IPAC2016 - Classification: 05 Beam Dynamics and Electromagnetic Fields / D06 Coherent and Incoherent Instabilities

Paper	Title	Page
TUOAB02	Conditions for CSR Microbunching Gain Suppression	1057
SUPSS056	use link to see paper's listing under its alternate paper code
	C.-Y. Tsai Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA S. Di Mitri Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy D. Douglas, R. Li, C. Tennant JLab, Newport News, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. The coherent synchrotron radiation (CSR) of a high brightness electron beam traversing a series of dipoles, such as transport arcs, may result in phase space degradation. On one hand, the CSR can perturb electron transverse motion in dispersive regions along the beamline, causing emittance growth. On the other hand, the CSR effect on the longitudinal beam dynamics could result in microbunching gain enhancement. For transport arcs, several schemes have been proposed* to suppress the CSR-induced emittance growth. Similarly, several scenarios have been introduced** to suppress CSR-induced microbunching gain, which however mostly aim for linac-based machines. In this paper we try to provide sufficient conditions for suppression of CSR-induced microbunching gain along a transport arc, analogous to. Several example lattices are presented, with the relevant microbunching analyses carried out by our semi-analytical Vlasov solver*. The simulation results show that lattices satisfying the proposed conditions indeed have microbunching gain suppressed. We expect this analysis can shed light on lattice design approach that could suppress the CSR-induced microbunching gain. D.Douglas et al, JLAB-ACP-14-1751, S.DiMitri et al, PRL (2013), R.Hajima, NIMA (2004), Y.Jiao et al, PRSTAB (2014) Z.Huang et al, PRSTAB (2004), Saldin et al, NIMA (2004) *C.Tsai et al, FEL'15
	Slides TUOAB02 [6.484 MB]
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TUPMW011	Current Status of Instability Threshold Measurements in the LHC at 6.5 TeV	1434
	L.R. Carver, J. Barranco, N. Biancacci, X. Buffat, W. Höfle, G. Kotzian, T. Lefèvre, T.E. Levens, E. Métral, T. Pieloni, B. Salvant, C. Tambasco CERN, Geneva, Switzerland N. Wang IHEP, Beijing, People's Republic of China M. Zobov INFN/LNF, Frascati (Roma), Italy
	Throughout 2015, many measurements of the minimum stabilizing octupole current required to prevent coherent transverse instabilities have been performed. These measurements allow the LHC impedance model at flat top to be verified and give good indicators of future performance and limitations. The results are summarized here, and compared to predictions from the simulation code DELPHI.
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TUPOR006	Systematic Studies of Short Bunch-Length Bursting at ANKA	1662
	M. Brosi, E. Blomley, E. Bründermann, N. Hiller, B. Kehrer, A.-S. Müller, M. Schedler, M. Schuh, P. Schönfeldt, J.L. Steinmann KIT, Karlsruhe, Germany
	Funding: Supported by the German Federal Ministry of Education and Research (05K13VKA), the Helmholtz Association (VH-NG-320) and by the Helmholtz International Research School for Teratronics (HIRST). At ANKA, the Karlsruhe synchrotron radiation source, the so called short bunch-length operation mode allows the reduction of the bunch length down to a few picoseconds. The micro-bunching instability resulting from the high degree of longitudinal compression leads to fluctuations in the emitted intensity in the THz regime, referred to as bursting. For extremely compressed bunches at ANKA bursting also occurs, in a certain current range, below the main bursting threshold. This contribution shows measurements of this short bunch-length bursting and makes first comparisons with theory.
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TUPOR009	Single Bunch Longitudinal Instability in the CERN SPS	1670
SUPSS055	use link to see paper's listing under its alternate paper code
	A. Lasheen, T. Bohl, S. Hancock, T. Roggen, E.N. Shaposhnikova CERN, Geneva, Switzerland E. Radvilas Gediminas Technical University, Vilnius, Lithuania
	The longitudinal single bunch instability observed in the SPS leads to uncontrolled emittance blow-up and limits the quality of high intensity beams required for the High Luminosity LHC and AWAKE projects at CERN. The present SPS impedance model developed from a thorough survey of machine elements was used in macro-particle simulations (with the code BLonD) of the bunch behavior through the acceleration cycle. Comparison of simulations with measurements of the synchrotron frequency shift, performed on the SPS flat bottom to probe the impedance, show a reasonable agreement. During extensive experimental studies various beam and machine parameters (bunch intensity, longitudinal emittance, RF voltage, with single and double RF systems) were scanned in order to further benchmark the SPS impedance model with measurements and to better understand the mechanism behind the instability. It was found that the dependence of instability threshold on longitudinal emittance and beam energy has an unexpected non-monotonic behavior, leading to islands of (in)stability. The results of this study are presented and can be used to define possible parameter settings for the future CERN projects.
