05 Beam Dynamics and Electromagnetic Fields
D05 Instabilities - Processes, Impedances, Countermeasures
Paper Title Page
MOPC057 Loss of Landau Damping in the LHC 211
 
  • E.N. Shaposhnikova, T. Argyropoulos, P. Baudrenghien, T. Bohl, A.C. Butterworth, J. Esteban Muller, T. Mastoridis, G. Papotti, J. Tückmantel, W. Venturini Delsolaro, U. Wehrle
    CERN, Geneva, Switzerland
  • C.M. Bhat
    Fermilab, Batavia, USA
  
  Loss of Landau damping leading to a single bunch longitudinal quadrupole instability has been observed in the LHC during the ramp and on the 3.5 TeV flat top for small injected longitudinal emittances. The first measurements are in good agreement with the threshold calculated for the expected longitudinal reactive impedance budget of the LHC as well as with the threshold dependence on beam energy. The cure is a controlled longitudinal emittance blow-up during the ramp which for constant threshold through the cycle should provide an emittance proportional to the square root of energy.  
 
MOPS007 Interference of CSR Fields in a Curved Waveguide 604
 
  • D.M. Zhou, K. Ohmi
    KEK, Ibaraki, Japan
  
  CSR fields generated by a bunched beam passing through a series of bending magnets may interfere with each other due the reflections of outer chamber wall. This kind of multi-bend interference causes sharp peaks and long-range tail in the CSR impedance and wake potentials, respectively. Using a dedicated computer code, CSRZ, we calculated the longitudinal CSR impedance in the SuperKEKB positron damping ring for purpose of demonstration. It was found that multi-bend interference may enhance the CSR fields within a distance comparable to the bunch length, which is typically in the order of several millimeters. A simple instability analysis was performed and it suggested that multi-bend interference might play a role in the single-bunch instabilities of small electron/positron rings.  
 
MOPS010 Experimental Studies with Low Transition Energy Optics in the SPS 613
 
  • H. Bartosik, T. Argyropoulos, T. Bohl, S. Cettour Cave, K. Cornelis, J. Esteban Muller, Y. Papaphilippou, G. Rumolo, B. Salvant, E.N. Shaposhnikova, J. Wenninger
    CERN, Geneva, Switzerland
  
  The optics of the SPS can be tuned to lower transition energy such that the slippage factor at injection is raised by a factor of almost 3. From theory, an increase of the intensity thresholds for transverse mode coupling, longitudinal coupled bunch and longitudinal instabilities due to the loss of Landau damping can be expected. In this paper, experimental studies in the SPS with single bunches of protons with intensities of up to 3.5·1011 p/b on the flat bottom and at 450 GeV/c are presented. Longitudinal instabilities were studied with LHC-type beams with 50~ns spacing and injected intensities up to 1.8·1011 p/b. The measurements address the increase of intensity thresholds and the achievable transverse emittances in the new low gamma transition optics with respect to the nominal SPS optics. The obtained results are compared with numerical simulations.  
 
MOPS011 Impact of Low Transition Energy Optics to the Electron Cloud Instability of LHC Beams in the SPS 616
 
  • H. Bartosik, E. Benedetto, K.S.B. Li, Y. Papaphilippou, G. Rumolo
    CERN, Geneva, Switzerland
  
  One of the main limitations for high intensity multi-bunch LHC proton beams in the SPS is imposed by electron cloud instabilities. A new optics of the SPS with lower transition energy was implemented and successfully tested in machine studies. The significant increase of the slippage factor that it provides at injection energy results in the expected increase of the single bunch instability thresholds. In this paper, the impact of this new optics on the electron cloud instability threshold is estimated by using numerical simulations, taking into account the change of the optics functions and the faster synchrotron motion due to the reduced transition energy.  
 
MOPS013 Transverse Low Frequency Broad-band Impedance Measurements in the CERN PS 622
 
  • S. Aumon
    EPFL, Lausanne, Switzerland
  • P. Freyermuth, S.S. Gilardoni, O. Hans, E. Métral, G. Rumolo
    CERN, Geneva, Switzerland
  
  The base-line scenario for the High-Luminosity LHC upgrade foresees an intensity increase delivered by the injectors. With its 53 years, the CERN PS would have to operate beyond the limit of its performances to match the future requirements. Beam instabilities driven by transverse impedance are an important issue for the operation of high intensity beams as for the high-brightness LHC beams. Measurements of transverse tune dependence with beam intensity were performed at injection kinetic energy 1.4~GeV and at LHC beam extraction momentum 26~GeV/c. This allows deducing the low frequency inductive broad-band impedance of the machine. Then an estimation of the real part of the impedance is made by the rise time measurement of a fast transverse instability believed to be a TMCI type. Those are the first step towards a global machine impedance characterization in order to push forward the performances of the accelerator.  
 
