IPAC2017 - Classification: 05 Beam Dynamics and Electromagnetic Fields / D04 Beam Coupling Impedance - Theory, Simulations, Measurements, Code Developments

Paper	Title	Page
WEPIK014	Coupled Bunch Instability and Its Cure at J-PARC RCS	2946
	Y. Shobuda, H. Harada, H. Hotchi, P.K. Saha, T. Takayanagi, F. Tamura, N. Tani, T. Togashi, Y. Watanabe, K. Yamamoto, M. Yamamoto JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken, Japan Y.H. Chin, Y. Irie, T. Toyama KEK, Ibaraki, Japan
	The RCS at J-PARC is a kicker-impedance dominant machine, which violates the impedance budget from a classical viewpoint. Nevertheless, we have recently succeeded to accelerate a 1-MW equivalent beam by making maximum use of the space charge effect on the beam instabilities. In this report, we explain the manipulation to suppress the beam instability, at first. Then, we discuss some issues to suppress the beam instabilities for beams with much smaller transverse emittance, as well as the present status of our efforts to reduce the kicker impedance toward the realization of the higher beam power at the RCS.
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WEPIK054	Evaluation and Attenuation of Sirius Components Impedance	3048
	H.O.C. Duarte, L. Liu, S.R. Marques LNLS, Campinas, Brazil
	The Sirius in-vacuum components have their design improvements, possibilities and choices presented, where wake heating, single-bunch and multi-bunch effects and mechanical aspects were taken into account. The results were finally evaluated and added to the Sirius impedance budget.
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WEPIK055	Analysis and Countermeasures of Wakefield Heat Losses for Sirius	3052
	H.O.C. Duarte, L. Liu, S.R. Marques, T.M. da Rocha, F.H. de Sá LNLS, Campinas, Brazil
	Design evaluation and possible solutions for several in-vacuum components of Sirius are presented, having their impedance analysis focused on mitigating the wake heating impact. Thermal and/or structural simulation of the models are carried out by considering the heat load directly obtained from wakefield simulations with resistive wall boundary conditions.
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WEPIK058	Preliminary Longitudinal Impedance Model for the ESRF-EBS	3063
	S.M. White ESRF, Grenoble, France
	In light sources, longitudinal beam coupling impedance can deteriorate performance through bunch lengthening or increased longitudinal emittance due to the microwave instability. Simulation estimates are therefore required to devise the appropriate counter-measures if necessary. The main contributors to the longitudinal impedance model of the new ESRF-EBS storage ring were simulated. A preliminary longitudinal impedance model is presented and preliminary tracking simulations are shown.
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WEPIK066	Calculation of Wakefields and Higher Order Modes for the Vacuum Chamber of the CMS, ATLAS, ALICE and LHCb Experiments for the HL-LHC	3081
	R. Wanzenberg, O. Zagorodnova DESY, Hamburg, Germany E. Métral, B. Salvant CERN, Geneva, Switzerland
	Funding: Partly funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement 284404. The High Luminosity Large Hadron Collider (HL-LHC) project was started with the goal to extend the discovery potential of the Large Hadron Collider (LHC). The HL-LHC study implies also upgraded dimensions of the experimental beam pipes of the CMS, ATLAS, ALICE and LHCb experiments. The trapped monopole and dipole Higher Order Modes (HOMs) and the short range wakefields for the new design of the vacuum chambers were calculated with help of the computer codes MAFIA and ECHO2D. The results of the short range wakefields calculations and the HOMs calculations are presented in this report. The short range wakefields are presented in terms of longitudinal and transverse wake potentials and also in terms of loss and kick parameters. Selected results from the HOMs calculations , including the the frequency, the loss parameter, the R/Q and the Q value are presented.
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WEPIK071	Resistive-Wall Impedance Effects for the New KEK Light Source	3095
	N. Nakamura KEK, Ibaraki, Japan
	KEK Light Source (KEK-LS) is a 3-GeV storage ring of 20-cell HMBA (Hybrid Multi-Bend Achromat) lattice, which is planned to be constructed as a successor of the two existing Photon Factory storage rings (PF ring and PF-AR) in the KEK Tsukuba Campus. In this ring, a lot of in-vacuum undulators with a small magnetic gap (4 mm at minimum) will be installed and the vacuum pipe of a small aperture (25 mm in diameter) will be used. In addition, NEG coating, having a low electric conductivity, will be utilized for the vacuum pipe to ensure a sufficient beam lifetime early in the machine commissioning. In this paper, the heating power due to the longitudinal RW impedance and the growth rate of coupled-bunch instability caused by the transverse RW impedance are calculated and the effects of the RW impedance on KEK-LS are presented. K. Harada et al., Proc. of IPAC2016, Busan, Korea, pp.3251-3253; K. Harada et al., these proceedings.
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WEPIK078	Development of the Impedance Model in HEPS	3110
	N. Wang, Z. Duan, X.Y. Li, H. Shi, S.K. Tian, G. Xu IHEP, Beijing, People's Republic of China
	The High Energy Photon Source (HEPS) is a new designed photon source at beam energy of 6 GeV. Due to the small beam pipe aperture and a large number of insertion devices in the machine, the impedance can drive collective instabilities and limit the machine performance. Therefore, a thorough estimation of the coupling impedance is necessary in controlling the total impedance of the whole machine. A primary impedance model is obtained for the storage ring.
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WEPIK081	The Logitudinal Broadband Impedance and Energy Spread Measurements at the VEPP-4M Electron-Positron Collider	3117
	V.M. Borin, V.A. Kiselev, G.Y. Kurkin, S.A. Nikitin, P.A. Piminov, S.V. Sinyatkin BINP SB RAS, Novosibirsk, Russia V.L. Dorokhov, O.I. Meshkov BINP, Novosibirsk, Russia
