IPAC2014 - Classification: 05 Beam Dynamics and Electromagnetic Fields / D05 Instabilities

Paper	Title	Page
TUPME026	TMCI Thresholds for LHC Single Bunches in the CERN-SPS and Comparison with Simulations	1407
	H. Bartosik, G. Iadarola, Y. Papaphilippou, G. Rumolo, B. Salvant, C. Zannini CERN, Geneva, Switzerland
	At the beginning of 2013 an extensive measurement campaign was carried out at the SPS in order to determine the Transverse Mode Coupling Instability thresholds of LHC-type bunches in a wide range of intensities and longitudinal emittances. The measurements were performed in two different configurations of machine optics (nominal and low gamma transition) with the goal to characterize the differences in behavior and performance. The purpose of this paper is to describe in detail the measurement procedure and results, as well as the comparison of the experimental data with HEADTAIL simulations based on the latest SPS impedance model. Beside the impedances of the resistive wall, the beam position monitors (BPMs), the RF cavities, and the flanges, an advanced model of the impedance of the kicker magnets is included, which are found to play a major role in the definition of the stability region of the LHC-type bunches in the two optics configurations studied.
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TUPME029	Identification of High-frequency Resonant Impedance in the CERN SPS	1416
	E.N. Shaposhnikova, T. Argyropoulos, T. Bohl, J.V. Campelo, F. Caspers, J.F. Esteban Müller, A. Lasheen, B. Salvant, H. Timko CERN, Geneva, Switzerland
	The spectrum of long bunches injected into the ring with RF switched off has been used in the SPS in the past to probe the longitudinal coupling impedance. After a large campaign of shielding of 800 inter-magnet vacuum ports in 1999 - 2001, the microwave instability threshold was significantly increased and the high-frequency spectrum of the beam became practically flat, apart from a prominent peak at around 1.4 GHz. As corresponding high-frequency impedance could potentially lead to microwave instability of high intensity bunches observed now at high energies in the SPS, a search of the source of this impedance was launched. Using a combination of impedance simulations and measurements, vacuum flanges that are present in a large quantity in the machine have been identified as a main source of impedance at this frequency. Particle simulations based on the SPS impedance model, which includes this previously unknown impedance, are able to reproduce the characteristics of the bunch spectrum and amplitude growth rates and hence, confirm that the impedance of the vacuum flanges is responsible for the observed spectral peak.
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TUPRI021	Impedance Calculation and Simulation of Microwave Instability for the Main Rings of SuperKEKB	1600
	D. Zhou, T. Abe, T. Ishibashi, Y. Morita, K. Ohmi, K. Shibata, Y. Suetsugu, M. Tobiyama KEK, Ibaraki, Japan
	The SuperKEKB B-factory is now under construction. The designs of the components for the SuperKEKB have mostly been finished. This paper summarises the updated results of longitudinal impedance calculations for various components of the main rings. By summing up all available impedances, a pseudo-Green wake function with bunch length of σ_z=0.5 mm is constructed as an impedance model for consequent studies of collective effects. The results of these studies are also reported in this paper.
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TUPRI028	Review of Rest Gas Interaction at Very Low Energies applied to the Extra Low ENergy Antiproton ring ELENA	1621
	C. Carli, T.L. Rijoff CERN, Geneva, Switzerland O. Karamyshev, C.P. Welsch Cockcroft Institute, Warrington, Cheshire, United Kingdom O. Karamyshev, C.P. Welsch The University of Liverpool, Liverpool, United Kingdom
	The Extremely Low ENergy Antiproton ring (ELENA) is a small synchrotron equipped with an electron cooler, which shall be constructed at CERN to decelerate antiprotons to energies as low as 100 keV. Scattering of beam particles on rest gas molecules may have a detrimental effect at such low energies and leads to stringent vacuum requirements. Within this contribution scattering of the stored beam on rest gas molecules is discussed for very low beam energies. It is important to carefully distinguish between antiprotons scattered out of the acceptance and lost, and those remaining inside the aperture to avoid overestimation of emittance blow-up. Furthermore, many antiprotons do not interact at all during the time they are stored in ELENA and hence this is not a multiple scattering process
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TUPRI036	Fast Ion Instability at CESR-TA	1638
	A. Chatterjee, K.J. Blaser, M. P. Ehrlichman, D. L. Rubin, J.P. Shanks Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: Work supported by NSF and DOE Contracts No. PHY-0734867, No. PHY-1002467, No. PHYS-1068662, No. DE-FC02-08ER41538, No. DE-SC0006505, and the Japan/U.S. Cooperation Program. Fast Ion Instability can lead to deterioration of an electron beam (increasing emittance and instability of a train of bunches) in storage rings and linacs. We study this at the Cornell Electron Storage Ring Test Accelerator using a 2.1 GeV low emittance beam. As the source of ions is residual gas, our measurements are conducted at various pressures, including nominal vacuum as well as injected gas (Ar, Kr). We measure turn-by-turn vertical bunch size and position, as well as the multi-bunch power spectrum. A detailed simulation is then used to compare theory with observations.
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TUPRI037	Some Features of Wave Distribution in the Thin-Wall Waveguide	1641
	M. Ivanyan, L.V. Hovakimyan, A. Sargsyan CANDLE SRI, Yerevan, Armenia
	In this report we derive rigorous and approximate dispersion relations for the round resistive thin-wall waveguide. The features of the distributions of dispersion curves of the waveguide axisymmetric TM modes are obtained. Cases of splitting and degeneracy of modes under consideration are detected and regularities of their behaviours are established.
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TUPRI038	The Low Energy Particle Wakefield Radiation From the Open End of Internally Coated Metallic Tube	1644
	M. Ivanyan, A. Grigoryan, A. Sargsyan, A.V. Tsakanian CANDLE SRI, Yerevan, Armenia
	The radiation of the non-relativistic electron beam from the open end of the resistive circular waveguide is presented. The angular and spectral characteristics of the radiation are determined. The possibility of producing the focused guasi-monochromatic radiation is discussed. The principal scheme of the experiments for 5 and 20 MeV AREAL RF photogun linac is presented.
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TUPRI041	Study of Collective Beam Instabilities for Sirius	1653
	F.H. de Sá, H.O.C. Duarte, L. Liu, N. Milas, X.R. Resende LNLS, Campinas, Brazil
	In this paper we present the on going work of construction of the Sirius impedance budget and instability threshold estimates for several machine operation scenarios.
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TUPRI042	Numerical Study of the Microbunching Instability at UVSOR-III: Influence of the Resistive and Inductive Impedances	1656
SUSPSNE061	use link to see paper's listing under its alternate paper code
	E. Roussel, S. Bielawski, C. Evain, C. Szwaj PhLAM/CERCLA, Villeneuve d'Ascq Cedex, France M. Adachi, M. Katoh, S.I. Kimura, T. Konomi UVSOR, Okazaki, Japan M. Hosaka, Y. Takashima, N. Yamamoto Nagoya University, Nagoya, Japan K.S. Ilin, J. Raasch, A. Scheuring, M. Siegel, P. Thoma KIT, Karlsruhe, Germany H. Zen Kyoto University, Kyoto, Japan
	At high charge, relativistic electron bunches circulating in storage rings undergo an instability, the so-called microbunching or the CSR (Coherent Synchrotron Radiation) instability. This instability is due to the interaction of the electrons with their own radiation and leads to the formation of microstructures (at millimeter scale) in the longitudinal phase space. Thanks to a new type of detector, based on superconducting thin film YBCO, it is now possible to observe directly these microstructures and follow their temporal evolution. These experimental observations open a new way to make severe comparisons with theory. Here we present results of the modeling of the dynamics at UVSOR-III using a one dimensional Vlasov-Fokker-Planck equation. We show that to obtain a relatively good agreement between numerical simulations and experiments, we have to take into account several types of impedance such as the shielded CSR impedance but also the resistive and inductive impedances. First Direct, Real Time, Recording of the CSR Pulses Emitted During the Microbunching Instability, using Thin Film YBCO Detectors at UVSOR-III, IPAC2014
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TUPRI043	Analysis of Coupled Bunch Instabilities in BESSY-VSR	1659
	M. Ruprecht, P. Goslawski, A. Jankowiak, M. Ries, A. Schälicke, G. Wüstefeld HZB, Berlin, Germany T. Weis DELTA, Dortmund, Germany
	BESSY-VSR, a scheme where 1.5 ps and 15 ps long bunches (rms) can be stored simultaneously in the BESSY II storage ring has recently been proposed. The strong longitudinal bunch focusing is achieved by superconducting high gradient RF cavities. This paper presents investigations of coupled bunch instabilities driven by HOMs of superconducting multi cell cavities in BESSY-VSR. Analytical calculations and tracking simulations in time domain are performed in the longitudinal and the transverse planes and factors that influence the threshold currents are being discussed. Suitable candidates of cavities which are presently available or in the phase of design are compared with respect to their instability thresholds. G. Wüstefeld, A. Jankowiak, J. Knobloch, M. Ries, Simultaneous Long and Short Electron Bunches in the BESSY II Storage Ring, Proceedings of IPAC2011, San Sebastián, Spain