MOPS045 Coupling Impedance of Rough Resistive Pipe* 700
 
  • M. Ivanyan, V.M. Tsakanov
    CANDLE, Yerevan, Armenia
  
  A new version of modelling of the surface roughness impact by thin dielectric layer in the round resistive beam pipe is suggested. The calculation method of coupled resistive-roughness impedance is developed.  
 
MOPS046 Impedances and Wakes in Round Three-layer Ceramic Waveguide 703
 
  • M. Ivanyan, A.V. Tsakanian
    CANDLE, Yerevan, Armenia
  
  The round ceramic waveguide with inner and outer thin metal coating is considered. Using the exact methods the longitudinal impedances and potentials are calculated. Identification of the main patterns of changes in their properties by varying the electrodynamic and geometric parameters of the waveguide is performed as well. The possibility of optimizing the parameters of the waveguide for the effective implementation of two-beam acceleration is discussed.  
 
MOPS047 Studies of Transverse Single-pass Beam Breakup in E-Linac 706
 
  • D. Kaltchev, R.A. Baartman, Y.-C. Chao, P. Kolb, S.R. Koscielniak, L. Merminga, A.K. Mitra, V. Zvyagintsev
    TRIUMF, Canada's National Laboratory for Particle and Nuclear Physics, Vancouver, Canada
  
  Time-domain simulations of single-pass transverse beam-breakup (BBU) effects in E-linac are described. We use dipole-HOM parameters for the 9-cell cavity obtained with Particle Studio to evaluate the rms bunch orbit offsets at linac exit. Finding the multi-bunch orbit contribution to machine emittance as a function of the average beam current allows to evaluate the performance of two cavity models for two different modes of machine operation.  
 
MOPS048 Microbunching Instability Studies at SOLEIL 709
 
  • C. Evain, J. Barros, J.B. Brubach, L. Cassinari, M.-E. Couprie, G. Creff, M. Labat, A. Loulergue, L. Manceron, R. Nagaoka, P. Roy, M.-A. Tordeux
    SOLEIL, Gif-sur-Yvette, France
  
  Microbunching instability arises in storage rings when the number of electrons in a bunch exceeds a threshold value. Its signature, i.e. a strong and irregular emission of Coherent Synchrotron Radiation (CSR) in the Terahertz (THz) domain, is studied at SOLEIL on the AILES infrared beamline, with the storage ring tuned in a low-alpha configuration (used to get shorter electron bunch). The comparison of this observed THz CSR with numerical simulations of the longitudinal electron bunch dynamics, permits to put in evidence that during the instability a modulation appears and drifts in the longitudinal profile of the electron bunch. The understanding of this instability is important as it limits some operation of the storage rings. Indeed the induced fluctuations prevent the use of THz on the far IR beamline at high current per bunch. And in normal alpha operation this instability may spoil the electron/laser interaction effects used to get femtosecond and/or coherent pulse in storage rings (with slicing, Coherent Harmonic Generation or EEHG schemes on storage ring).  
 
MOPS049 Study of Ion-induced Instabilities and Transverse Feedback Performance at SOLEIL 712
 
  • R. Nagaoka, L. Cassinari, M.D. Diop, J.-M. Filhol, M.-P. Level, A. Loulergue, P. Marchand, R. Sreedharan
    SOLEIL, Gif-sur-Yvette, France
  
  Experimental studies indicate that the SOLEIL storage ring at its maximum designed current of 500 mA is under a large influence of ions, potentially capable of inducing the so called fast beam-ion instability. To avoid it, the following three conditions have been empirically found effective: A reduced RF voltage, uniform filling and a large vertical chromaticity. While the choice of uniform filling appears contradictory to raising the ion instability threshold, it goes well with lowering of the RF voltage if outgassing due to beam-induced heating of the vacuum components is the primary source of ions. Additional difficulties associated are frequent occurrence of sudden beam blowups despite the presence of transverse feedback, which are large enough to trigger machine interlocks leading to complete beam losses. These blow ups may even take place horizontally inside in-vacuum insertion devices. The present paper reports on the results and findings obtained through experimental and simulation studies carried out on the collective beam dynamics and the transverse feedback performance, which are deeply interlinked, in order to clarify the mechanism of the encountered phenomena.  
 