	The paper presents measurements of the longitudinal broadband impedance and beam energy spread of the beam at the VEPP-4M electron-positron collider in an energy range of 1.0 - 3.5 GeV. In order to measure the longitudinal bunch size at various beam currents we used PS-1/S1 streak camera with picosecond temporal resolution. The dependence of bunch length from the bunch current at different energies demonstrates a microwave instability threshold. The bunch lengthening was caused by potential well distortion as well. Potential well distortion lengthening was used to estimate a value of the reactive part of the longitudinal impedance of the vacuum chamber of the collider Observed microwave instability thresholds was used to measure the value of the broadband impedance. The impact of the Touschek effect in the beam energy spread is discussed.
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WEPIK089	Characterization of Resonant Impedances of CERN-SPS Gate Valves	3139
	T. Kaltenbacher, J. Repond, C. Vollinger CERN, Geneva, Switzerland
	For the CERN High Luminosity LHC project, a doubling of bunch intensity is foreseen. However, this intensity increase is currently limited by the LHC injector chain, in part due to longitudinal multi-bunch instabilities in the SPS. Therefore, the implementation of an accurate SPS impedance model was started some time ago in order to obtain a better understanding of instability sources and develop mitigation measures. In this paper, we present the electromagnetic characterization of commonly used all-metal gate valves with respect to their contribution to the SPS longitudinal impedance. The valve impedance was evaluated with commercially available EM-field simulation programs and verified with RF-bench measurements. Using this input, it was possible to obtain in particle simulations the dependence of the multi-bunch stability threshold on the number of these valves. A practical means of mitigation is to use a commercially available impedance shielded version of these gate valves. We also present the associated reduction in beam coupling impedance and the expected gain in beam stability if all existing unshielded valves are replaced by shielded valves.
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WEPIK090	Characterization of Shielding for the CERN-SPS Vacuum Flanges With Respect to Beam Coupling Impedance	3143
	T. Kaltenbacher, C. Vollinger CERN, Geneva, Switzerland
	Longitudinal multi-bunch instabilities in the CERN-SPS pose a serious limitation for future beam intensities required for high luminosity LHC. Hence, an impedance model for the SPS accelerator was developed from which one group of vacuum flanges could be identified as being a major culprit for these instabilities. These flanges support high impedance modes and their impact on beam stability was traced to a longitudinal mode at about 1.4GHz. For improvement of multi-bunch stability threshold, this group of flanges will be shielded as part of an impedance reduction campaign. We describe the evaluation of different impedance shielding designs proposed to reduce the longitudinal beam coupling impedance of this group of vacuum flanges in the SPS. EM-field simulations were performed to identify remaining resonances in these vacuum flanges with impedance shield prototypes installed, and the simulation models were benchmarked with RF-measurements. Depending on the performance and other parameters, the most suitable shield design will be selected, built and installed. As a first step, the installation of one shielding design in some positions in the SPS is planned for the beginning of 2017.
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WEPIK094	LEIR Impedance Model and Coherent Beam Instability Observations	3159
	N. Biancacci, H. Bartosik, A. Huschauer, E. Métral, T.L. Rijoff, B. Salvant, R. Scrivens CERN, Geneva, Switzerland M. Migliorati University of Rome La Sapienza, Rome, Italy
	The LEIR machine is the first synchrotron in the ion acceleration chain at CERN and it is responsible to deliver high intensity ion beams to the LHC. Following the recent progress in the understanding of the intensity limitations, detailed studies of the machine impedance started. In this work we describe the present LEIR impedance model, detailing the contribution to the total longitudinal and transverse impedance of several machine element. We then compare the machine tune shift versus intensity predictions against measurements at injection energy and summarize the coherent instability observations in absence of transverse damper feedback.
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WEPIK095	Evaluation of Longitudinal Beam Impedance in the Beam Gas Ionization Monitor of the CERN-PS Accelerator	3163
	N. Nasr Esfahani, T. Kaltenbacher, J.W. Storey, C. Vollinger CERN, Geneva, Switzerland
	The recently observed beam induced heating issues in the BGI monitors of the LHC which could have been occurred due to a strong coupling between the beam and the localized modes at the sensor location showed the general importance of a thorough evaluation of the beam coupling impedance and the corresponding heat deposit in beam monitoring equipments. This paper is devoted to the examination of the beam coupling impedance and beam induced heating for a currently under development beam gas ionization (BGI) monitor which is intended to be a part of the CERN Proton Synchrotron (PS) beam monitoring equipment. Details of the EM and wake field simulations for this BGI monitor together with the RF measurement results and power loss calculations will be presented.
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WEPIK096	Assessment of Beam Impedance for the CERN-PS Booster Wire Scanner	3167
	T. Kaltenbacher, N. Nasr Esfahani, C. Vollinger CERN, Geneva, Switzerland