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TUPRI044	Investigation of Microbunching-instability in BERLinPro	1662
	S.D. Rädel, A. Jankowiak, A. Meseck HZB, Berlin, Germany
	Funding: Work supported by German Bundesministerium für Bildung und Forschung, Land Berlin, and grants of Helmholtz Association BERLinPro is using the new energy recovery linac technology. As, maintaining the low emittance and energy spread is of major importance in an ERL, the deep understanding and control of effects which can degrade the emittance and energy spread such as space charge effects are of interest. The microbunching caused by the longitudinal space charge forces can lead to an increase in emittance and energy spread in the arcs of the loop. In this contribution, the impacts of the microbunching instability on the beam quality and its implication for BERLinPro are discussed.
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TUPRI045	Beam Coupling Impedance Simulation in the Frequency Domain for the SIS100 Synchrotron	1665
	U. Niedermayer, O. Boine-Frankenheim TEMF, TU Darmstadt, Darmstadt, Germany O. Boine-Frankenheim GSI, Darmstadt, Germany
	For the quantification of intensity thresholds due to coherent instabilities and beam induced heating in the FAIR synchrotron SIS100 a detailed knowledge of transverse and longitudinal beam coupling impedance is required. Due to the rather long proton and heavy-ion bunches, the relevant spectrum is below 100MHz. For the computation of beam coupling impedances in the low frequency regime, frequency domain methods are more advantageous than (explicit) time domain methods. We show the setup of a 2D finite element code that allows to compute the impedance for arbitrary longituninally homogeneous beam and structure shapes. Perfectly conducting pipes, a dispersive ferrite tube, and thin resistive beam pipe serve as test cases. The influence of the beam velocity on the coupling impedance is studied.
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TUPRI046	Dynamics of Ion Distributions in Beam Guiding Magnets	1668
	A. Markoviḱ, G. Pöplau, U. van Rienen Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany W. Hillert, D. Sauerland ELSA, Bonn, Germany A. Meseck HZB, Berlin, Germany
	Funding: Supported by the German Federal Ministry of Education and Research (BMBF) under contract number 05K13HRC. Ions generated by synchrotron radiation and collisions of the beam with the rest gas in the vacuum chamber could be a limiting factor for the operation of electron storage rings and Energy Recovery Linacs (ERL). In order to develop beam instability mitigation strategies, a deeper understanding of the ion-cloud behaviour is needed. Numerical simulations of the interaction between electron beams and parasitic ions verified with dedicated measurements can help to acquire that knowledge. This paper presents results of detailed simulations of the interaction in quadrupole magnets and drift sections of the Electron Stretcher Accelerator ELSA in Bonn. The focus is on the evaluation of the dynamics of different ion species and their characteristic distribution in quadrupole magnets.
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TUPRI047	Electromagnetic Modeling of Open Cell Conductive Foams for High Synchrotron Radiation Rings	1671
	S. Petracca, A. Stabile U. Sannio, Benevento, Italy A. Stabile INFN-Salerno, Baronissi, Salerno, Italy
	Open cell conductive foams (OCMF) have been recently suggested as an alternative to perforated metal patches for efficiently handling gas desorption from the beam pipe wall due to intense synchrotron radiation, yielding superior performance in terms of residual gas concentration and beam shielding. Experimental work is ongoing to assess their properties, including secondary emission yields and beam coupling impedances. In this communication we attempt a review of the Literature about electromagnetic modeling of OCMF, and outline a general framework for computing the surface impedance of OCMF walls and deriving the longitudinal and transverse beam coupling impedances thereof, based on effective medium theory and electromagnetic reciprocity. A critical analysis of the relevant modeling approximations is included.
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TUPRI048	A Map Approach for Electron Cloud Density in a Strong LHC Dipole	1674
	S. Petracca, A. Stabile U. Sannio, Benevento, Italy A. Stabile INFN-Salerno, Baronissi, Salerno, Italy
	The luminosity is limited by the electron cloud effects in presently running and proposed future storage rings. The evolution of the electron density during the electron cloud formation can be reproduced using a bunch-to-bunch iterative map formalism. By performing simulation codes this approach has been used to obtain a numerical prediction of the coefficients in the map, while in the presence of a magnetic field an analytic formula has been obtained for the linear coefficient. The next goal is finding a theoretical prescription of the quadratic coefficient at least in the presence of magnetic dipole. Then it will be possible to reproduce, by using the map formalism, the dynamics of electron cloud without performing the simulations.
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TUPRI049	Geometric Beam Coupling Impedance of LHC Secondary Collimators	1677
SUSPSNE059	use link to see paper's listing under its alternate paper code
	O. Frasciello, S. Tomassini, M. Zobov INFN/LNF, Frascati (Roma), Italy A. Grudiev, N. Mounet, B. Salvant CERN, Geneva, Switzerland
	Funding: Work supported by European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement 284404 The High Luminosity LHC project is aimed at increasing the LHC luminosity by an order of magnitude. One of the key ingredients to achieve the luminosity goal is the beam intensity increase. In order to keep under control beam instabilities and to avoid excessive power losses a careful design of new vacuum chamber components and an improvement of the present LHC impedance model are required. Collimators are the main impedance contributors. Measurements with beam have revealed that the betatron coherent tune shifts were by about a factor of 2 higher with respect to the theoretical predictions based on the current model. Up to now the resistive wall impedance has been considered as the major impedance contribution for collimators. By carefully simulating their geometric impedance we show that for the graphite collimators with half-gaps higher than 10 mm the geometric impedance exceeds the resistive wall one. In turn, for the tungsten collimators the geometric impedance dominates for all used gap values. Hence, including the geometric collimator impedance into the LHC impedance model enabled us to reach a better agreement between the measured and simulated collimator tune shifts.
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TUPRI050	Numerical Calculation and Experiment of Ion Related Phenomenon in SPring-8 Storage Ring	1680
	A. Mochihashi, M. Takao JASRI/SPring-8, Hyogo-ken, Japan
	In the SPring-8 storage ring, various kinds of bunch filling pattern are available. Under some bunch filling patterns, residual gas ions created by scattering process between high energy electrons and residual gas molecules can be trapped stably around the electron beam and disturb the original motion of the beam. We have considered the stability of the electron beam due to the ion related phenomenon under several bunch filling patterns by computer simulation. In the simulation, we have modeled the electron bunch as single particle and the residual gas ions as macroparticles. The number of the trapped ions, size of the ion cloud and change in betatron oscillation amplitude of the beam under several filling pattern conditions will be discussed. We have also performed experiments for stability of the beam under equally spaced bunch filling patterns which give severe condition for the ion related instability. The numerical calculations and the experimental results will be discussed in the presentation.
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TUPRI051	Comparison between Measurements and Orbit Code Simulations for Beam Instabilities due to Kicker Impedance in the 3-GeV RCS of J-PARC	1683
	P.K. Saha, H. Harada, N. Hayashi, H. Hotchi, Y. Shobuda, F. Tamura, M. Yamamoto JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken, Japan
	The transverse impedance of the extraction kicker magnets is the most dominant beam instability source in the 3-GeV Rapid Cycling Synchrotron of J-PARC. The instability occurs when chromaticity is fully corrected during acceleration but on the other hand no instabilities are observed for a full chromatic correction only at the injection energy even for a beam power up to 500 kW. However, the situation may change for a beam power of 1 MW and also for the upgraded injection beam energy from the present 181 MeV to the 400 MeV, as space charge effect in the non-relativistic region is believed to suppress the growth rate of beam instability. In order to study the kicker impedance in detail, recently we have introduced measured time dependent impedance source in the ORBIT simulation code in a realistic manner. The ORBIT code itself has also been well upgraded and given realistic features for application to synchrotrons. We have also carried out a systematic experimental study for a maximum beam power of 500 kW. In this paper, a detail comparison between measurements and corresponding simulations including 1 MW simulation results are presented.
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TUPRI052	Analysis of Single Bunch Measurements at the ALBA Storage Ring	1686