MOPS050 Electron Beam Dynamics in the 50 MeV ThomX Compact Storage Ring 715
 
  • C. Bruni, J. Haissinski
    LAL, Orsay, France
  • A. Loulergue, R. Nagaoka
    SOLEIL, Gif-sur-Yvette, France
  
  ThomX is a high flux compact X-ray source based on Compton back scattering between a relativistic electron beam and an intense laser pulse. To increase the repetition rate, the electron beam is stored in a ring. The main drawback of such a scheme is the low energy of the electrons regarding collective effects and intrabeam scattering. These effects tend to enlarge or even disrupt the stored bunch and they limit its charge, especially in a system where damping plays a negligible role. Thus such collective effects reduce the maximum X-ray flux and it is important to investigate them to predict the performance of this type of X-ray source. In addition, the Compton back scattering acts on the electron beam by increasing its energy spread. This presentation will show firstly the impact of collective effects on the electron beam, essentially during the first turns when they are the most harmful. Then, the reduction of the X-ray flux due to Compton back scattering and intrabeam scattering will be investigated on a longer time scale.  
 
MOPS051 Modeling of the Beam Break Up Instability for BERLinPro* 718
 
  • Y. Petenev, A.V. Bondarenko, A.N. Matveenko
    HZB, Berlin, Germany
  
  Following funding approval late 2010, Helmholtz-Zentrum Berlin officially started Jan. 2011 the design and construction of the Berlin Energy Recovery Linac Project BERLinPro. The initial goal of this compact ERL is to develop the ERL accelerator physics and technology required to accelerate a high-current low emittance beam. In this work the threshold current of the Beam Break Up (BBU) instability was calculated for the BERLinPro. The comparison of two 100 MeV linacs based on different type of superconducting cavities is made. Different methods of BBU suppression are investigated (e.g. the influence of solenoid, pseudo-reflector and quadruple triplets in the linac structure on the BBU threshold).  
 
MOPS052 Analytical and Numerical Calculations of Beam Pipe Impedances at Low Frequencies with Application to Thin SIS100 Pipe 721
 
  • U. Niedermayer, O. Boine-Frankenheim, L. Hänichen
    TEMF, TU Darmstadt, Darmstadt, Germany
  
  The projected fast ramped synchrotron SIS100 for FAIR uses an elliptical stainless steel beam pipe of 0.3 mm thickness. The lowest coherent betatron sidebands reach down to 100 kHz which demands accurate impedance calculations in the low frequency (LF) regime. For these frequencies, i.e. skin depth greater than wall thickness, structures behind the pipe may contribute to the impedance. Due to the extremely large wake length numerical methods in the time domain are not applicable. The longitudinal and transverse impedance of the thin SIS100 beam pipe including structures behind the pipe are obtained numerically by a method using power loss in the frequency domain. We compare different analytical models for simplified pipe structures to the numerical results. The dc and ultra-relativistic limits are investigated. The interpretation of bench measurements in the LF regime is discussed.  
 
MOPS053 Electron Cloud Effects in Coasting Heavy-ion Beams* 724
 
  • F.B. Petrov, T. Weiland
    TEMF, TU Darmstadt, Darmstadt, Germany
  • O. Boine-Frankenheim
    GSI, Darmstadt, Germany
  
  Funding: Work supported by BMBF under contract 06DA9022I.
During slow extraction of intense ion beams electron clouds (EC) can accumulate in the circulating coasting beam and reduce the extraction efficiency. This is a concern for the existing SIS-18 heavy ion synchrotron at GSI and for the projected SIS-100 as part of the FAIR project. For medium energy heavy-ion beams the production of electrons from residual gas ionization is very effective. The electron density is limited due to Coulomb scattering by the beam ions. Above a threshold beam intensity the two-stream instability and the resulting coherent beam oscillations limit the electron density. Below this threshold the electron cloud can lead to observable deformations of the Schottky side-bands. To avoid EC build-up one can introduce a gap in the beam using barrier rf bucket. The reduction of the build-up efficiency caused by the gap is studied in details based on the solution of the Hill's equation for electrons. Finally we estimate the saturation level for the electron cloud density. 		 
 
MOPS054 Impedance of the Pulse Power Converter for the SIS100 Bipolar Extraction Kicker System 727
 
  • K. Samuelsson, V. Hinrichsen
    TU Darmstadt, Darmstadt, Germany
  • U. Blell, P.J. Spiller
    GSI, Darmstadt, Germany
  
  SIS100 will be operated with high intensity heavy-ion and proton beams. The reduction of ring impedances is therefore of great importance in order to avoid coherent beam instabilities. The kicker system is one of the main contributors to the overall ring impedance in SIS100. This paper will focus on the contribution of the external network to the kicker impedance. Calculations as well as experimental impedance measurements of the network contribution have already been carried out for the SIS18 and ESR kickers. The SIS100 will be equipped with a bipolar kicker system, which uses a Pulse Forming Network (PFN) as energy storage. For potential detachment purposes an insulation transformer will be installed. Since this setup is new in several ways it is important to know its contribution to the coupling impedance of the kicker system. In this contribution the corresponding numerical calculation is presented.  
 