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TUPOR013	Analysis of Multi-bunch Instabilities at the Diamond Storage Ring	1685
	R. Bartolini, R.T. Fielder, G. Rehm DLS, Oxfordshire, United Kingdom V.V. Smaluk BNL, Upton, Long Island, New York, USA
	We present recent results of analytical, numerical and experimental analysis of multi-bunch instabilities at the Diamond storage ring. The works compares the impedance estimates from numerical modelling with the analysis of the growth rates of the excited multi-bunch modes in different machine configurations. The contribution of a number of wakefield sources has been identified with very high precision thanks to high quality data provided by the existing Transverse multi-bunch feedback diagnostics
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TUPOR020	Combination of Density and Energy Modulation in Microbunching Analysis	1703
	C.-Y. Tsai Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA R. Li JLab, Newport News, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Microbunching instability (MBI) has been one of the most challenging issues in the transport of high-brightness electron beams for modern recirculating or energy recovery linac machines. Recently we have developed and implemented a Vlasov solver* to calculate microbunching gain for an arbitrary beamline lattice design, based on the extension of early theoretical formulation for the microbunching amplification from an initial density perturbation to the final density modulation. For more thorough analyses, in addition to the case of (initial) density to (final) density amplification, we in this paper extend the previous formulation to more general cases, including energy-to-density, density-to-energy and energy-to-energy amplifications for a recirculation machine. Such semi-analytical formulae are then incorporated into our Vlasov solver, and reasonable agreement is obtained when the semi-analytical results are benchmarked with particle tracking simulation using ELEGANT. C.Y. Tsai et al, FEL'15 S. Heifets et al, PRSTAB 5, 064401 (2002), Z. Huang and K. Kim, PRSTAB 5, 074401 (2002), M. Vneturini, PRSTAB 10, 104401 (2007) * M. Borland, APS LS-287, 2000
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TUPOR023	Investigation of Trapped Resonant Modes in Insertion Devices at the Australian Synchrotron	1710
	R.T. Dowd, M.P. Atkinson, M.J. Boland, G. LeBlanc, Y.E. Tan SLSA, Clayton, Australia D. Teytelman Dimtel, San Jose, USA
	The Australian Synchrotron light Source has 3 variable gap in-vacuum undulators (IVU) in the storage ring. Since installation, these devices have been the source of strong beam instabilities. These instabilities seem to behave as trapped resonant modes of very high Q and high frequency, although a definite source has not been identified. The presence of these instabilities has necessitated operating at unusually high chromaticity for much of the light source's operations. More recently transverse feedback has been able to control the instabilities and recent developments in diagnostics have allowed some investigation of the frequency and mode response of these resonances. The results of this investigation will be presented in this paper.
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TUPOR025	Beam Break-up Measurements at the Recirculating Electron Accelerator S-DALINAC	1714
	T. Kürzeder, M. Arnold, L.E. Jürgensen, J. Pforr, N. Pietralla TU Darmstadt, Darmstadt, Germany F. Hug IKP, Mainz, Germany
	Funding: Supported by the German Federal Ministry for Education and Research (BMBF) under Grant No. 05K13RDA Beam break-up (BBU) instability is an important limitation to the current which can be accelerated in a superconducting linac. In particular recirculating machines and Energy Recovery Linacs have to deal with that problem. Therefore, it is important to find strategies for increasing the threshold currents of these machines. The superconducting accelerator S-DALINAC at the Technische Universität Darmstadt provides electron beams in c.w. for nuclear physics experiments since 1991. It consists of a 10 MeV injector and a 40 MeV main linac where two and eight 20-cell elliptical 3-GHz cavities are operated in a liquid helium bath at 2 K. Using two recirculation beam lines the main accelerator can be used up to 3 times. Operational experiences have shown that the design-beam current of 20 μA could not be reached. One reason is the occurrence of BBU. We will report on measurements of the threshold current at various energy settings of the S-DALINAC. The results of a first test to increase the BBU limit by using skew quadrupole magnets in the first recirculation beam line will be presented.
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TUPOR027	Interaction of RF Phase Modulation and Coupled-Bunch Instabilities at the DELTA Storage Ring	1720
	M. Sommer, B.D. Isbarn, B. Riemann, T. Weis DELTA, Dortmund, Germany