	T.F.G. Günzel, U. Iriso, F. Pérez CELLS-ALBA Synchrotron, Cerdanyola del Vallès, Spain E. Koukovini-Platia, G. Rumolo CERN, Geneva, Switzerland
	Measurements of the vertical single bunch mode detuning and the TMCI threshold at zero chromaticity were carried out and their results were compared to the theoretical expectation. Around 65% of the found mode detuning can be explained by a developed transverse impedance model. A good bunch length parametrisation with current contributed essentially to this result. The analysis of single bunch measurements at non-zero chromaticity will also be presented.
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TUPRI053	Transverse Beam Instabilities in the MAX IV 3 GeV ring	1689
	G. Skripka, P.F. Tavares MAX-lab, Lund, Sweden M. Klein, R. Nagaoka SOLEIL, Gif-sur-Yvette, France
	Collective effects in MAX IV 3 GeV storage ring are strongly enhanced by the combination of low emittance, high current and small effective aperture. Three passive harmonic cavities (HC) are introduced to lengthen the bunches, by which beam stabilization is anticipated via decoupling to high frequency wakes, along with Landau damping. The role of the ransverse impedance budget of the MAX IV 3 GeV storage ring as a source of collective beam instabilities was determined. With the help of the macroparticle multi-bunch tracking code mbtrack that directly uses the former as input, we studied the influence of geometric and resistive wall impedance in both transverse planes, as well as that of chromaticity shifting. A fully dynamic treatment of the passive harmonic cavities developed for this study allowed us to evaluate their effectiveness under varying beam conditions.
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TUPRI054	FEM Analysis of Beam-coupling Impedance and RF Contacts Criticality on the LHC UA9 Piezo Goniometer	1692
	A. Danisi, R. Losito, A. Masi, A. Passarelli, B. Salvant, C. Zannini CERN, Geneva, Switzerland
	The UA9 piezo-goniometer has been designed to guarantee micro-radians-accuracy angular positioning of a silicon crystal for a crystal collimation experiment in the LHC, and to minimize the impact on the LHC beam-coupling impedance. This paper presents a Finite Element Method (FEM) study of the device, in both parking and operational positions, to evaluate its impact on the LHC impedance budget. The study has been a progressive simulation work, started from the simplification of the original detailed design, and aimed at highlighting the effect of single details (e.g. objects in confining chambers) on the longitudinal and transverse components of beam-coupling impedance. In addition, the shielding contribution of the RF gaskets has been carefully evaluated, with the objective to assess the consequences for operation in case of their failure. Sensitivity analyses to simulation parameters are also performed, in order to test the FEM model robustness. A final word is drawn on the overall device impedance criticality.
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TUPRI055	Theoretical Analysis of Metamaterial Insertions for Resistive-wall Beam-coupling Impedance Reduction	1695
	A. Danisi, R. Losito, A. Masi, B. Salvant, C. Zannini CERN, Geneva, Switzerland
	Resistive-wall impedance usually constitutes a significant percentage of the total beam-coupling impedance budget of many accelerator structures (e.g. for LHC, it can be more than 50%). Reduction techniques for resistive-wall components entail high electrical-conductivity coatings. This paper proposes the use of metamaterials, having negative values of magnetic permeability or dielectric permittivity (or both), for sensibly reducing or theoretically nearly cancelling the resistive-wall component of beam-coupling impedance. The proposed approach is developed by means of an equivalent transmission-line model, whose results show the potential reduction of both longitudinal and transverse impedance when using metamaterial insertions. The effects on the real and imaginary part have been singled out. The effectiveness of such materials is discussed both for negative-permittivity and for negative-permeability cases, which actually show different impacts and can be then target of proper engineering. This first-stage study opens the possibility of considering metamaterials for impedance mitigation or for setting up proper experimental setups.
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TUPRI056	Beam Measurements of the LHC Impedance and Validation of the Impedance Model	1698
	J.F. Esteban Müller, T. Argyropoulos, T. Bohl, N. Mounet, E.N. Shaposhnikova, H. Timko CERN, Geneva, Switzerland
	Different measurements of the longitudinal impedance of the LHC done with single bunches with various intensities and longitudinal emittances during measurement sessions in 2011-2012 are compared with particle simulations based on the existing LHC impedance model. The very low reactive impedance of the LHC, with Im(Z/n) around 0.1 Ohm, is not easy to measure. The most sensitive observation is the loss of Landau damping during acceleration, which shows at which energy bunches become unstable depending on their parameters. In addition, the synchrotron frequency shift due to the reactive impedance was estimated following two methods. Firstly, it was obtained from the peak-detected Schottky spectrum. Secondly, a sine modulation in the RF phase was applied to the bunches with different intensities and the modulation frequency was scanned. In both cases, the synchrotron frequency shift was of the order of the measurement precision.
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TUPRI057	Review of the Transverse Impedance Budget for the CLIC Damping Rings	1701
	E. Koukovini-Platia, G. Rumolo CERN, Geneva, Switzerland
	Single bunch instability thresholds and the associated coherent tune shifts have been evaluated in the transverse plane for the damping rings (DR) of the Compact Linear Collider (CLIC). A multi-kick version of the HEADTAIL code was used to study the instability thresholds in the case where different impedance contributions are taken into account such as the broad-band resonator model in combination with the resistive wall contribution from the arcs and the wigglers of the DR. Simulations performed for positive values of chromaticity showed that higher order bunch modes can be potentially dangerous for the beam stability.
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TUPRI058	Impedance Studies of the Dummy Septum for CERN PS Multi-turn Extraction	1704
	S. Persichelli, O.E. Berrig, M. Giovannozzi, J. Herbst, J. Kuczerowski, M. Migliorati, B. Salvant CERN, Geneva, Switzerland
	A protection septum has been installed in the CERN PS section 15 in order to mitigate irradiation of the magnetic septum 16 for fast extractions towards the SPS. Impedance studies have been performed, showing that beams circulating in the septum during extraction generate sharp resonances in the coupling impedance. Impedance measurements with the wire technique have been performed, showing a good agreement with simulations. Instability rise times of trapped modes have been evaluated and compared to extraction duration. Solutions for reducing the impact on the stability of the beam have been considered.
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TUPRI059	The Proton Synchrotron Transverse impedance model	4096
SUSPSNE060	use link to see paper's listing under its alternate paper code
	S. Persichelli, N. Biancacci, S.S. Gilardoni, M. Migliorati, E. Métral, B. Salvant CERN, Geneva, Switzerland
	The current knowledge of the transverse impedance of the CERN Proton Synchrotron (PS) has been established by theoretical computations, electromagnetic simulations and beam-based measurements at different energies. The transverse coherent tune and phase advance shifts as a function of intensity have been measured in order to evaluate the total effective transverse impedance and its distribution in the accelerator. In order to understand the beam dynamics, the frequency dependence of the impedance budget has also been evaluated considering the individual contribution of several machine devices. 3D models of many PS elements have been realized to perform accurate impedance simulations, while resistive wall and indirect space charge impedances have been evaluated with theoretical and numerical computations. Finally comparisons between the total budget and the measurement results are presented.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2014-TUPRI059
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TUPRI060	Impedance Studies for the PS Finemet® Loaded Longitudinal Damper	1708
	S. Persichelli, M. Migliorati, M.M. Paoluzzi, B. Salvant CERN, Geneva, Switzerland
	The impedance of the Finemet® loaded longitudinal damper cavity, installed in the CERN Proton Synchrotron straight section 02 during the Long Shutdown 2013-2014, has been evaluated. Time domain simulations with CST Particle Studio have been performed in order to get the longitudinal and transverse impedance of the device and make a comparison with the longitudinal impedance that was measured for a single cell prototype.
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TUPRI061	Power Loss Calculation in Separated and Common Beam Chambers of the LHC	1711
	C. Zannini, G. Iadarola, G. Rumolo CERN, Geneva, Switzerland G. Iadarola Naples University Federico II, Science and Technology Pole, Napoli, Italy
	The performance of 25 ns beams in the LHC is strongly limited by the electron cloud. To determine the amount electron cloud in the cold sections of the machine, it is very important to be able to disentangle the beam induced heating due to the beam coupling impedance from that attributable to electron cloud. This paper will focus on the calculation of the first contribution. First, the impedance model used for the calculation of the beam induced power loss is briefly discussed. Then, the methods for the calculation of the beam induced power loss in regions with one or two beams are also described. Finally, the calculated power loss is compared with the measured heat loads for both 25 and 50 ns beams in both the LHC arcs and in the inner triplets (ITs).