MOPS055 Observation of Intensity Dependent Single Bunch Effects at the Synchrotron Light Source PETRA III 730
 
  • R. Wanzenberg, K. Balewski
    DESY, Hamburg, Germany
  
  At DESY the PETRA ring is operated as a synchrotron radiation facility with a very low emittance of 1 nm. Regular user operation has started in summer 2010. A summary of observations and measurements of intensity dependent single bunch effects is presented in this report. The longitudinal impedance of the ring is estimated from the measured bunch length versus beam intensity. The results are compared with predictions from the impedance model. Furthermore measurements of the single bunch intensity limit due to the transverse mode coupling instability (TMCI) are reported. The tune and phase shift around the ring has been measured as a function of the beam intensity. At PETRA III tune spectra have been observed with some characteristics which have been observed at other storage rings in connection with electron cloud effects. The present status of the observations of potential electron cloud effects is also discussed.  
 
MOPS056 An Analytical Formula of the Electron Cloud Linear Map Coefficient in a Strong Dipole 733
 
  • T. Demma
    INFN/LNF, Frascati (Roma), Italy
  • S. Petracca, A. Stabile
    U. Sannio, Benevento, Italy
  • G. Rumolo
    CERN, Geneva, Switzerland
  
  Electron cloud effects have been indentified as one of the most serious bottleneck for reaching design performances in presently running and proposed future storage rings. The analysis of these effects is usually performed with very time consuming simulation codes. An alternative analytic approach, based on a cubic map model for the bunch-to-bunch evolution of the electron cloud density, could be useful to determine regions in parameters space compatible with safe machine operations. In this communication we derive a simple approximate formula relating the quadratic coefficient in the electron cloud density map to the parameters relevant for the electron cloud evolution in a strong vertical magnetic field. Results are compared with simulations with particular reference to the LHC dipoles.  
 
MOPS057 Beam-beam Interaction under External Force Oscillation 736
 
  • K. Ohmi
    KEK, Ibaraki, Japan
  
  Beam-ion interaction is strongly nonlinear. Response for external oscillation applied to beam shows characteristic feature. Simulations for external frequency scan becomes feasible for the recent computer power. We show the frequency response for beam-ion system in KEK-PF and recent low emittance rings.  
 
MOPS058 KEKB Linac Wakefield Studies of Comparing Theoretical Calculation, Simulation and Experimental Measurement* 739
 
  • L. Zang, N. Iida, Y. Ogawa, M. Satoh, M. Yoshida, D.M. Zhou
    KEK, Ibaraki, Japan
  
  For superKEKB, in order to achieve aiming luminosity machine need to run with a nano-beam scheme so that a small beam emittance is critical important. During the beam propagation, the short-range wake field in the accelerating structure will cause the beam instability and emittance growth. In practical, injecting beam with certain offset could compensate wakfield. And beam emittance could be measured by tuning the quadruple known as quadscan method. In this paper, wakefield theoretical calculation, simulation results will be presented. And then the wakefield impact to beam emittance and wakefield compensation will be discussed. Finally, we will show the comparison of the results getting from theoretical calculation and experimental measurement.  
 
MOPS059 Transverse Impedance Calculation for Simplified Model of Ferrite Kicker Magnet with Beta < 1 742
 
  • N. Wang, Q. Qin
    IHEP Beijing, Beijing, People's Republic of China
  
  In high intensity rings, kicker magnet is usually considered as a main source to the total impedance. Transverse coupling impedance of a simplified kicker model has been derived analytically in the ultrarelativistic limit. We extend the result to the general case of v < c, and present the analytical formulae of both horizontal and vertical transverse impedances. Numerical results are given for the CSNS extraction kicker magnets.  
 
MOPS060 Study on Resistive Wall Instability in CSNS/RCS 745
 
  • L. Huang, Y.D. Liu, S. Wang
    IHEP Beijing, Bejing, People's Republic of China
  
  Rapid Cycling Synchrotron of the China Spallation Neutron Source is a high intensity proton accelerator, with average beam power of 100kW. The collective effects caused by the coupling impedance may be the limit to beam power. The impedance estimation for components on beam line shows that the resistive wall impedance and its instability are more serious than any others. Based on the impedance budget, the instability is theoretically estimated. And a simple resistive wall wake field model is used to simulate the bunch oscillation and the growth rate instability. In this model, the continuous resistive wall wake field is equivalent to a point wake field and long bunch is sliced into many micro-bunches. By tracking the dynamics of the macro-bunches, the transverse growth rate are obtained and the result are analyzed.  
 