	It is well known that performance of accelerators critically depends on the interaction of high intensity beams with the surrounding structures. As a result of these beam interactions, it is required at CERN to characterize the beam coupling impedance of each new machine element that is to be installed in the accelerator ring. In the framework of the LIU (LHC Injectors Upgrade) project, a new design of rotational wire scanner to be used in the PS Booster is currently under development. As an intermediate step, the prototype of this wire scanner was evaluated with respect to its longitudinal beam coupling impedance. Depending on the performance of this machine element, it is planned to replace existing wire scanners in other machines at CERN (e.g. PS-Booster, PS and SPS) with very similar designs. This paper presents the simulations and describes the measurement methods used for benchmarking electromagnetic simulations performed for the impedance evaluation of the LIU wire scanner for the PS-Booster. Additionally, the device was fitted with an RF feed-through in order to monitor and attenuate certain undesired modes supported by this structure.
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WEPIK100	The Applicability of NEG Coated Undulator Vessels for the CLARA FEL Test Facility	3181
	O.B. Malyshev, K.B. Marinov, K.J. Middleman, N. Thompson, R. Valizadeh, P.H. Williams STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom O.B. Malyshev, K.J. Middleman, N. Thompson, R. Valizadeh, P.H. Williams Cockcroft Institute, Warrington, Cheshire, United Kingdom
	CLARA is a FEL test facility at Daresbury Laboratory (DL), UK. The undulator vacuum chamber is 20 m long with inner diameter 6 mm and its vacuum performance can benefit from a NEG coating. The thickness of the coating layer must be carefully optimised. A layer ~ 1 um would help the vacuum but a thinner layer would be partially transparent for the EM field reducing the resistive wall wakefields due to the NEG. A very thin layer, however, may not yield the necessary vacuum performance. Two types of NEG coatings produced at DL - dense and columnar - were considered. Their bulk conductivities were measured in a separate study. The resistive wall wakefield impedance was calculated following the standard approach for multilayer vessels. A 250 fs rms electron bunch was generated in ASTRA and its wakefield was obtained from the vessel impedance. The FEL performance was then studied through GENESIS simulations and the result compared to the case with no wakefields. It was found that NEG layers thicker than 100 nm give an unacceptable reduction of the FEL power and the vacuum performance of such thin coatings is unknown. Possible solutions to this problem are discussed.
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WEPIK102	Measurement of RF Resonances and Measured Impact on Transverse Multibunch Instabilities from In-vacuum Insertions Devices	3188
	G. Rehm DLS, Oxfordshire, United Kingdom
	Diamond Light Source has currently 15 in-vacuum insertion devices (ID) installed, mostly built in-house. Their measured impact on multi-bunch mode damping as a result of varying magnet gap was shown before, now we augment these with measurements of broadband frequency spectra with stored beam obtained using an antenna placed in the ID vacuum. Finally, we present off-line measurements of resonances in the ID vessel acquired using a vector network analyser and two antennae installed in-vacuum.
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WEPIK106	Impedance and Collective Effects for the Advanced Light Source Upgrade at LBNL	3192
	S. Persichelli, J.M. Byrd, S. De Santis, D. Li, T.H. Luo, C. Steier, M. Venturini LBNL, Berkeley, California, USA
	Funding: Work supported by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 The upgrade of the Advanced Light Source (ALS-U) consists of a multiband achromat ultralow emittance lattice for the production of diffraction-limited soft x-rays. A very important issue for ALS-U is represented by instabilities induced by wakefields, that may limit the peak current of individual bunches and the total beam current. In addition, vacuum chamber apertures of few millimeters, that are a key feature of low-emittance machines, can result in a significant increase in the Resistive Wall (RW) impedance. In this paper we present progress on establishing short range wakefield model for ALS-U and evaluating the impact on the longitudinal and transverse single-bunch dynamics.
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WEPIK109	Experimental Study of Vertical-Longitudinal Coupling Induced by Wakefields at CesrTA	3200
	S. Wang, J.D. Perrin, S. Poprocki, D. L. Rubin Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: Work supported by NSF PHY-1416318, PHYS-1068662 Transverse vertical wakefields can cause vertical beam size growth in accelerators. Here we report recent measurements and simulations of wakefields from movable scrapers at the CesrTA. The charge dependent vertical beam size growth was observed while a single scraper was inserted through the top of the chamber. No change in the beam size was observed with top and bottom scrapers inserted symmetrically. The apparent growth in the vertical beam size was due in large part to the y-z coupling (vertical crabbing) induced by the monopole wake of the asymmetric scraper configuration. We explored this y-z coupling by varying vertical betatron phase advance between the vertical beam size monitor and the scrapers. In addition, we found that existing residual, current independent y-z coupling could be compensated by the scraper wake. Predictions of a tracking simulation are in good agreement with the measurements.
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WEPIK110	Resistive Wall Instability and Impedance Studies of Narrow Undulator Chamber in CHESS-U	3204
	S. Wang, M.G. Billing, S. Poprocki, D. L. Rubin, D. Sagan Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: Work supported by NSF DMR-0936384 and NSF DMR-1332208 In a major upgrade of the Cornell Electron Storage Ring (CESR) one sextant of ring will be replaced with double bend achromats (DBAs) and undulator straights for x-ray users. The resistive wall impedance from the narrow gap (4.5 mm) undulator chambers (5 m per straight) may limit total beam. Here we report recent results of modelling and calculation of multibunch instabilities due to the impedance of chamber walls and transition tapers. The short range wakefields and resistive wall impedance are modelled and incorporated in a tracking simulation. The coupled-bunch growth rate found with the tracking study is in good agreement with the analytic approximation. We find that the resistive wall instability can be readily damped by our existing bunch-by-bunch feedback system.