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TUPRI062	The Mode Matching Technique Applied to the Transverse Beam Coupling Impedance Calculation of Azimuthally Symmetric Devices of Finite Length	1714
	N. Biancacci, E. Métral, B. Salvant, C. Zannini CERN, Geneva, Switzerland M. Migliorati, L. Palumbo URLS, Rome, Italy V.G. Vaccaro Naples University Federico II and INFN, Napoli, Italy
	The infinite length approximation is often used to simplify the calculation of the beam coupling impedance of accelerator elements. This is expected to be a reasonable assumption for devices whose length is greater than the transverse dimension but may be a less accurate approximation for segmented devices. In this contribution we present the extension of the study of the beam coupling impedance of a finite length device to the transverse plane. In order to take into account the finite length, we decompose the fields in the cavity and in the beam pipe into a set of orthonormal modes and apply the Mode Matching method to obtain the impedance. To validate our method, we will present comparisons between analytical formulas and 3D electromagnetic CST simulations.
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TUPRI063	Electromagnetic Simulations for Non-ultrarelativistic Beams and Application to the CERN Low Energy Machines	1718
	C. Zannini, N. Biancacci, T.L. Rijoff, G. Rumolo CERN, Geneva, Switzerland T.L. Rijoff TU Darmstadt, Darmstadt, Germany
	In the framework of the PS-Booster upgrade project an accurate impedance model is needed in order to determine the effect on the beam stability and assess the impact of the new devices to be installed in the machine. CST 3-D EM simulations are widely used to estimate the impedance contribution of the different devices along the CERN accelerator complex. Unlike the highly relativistic case, in which the reliability of the EM solver has been proved in many specific cases by comparing simulations with analytical results, the nonrelativistic case has been so far not yet benchmarked. In order to use systematically CST 3-D EM simulations for the PS-Booster, or even lower energy machines like the antiproton decelerator ELENA, a validation campaign has been carried out. The main complication to single out the beam coupling impedance, as resulting from the interaction of the beam with the surroundings, consisted of removing reliably the strong contribution of the direct space charge of the source bunch, which is included in the EM calculation. The simulation results were then benchmarked with the analytical results for the case of a PEC cylindrical tube and of a ferrite loaded kicker.
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TUPRI067	Recent Results for the Dependence of Beam Instabilities caused by Electron Clouds at CesrTA due to Variations in Bunch Spacing and Chromaticity	1721
	M.G. Billing, K.R. Butler, G. Dugan, M.J. Forster, G. Ramirez, N.T. Rider, K.G. Sonnad, H.A. Williams Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA J.W. Flanagan KEK, Ibaraki, Japan R. Holtzapple, K.E. McArdle, M.I. Miller, M.M. Totten CalPoly, San Luis Obispo, California, USA
	Funding: Work supported by DOE Award DE-FC02-08ER41538, NSF Award PHY-0734867, PHY-1068662 and the Lepton Collider R&D, Coop Agreement: NSF Award PHY-1002467 At the Cornell Electron-Positron Storage Ring Test Accelerator (CesrTA) experiments have been studying the interaction of the electron cloud (EC) with 2.1 GeV stored electron and positron beams. These experiments are intended to characterize the dependence of beam–EC interactions on various beam parameters, such as bunch spacing and vertical chromaticity. Most experiments were performed with 30 or 45-bunch trains, at a fixed current of 0.75 mA/bunch. Earlier experiments with positrons had varied the bunch spacing between 4 and 56 ns at three different vertical chromaticity settings. More recent measurements have included electron-bunch trains to contrast the build up of EC between electron and positron beams. The dynamics of the stored beam was quantified using: a gated Beam Position Monitor (BPM) and spectrum analyzer to measure the frequency spectrum of bunches in the trains; an x-ray beam size monitor to record the bunch-by-bunch, turn-by-turn vertical size of each bunch within the trains. We report on recent ob-servations from these experiments and additional studies, using witness bunches trailing 30 or 45-bunch positron trains, which were used for the generation of the ECs.
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TUPRI069	NSLS-II Commissioning with 500 MHZ 7-CELL PETRA-III Cavity	1724
	A. Blednykh, G. Bassi, W.X. Cheng, J. Choi, Y. Hidaka, S.L. Kramer, Y. Li, B. Podobedov, J. Rose, T.V. Shaftan, G.M. Wang, F.J. Willeke, L.-H. Yu BNL, Upton, Long Island, New York, USA
	The NSLS-II storage ring has been commissioned during Phase 1 with 500 MHz 7-cell PETRA-III RF cavity. In this paper we present our first beam-measured data on instabilities and collective effects with a normal conducting RF system.
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TUPRI070	Analysis of Coupled-bunch Instabilities in the NSLS-II Storage Ring	1727
	G. Bassi, A. Blednykh, F. Gao, J. Rose BNL, Upton, Long Island, New York, USA
	We discuss coupled-bunch instabilities thresholds for the NSLS-II Storage Ring. In particular, we analyze thresholds from the High Order Modes (HOMs) of the PETRA-III 7-cell cavity. Beam dynamics simulations with the code OASIS, using the measured HOMs, will be compared with machine studies.
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TUPRI071	Transverse Impedance Measurement in RHIC and the AGS	1730
	N. Biancacci CERN, Geneva, Switzerland M. Blaskiewicz, Y. Dutheil, C. Liu, K. Mernick, M.G. Minty, S.M. White BNL, Upton, Long Island, New York, USA
	The RHIC luminosity upgrade program aims for an increase of the polarized proton luminosity by a factor 2. To achieve this goal a significant increase in the beam intensity is foreseen. The beam coupling impedance represents a source of detrimental effects for beam quality and stability at high bunch intensities. In this paper, we evaluate a new global transverse impedance in both RHIC and the AGS with recent measurements of tune shift as a function of bunch intensity. The results are compared to past measurements and present impedance model.
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THYA01	Control and Application of Beam Microbunching in High Brightness Linac-driven Free Electron Lasers	2789
	G.V. Stupakov SLAC, Menlo Park, California, USA
	The remarkable properties of coherent radiation from free-electron lasers (FELs) are due to the current modulation in the beam with the modulation period equal to the radiation wavelength. This modulation is developed as a result of a beam instability when the beam propagates in a long FEL undulator, and requires a beam with a high-peak current, small emittance and a small energy spread. Unfortunately the same beam qualities make it a subject to a so-called microbunching instability at a much longer scale than the radiation wavelength. It is driven by the space charge and CSR impedances in the machine and develops during the beam acceleration in the linac, compression, and transport to the undulator. If not controlled, the microbunching instability influences dramatically the FEL performance and in the worst case can even ruin the lasing. In the presentation we will review the mechanism behind the microbunching instability, the suppression methods used in existing facilities as well as possible future developments and concepts.
	Slides THYA01 [5.631 MB]
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THOBA01	A New Scheme for Electro-optic Sampling at Record Repetition Rates : Principle and Application to the First (turn-by-turn) Recordings of THz CSR Bursts at SOLEIL	2794
	E. Roussel, S. Bielawski, C. Evain, M. Le Parquier, C. Szwaj PhLAM/CERCLA, Villeneuve d'Ascq Cedex, France J.B. Brubach, L. Cassinari, M.-E. Couprie, M. Labat, L. Manceron, J.P. Ricaud, P. Roy, M.-A. Tordeux SOLEIL, Gif-sur-Yvette, France
	The microbunching instability is an ubiquitous problem in storage rings at high current density. However, the involved fast time-scales hampered the possibility to make direct real-time recordings of theses structures. When the structures occur at a cm scale, recent works at UVSOR, revealed that direct recording of the CSR electric field with ultra-high speed electronics (17 ps) provides extremely precious informations on the microbunching dynamics. However, when CSR occurs at THz frequencies (and is thus out of reach of electronics), the problem remained largely open. Here we present a new opto-electronic strategy that enabled to record series of successive electric field pulses shapes with picosecond resolution (including carrier and envelope), every 12 ns, over a total duration of several milliseconds. We also present the first experimental results obtained with this method at Synchrotron SOLEIL, above the microbunching instability threshold, and we present direct tests of Vlasov-Fokker-Planck and macroparticle models. The method can be applied to the detection of ps electric fields in other situations where high repetition rate is also an issue. First Direct, Real Time, Recording of the CSR Pulses Emitted During the Microbunching Instability, using Thin Film YBCO Detectors at UVSOR-III, IPAC2014
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Paper