MOPS064 Longitudinal Beam Stability and Related Effects at the ALBA Storage Ring 748
 
  • T.F. Günzel
    CELLS-ALBA Synchrotron, Cerdanyola del Vallès, Spain
  
  The risk of longitudinal instabilities excited by narrowband and broadband resonator impedance was studied. A campaign for the search of modes trapped in vacuum chamber elements of the ALBA storage ring via electromagnetic simulation was initiated. Several critical vacuum elements in the ring like the vertical scraper, the injection and feedback kickers were identified. The outlets of the injection kicker had to be protected with RF-fingers whereas the scraper only produces dangerous modes in the withdrawn state, both do not pose a real problem. However, the calculated power distribution generated in the feedback kickers could be an obstacle for reaching the nominal current of 400mA. Furthermore, the budget of Z(n)/n of the storage ring was computed and checked on the risk of microwave instability using the Boussard criterion.  
 
MOPS065 Transverse Instability Studies at the ALBA Storage Ring 751
 
  • T.F. Günzel
    CELLS-ALBA Synchrotron, Cerdanyola del Vallès, Spain
  
  In the first phase of the ALBA storage ring operation 3 NEG-coated aluminum chambers, 2 in-vacuum undulators and one wiggler chamber will be installed. Under particular consideration of the multilayer character of these chambers and the injection kickers the thresholds of the transverse mode coupled instability(TMCI) were calculated using MOSES*. The thresholds 17.5mA/40.5mA vertical/horizontal leave a rather large operative margin. The detrimental effect of the NEG-coating on the TMCI is relatively limited and on the resistive wall instability is even negligible. As well the thresholds of the head-tail instability were computed as function of chromaticity. Also the incoherent tune shifts generated by the quadrupolar resistive wall wake fields due to the flatness of the vacuum chambers were calculated. The computed results have been compared to first measurements of the storage ring commissioning.
* Y.H.Chin, MOSES 2.0, CERN/LEP-TH/88-05 		 
 
MOPS066 Collective Effects in the MAX IV 3 GeV Ring 754
 
  • P.F. Tavares
    MAX-lab, Lund, Sweden
  • T.F. Günzel
    CELLS-ALBA Synchrotron, Cerdanyola del Vallès, Spain
  • R. Nagaoka
    SOLEIL, Gif-sur-Yvette, France
  
  We present calculations of collective instability effects in the 3 GeV electron storage ring of the MAX IV facility currently under construction in Lund, Sweden. The storage ring is designed to deliver ultra-low emittance down to 0.24 nm rad so as to provide high brightness synchrotron radiation from undulators. This is achieved in a comparatively small machine (528 m circumference) through the use of a multi-bend achromat lattice and a compact magnet design featuring multi-purpose narrow gap magnet blocks. This design features small dispersion leading to low momentum compaction, which, together with the small circular (11 mm radius) chambers, poses a challenge to reach the design current (500 mA in 176 bunches) without exciting instabilities and degrading beam parameters due to the interaction with the machine impedance. Particularly important are multi-bunch resistive wall effects in the NEG coated copper chamber as well single-bunch instabilities driven by the broad-band impedance. A low RF frequency (100 MHz) and harmonic cavities are foreseen to lengthen the bunches and increase instability thresholds.  
 
MOPS068 Localization of Transverse Impedance Sources in the SPS using HEADTAIL Macroparticle Simulations 757
 
  • N. Biancacci, G. Arduini, E. Métral, D. Quatraro, G. Rumolo, B. Salvant, R. Tomás
    CERN, Geneva, Switzerland
  • N. Biancacci, M. Migliorati, L. Palumbo
    Rome University La Sapienza, Roma, Italy
  • R. Calaga
    BNL, Upton, Long Island, New York, USA
  
  In particle accelerators, beam coupling impedance is one of the main contributors to instability phenomena that lead to particle losses and beam quality deterioration. For this reason these machines are continuously monitored and the global and local amount of impedance needs to be evaluated. In this work we present our studies on the local transverse impedance detection algorithm. The main assumptions behind the algorithm are described in order to understand limits in reconstructing the impedance location. The phase advance response matrix is analyzed in particular for the SPS lattice, studying the different response from 90,180,270 degrees phase advance sections. The thin lenses scheme is also implemented and new analytical formulas for phase advance beating were derived. This avails us to put reconstructing lenses everywhere in the lattice, and to study their positioning scheme. Limits in linear response are analyzed. This sets the upper and lower limits in reconstruction to the phase advance measurement accuracy and the linear response regime limit.  
 
MOPS069 Review of Beam Instabilities in the Presence of Electron Clouds in the LHC 760
 
  • K.S.B. Li, G. Rumolo
    CERN, Geneva, Switzerland
  
  Recent observations at the LHC indicate the build-up of electron clouds when 50 ns spaced beams are injected into the machine at nominal intensity. These electron clouds are a source of coherent beam instabilities and incoherent emittance growth and limit the achievable luminosity. To better understand the influence of electron clouds on the beam dynamics, simulations have been carried out to study both the coherent and the incoherent effects on the beam. The simulations are performed with the HeadTail tracking code; the usage of new post-processing software allows determining not only the beam intensity thresholds in terms of the central electron cloud density but also the footprint of the beam in tune space. In this paper we review instability thresholds and tune footprints for beams with different emittances and interacting with an electron cloud in field-free or dipole regions.  
 