	Funding: Work supported by the BMBF under contract no. 05K13PEB. Analyzing the interaction of RF phase modulation and coupled-bunch instabilities requires a method to determine damping rates of coupled-bunch modes at presence of RF phase modulation. This paper shows, that the common way of using exponential fits to determine damping rates is not viable for high modulation amplitudes. It presents a new method, which is capable of acquiring damping rates of coupled-bunch modes for phase shifts up to 5°, using a bunch-by-bunch feedback system. For this purpose a specific mode is excited by the feedback system and the saturation value, i.e. the maximum excitation, is measured to calculate the damping rate. With this new method, the modulation amplitude of the RF phase modulation is swept from 0° to 5° and it can be shown, that the damping rate is proportional to the square of the modulation amplitude.
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TUPOR028	Excitation of Longitudinal Coupled-bunch Oscillations with the Wide-band Cavity in the CERN PS	1724
	L. Ventura, M. Migliorati INFN-Roma1, Rome, Italy H. Damerau, M. Migliorati, G. Sterbini CERN, Geneva, Switzerland M. Migliorati University of Rome "La Sapienza", Rome, Italy
	Longitudinal coupled-bunch oscillations in the CERN Proton Synchrotron have been studied in the past years and they have been recognized as one of the major challenges to reach the high brightness beam required by the High Luminosity LHC project. In the frame of the LHC Injectors Upgrade project in 2014 a new wide-band Finemet cavity has been installed in the Proton Synchrotron as a part of the coupled-bunch feedback system. To explore the functionality of the Finemet cavity during 2015 a dedicated measurement campaign has been performed. Coupled-bunch oscillations have been excited with the cavity around each harmonic of the revolution frequency with both a uniform and nominal filling pattern. In the following the measurements procedure and results are presented.
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TUPOR029	Study of Fast Instability in Fermilab Recycler	1728
	S. A. Antipov University of Chicago, Chicago, Illinois, USA P. Adamson, S. Nagaitsev, M.-J. Yang Fermilab, Batavia, Illinois, USA
	One of the factors which may limit the intensity in the Fermilab Recycler is a fast transverse instability. It develops within a hundred turns and, in certain conditions, may lead to a beam loss. Various peculiar features of the instability: its occurrence only above a certain intensity threshold, and only in horizontal plane, as well as the rate of the instability, suggest that its cause is electron cloud. We studied the phenomena by observing the dynamics of stable and unstable beam. We found that beam motion can be stabilized by a clearing bunch, which confirms the electron cloud nature of the instability. The findings suggest electron cloud trapping in Recycler combined function mag-nets. Bunch-by-bunch measurements of betatron tune show a tune shift towards the end of the bunch train and allow the estimation of the density of electron cloud and the rate of its build-up. The experimental results are in agreement with numerical simulations of electron cloud build-up and its interaction with the beam.
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TUPOR031	Trapped Ion Effects and Mitigation During High Current Operation in the Cornell DC Photoinjector	1735
SUPSS054	use link to see paper's listing under its alternate paper code
	S.J. Full, A.C. Bartnik, I.V. Bazarov, J. Dobbins, B.M. Dunham, G.H. Hoffstaetter Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: U.S. Department of Energy (Grant No. DE-SC0012493), National Science Foundation (Award No. NSF-DMR 0807731) The Cornell high intensity photoinjector reaches a new regime of linac beam parameters where high continuous-wave electron beam currents lead to ion trapping. Above 10 mA, we have observed beam trips that limit stable machine operation to approximately 10^-15 minutes. By applying known ion clearing methods, the machine lifetime increases to at least 24 hours of continuous operation, suggesting that trapped ions are the most likely cause of the trips. In this paper we share some of our observations ion trapping in the photoinjector, as well as experimental tests of three common ion mitigation methods: clearing electrodes, beam shaking and bunch gaps.
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TUPOR033	Experimental Study of Single Bunch Instabilities at NSLS-II Storage Ring	1738
	W.X. Cheng, B. Bacha, G. Bassi, A. Blednykh, B. Podobedov, O. Singh, V. Smalyuk BNL, Upton, Long Island, New York, USA
	Single bunch instabilities have been observed since the early stage of NSLS-II storage ring commissioning. After installing the super-conducting cavity, the single bunch instability threshold current was similar at 0.7mA. The instability was eventually determined to be due to transverse mode coupling. Microwave instability has been characterized using streak camera bunch profile, horizontal beam sizes at dispersion location and beam spectrums. Microwave instability threshold current dependency on bunch lengths and IUV gaps has been studied. Most recent experimental results will be presented in this paper.
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Paper