Title

Page

TMCI Thresholds for LHC Single Bunches in the CERN-SPS and Comparison with Simulations

1407

H. Bartosik, G. Iadarola, Y. Papaphilippou, G. Rumolo, B. Salvant, C. Zannini
CERN, Geneva, Switzerland

At the beginning of 2013 an extensive measurement campaign was carried out at the SPS in order to determine the Transverse Mode Coupling Instability thresholds of LHC-type bunches in a wide range of intensities and longitudinal emittances. The measurements were performed in two different configurations of machine optics (nominal and low gamma transition) with the goal to characterize the differences in behavior and performance. The purpose of this paper is to describe in detail the measurement procedure and results, as well as the comparison of the experimental data with HEADTAIL simulations based on the latest SPS impedance model. Beside the impedances of the resistive wall, the beam position monitors (BPMs), the RF cavities, and the flanges, an advanced model of the impedance of the kicker magnets is included, which are found to play a major role in the definition of the stability region of the LHC-type bunches in the two optics configurations studied.

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TUPME029

Identification of High-frequency Resonant Impedance in the CERN SPS

1416

E.N. Shaposhnikova, T. Argyropoulos, T. Bohl, J.V. Campelo, F. Caspers, J.F. Esteban Müller, A. Lasheen, B. Salvant, H. Timko
CERN, Geneva, Switzerland

The spectrum of long bunches injected into the ring with RF switched off has been used in the SPS in the past to probe the longitudinal coupling impedance. After a large campaign of shielding of 800 inter-magnet vacuum ports in 1999 - 2001, the microwave instability threshold was significantly increased and the high-frequency spectrum of the beam became practically flat, apart from a prominent peak at around 1.4 GHz. As corresponding high-frequency impedance could potentially lead to microwave instability of high intensity bunches observed now at high energies in the SPS, a search of the source of this impedance was launched. Using a combination of impedance simulations and measurements, vacuum flanges that are present in a large quantity in the machine have been identified as a main source of impedance at this frequency. Particle simulations based on the SPS impedance model, which includes this previously unknown impedance, are able to reproduce the characteristics of the bunch spectrum and amplitude growth rates and hence, confirm that the impedance of the vacuum flanges is responsible for the observed spectral peak.

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TUPRI021

Impedance Calculation and Simulation of Microwave Instability for the Main Rings of SuperKEKB

1600

D. Zhou, T. Abe, T. Ishibashi, Y. Morita, K. Ohmi, K. Shibata, Y. Suetsugu, M. Tobiyama
KEK, Ibaraki, Japan

The SuperKEKB B-factory is now under construction. The designs of the components for the SuperKEKB have mostly been finished. This paper summarises the updated results of longitudinal impedance calculations for various components of the main rings. By summing up all available impedances, a pseudo-Green wake function with bunch length of σ_z=0.5 mm is constructed as an impedance model for consequent studies of collective effects. The results of these studies are also reported in this paper.

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TUPRI028

Review of Rest Gas Interaction at Very Low Energies applied to the Extra Low ENergy Antiproton ring ELENA

1621

C. Carli, T.L. Rijoff
CERN, Geneva, Switzerland
O. Karamyshev, C.P. Welsch
Cockcroft Institute, Warrington, Cheshire, United Kingdom
O. Karamyshev, C.P. Welsch
The University of Liverpool, Liverpool, United Kingdom

The Extremely Low ENergy Antiproton ring (ELENA) is a small synchrotron equipped with an electron cooler, which shall be constructed at CERN to decelerate antiprotons to energies as low as 100 keV. Scattering of beam particles on rest gas molecules may have a detrimental effect at such low energies and leads to stringent vacuum requirements. Within this contribution scattering of the stored beam on rest gas molecules is discussed for very low beam energies. It is important to carefully distinguish between antiprotons scattered out of the acceptance and lost, and those remaining inside the aperture to avoid overestimation of emittance blow-up. Furthermore, many antiprotons do not interact at all during the time they are stored in ELENA and hence this is not a multiple scattering process

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TUPRI036

Fast Ion Instability at CESR-TA

1638

A. Chatterjee, K.J. Blaser, M. P. Ehrlichman, D. L. Rubin, J.P. Shanks
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: Work supported by NSF and DOE Contracts No. PHY-0734867, No. PHY-1002467, No. PHYS-1068662, No. DE-FC02-08ER41538, No. DE-SC0006505, and the Japan/U.S. Cooperation Program.
Fast Ion Instability can lead to deterioration of an electron beam (increasing emittance and instability of a train of bunches) in storage rings and linacs. We study this at the Cornell Electron Storage Ring Test Accelerator using a 2.1 GeV low emittance beam. As the source of ions is residual gas, our measurements are conducted at various pressures, including nominal vacuum as well as injected gas (Ar, Kr). We measure turn-by-turn vertical bunch size and position, as well as the multi-bunch power spectrum. A detailed simulation is then used to compare theory with observations.

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TUPRI037

Some Features of Wave Distribution in the Thin-Wall Waveguide

1641

M. Ivanyan, L.V. Hovakimyan, A. Sargsyan
CANDLE SRI, Yerevan, Armenia

In this report we derive rigorous and approximate dispersion relations for the round resistive thin-wall waveguide. The features of the distributions of dispersion curves of the waveguide axisymmetric TM modes are obtained. Cases of splitting and degeneracy of modes under consideration are detected and regularities of their behaviours are established.

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TUPRI038

The Low Energy Particle Wakefield Radiation From the Open End of Internally Coated Metallic Tube

1644

M. Ivanyan, A. Grigoryan, A. Sargsyan, A.V. Tsakanian
CANDLE SRI, Yerevan, Armenia

The radiation of the non-relativistic electron beam from the open end of the resistive circular waveguide is presented. The angular and spectral characteristics of the radiation are determined. The possibility of producing the focused guasi-monochromatic radiation is discussed. The principal scheme of the experiments for 5 and 20 MeV AREAL RF photogun linac is presented.

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TUPRI041

Study of Collective Beam Instabilities for Sirius

1653

F.H. de Sá, H.O.C. Duarte, L. Liu, N. Milas, X.R. Resende
LNLS, Campinas, Brazil

In this paper we present the on going work of construction of the Sirius impedance budget and instability threshold estimates for several machine operation scenarios.

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TUPRI042

Numerical Study of the Microbunching Instability at UVSOR-III: Influence of the Resistive and Inductive Impedances

1656

SUSPSNE061

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

E. Roussel, S. Bielawski, C. Evain, C. Szwaj
PhLAM/CERCLA, Villeneuve d'Ascq Cedex, France
M. Adachi, M. Katoh, S.I. Kimura, T. Konomi
UVSOR, Okazaki, Japan
M. Hosaka, Y. Takashima, N. Yamamoto
Nagoya University, Nagoya, Japan
K.S. Ilin, J. Raasch, A. Scheuring, M. Siegel, P. Thoma
KIT, Karlsruhe, Germany
H. Zen
Kyoto University, Kyoto, Japan

At high charge, relativistic electron bunches circulating in storage rings undergo an instability, the so-called microbunching or the CSR (Coherent Synchrotron Radiation) instability. This instability is due to the interaction of the electrons with their own radiation and leads to the formation of microstructures (at millimeter scale) in the longitudinal phase space. Thanks to a new type of detector, based on superconducting thin film YBCO, it is now possible to observe directly these microstructures and follow their temporal evolution*. These experimental observations open a new way to make severe comparisons with theory. Here we present results of the modeling of the dynamics at UVSOR-III using a one dimensional Vlasov-Fokker-Planck equation. We show that to obtain a relatively good agreement between numerical simulations and experiments, we have to take into account several types of impedance such as the shielded CSR impedance but also the resistive and inductive impedances.
* First Direct, Real Time, Recording of the CSR Pulses Emitted During the Microbunching Instability, using Thin Film YBCO Detectors at UVSOR-III, IPAC2014

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TUPRI043

Analysis of Coupled Bunch Instabilities in BESSY-VSR

1659

M. Ruprecht, P. Goslawski, A. Jankowiak, M. Ries, A. Schälicke, G. Wüstefeld
HZB, Berlin, Germany
T. Weis
DELTA, Dortmund, Germany