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WEPIK113	Entrance and Exit CSR Impedance for Non-Ultrarelativistic Beam	3214
	R. Li JLab, Newport News, Virginia, USA C.-Y. Tsai Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
	Funding: Work supported by the Department of Energy, Laboratory Directed Research and Development Funding, under Contract No. DE-AC05-06OR23177 For a high-brightness electron beam being transported through beamlines involving bending systems, the coherent synchrotron radiation (CSR) and longitudinal space charge (LSC) interaction could often cause microbunching instability. The semi-analytical Vlasov solver for microbunching gain* depends on the impedances for the relevant collective effects. The existing results for CSR impedances are usually obtained for the ultrarelativistic limit. To extend the microbunching analysis to cases of low energies, such as the case of an ERL merger, or to density modulations at extremely small wavelength, it is necessary to extend the impedance analysis to the non-ultrarelativistic regime. In this study, we present the impedance analysis for the transient CSR interaction in the non-ultrarelativistic regime, for transients including both entrance to and exit from a magnetic dipole. These impedance results will be compared to their ultra-relativistic counterparts, and the corresponding wakefield obtained from the impedance for low-energy beams will be compared with the existing results of transient CSR wakefield for general beam energies. C.-Y. Tsai et al., Proc. of IPAC'15, 596 (2015). C. Mitchell et al ., Proc. of IPAC'13, 1832 (2014). * E. L. Saldin et al ., Nucl. Instrum. Meth. A 398, 373 (1997).
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WEPIK117	A Comprehensive Study of the Microwave Instability	3224
	A. Blednykh, B. Bacha, G. Bassi, O.V. Chubar, M.S. Rakitin, V.V. Smaluk, M. Zhernenkov BNL, Upton, Long Island, New York, USA
	Funding: Work supported by DOE contract DE-SC0012704 Several instability thresholds and special waveform beam pattern have been observed during measurements of the horizontal beam size change vs single bunch current by the synchrotron light monitor (SLM) camera installed in a low dispersion area of the NSLS-II storage ring. The electron beam energy spread from In-Vacuum Undulator (IVU) of the Soft Matter Interfaces (SMI) beam line confirmed the microwave beam pattern behavior as a current dependent effect. The numerically obtained total longitudinal wakepotential by the GdfidL code allowed us to compare the measured results with particle tracking simulations using the SPACE code. The instability thresholds behavior at different RF voltages are in some sort of overarching agreement.
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WEPIK118	Synchronous Phase Shift from Beam Loading Analysis	3227
	G. Bassi, A. Blednykh, J. Rose, V.V. Smaluk, J. Tagger BNL, Upton, Long Island, New York, USA
	We discuss measurements, performed in the NSLS-II storage ring, of the synchronous phase shift as a function of single bunch current from beam loading parameters. The synchronous phase is calculated from the forward and reflected power measured in the RF cavities. The comparison with direct synchronous phase measurements shows good agreement.
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WEPVA101	Review of Stripline Beam Impedance: Application to the Extraction Kicker for the CLIC Damping Rings	3499
	C. Belver-Aguilar, M.J. Barnes CERN, Geneva, Switzerland
	The beam coupling impedance of the stripline kicker for beam extraction from the CLIC Damping Rings (DRs) has been studied analytically, numerically with CST Particle Studio (PS) and measured in the laboratory, although not all the results were understood. In order to have a better knowledge about the beam coupling impedance of a stripline kicker, a simple model has been first studied, with flat electrodes and a cylindrical beam pipe. From this preliminary study, a new approach for the dipolar component of the horizontal impedance has been derived, when considering both odd and even operating modes of the striplines. This new approach has been used to understand the differences found between the predicted transverse impedance and the two wire measurements carried out in the laboratory for the prototype CLIC DR striplines. Future tests of beam coupling impedance with beam in the ALBA Synchrotron Ligth Source will complete this study.
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WEPVA134	Impedance Measurement of Vacuum Chamber Components for the Advance Photon Source (APS) Upgrade	3583
SUSPSIK052	use link to see paper's listing under its alternate paper code
	M.P. Sangroula IIT, Chicago, Illinois, USA R.M. Lill, R.R. Lindberg, X. Sun ANL, Argonne, Illinois, USA
	Funding: U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. The proposed Advance Photon Source Upgrade (APS-U) employs a multi-bend achromat (MBA) lattice to increase the photon brightness by two to three orders of magnitude. One of the main design challenges of the upgrade is to minimize rf heating and collective instabilities associated with the impedance of small-aperture vacuum components. As part of this effort, my research focuses on impedance measurement and simulation of various MBA vacuum components. Here, we present the summary of the impedance contributions for the APS-U and describe our planned impedance measurement technique, including some measurement results for the non-evaporative getter (NEG)-coated copper chamber and simulation results for other critical components using a novel Goubau line (G-line) set up.
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Paper