Title

Page

TUOAB02

Conditions for CSR Microbunching Gain Suppression

1057

SUPSS056

use link to see paper's listing under its alternate paper code

C.-Y. Tsai
Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
S. Di Mitri
Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
D. Douglas, R. Li, C. Tennant
JLab, Newport News, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
The coherent synchrotron radiation (CSR) of a high brightness electron beam traversing a series of dipoles, such as transport arcs, may result in phase space degradation. On one hand, the CSR can perturb electron transverse motion in dispersive regions along the beamline, causing emittance growth. On the other hand, the CSR effect on the longitudinal beam dynamics could result in microbunching gain enhancement. For transport arcs, several schemes have been proposed* to suppress the CSR-induced emittance growth. Similarly, several scenarios have been introduced** to suppress CSR-induced microbunching gain, which however mostly aim for linac-based machines. In this paper we try to provide sufficient conditions for suppression of CSR-induced microbunching gain along a transport arc, analogous to*. Several example lattices are presented, with the relevant microbunching analyses carried out by our semi-analytical Vlasov solver***. The simulation results show that lattices satisfying the proposed conditions indeed have microbunching gain suppressed. We expect this analysis can shed light on lattice design approach that could suppress the CSR-induced microbunching gain.
*D.Douglas et al, JLAB-ACP-14-1751, S.DiMitri et al, PRL (2013), R.Hajima, NIMA (2004), Y.Jiao et al, PRSTAB (2014)
**Z.Huang et al, PRSTAB (2004), Saldin et al, NIMA (2004)
***C.Tsai et al, FEL'15

Slides TUOAB02 [6.484 MB]

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TUPMW011

Current Status of Instability Threshold Measurements in the LHC at 6.5 TeV

1434

L.R. Carver, J. Barranco, N. Biancacci, X. Buffat, W. Höfle, G. Kotzian, T. Lefèvre, T.E. Levens, E. Métral, T. Pieloni, B. Salvant, C. Tambasco
CERN, Geneva, Switzerland
N. Wang
IHEP, Beijing, People's Republic of China
M. Zobov
INFN/LNF, Frascati (Roma), Italy

Throughout 2015, many measurements of the minimum stabilizing octupole current required to prevent coherent transverse instabilities have been performed. These measurements allow the LHC impedance model at flat top to be verified and give good indicators of future performance and limitations. The results are summarized here, and compared to predictions from the simulation code DELPHI.