BESSY-VSR, a scheme where 1.5 ps and 15 ps long bunches (rms) can be stored simultaneously in the BESSY II storage ring has recently been proposed*. The strong longitudinal bunch focusing is achieved by superconducting high gradient RF cavities. This paper presents investigations of coupled bunch instabilities driven by HOMs of superconducting multi cell cavities in BESSY-VSR. Analytical calculations and tracking simulations in time domain are performed in the longitudinal and the transverse planes and factors that influence the threshold currents are being discussed. Suitable candidates of cavities which are presently available or in the phase of design are compared with respect to their instability thresholds.
* G. Wüstefeld, A. Jankowiak, J. Knobloch, M. Ries, Simultaneous Long and Short Electron Bunches in the BESSY II Storage Ring, Proceedings of IPAC2011, San Sebastián, Spain

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TUPRI044

Investigation of Microbunching-instability in BERLinPro

1662

S.D. Rädel, A. Jankowiak, A. Meseck
HZB, Berlin, Germany

Funding: Work supported by German Bundesministerium für Bildung und Forschung, Land Berlin, and grants of Helmholtz Association
BERLinPro is using the new energy recovery linac technology. As, maintaining the low emittance and energy spread is of major importance in an ERL, the deep understanding and control of effects which can degrade the emittance and energy spread such as space charge effects are of interest. The microbunching caused by the longitudinal space charge forces can lead to an increase in emittance and energy spread in the arcs of the loop. In this contribution, the impacts of the microbunching instability on the beam quality and its implication for BERLinPro are discussed.

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TUPRI045

Beam Coupling Impedance Simulation in the Frequency Domain for the SIS100 Synchrotron

1665

U. Niedermayer, O. Boine-Frankenheim
TEMF, TU Darmstadt, Darmstadt, Germany
O. Boine-Frankenheim
GSI, Darmstadt, Germany

For the quantification of intensity thresholds due to coherent instabilities and beam induced heating in the FAIR synchrotron SIS100 a detailed knowledge of transverse and longitudinal beam coupling impedance is required. Due to the rather long proton and heavy-ion bunches, the relevant spectrum is below 100MHz. For the computation of beam coupling impedances in the low frequency regime, frequency domain methods are more advantageous than (explicit) time domain methods. We show the setup of a 2D finite element code that allows to compute the impedance for arbitrary longituninally homogeneous beam and structure shapes. Perfectly conducting pipes, a dispersive ferrite tube, and thin resistive beam pipe serve as test cases. The influence of the beam velocity on the coupling impedance is studied.

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TUPRI046

Dynamics of Ion Distributions in Beam Guiding Magnets

1668

A. Markoviḱ, G. Pöplau, U. van Rienen
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
W. Hillert, D. Sauerland
ELSA, Bonn, Germany
A. Meseck
HZB, Berlin, Germany

Funding: Supported by the German Federal Ministry of Education and Research (BMBF) under contract number 05K13HRC.
Ions generated by synchrotron radiation and collisions of the beam with the rest gas in the vacuum chamber could be a limiting factor for the operation of electron storage rings and Energy Recovery Linacs (ERL). In order to develop beam instability mitigation strategies, a deeper understanding of the ion-cloud behaviour is needed. Numerical simulations of the interaction between electron beams and parasitic ions verified with dedicated measurements can help to acquire that knowledge. This paper presents results of detailed simulations of the interaction in quadrupole magnets and drift sections of the Electron Stretcher Accelerator ELSA in Bonn. The focus is on the evaluation of the dynamics of different ion species and their characteristic distribution in quadrupole magnets.

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TUPRI047

Electromagnetic Modeling of Open Cell Conductive Foams for High Synchrotron Radiation Rings

1671

S. Petracca, A. Stabile
U. Sannio, Benevento, Italy
A. Stabile
INFN-Salerno, Baronissi, Salerno, Italy

Open cell conductive foams (OCMF) have been recently suggested as an alternative to perforated metal patches for efficiently handling gas desorption from the beam pipe wall due to intense synchrotron radiation, yielding superior performance in terms of residual gas concentration and beam shielding. Experimental work is ongoing to assess their properties, including secondary emission yields and beam coupling impedances. In this communication we attempt a review of the Literature about electromagnetic modeling of OCMF, and outline a general framework for computing the surface impedance of OCMF walls and deriving the longitudinal and transverse beam coupling impedances thereof, based on effective medium theory and electromagnetic reciprocity. A critical analysis of the relevant modeling approximations is included.

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TUPRI048

A Map Approach for Electron Cloud Density in a Strong LHC Dipole

1674

S. Petracca, A. Stabile
U. Sannio, Benevento, Italy
A. Stabile
INFN-Salerno, Baronissi, Salerno, Italy

The luminosity is limited by the electron cloud effects in presently running and proposed future storage rings. The evolution of the electron density during the electron cloud formation can be reproduced using a bunch-to-bunch iterative map formalism. By performing simulation codes this approach has been used to obtain a numerical prediction of the coefficients in the map, while in the presence of a magnetic field an analytic formula has been obtained for the linear coefficient. The next goal is finding a theoretical prescription of the quadratic coefficient at least in the presence of magnetic dipole. Then it will be possible to reproduce, by using the map formalism, the dynamics of electron cloud without performing the simulations.

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TUPRI049

Geometric Beam Coupling Impedance of LHC Secondary Collimators

1677

SUSPSNE059

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O. Frasciello, S. Tomassini, M. Zobov
INFN/LNF, Frascati (Roma), Italy
A. Grudiev, N. Mounet, B. Salvant
CERN, Geneva, Switzerland

Funding: Work supported by European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement 284404
The High Luminosity LHC project is aimed at increasing the LHC luminosity by an order of magnitude. One of the key ingredients to achieve the luminosity goal is the beam intensity increase. In order to keep under control beam instabilities and to avoid excessive power losses a careful design of new vacuum chamber components and an improvement of the present LHC impedance model are required. Collimators are the main impedance contributors. Measurements with beam have revealed that the betatron coherent tune shifts were by about a factor of 2 higher with respect to the theoretical predictions based on the current model. Up to now the resistive wall impedance has been considered as the major impedance contribution for collimators. By carefully simulating their geometric impedance we show that for the graphite collimators with half-gaps higher than 10 mm the geometric impedance exceeds the resistive wall one. In turn, for the tungsten collimators the geometric impedance dominates for all used gap values. Hence, including the geometric collimator impedance into the LHC impedance model enabled us to reach a better agreement between the measured and simulated collimator tune shifts.

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TUPRI050

Numerical Calculation and Experiment of Ion Related Phenomenon in SPring-8 Storage Ring

1680

A. Mochihashi, M. Takao
JASRI/SPring-8, Hyogo-ken, Japan

In the SPring-8 storage ring, various kinds of bunch filling pattern are available. Under some bunch filling patterns, residual gas ions created by scattering process between high energy electrons and residual gas molecules can be trapped stably around the electron beam and disturb the original motion of the beam. We have considered the stability of the electron beam due to the ion related phenomenon under several bunch filling patterns by computer simulation. In the simulation, we have modeled the electron bunch as single particle and the residual gas ions as macroparticles. The number of the trapped ions, size of the ion cloud and change in betatron oscillation amplitude of the beam under several filling pattern conditions will be discussed. We have also performed experiments for stability of the beam under equally spaced bunch filling patterns which give severe condition for the ion related instability. The numerical calculations and the experimental results will be discussed in the presentation.

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TUPRI051

Comparison between Measurements and Orbit Code Simulations for Beam Instabilities due to Kicker Impedance in the 3-GeV RCS of J-PARC

1683

P.K. Saha, H. Harada, N. Hayashi, H. Hotchi, Y. Shobuda, F. Tamura, M. Yamamoto
JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken, Japan

The transverse impedance of the extraction kicker magnets is the most dominant beam instability source in the 3-GeV Rapid Cycling Synchrotron of J-PARC. The instability occurs when chromaticity is fully corrected during acceleration but on the other hand no instabilities are observed for a full chromatic correction only at the injection energy even for a beam power up to 500 kW. However, the situation may change for a beam power of 1 MW and also for the upgraded injection beam energy from the present 181 MeV to the 400 MeV, as space charge effect in the non-relativistic region is believed to suppress the growth rate of beam instability. In order to study the kicker impedance in detail, recently we have introduced measured time dependent impedance source in the ORBIT simulation code in a realistic manner. The ORBIT code itself has also been well upgraded and given realistic features for application to synchrotrons. We have also carried out a systematic experimental study for a maximum beam power of 500 kW. In this paper, a detail comparison between measurements and corresponding simulations including 1 MW simulation results are presented.