Title

Page

Coupled Bunch Instability and Its Cure at J-PARC RCS

2946

Y. Shobuda, H. Harada, H. Hotchi, P.K. Saha, T. Takayanagi, F. Tamura, N. Tani, T. Togashi, Y. Watanabe, K. Yamamoto, M. Yamamoto
JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken, Japan
Y.H. Chin, Y. Irie, T. Toyama
KEK, Ibaraki, Japan

The RCS at J-PARC is a kicker-impedance dominant machine, which violates the impedance budget from a classical viewpoint. Nevertheless, we have recently succeeded to accelerate a 1-MW equivalent beam by making maximum use of the space charge effect on the beam instabilities. In this report, we explain the manipulation to suppress the beam instability, at first. Then, we discuss some issues to suppress the beam instabilities for beams with much smaller transverse emittance, as well as the present status of our efforts to reduce the kicker impedance toward the realization of the higher beam power at the RCS.

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WEPIK054

Evaluation and Attenuation of Sirius Components Impedance

3048

H.O.C. Duarte, L. Liu, S.R. Marques
LNLS, Campinas, Brazil

The Sirius in-vacuum components have their design improvements, possibilities and choices presented, where wake heating, single-bunch and multi-bunch effects and mechanical aspects were taken into account. The results were finally evaluated and added to the Sirius impedance budget.

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WEPIK055

Analysis and Countermeasures of Wakefield Heat Losses for Sirius

3052

H.O.C. Duarte, L. Liu, S.R. Marques, T.M. da Rocha, F.H. de Sá
LNLS, Campinas, Brazil

Design evaluation and possible solutions for several in-vacuum components of Sirius are presented, having their impedance analysis focused on mitigating the wake heating impact. Thermal and/or structural simulation of the models are carried out by considering the heat load directly obtained from wakefield simulations with resistive wall boundary conditions.

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WEPIK058

Preliminary Longitudinal Impedance Model for the ESRF-EBS

3063

S.M. White
ESRF, Grenoble, France

In light sources, longitudinal beam coupling impedance can deteriorate performance through bunch lengthening or increased longitudinal emittance due to the microwave instability. Simulation estimates are therefore required to devise the appropriate counter-measures if necessary. The main contributors to the longitudinal impedance model of the new ESRF-EBS storage ring were simulated. A preliminary longitudinal impedance model is presented and preliminary tracking simulations are shown.

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WEPIK066

Calculation of Wakefields and Higher Order Modes for the Vacuum Chamber of the CMS, ATLAS, ALICE and LHCb Experiments for the HL-LHC

3081

R. Wanzenberg, O. Zagorodnova
DESY, Hamburg, Germany
E. Métral, B. Salvant
CERN, Geneva, Switzerland

Funding: Partly funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement 284404.
The High Luminosity Large Hadron Collider (HL-LHC) project was started with the goal to extend the discovery potential of the Large Hadron Collider (LHC). The HL-LHC study implies also upgraded dimensions of the experimental beam pipes of the CMS, ATLAS, ALICE and LHCb experiments. The trapped monopole and dipole Higher Order Modes (HOMs) and the short range wakefields for the new design of the vacuum chambers were calculated with help of the computer codes MAFIA and ECHO2D. The results of the short range wakefields calculations and the HOMs calculations are presented in this report. The short range wakefields are presented in terms of longitudinal and transverse wake potentials and also in terms of loss and kick parameters. Selected results from the HOMs calculations , including the the frequency, the loss parameter, the R/Q and the Q value are presented.

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WEPIK071

Resistive-Wall Impedance Effects for the New KEK Light Source

3095

N. Nakamura
KEK, Ibaraki, Japan

KEK Light Source (KEK-LS) is a 3-GeV storage ring of 20-cell HMBA (Hybrid Multi-Bend Achromat) lattice*, which is planned to be constructed as a successor of the two existing Photon Factory storage rings (PF ring and PF-AR) in the KEK Tsukuba Campus. In this ring, a lot of in-vacuum undulators with a small magnetic gap (4 mm at minimum) will be installed and the vacuum pipe of a small aperture (25 mm in diameter) will be used. In addition, NEG coating, having a low electric conductivity, will be utilized for the vacuum pipe to ensure a sufficient beam lifetime early in the machine commissioning. In this paper, the heating power due to the longitudinal RW impedance and the growth rate of coupled-bunch instability caused by the transverse RW impedance are calculated and the effects of the RW impedance on KEK-LS are presented.
* K. Harada et al., Proc. of IPAC2016, Busan, Korea, pp.3251-3253; K. Harada et al., these proceedings.

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WEPIK078

Development of the Impedance Model in HEPS

3110

N. Wang, Z. Duan, X.Y. Li, H. Shi, S.K. Tian, G. Xu
IHEP, Beijing, People's Republic of China

The High Energy Photon Source (HEPS) is a new designed photon source at beam energy of 6 GeV. Due to the small beam pipe aperture and a large number of insertion devices in the machine, the impedance can drive collective instabilities and limit the machine performance. Therefore, a thorough estimation of the coupling impedance is necessary in controlling the total impedance of the whole machine. A primary impedance model is obtained for the storage ring.