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TUPOR006

Systematic Studies of Short Bunch-Length Bursting at ANKA

1662

M. Brosi, E. Blomley, E. Bründermann, N. Hiller, B. Kehrer, A.-S. Müller, M. Schedler, M. Schuh, P. Schönfeldt, J.L. Steinmann
KIT, Karlsruhe, Germany

Funding: Supported by the German Federal Ministry of Education and Research (05K13VKA), the Helmholtz Association (VH-NG-320) and by the Helmholtz International Research School for Teratronics (HIRST).
At ANKA, the Karlsruhe synchrotron radiation source, the so called short bunch-length operation mode allows the reduction of the bunch length down to a few picoseconds. The micro-bunching instability resulting from the high degree of longitudinal compression leads to fluctuations in the emitted intensity in the THz regime, referred to as bursting. For extremely compressed bunches at ANKA bursting also occurs, in a certain current range, below the main bursting threshold. This contribution shows measurements of this short bunch-length bursting and makes first comparisons with theory.

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TUPOR009

Single Bunch Longitudinal Instability in the CERN SPS

1670

SUPSS055

use link to see paper's listing under its alternate paper code

A. Lasheen, T. Bohl, S. Hancock, T. Roggen, E.N. Shaposhnikova
CERN, Geneva, Switzerland
E. Radvilas
Gediminas Technical University, Vilnius, Lithuania

The longitudinal single bunch instability observed in the SPS leads to uncontrolled emittance blow-up and limits the quality of high intensity beams required for the High Luminosity LHC and AWAKE projects at CERN. The present SPS impedance model developed from a thorough survey of machine elements was used in macro-particle simulations (with the code BLonD) of the bunch behavior through the acceleration cycle. Comparison of simulations with measurements of the synchrotron frequency shift, performed on the SPS flat bottom to probe the impedance, show a reasonable agreement. During extensive experimental studies various beam and machine parameters (bunch intensity, longitudinal emittance, RF voltage, with single and double RF systems) were scanned in order to further benchmark the SPS impedance model with measurements and to better understand the mechanism behind the instability. It was found that the dependence of instability threshold on longitudinal emittance and beam energy has an unexpected non-monotonic behavior, leading to islands of (in)stability. The results of this study are presented and can be used to define possible parameter settings for the future CERN projects.

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TUPOR013

Analysis of Multi-bunch Instabilities at the Diamond Storage Ring

1685

R. Bartolini, R.T. Fielder, G. Rehm
DLS, Oxfordshire, United Kingdom
V.V. Smaluk
BNL, Upton, Long Island, New York, USA

We present recent results of analytical, numerical and experimental analysis of multi-bunch instabilities at the Diamond storage ring. The works compares the impedance estimates from numerical modelling with the analysis of the growth rates of the excited multi-bunch modes in different machine configurations. The contribution of a number of wakefield sources has been identified with very high precision thanks to high quality data provided by the existing Transverse multi-bunch feedback diagnostics