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TUPRI052

Analysis of Single Bunch Measurements at the ALBA Storage Ring

1686

T.F.G. Günzel, U. Iriso, F. Pérez
CELLS-ALBA Synchrotron, Cerdanyola del Vallès, Spain
E. Koukovini-Platia, G. Rumolo
CERN, Geneva, Switzerland

Measurements of the vertical single bunch mode detuning and the TMCI threshold at zero chromaticity were carried out and their results were compared to the theoretical expectation. Around 65% of the found mode detuning can be explained by a developed transverse impedance model. A good bunch length parametrisation with current contributed essentially to this result. The analysis of single bunch measurements at non-zero chromaticity will also be presented.

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TUPRI053

Transverse Beam Instabilities in the MAX IV 3 GeV ring

1689

G. Skripka, P.F. Tavares
MAX-lab, Lund, Sweden
M. Klein, R. Nagaoka
SOLEIL, Gif-sur-Yvette, France

Collective effects in MAX IV 3 GeV storage ring are strongly enhanced by the combination of low emittance, high current and small effective aperture. Three passive harmonic cavities (HC) are introduced to lengthen the bunches, by which beam stabilization is anticipated via decoupling to high frequency wakes, along with Landau damping. The role of the ransverse impedance budget of the MAX IV 3 GeV storage ring as a source of collective beam instabilities was determined. With the help of the macroparticle multi-bunch tracking code mbtrack that directly uses the former as input, we studied the influence of geometric and resistive wall impedance in both transverse planes, as well as that of chromaticity shifting. A fully dynamic treatment of the passive harmonic cavities developed for this study allowed us to evaluate their effectiveness under varying beam conditions.

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TUPRI054

FEM Analysis of Beam-coupling Impedance and RF Contacts Criticality on the LHC UA9 Piezo Goniometer

1692

A. Danisi, R. Losito, A. Masi, A. Passarelli, B. Salvant, C. Zannini
CERN, Geneva, Switzerland

The UA9 piezo-goniometer has been designed to guarantee micro-radians-accuracy angular positioning of a silicon crystal for a crystal collimation experiment in the LHC, and to minimize the impact on the LHC beam-coupling impedance. This paper presents a Finite Element Method (FEM) study of the device, in both parking and operational positions, to evaluate its impact on the LHC impedance budget. The study has been a progressive simulation work, started from the simplification of the original detailed design, and aimed at highlighting the effect of single details (e.g. objects in confining chambers) on the longitudinal and transverse components of beam-coupling impedance. In addition, the shielding contribution of the RF gaskets has been carefully evaluated, with the objective to assess the consequences for operation in case of their failure. Sensitivity analyses to simulation parameters are also performed, in order to test the FEM model robustness. A final word is drawn on the overall device impedance criticality.

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TUPRI055

Theoretical Analysis of Metamaterial Insertions for Resistive-wall Beam-coupling Impedance Reduction

1695

A. Danisi, R. Losito, A. Masi, B. Salvant, C. Zannini
CERN, Geneva, Switzerland

Resistive-wall impedance usually constitutes a significant percentage of the total beam-coupling impedance budget of many accelerator structures (e.g. for LHC, it can be more than 50%). Reduction techniques for resistive-wall components entail high electrical-conductivity coatings. This paper proposes the use of metamaterials, having negative values of magnetic permeability or dielectric permittivity (or both), for sensibly reducing or theoretically nearly cancelling the resistive-wall component of beam-coupling impedance. The proposed approach is developed by means of an equivalent transmission-line model, whose results show the potential reduction of both longitudinal and transverse impedance when using metamaterial insertions. The effects on the real and imaginary part have been singled out. The effectiveness of such materials is discussed both for negative-permittivity and for negative-permeability cases, which actually show different impacts and can be then target of proper engineering. This first-stage study opens the possibility of considering metamaterials for impedance mitigation or for setting up proper experimental setups.

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TUPRI056

Beam Measurements of the LHC Impedance and Validation of the Impedance Model

1698

J.F. Esteban Müller, T. Argyropoulos, T. Bohl, N. Mounet, E.N. Shaposhnikova, H. Timko
CERN, Geneva, Switzerland

Different measurements of the longitudinal impedance of the LHC done with single bunches with various intensities and longitudinal emittances during measurement sessions in 2011-2012 are compared with particle simulations based on the existing LHC impedance model. The very low reactive impedance of the LHC, with Im(Z/n) around 0.1 Ohm, is not easy to measure. The most sensitive observation is the loss of Landau damping during acceleration, which shows at which energy bunches become unstable depending on their parameters. In addition, the synchrotron frequency shift due to the reactive impedance was estimated following two methods. Firstly, it was obtained from the peak-detected Schottky spectrum. Secondly, a sine modulation in the RF phase was applied to the bunches with different intensities and the modulation frequency was scanned. In both cases, the synchrotron frequency shift was of the order of the measurement precision.

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TUPRI057

Review of the Transverse Impedance Budget for the CLIC Damping Rings

1701

E. Koukovini-Platia, G. Rumolo
CERN, Geneva, Switzerland

Single bunch instability thresholds and the associated coherent tune shifts have been evaluated in the transverse plane for the damping rings (DR) of the Compact Linear Collider (CLIC). A multi-kick version of the HEADTAIL code was used to study the instability thresholds in the case where different impedance contributions are taken into account such as the broad-band resonator model in combination with the resistive wall contribution from the arcs and the wigglers of the DR. Simulations performed for positive values of chromaticity showed that higher order bunch modes can be potentially dangerous for the beam stability.

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TUPRI058

Impedance Studies of the Dummy Septum for CERN PS Multi-turn Extraction

1704

S. Persichelli, O.E. Berrig, M. Giovannozzi, J. Herbst, J. Kuczerowski, M. Migliorati, B. Salvant
CERN, Geneva, Switzerland

A protection septum has been installed in the CERN PS section 15 in order to mitigate irradiation of the magnetic septum 16 for fast extractions towards the SPS. Impedance studies have been performed, showing that beams circulating in the septum during extraction generate sharp resonances in the coupling impedance. Impedance measurements with the wire technique have been performed, showing a good agreement with simulations. Instability rise times of trapped modes have been evaluated and compared to extraction duration. Solutions for reducing the impact on the stability of the beam have been considered.

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TUPRI059

The Proton Synchrotron Transverse impedance model

4096

SUSPSNE060

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S. Persichelli, N. Biancacci, S.S. Gilardoni, M. Migliorati, E. Métral, B. Salvant
CERN, Geneva, Switzerland

The current knowledge of the transverse impedance of the CERN Proton Synchrotron (PS) has been established by theoretical computations, electromagnetic simulations and beam-based measurements at different energies. The transverse coherent tune and phase advance shifts as a function of intensity have been measured in order to evaluate the total effective transverse impedance and its distribution in the accelerator. In order to understand the beam dynamics, the frequency dependence of the impedance budget has also been evaluated considering the individual contribution of several machine devices. 3D models of many PS elements have been realized to perform accurate impedance simulations, while resistive wall and indirect space charge impedances have been evaluated with theoretical and numerical computations. Finally comparisons between the total budget and the measurement results are presented.

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TUPRI060

Impedance Studies for the PS Finemet® Loaded Longitudinal Damper

1708

S. Persichelli, M. Migliorati, M.M. Paoluzzi, B. Salvant
CERN, Geneva, Switzerland

The impedance of the Finemet® loaded longitudinal damper cavity, installed in the CERN Proton Synchrotron straight section 02 during the Long Shutdown 2013-2014, has been evaluated. Time domain simulations with CST Particle Studio have been performed in order to get the longitudinal and transverse impedance of the device and make a comparison with the longitudinal impedance that was measured for a single cell prototype.

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TUPRI061

Power Loss Calculation in Separated and Common Beam Chambers of the LHC

1711

C. Zannini, G. Iadarola, G. Rumolo
CERN, Geneva, Switzerland
G. Iadarola
Naples University Federico II, Science and Technology Pole, Napoli, Italy

The performance of 25 ns beams in the LHC is strongly limited by the electron cloud. To determine the amount electron cloud in the cold sections of the machine, it is very important to be able to disentangle the beam induced heating due to the beam coupling impedance from that attributable to electron cloud. This paper will focus on the calculation of the first contribution. First, the impedance model used for the calculation of the beam induced power loss is briefly discussed. Then, the methods for the calculation of the beam induced power loss in regions with one or two beams are also described. Finally, the calculated power loss is compared with the measured heat loads for both 25 and 50 ns beams in both the LHC arcs and in the inner triplets (ITs).