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WEPIK081

The Logitudinal Broadband Impedance and Energy Spread Measurements at the VEPP-4M Electron-Positron Collider

3117

V.M. Borin, V.A. Kiselev, G.Y. Kurkin, S.A. Nikitin, P.A. Piminov, S.V. Sinyatkin
BINP SB RAS, Novosibirsk, Russia
V.L. Dorokhov, O.I. Meshkov
BINP, Novosibirsk, Russia

The paper presents measurements of the longitudinal broadband impedance and beam energy spread of the beam at the VEPP-4M electron-positron collider in an energy range of 1.0 - 3.5 GeV. In order to measure the longitudinal bunch size at various beam currents we used PS-1/S1 streak camera with picosecond temporal resolution. The dependence of bunch length from the bunch current at different energies demonstrates a microwave instability threshold. The bunch lengthening was caused by potential well distortion as well. Potential well distortion lengthening was used to estimate a value of the reactive part of the longitudinal impedance of the vacuum chamber of the collider Observed microwave instability thresholds was used to measure the value of the broadband impedance. The impact of the Touschek effect in the beam energy spread is discussed.

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WEPIK089

Characterization of Resonant Impedances of CERN-SPS Gate Valves

3139

T. Kaltenbacher, J. Repond, C. Vollinger
CERN, Geneva, Switzerland

For the CERN High Luminosity LHC project, a doubling of bunch intensity is foreseen. However, this intensity increase is currently limited by the LHC injector chain, in part due to longitudinal multi-bunch instabilities in the SPS. Therefore, the implementation of an accurate SPS impedance model was started some time ago in order to obtain a better understanding of instability sources and develop mitigation measures. In this paper, we present the electromagnetic characterization of commonly used all-metal gate valves with respect to their contribution to the SPS longitudinal impedance. The valve impedance was evaluated with commercially available EM-field simulation programs and verified with RF-bench measurements. Using this input, it was possible to obtain in particle simulations the dependence of the multi-bunch stability threshold on the number of these valves. A practical means of mitigation is to use a commercially available impedance shielded version of these gate valves. We also present the associated reduction in beam coupling impedance and the expected gain in beam stability if all existing unshielded valves are replaced by shielded valves.

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WEPIK090

Characterization of Shielding for the CERN-SPS Vacuum Flanges With Respect to Beam Coupling Impedance

3143

T. Kaltenbacher, C. Vollinger
CERN, Geneva, Switzerland

Longitudinal multi-bunch instabilities in the CERN-SPS pose a serious limitation for future beam intensities required for high luminosity LHC. Hence, an impedance model for the SPS accelerator was developed from which one group of vacuum flanges could be identified as being a major culprit for these instabilities. These flanges support high impedance modes and their impact on beam stability was traced to a longitudinal mode at about 1.4GHz. For improvement of multi-bunch stability threshold, this group of flanges will be shielded as part of an impedance reduction campaign. We describe the evaluation of different impedance shielding designs proposed to reduce the longitudinal beam coupling impedance of this group of vacuum flanges in the SPS. EM-field simulations were performed to identify remaining resonances in these vacuum flanges with impedance shield prototypes installed, and the simulation models were benchmarked with RF-measurements. Depending on the performance and other parameters, the most suitable shield design will be selected, built and installed. As a first step, the installation of one shielding design in some positions in the SPS is planned for the beginning of 2017.

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WEPIK094

LEIR Impedance Model and Coherent Beam Instability Observations

3159

N. Biancacci, H. Bartosik, A. Huschauer, E. Métral, T.L. Rijoff, B. Salvant, R. Scrivens
CERN, Geneva, Switzerland
M. Migliorati
University of Rome La Sapienza, Rome, Italy

The LEIR machine is the first synchrotron in the ion acceleration chain at CERN and it is responsible to deliver high intensity ion beams to the LHC. Following the recent progress in the understanding of the intensity limitations, detailed studies of the machine impedance started. In this work we describe the present LEIR impedance model, detailing the contribution to the total longitudinal and transverse impedance of several machine element. We then compare the machine tune shift versus intensity predictions against measurements at injection energy and summarize the coherent instability observations in absence of transverse damper feedback.

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WEPIK095

Evaluation of Longitudinal Beam Impedance in the Beam Gas Ionization Monitor of the CERN-PS Accelerator

3163

N. Nasr Esfahani, T. Kaltenbacher, J.W. Storey, C. Vollinger
CERN, Geneva, Switzerland

The recently observed beam induced heating issues in the BGI monitors of the LHC which could have been occurred due to a strong coupling between the beam and the localized modes at the sensor location showed the general importance of a thorough evaluation of the beam coupling impedance and the corresponding heat deposit in beam monitoring equipments. This paper is devoted to the examination of the beam coupling impedance and beam induced heating for a currently under development beam gas ionization (BGI) monitor which is intended to be a part of the CERN Proton Synchrotron (PS) beam monitoring equipment. Details of the EM and wake field simulations for this BGI monitor together with the RF measurement results and power loss calculations will be presented.