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TUPOR020

Combination of Density and Energy Modulation in Microbunching Analysis

1703

C.-Y. Tsai
Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
R. Li
JLab, Newport News, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Microbunching instability (MBI) has been one of the most challenging issues in the transport of high-brightness electron beams for modern recirculating or energy recovery linac machines. Recently we have developed and implemented a Vlasov solver* to calculate microbunching gain for an arbitrary beamline lattice design, based on the extension of early theoretical formulation** for the microbunching amplification from an initial density perturbation to the final density modulation. For more thorough analyses, in addition to the case of (initial) density to (final) density amplification, we in this paper extend the previous formulation to more general cases, including energy-to-density, density-to-energy and energy-to-energy amplifications for a recirculation machine. Such semi-analytical formulae are then incorporated into our Vlasov solver, and reasonable agreement is obtained when the semi-analytical results are benchmarked with particle tracking simulation using ELEGANT***.
* C.Y. Tsai et al, FEL'15
** S. Heifets et al, PRSTAB 5, 064401 (2002), Z. Huang and K. Kim, PRSTAB 5, 074401 (2002), M. Vneturini, PRSTAB 10, 104401 (2007)
*** M. Borland, APS LS-287, 2000

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TUPOR023

Investigation of Trapped Resonant Modes in Insertion Devices at the Australian Synchrotron

1710

R.T. Dowd, M.P. Atkinson, M.J. Boland, G. LeBlanc, Y.E. Tan
SLSA, Clayton, Australia
D. Teytelman
Dimtel, San Jose, USA

The Australian Synchrotron light Source has 3 variable gap in-vacuum undulators (IVU) in the storage ring. Since installation, these devices have been the source of strong beam instabilities. These instabilities seem to behave as trapped resonant modes of very high Q and high frequency, although a definite source has not been identified. The presence of these instabilities has necessitated operating at unusually high chromaticity for much of the light source's operations. More recently transverse feedback has been able to control the instabilities and recent developments in diagnostics have allowed some investigation of the frequency and mode response of these resonances. The results of this investigation will be presented in this paper.

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TUPOR025

Beam Break-up Measurements at the Recirculating Electron Accelerator S-DALINAC

1714

T. Kürzeder, M. Arnold, L.E. Jürgensen, J. Pforr, N. Pietralla
TU Darmstadt, Darmstadt, Germany
F. Hug
IKP, Mainz, Germany

Funding: Supported by the German Federal Ministry for Education and Research (BMBF) under Grant No. 05K13RDA
Beam break-up (BBU) instability is an important limitation to the current which can be accelerated in a superconducting linac. In particular recirculating machines and Energy Recovery Linacs have to deal with that problem. Therefore, it is important to find strategies for increasing the threshold currents of these machines. The superconducting accelerator S-DALINAC at the Technische Universität Darmstadt provides electron beams in c.w. for nuclear physics experiments since 1991. It consists of a 10 MeV injector and a 40 MeV main linac where two and eight 20-cell elliptical 3-GHz cavities are operated in a liquid helium bath at 2 K. Using two recirculation beam lines the main accelerator can be used up to 3 times. Operational experiences have shown that the design-beam current of 20 μA could not be reached. One reason is the occurrence of BBU. We will report on measurements of the threshold current at various energy settings of the S-DALINAC. The results of a first test to increase the BBU limit by using skew quadrupole magnets in the first recirculation beam line will be presented.

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TUPOR027

Interaction of RF Phase Modulation and Coupled-Bunch Instabilities at the DELTA Storage Ring

1720

M. Sommer, B.D. Isbarn, B. Riemann, T. Weis
DELTA, Dortmund, Germany

Funding: Work supported by the BMBF under contract no. 05K13PEB.
Analyzing the interaction of RF phase modulation and coupled-bunch instabilities requires a method to determine damping rates of coupled-bunch modes at presence of RF phase modulation. This paper shows, that the common way of using exponential fits to determine damping rates is not viable for high modulation amplitudes. It presents a new method, which is capable of acquiring damping rates of coupled-bunch modes for phase shifts up to 5°, using a bunch-by-bunch feedback system. For this purpose a specific mode is excited by the feedback system and the saturation value, i.e. the maximum excitation, is measured to calculate the damping rate. With this new method, the modulation amplitude of the RF phase modulation is swept from 0° to 5° and it can be shown, that the damping rate is proportional to the square of the modulation amplitude.