MOPS070 Electromagnetic Modeling of C Shape Ferrite Loaded Kickers 763
 
  • C. Zannini
    EPFL, Lausanne, Switzerland
  • E. Métral, G. Rumolo, B. Salvant, V.G. Vaccaro, C. Zannini
    CERN, Geneva, Switzerland
  
  The kickers are major contributors to the CERN SPS beam coupling impedance. As such, they may represent a limitation to increasing the SPS bunch current in the frame of an intensity upgrade of the LHC. In this paper, analytical approach and CST Particle Studio time domain electromagnetic simulations are performed to obtain the longitudinal and transverse impedances/wake potentials of models of ferrite loaded kickers. It turns out that the existing models are not sufficient to characterize correctly these components from the coupling impedance point of view. In particular the results show that below few hundred MHz the real C-structure of the magnet cannot be neglected. Therefore an analytical model was developed and benchmarked with EM simulations to take into account the C-shape of the magnet.  
 
MOPS071 Simulations of the Impedance of the New PS Wire Scanner Tank 766
 
  • B. Salvant
    EPFL, Lausanne, Switzerland
  • W. Andreazza, F. Caspers, A. Grudiev, J.F. Herranz Alvarez, E. Métral, G. Rumolo
    CERN, Geneva, Switzerland
  
  The CERN PS is equipped with 4 wire scanners. It was identified that the small aperture of the current wire scanner tank causes beam losses and a new tank design was needed. The interaction of the PS bunches with the beam coupling impedance of this new tank may lead to beam degradation and wire damage. This contribution presents impedance studies of the current PS tank as well as the new design in order to assess the need to modify the design and/or install lossy materials plates dedicated to damp higher order cavity modes and reduce the total power deposited by the beam in the tank.  
 
MOPS072 Broadband Electromagnetic Characterization of Materials for Accelerator Components 769
 
  • C. Zannini, A. Grudiev, E. Métral, T. Pieloni, G. Rumolo
    CERN, Geneva, Switzerland
  • G. De Michele
    PSI, Villigen, Switzerland
  • C. Zannini
    EPFL, Lausanne, Switzerland
  
  Electromagnetic (EM) characterization of materials up to high frequencies is a major requirement for the correct modeling of many accelerator components: collimators, kickers, high order modes damping devices for accelerating cavities. In this scenario, the coaxial line method has gained much importance compared to other methods because of its applicability in a wide range of frequencies. In this paper we describe a new coaxial line method that allows using only one measurement setup to characterize the material in a range of frequency from few MHz up to several GHz. A coaxial cable fed at one side is filled with the material under test and closed on a known load on the other side. The properties of the material are obtained from the measured reflection coefficient by using it as input for a transmission line (TL) model or for 3D EM simulations, which describe the measurements setup. We have applied this method to characterize samples of SiC (Silicon Carbide) which could be used for LHC collimators and for CLIC accelerating structures and NiZn ferrite used for kicker magnets.  
 
MOPS073 Impedance Calculation for Simple Models of Kickers in the Non-ultrarelativistic Regime 772
 
  • N. Biancacci, N. Mounet, E. Métral, B. Salvant, C. Zannini
    CERN, Geneva, Switzerland
  • N. Biancacci, M. Migliorati, A. Mostacci, L. Palumbo
    Rome University La Sapienza, Roma, Italy
  • Q. Qin, N. Wang
    IHEP Beijing, Beijing, People's Republic of China
  
  Kicker magnets are usually significant contributors to the beam coupling impedance of particle accelerators. An accurate understanding of their impedance is required in order to correctly assess the machine intensity limitations. The field matching method derived by H. Tsutsui for the longitudinal and transverse dipolar (driving) impedance of simple models of kickers in the ultrarelativistic regime was already extended to the non-ultrarelativistic case, and to the quadrupolar (detuning) impedance in the ultrarelativistic case. This contribution presents the extension to the quadrupolar impedance in the non-ultrarelativistic case, as well as benchmarks with other available methods to compute the impedance. In particular, all the components of the impedances are benchmarked with Tsutsui's model, i.e. in the ultrarelativistic limit, with the model for a flat chamber impedance recently computed by N. Mounet and E. Métral, in the case of finite relativistic gamma, and with CST Particle Studio simulations.  
 