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WEPIK096

Assessment of Beam Impedance for the CERN-PS Booster Wire Scanner

3167

T. Kaltenbacher, N. Nasr Esfahani, C. Vollinger
CERN, Geneva, Switzerland

It is well known that performance of accelerators critically depends on the interaction of high intensity beams with the surrounding structures. As a result of these beam interactions, it is required at CERN to characterize the beam coupling impedance of each new machine element that is to be installed in the accelerator ring. In the framework of the LIU (LHC Injectors Upgrade) project, a new design of rotational wire scanner to be used in the PS Booster is currently under development. As an intermediate step, the prototype of this wire scanner was evaluated with respect to its longitudinal beam coupling impedance. Depending on the performance of this machine element, it is planned to replace existing wire scanners in other machines at CERN (e.g. PS-Booster, PS and SPS) with very similar designs. This paper presents the simulations and describes the measurement methods used for benchmarking electromagnetic simulations performed for the impedance evaluation of the LIU wire scanner for the PS-Booster. Additionally, the device was fitted with an RF feed-through in order to monitor and attenuate certain undesired modes supported by this structure.

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WEPIK100

The Applicability of NEG Coated Undulator Vessels for the CLARA FEL Test Facility

3181

O.B. Malyshev, K.B. Marinov, K.J. Middleman, N. Thompson, R. Valizadeh, P.H. Williams
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
O.B. Malyshev, K.J. Middleman, N. Thompson, R. Valizadeh, P.H. Williams
Cockcroft Institute, Warrington, Cheshire, United Kingdom

CLARA is a FEL test facility at Daresbury Laboratory (DL), UK. The undulator vacuum chamber is 20 m long with inner diameter 6 mm and its vacuum performance can benefit from a NEG coating. The thickness of the coating layer must be carefully optimised. A layer ~ 1 um would help the vacuum but a thinner layer would be partially transparent for the EM field reducing the resistive wall wakefields due to the NEG. A very thin layer, however, may not yield the necessary vacuum performance. Two types of NEG coatings produced at DL - dense and columnar - were considered. Their bulk conductivities were measured in a separate study. The resistive wall wakefield impedance was calculated following the standard approach for multilayer vessels. A 250 fs rms electron bunch was generated in ASTRA and its wakefield was obtained from the vessel impedance. The FEL performance was then studied through GENESIS simulations and the result compared to the case with no wakefields. It was found that NEG layers thicker than 100 nm give an unacceptable reduction of the FEL power and the vacuum performance of such thin coatings is unknown. Possible solutions to this problem are discussed.

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WEPIK102

Measurement of RF Resonances and Measured Impact on Transverse Multibunch Instabilities from In-vacuum Insertions Devices

3188

G. Rehm
DLS, Oxfordshire, United Kingdom

Diamond Light Source has currently 15 in-vacuum insertion devices (ID) installed, mostly built in-house. Their measured impact on multi-bunch mode damping as a result of varying magnet gap was shown before, now we augment these with measurements of broadband frequency spectra with stored beam obtained using an antenna placed in the ID vacuum. Finally, we present off-line measurements of resonances in the ID vessel acquired using a vector network analyser and two antennae installed in-vacuum.

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WEPIK106

Impedance and Collective Effects for the Advanced Light Source Upgrade at LBNL

3192

S. Persichelli, J.M. Byrd, S. De Santis, D. Li, T.H. Luo, C. Steier, M. Venturini
LBNL, Berkeley, California, USA

Funding: Work supported by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231
The upgrade of the Advanced Light Source (ALS-U) consists of a multiband achromat ultralow emittance lattice for the production of diffraction-limited soft x-rays. A very important issue for ALS-U is represented by instabilities induced by wakefields, that may limit the peak current of individual bunches and the total beam current. In addition, vacuum chamber apertures of few millimeters, that are a key feature of low-emittance machines, can result in a significant increase in the Resistive Wall (RW) impedance. In this paper we present progress on establishing short range wakefield model for ALS-U and evaluating the impact on the longitudinal and transverse single-bunch dynamics.

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WEPIK109

Experimental Study of Vertical-Longitudinal Coupling Induced by Wakefields at CesrTA

3200

S. Wang, J.D. Perrin, S. Poprocki, D. L. Rubin
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: Work supported by NSF PHY-1416318, PHYS-1068662
Transverse vertical wakefields can cause vertical beam size growth in accelerators. Here we report recent measurements and simulations of wakefields from movable scrapers at the CesrTA. The charge dependent vertical beam size growth was observed while a single scraper was inserted through the top of the chamber. No change in the beam size was observed with top and bottom scrapers inserted symmetrically. The apparent growth in the vertical beam size was due in large part to the y-z coupling (vertical crabbing) induced by the monopole wake of the asymmetric scraper configuration. We explored this y-z coupling by varying vertical betatron phase advance between the vertical beam size monitor and the scrapers. In addition, we found that existing residual, current independent y-z coupling could be compensated by the scraper wake. Predictions of a tracking simulation are in good agreement with the measurements.

