Loughborough, Leicestershire, United Kingdom
B.S. Sian, G.X. Xia
UMAN, Manchester, United Kingdom

Developing a surface with low Secondary Electron Yield (SEY) is one of the principal methods of mitigating the beam-induced electron multipacting and electron cloud in high-energy charged particle accelerators. Since the wall material, surface chemistry, topography and electron energy are the parameters that influence the SEY, common mitigation mechanisms are based on engineering the above parameters. Recently ASTeC has demonstrated that nano- and microstructures engineered on Cu, Al and Stainless steel surface reduces SEY to less than 1. Such structures can be readily produced by nano- and sub-nanosecond pulsed laser. SEY can be further reduced to an even lower value by bake-out and/or photon and/or electron bombardment. A systematic analysis of surface composition and chemistry (using XPS), the surface topography (using SEM), and SEY measurements with primary electron energies ranging from 50 to 1000 eV is reported and correlated to the laser treatment conditions. A test liner was prepared by ASTeC for a first machine test of such new anti-e-cloud technique. The liner has been installed at SPS for testing in the LHC-like beam conditions.

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUT3AH1

Collective Effects Issues for FCC-ee

100

M. Migliorati
University of Rome La Sapienza, Rome, Italy
E. Belli
CERN, Geneva, Switzerland
G. Castorina, B. Spataro, M. Zobov
INFN/LNF, Frascati (Roma), Italy
S. Persichelli
LBNL, Berkeley, California, USA

The Future Circular Collider study, hosted by CERN to design post-LHC particle accelerator options in a worldwide context, represents a great challenge under several aspects, which require R&D on beam dynamics and new technologies. One very critical point is represented by collective effects, generated by the interaction of the beam with self-induced electromagnetic fields, called wake fields, which could produce beam instabilities, thus reducing the machines performance and limiting the maximum stored current. It is therefore very important to be able to predict these effects and to study in detail potential solutions to counteract them. In this paper the resistive wall and some other important geometrical sources of impedance for the FCC electron-positron accelerator are identified and evaluated, and their impact on the beam dynamics, which could lead to unwanted instabilities, is discussed.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH1

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reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUT3AH2

Instability Issues in CEPC

108

N. Wang, W. Chou, J. Gao, D.J. Gong, J. He, Q. Qin, Y.S. Sun, D. Wang, Y. Wang, G. Xu, Y. Zhang, H.J. Zheng
IHEP, Beijing, People's Republic of China
K. Ohmi, D. Zhou
KEK, Ibaraki, Japan

The CEPC is a high energy circular electron-positron collider under design. Large bunch population is required to achieve the design luminosity. Instabilities driven by the coupling impedance are possible limitations for reaching high machine performance. An updated impedance model, including the resistive wall and the main vacuum components, has been obtained for the main ring. Based on the impedance model, the collective instability issues of the beam with the partial-double ring design are discussed.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH2

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reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUT3AH3

Beam-based Impedance Measurement Techniques

112

V.V. Smaluk
BNL, Upton, Long Island, New York, USA

Funding: Work supported by DOE under contract No.DE-AC02- 98CH10886
Characterization of a vacuum chamber impedance is necessary to estimate stability conditions of a particle beam motion, to find a limit of the beam intensity and characteristic times of single-bunch and multi-bunch instabilities. For new accelerator projects, minimization of the vacuum chamber impedance is now the mandatory requirement. For an accelerator in operation, the impedance can be measured experimentally using various beam-based techniques. The beam-impedance interaction manifests itself in measurable beam parameters, such as betatron tunes, closed orbit, growth rates of instabilities, bunch length and synchronous phase. The beam-based techniques developed for measurement of the longitudinal and transverse impedance are discussed, including theoretical basics and experimental results.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH3

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reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUT3AH4

Coherent Wave Excitation in a High Current Storage Ring

A. Novokhatski
SLAC, Menlo Park, California, USA

Funding: Work supported by the U.S. Department of Energy under contract number DE-AC02-76SF00515
We will discuss the fine structure of the forces acting on a particle in a bunch moving in a storage ring. These forces are due to coherent electromagnetic fields, which are excited by a charged bunch, moving along a curved path; or due to diffraction of its own electromagnetic field on irregularities of the metal beam pipe. Transverse forces have much in common, in spite of the different way of excitation. The strength of coherent fields is proportional to the square of the number of particles in the bunch. In case of a large beam current backward reaction of these fields will play an important role in the dynamics of the beam on one side and on the other side it leads to heating of the various elements of the beam pipe. Some part of these fields, electromagnetic waves with frequencies above the cut-out frequency can propagate over long distances in the ring and to be absorbed by some materials with low conductivity. Without proper cooling, the temperature rise can be so great that small pieces of metal (e.g., shielding fingers) can be melted. Additional effects can the spark and increasing the vacuum pressure.