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TUPRI062

The Mode Matching Technique Applied to the Transverse Beam Coupling Impedance Calculation of Azimuthally Symmetric Devices of Finite Length

1714

N. Biancacci, E. Métral, B. Salvant, C. Zannini
CERN, Geneva, Switzerland
M. Migliorati, L. Palumbo
URLS, Rome, Italy
V.G. Vaccaro
Naples University Federico II and INFN, Napoli, Italy

The infinite length approximation is often used to simplify the calculation of the beam coupling impedance of accelerator elements. This is expected to be a reasonable assumption for devices whose length is greater than the transverse dimension but may be a less accurate approximation for segmented devices. In this contribution we present the extension of the study of the beam coupling impedance of a finite length device to the transverse plane. In order to take into account the finite length, we decompose the fields in the cavity and in the beam pipe into a set of orthonormal modes and apply the Mode Matching method to obtain the impedance. To validate our method, we will present comparisons between analytical formulas and 3D electromagnetic CST simulations.

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TUPRI063

Electromagnetic Simulations for Non-ultrarelativistic Beams and Application to the CERN Low Energy Machines

1718

C. Zannini, N. Biancacci, T.L. Rijoff, G. Rumolo
CERN, Geneva, Switzerland
T.L. Rijoff
TU Darmstadt, Darmstadt, Germany

In the framework of the PS-Booster upgrade project an accurate impedance model is needed in order to determine the effect on the beam stability and assess the impact of the new devices to be installed in the machine. CST 3-D EM simulations are widely used to estimate the impedance contribution of the different devices along the CERN accelerator complex. Unlike the highly relativistic case, in which the reliability of the EM solver has been proved in many specific cases by comparing simulations with analytical results, the nonrelativistic case has been so far not yet benchmarked. In order to use systematically CST 3-D EM simulations for the PS-Booster, or even lower energy machines like the antiproton decelerator ELENA, a validation campaign has been carried out. The main complication to single out the beam coupling impedance, as resulting from the interaction of the beam with the surroundings, consisted of removing reliably the strong contribution of the direct space charge of the source bunch, which is included in the EM calculation. The simulation results were then benchmarked with the analytical results for the case of a PEC cylindrical tube and of a ferrite loaded kicker.

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TUPRI067

Recent Results for the Dependence of Beam Instabilities caused by Electron Clouds at CesrTA due to Variations in Bunch Spacing and Chromaticity

1721

M.G. Billing, K.R. Butler, G. Dugan, M.J. Forster, G. Ramirez, N.T. Rider, K.G. Sonnad, H.A. Williams
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
J.W. Flanagan
KEK, Ibaraki, Japan
R. Holtzapple, K.E. McArdle, M.I. Miller, M.M. Totten
CalPoly, San Luis Obispo, California, USA

Funding: Work supported by DOE Award DE-FC02-08ER41538, NSF Award PHY-0734867, PHY-1068662 and the Lepton Collider R&D, Coop Agreement: NSF Award PHY-1002467
At the Cornell Electron-Positron Storage Ring Test Accelerator (CesrTA) experiments have been studying the interaction of the electron cloud (EC) with 2.1 GeV stored electron and positron beams. These experiments are intended to characterize the dependence of beam–EC interactions on various beam parameters, such as bunch spacing and vertical chromaticity. Most experiments were performed with 30 or 45-bunch trains, at a fixed current of 0.75 mA/bunch. Earlier experiments with positrons had varied the bunch spacing between 4 and 56 ns at three different vertical chromaticity settings. More recent measurements have included electron-bunch trains to contrast the build up of EC between electron and positron beams. The dynamics of the stored beam was quantified using: a gated Beam Position Monitor (BPM) and spectrum analyzer to measure the frequency spectrum of bunches in the trains; an x-ray beam size monitor to record the bunch-by-bunch, turn-by-turn vertical size of each bunch within the trains. We report on recent ob-servations from these experiments and additional studies, using witness bunches trailing 30 or 45-bunch positron trains, which were used for the generation of the ECs.

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TUPRI069

NSLS-II Commissioning with 500 MHZ 7-CELL PETRA-III Cavity

1724

A. Blednykh, G. Bassi, W.X. Cheng, J. Choi, Y. Hidaka, S.L. Kramer, Y. Li, B. Podobedov, J. Rose, T.V. Shaftan, G.M. Wang, F.J. Willeke, L.-H. Yu
BNL, Upton, Long Island, New York, USA

The NSLS-II storage ring has been commissioned during Phase 1 with 500 MHz 7-cell PETRA-III RF cavity. In this paper we present our first beam-measured data on instabilities and collective effects with a normal conducting RF system.

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TUPRI070

Analysis of Coupled-bunch Instabilities in the NSLS-II Storage Ring

1727

G. Bassi, A. Blednykh, F. Gao, J. Rose
BNL, Upton, Long Island, New York, USA

We discuss coupled-bunch instabilities thresholds for the NSLS-II Storage Ring. In particular, we analyze thresholds from the High Order Modes (HOMs) of the PETRA-III 7-cell cavity. Beam dynamics simulations with the code OASIS, using the measured HOMs, will be compared with machine studies.

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TUPRI071

Transverse Impedance Measurement in RHIC and the AGS

1730

N. Biancacci
CERN, Geneva, Switzerland
M. Blaskiewicz, Y. Dutheil, C. Liu, K. Mernick, M.G. Minty, S.M. White
BNL, Upton, Long Island, New York, USA

The RHIC luminosity upgrade program aims for an increase of the polarized proton luminosity by a factor 2. To achieve this goal a significant increase in the beam intensity is foreseen. The beam coupling impedance represents a source of detrimental effects for beam quality and stability at high bunch intensities. In this paper, we evaluate a new global transverse impedance in both RHIC and the AGS with recent measurements of tune shift as a function of bunch intensity. The results are compared to past measurements and present impedance model.

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THYA01

Control and Application of Beam Microbunching in High Brightness Linac-driven Free Electron Lasers

2789

G.V. Stupakov
SLAC, Menlo Park, California, USA

The remarkable properties of coherent radiation from free-electron lasers (FELs) are due to the current modulation in the beam with the modulation period equal to the radiation wavelength. This modulation is developed as a result of a beam instability when the beam propagates in a long FEL undulator, and requires a beam with a high-peak current, small emittance and a small energy spread. Unfortunately the same beam qualities make it a subject to a so-called microbunching instability at a much longer scale than the radiation wavelength. It is driven by the space charge and CSR impedances in the machine and develops during the beam acceleration in the linac, compression, and transport to the undulator. If not controlled, the microbunching instability influences dramatically the FEL performance and in the worst case can even ruin the lasing. In the presentation we will review the mechanism behind the microbunching instability, the suppression methods used in existing facilities as well as possible future developments and concepts.

Slides THYA01 [5.631 MB]

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THOBA01

A New Scheme for Electro-optic Sampling at Record Repetition Rates : Principle and Application to the First (turn-by-turn) Recordings of THz CSR Bursts at SOLEIL

2794

E. Roussel, S. Bielawski, C. Evain, M. Le Parquier, C. Szwaj
PhLAM/CERCLA, Villeneuve d'Ascq Cedex, France
J.B. Brubach, L. Cassinari, M.-E. Couprie, M. Labat, L. Manceron, J.P. Ricaud, P. Roy, M.-A. Tordeux
SOLEIL, Gif-sur-Yvette, France

The microbunching instability is an ubiquitous problem in storage rings at high current density. However, the involved fast time-scales hampered the possibility to make direct real-time recordings of theses structures. When the structures occur at a cm scale, recent works at UVSOR*, revealed that direct recording of the CSR electric field with ultra-high speed electronics (17 ps) provides extremely precious informations on the microbunching dynamics. However, when CSR occurs at THz frequencies (and is thus out of reach of electronics), the problem remained largely open. Here we present a new opto-electronic strategy that enabled to record series of successive electric field pulses shapes with picosecond resolution (including carrier and envelope), every 12 ns, over a total duration of several milliseconds. We also present the first experimental results obtained with this method at Synchrotron SOLEIL, above the microbunching instability threshold, and we present direct tests of Vlasov-Fokker-Planck and macroparticle models. The method can be applied to the detection of ps electric fields in other situations where high repetition rate is also an issue.
* First Direct, Real Time, Recording of the CSR Pulses Emitted During the
Microbunching Instability, using Thin Film YBCO Detectors at UVSOR-III, IPAC2014

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