MOPS074 Stabilization of the LHC Single-bunch Transverse Instability at High-energy by Landau Octupoles 775
 
  • E. Métral, B. Salvant
    CERN, Geneva, Switzerland
  • N. Mounet
    EPFL, Lausanne, Switzerland
  
  When the first ramp was tried on Saturday 15/05/2010 with a single bunch of about nominal intensity (i.e. ~ 1011 p/b), the bunch became unstable in the horizontal plane at ~ 2 TeV. The three main observations were: (i) a “Christmas tree” in the transverse tune measurement application (with many synchrotron sidebands excited), (ii) beam losses (few tens of percents) in IR7, and (iii) an increase of the bunch length. This transverse coherent instability has been stabilized successfully with Landau octupoles. Comparing all the measurements performed during this first year of LHC commissioning with the theoretical and simulation predictions reveals a good agreement.  
 
MOPS075 Simulation of Multibunch Motion with the HEADTAIL Code and Application to the CERN SPS and LHC 778
 
  • N. Mounet
    EPFL, Lausanne, Switzerland
  • N. Mounet, E. Métral, G. Rumolo
    CERN, Geneva, Switzerland
  
  Multibunch instabilities due to beam-coupling impedance can be a critical limitation for synchrotrons operating with many bunches. It is particularly true for the LHC under nominal conditions, where according to theoretical predictions the 2808 bunches rely entirely on the performance of the transverse feedback system to remain stable. To study these instabilities, the HEADTAIL code has been extended to simulate the motion of many bunches under the action of wake fields. All the features already present in the single-bunch version of the code, such as synchrotron motion, chromaticity, amplitude detuning due to octupoles and the ability to load any kind of wake fields through tables, have remained available. This new code has been then parallelized in order to track thousands of bunches in a reasonable amount of time. The code was benchmarked against theory and exhibited a good agreement. We also show results for bunch trains in the LHC and compare them with beam-based measurements.  
 
MOPS076 Long Range Wakefields in the SwissFEL C-band Linac 781
 
  • A. Citterio, M. Aiba, R. Zennaro
    PSI, Villigen, Switzerland
  
  The SwissFEL main linac consists of more than hundred constant gradient C-band accelerating structures which boost the beam energy from 410 MeV at the injector to the final nominal energy of 5.8 GeV. With a repetition rate of 100 Hz, two bunches per pulse can be accelerated with a spacing of 28 ns to feed simultaneously two different FEL arms*. Rising of the long range wakefields, both longitudinal and transverse, could affect this multibunch operation, causing degenerative effects on the quality of the second bunch. A direct computation of the longitudinal and transverse wakes by means of time domain simulations is compared with a model based on the computation of the dispersion curves of the wake modes by frequency domain simulations. A good agreement is obtained for both the synchronous frequency and impedance of all the main modes contributing to the wakefields. Moreover, the total longitudinal wake at 28 ns is below the thighter tolerances required by the beam dynamics, so that neither Higher Order Modes (HOMs) either beam loading require compensation. The effects on the beam of the long range transverse wakefields are also negligeable.
*R. Ganter et al, SwissFEL CDR, PSI report n. 10-04; http://www.psi.ch/swissfel/CurrentSwissFELPublicationsEN/SwissFELCDR_v1903.03.11-small.pdf 		 
 
MOPS078 Coaxial Wire Measurements of Ferrite Kicker Magnets 784
 
  • H.A. Day, R.M. Jones
    UMAN, Manchester, United Kingdom
  • M.J. Barnes, F. Caspers, H.A. Day, E. Métral, B. Salvant, C. Zannini
    CERN, Geneva, Switzerland
  
  Fast kicker magnets are used to inject beam into and eject beam out of the CERN accelerator rings. These kickers are generally transmission line type magnets with a rectangular shaped aperture through which the beam passes. Unless special precautions are taken the impedance of the yoke can provoke significant beam induced heating, especially for high intensities. In addition the impedance may contribute to beam instabilities. The results of longitudinal and transverse impedance measurements, for various kicker magnets, are presented and compared with analytical calculations: in addition predictions from a numerical analysis are discussed.  
 
MOPS079 Simulations of Coaxial Wire Measurements of the Impedance of Asymmetric Structures 787
 
  • H.A. Day, R.M. Jones
    UMAN, Manchester, United Kingdom
  • F. Caspers, H.A. Day, E. Métral
    CERN, Geneva, Switzerland
  
  Coaxial wire measurements have provided a simple and effective way to measure the beam coupling impedance of accelerator structures for a number of years. It has been known how to measure the longitudinal and dipolar transverse impedance using one and two wires for some time. Recently the ability to measure the quadrupolar impedance of structures exhibiting top/bottom and left/right symmetry has been demonstrated. A method for measuring the beam coupling impedance of asymmetric structures using displaced single wires and two wire measurements is proposed. Simulations of the measurement system are presented with further work proposed.  
 