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TUT3AH5

Electron Cloud and Ion Effects and Their Mitigation in FCC-ee

120

K. Ohmi, H. Fukuma, M. Tobiyama
KEK, Ibaraki, Japan

Electron cloud and fast-ion instabilities are serious issues for colliders operated with high current and many bunches. We discuss the instabilities in FCC-ee/hh based on simulations and experiences of KEKB/SuperKEKB.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH5

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reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUT3AH6

Electron Cloud at SuperKEKB

125

H. Fukuma, K. Ohmi, Y. Suetsugu, M. Tobiyama
KEK, Ibaraki, Japan

Several countermeasures such as ante-chambers, TiN coating, clearing electrodes, grooved structure and so on, have been taken to mitigate the electron cloud effects in the SuperKEKB positron ring. During phase 1 operation of SuperKEKB, which continued from this February to June, the electron cloud effects such as beam size blow-up, non-linear pressure increase, betatron tune shift along a bunch train and transverse coupled bunch instability were observed. Permanent magnets attached at bellows that generate longitudinal magnetic field in vacuum chambers were effective to reduce the electron cloud. This talk will cover following subjects about the electron cloud at SuperKEKB, 1) mitigation methods against the electron cloud, 2) observation of the electron cloud effects in phase 1 operation and 3) plan of further reduction of the electron cloud toward phase 2 operation which will start in the late FY2017.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH6

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reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUT3AH7

Electron Cloud and Collective Effects in the Interaction Region of FCC-ee

130

E. Belli, G. Rumolo
CERN, Geneva, Switzerland
E. Belli
University of Rome "La Sapienza", Rome, Italy
E. Belli
INFN-Roma, Roma, Italy
M. Migliorati
University of Rome La Sapienza, Rome, Italy
M. Migliorati
INFN-Roma1, Rome, Italy

The FCC-ee is an e⁺e⁻ circular collider designed to accommodate four different experiments in a beam energy range from 91 to 350 GeV and is a part of the Future Circular Collider (FCC) project at CERN. One of the most critical aspects of this new very challenging machine regards the collective effects which can produce instabilities, thus limiting the accelerator operation and reducing its performance. The following studies are focused on the Interaction Region of the machine. This talk will present preliminary simulation results of the power loss due to the wake fields generated by the electromagnetic interaction of the beam with the vacuum chamber. A preliminary estimation of the electron cloud build-up is also reported, whose effects have been recognized as one of the main limitations for the Large Hadron Collider at CERN.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH7

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUT3AH8

An Overview of Active Coupled-bunch Instability Control

D. Teytelman
Dimtel, San Jose, USA

Feedback control of coupled bunch instabilities has become an ubiquitous feature of modern electron and positron storage rings. Instabilities due to impedances (resistive wall, RF cavity fundamental and higher order modes, resonant modes elsewhere in the ring) as well as effects from ions and electron cloud have all been successfully suppressed by such systems. But feedback is not a panacea, like any physical system it has internal dynamics and noise which limit its effectiveness. With the new aggressive machine designs, it is especially important to understand these limitations. In this work I will describe the topology of the modern instability control system, emphasizing the limiting factors. Whenever possible, simplified scaling relations will be introduced in order to help in the machine design process. Presentation of the current methods will be followed by a discussion of promising future research directions that should help extend the reach of such feedback control.

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUT3AH9

Feedback Systems for FCC-ee

136

A. Drago
INFN/LNF, Frascati (Roma), Italy

In this paper, some preliminary considerations on the feedback systems for FCC-ee are developed. Bunch-by-bunch feedback systems have been de-signed in the last years for other e⁺/e⁻ colliders like PEP-II, KEKB, DAΦNE, SuperB and SuperKEKB. In all these cases, similar approaches have been implement-ed, even if some design variations have been suitable or necessary for different reasons. Bunch-by-bunch feedback systems are based on the concept that the barycenter of each bunch moves with harmonic motion around the equilibrium point in three planes (L, H, V). The feedback copes with the forcing excitation by producing damping correction for each individual bunch. This is possible managing every single bunch by a dedicated processing channel in real time. For FCC-ee the very high number of stored bunches re-quires much more power in terms of processing capa-bility for the feedback systems. Ring length (100 Km) and very low fractional tunes must be also considered requiring for a more effective strategy in the feedback system design.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-eeFACT2016-TUT3AH9

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)