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TUPOR028

Excitation of Longitudinal Coupled-bunch Oscillations with the Wide-band Cavity in the CERN PS

1724

L. Ventura, M. Migliorati
INFN-Roma1, Rome, Italy
H. Damerau, M. Migliorati, G. Sterbini
CERN, Geneva, Switzerland
M. Migliorati
University of Rome "La Sapienza", Rome, Italy

Longitudinal coupled-bunch oscillations in the CERN Proton Synchrotron have been studied in the past years and they have been recognized as one of the major challenges to reach the high brightness beam required by the High Luminosity LHC project. In the frame of the LHC Injectors Upgrade project in 2014 a new wide-band Finemet cavity has been installed in the Proton Synchrotron as a part of the coupled-bunch feedback system. To explore the functionality of the Finemet cavity during 2015 a dedicated measurement campaign has been performed. Coupled-bunch oscillations have been excited with the cavity around each harmonic of the revolution frequency with both a uniform and nominal filling pattern. In the following the measurements procedure and results are presented.

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TUPOR029

Study of Fast Instability in Fermilab Recycler

1728

S. A. Antipov
University of Chicago, Chicago, Illinois, USA
P. Adamson, S. Nagaitsev, M.-J. Yang
Fermilab, Batavia, Illinois, USA

One of the factors which may limit the intensity in the Fermilab Recycler is a fast transverse instability. It develops within a hundred turns and, in certain conditions, may lead to a beam loss. Various peculiar features of the instability: its occurrence only above a certain intensity threshold, and only in horizontal plane, as well as the rate of the instability, suggest that its cause is electron cloud. We studied the phenomena by observing the dynamics of stable and unstable beam. We found that beam motion can be stabilized by a clearing bunch, which confirms the electron cloud nature of the instability. The findings suggest electron cloud trapping in Recycler combined function mag-nets. Bunch-by-bunch measurements of betatron tune show a tune shift towards the end of the bunch train and allow the estimation of the density of electron cloud and the rate of its build-up. The experimental results are in agreement with numerical simulations of electron cloud build-up and its interaction with the beam.

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TUPOR031

Trapped Ion Effects and Mitigation During High Current Operation in the Cornell DC Photoinjector

1735

SUPSS054

use link to see paper's listing under its alternate paper code

S.J. Full, A.C. Bartnik, I.V. Bazarov, J. Dobbins, B.M. Dunham, G.H. Hoffstaetter
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: U.S. Department of Energy (Grant No. DE-SC0012493), National Science Foundation (Award No. NSF-DMR 0807731)
The Cornell high intensity photoinjector reaches a new regime of linac beam parameters where high continuous-wave electron beam currents lead to ion trapping. Above 10 mA, we have observed beam trips that limit stable machine operation to approximately 10^-15 minutes. By applying known ion clearing methods, the machine lifetime increases to at least 24 hours of continuous operation, suggesting that trapped ions are the most likely cause of the trips. In this paper we share some of our observations ion trapping in the photoinjector, as well as experimental tests of three common ion mitigation methods: clearing electrodes, beam shaking and bunch gaps.

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TUPOR033

Experimental Study of Single Bunch Instabilities at NSLS-II Storage Ring

1738

W.X. Cheng, B. Bacha, G. Bassi, A. Blednykh, B. Podobedov, O. Singh, V. Smalyuk
BNL, Upton, Long Island, New York, USA

Single bunch instabilities have been observed since the early stage of NSLS-II storage ring commissioning. After installing the super-conducting cavity, the single bunch instability threshold current was similar at 0.7mA. The instability was eventually determined to be due to transverse mode coupling. Microwave instability has been characterized using streak camera bunch profile, horizontal beam sizes at dispersion location and beam spectrums. Microwave instability threshold current dependency on bunch lengths and IUV gaps has been studied. Most recent experimental results will be presented in this paper.

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