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WEPIK110

Resistive Wall Instability and Impedance Studies of Narrow Undulator Chamber in CHESS-U

3204

S. Wang, M.G. Billing, S. Poprocki, D. L. Rubin, D. Sagan
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: Work supported by NSF DMR-0936384 and NSF DMR-1332208
In a major upgrade of the Cornell Electron Storage Ring (CESR) one sextant of ring will be replaced with double bend achromats (DBAs) and undulator straights for x-ray users. The resistive wall impedance from the narrow gap (4.5 mm) undulator chambers (5 m per straight) may limit total beam. Here we report recent results of modelling and calculation of multibunch instabilities due to the impedance of chamber walls and transition tapers. The short range wakefields and resistive wall impedance are modelled and incorporated in a tracking simulation. The coupled-bunch growth rate found with the tracking study is in good agreement with the analytic approximation. We find that the resistive wall instability can be readily damped by our existing bunch-by-bunch feedback system.

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WEPIK113

Entrance and Exit CSR Impedance for Non-Ultrarelativistic Beam

3214

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

Funding: Work supported by the Department of Energy, Laboratory Directed Research and Development Funding, under Contract No. DE-AC05-06OR23177
For a high-brightness electron beam being transported through beamlines involving bending systems, the coherent synchrotron radiation (CSR) and longitudinal space charge (LSC) interaction could often cause microbunching instability. The semi-analytical Vlasov solver for microbunching gain* depends on the impedances for the relevant collective effects. The existing results for CSR impedances are usually obtained for the ultrarelativistic limit. To extend the microbunching analysis to cases of low energies, such as the case of an ERL merger, or to density modulations at extremely small wavelength, it is necessary to extend the impedance analysis to the non-ultrarelativistic regime. In this study, we present the impedance analysis for the transient CSR interaction in the non-ultrarelativistic regime, for transients including both entrance to and exit from a magnetic dipole. These impedance results will be compared to their ultra-relativistic counterparts**, and the corresponding wakefield obtained from the impedance for low-energy beams will be compared with the existing results of transient CSR wakefield for general beam energies***.
* C.-Y. Tsai et al., Proc. of IPAC'15, 596 (2015).
** C. Mitchell et al ., Proc. of IPAC'13, 1832 (2014).
*** E. L. Saldin et al ., Nucl. Instrum. Meth. A 398, 373 (1997).

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WEPIK117

A Comprehensive Study of the Microwave Instability

3224

A. Blednykh, B. Bacha, G. Bassi, O.V. Chubar, M.S. Rakitin, V.V. Smaluk, M. Zhernenkov
BNL, Upton, Long Island, New York, USA

Funding: Work supported by DOE contract DE-SC0012704
Several instability thresholds and special waveform beam pattern have been observed during measurements of the horizontal beam size change vs single bunch current by the synchrotron light monitor (SLM) camera installed in a low dispersion area of the NSLS-II storage ring. The electron beam energy spread from In-Vacuum Undulator (IVU) of the Soft Matter Interfaces (SMI) beam line confirmed the microwave beam pattern behavior as a current dependent effect. The numerically obtained total longitudinal wakepotential by the GdfidL code allowed us to compare the measured results with particle tracking simulations using the SPACE code. The instability thresholds behavior at different RF voltages are in some sort of overarching agreement.

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WEPIK118

Synchronous Phase Shift from Beam Loading Analysis

3227

G. Bassi, A. Blednykh, J. Rose, V.V. Smaluk, J. Tagger
BNL, Upton, Long Island, New York, USA

We discuss measurements, performed in the NSLS-II storage ring, of the synchronous phase shift as a function of single bunch current from beam loading parameters. The synchronous phase is calculated from the forward and reflected power measured in the RF cavities. The comparison with direct synchronous phase measurements shows good agreement.

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WEPVA101

Review of Stripline Beam Impedance: Application to the Extraction Kicker for the CLIC Damping Rings

3499

C. Belver-Aguilar, M.J. Barnes
CERN, Geneva, Switzerland

The beam coupling impedance of the stripline kicker for beam extraction from the CLIC Damping Rings (DRs) has been studied analytically, numerically with CST Particle Studio (PS) and measured in the laboratory, although not all the results were understood. In order to have a better knowledge about the beam coupling impedance of a stripline kicker, a simple model has been first studied, with flat electrodes and a cylindrical beam pipe. From this preliminary study, a new approach for the dipolar component of the horizontal impedance has been derived, when considering both odd and even operating modes of the striplines. This new approach has been used to understand the differences found between the predicted transverse impedance and the two wire measurements carried out in the laboratory for the prototype CLIC DR striplines. Future tests of beam coupling impedance with beam in the ALBA Synchrotron Ligth Source will complete this study.

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WEPVA134

Impedance Measurement of Vacuum Chamber Components for the Advance Photon Source (APS) Upgrade

3583

SUSPSIK052

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M.P. Sangroula
IIT, Chicago, Illinois, USA
R.M. Lill, R.R. Lindberg, X. Sun
ANL, Argonne, Illinois, USA

Funding: U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357.
The proposed Advance Photon Source Upgrade (APS-U) employs a multi-bend achromat (MBA) lattice to increase the photon brightness by two to three orders of magnitude. One of the main design challenges of the upgrade is to minimize rf heating and collective instabilities associated with the impedance of small-aperture vacuum components. As part of this effort, my research focuses on impedance measurement and simulation of various MBA vacuum components. Here, we present the summary of the impedance contributions for the APS-U and describe our planned impedance measurement technique, including some measurement results for the non-evaporative getter (NEG)-coated copper chamber and simulation results for other critical components using a novel Goubau line (G-line) set up.

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