MOPS080 Comparison of the Current LHC Collimators and the SLAC Phase 2 Collimator Impedances 790
 
  • H.A. Day, R.M. Jones
    UMAN, Manchester, United Kingdom
  • F. Caspers, H.A. Day, E. Métral, B. Salvant
    CERN, Geneva, Switzerland
  
  One of the key sources of transverse impedance in the LHC has been the secondary graphite collimators that sit close to the beam at all energies. This limits the stable bunch intensity due to transverse coupled-bunch instabilities and transverse mode coupling instability. To counteract this, new secondary collimators have been proposed for the phase II upgrade of the LHC collimation system. A number of designs based on different jaw materials and mechanical designs have been proposed. A comparison of the beam coupling impedance of these different designs derived from simulations are presented, with reference to the existing phase I secondary collimator design.  
 
MOPS082 Some Considerations on the Choice of Frequency and Geometrical Beta in High Power Proton Linacs in the Context of Higher Order Modes 793
 
  • M. Schuh, F. Gerigk
    CERN, Geneva, Switzerland
  • M. Schuh
    MPI-K, Heidelberg, Germany
  • C.P. Welsch
    Cockcroft Institute, Warrington, Cheshire, United Kingdom
  
  Several high power superconducting (SC) proton linear accelerators are currently in the design stage around the world, such as for example the European Spallation Source (ESS) in Lund, Project X at Fermilab, the European ADS demonstrator MYRRAH in Mol and the Superconducting Proton linac (SPL) at CERN. In this contribution, the influence of Higher Order Modes (HOMs) in elliptical SC cavities is discussed as a function of the operation frequency, the number of cells and the geometrical beta of the cavity. Based on cavity design data beam dynamics simulations are executed for different linac layouts to quantify the influence of HOMs.  
 
MOPS083 Update on Electron Cloud Mitigation Studies at Cesr-TA* 796
 
  • J.R. Calvey, M.G. Billing, J.V. Conway, G. Dugan, S. Greenwald, Y. Li, X. Liu, J.A. Livezey, J. Makita, R.E. Meller, M.A. Palmer, S. Santos, R.M. Schwartz, J.P. Sikora, C.R. Strohman
    CLASSE, Ithaca, New York, USA
  • S. Calatroni, G. Rumolo
    CERN, Geneva, Switzerland
  • K. Kanazawa, Y. Suetsugu
    KEK, Ibaraki, Japan
  • M.T.F. Pivi, L. Wang
    SLAC, Menlo Park, California, USA
  
  Funding: Work supported by the US National Science Foundation (PHY-0734867) and Department of Energy (DE-FC02-08ER41538)
Over the course of the past three years, the Cornell Electron Storage Ring (CESR) has been reconfigured to serve as a test facility for next generation particle accelerators. A significant part of this program has been the installation of several diagnostic devices to measure and quantify the electron cloud effect, a potential limiting factor in these machines. In particular, more than 30 Retarding Field Analyzers (RFAs) have been installed in CESR. These devices measure the local electron cloud density and energy distribution, and can be used to evaluate the efficacy of different cloud mitigation techniques. This paper will provide an overview of RFA results obtained at CesrTA over the past year, including measurements taken as function of bunch spacing and wiggler magnetic field. Understanding these results provides a great deal of insight into the behavior of the electron cloud. 		 
 
MOPS084 Status of Electron Cloud Dynamics Measurements at CESRTA* 799
 
  • M.G. Billing, G. Dugan, M.J. Forster, D.L. Kreinick, R.E. Meller, M.A. Palmer, G. Ramirez, M.C. Rendina, N.T. Rider, J.P. Sikora, K.G. Sonnad, H.A. Williams
    CLASSE, Ithaca, New York, USA
  • J.Y. Chu
    CMU, Pittsburgh, Pennsylvania, USA
  • J.W. Flanagan
    KEK, Ibaraki, Japan
  • R. Holtzapple, M. Randazzo
    CalPoly, San Luis Obispo, California, USA
  
  Funding: Supported by US National Science Foundation (PHY-0734867) & Dept. of Energy (DE-FC02-08ER41538)
The study of electron cloud-related instabilities for the CESR-TA project permits the observation of the interaction of the electron cloud with the stored beam under a variety of accelerator conditions. These measurements are undertaken utilizing automatic and semi-automatic techniques for three basic observations: the measurement of tune shifts of individual bunches along a train, the detection of the coherent self-excited spectrum for each bunch within a train and the pulsed excitation of either the betatron dipole or head-tail mode for each individual bunch within the train, followed by the observation of the damping of its coherent motion. These techniques are employed to study the electron cloud-related interactions in a number of conditions, such as trains of bunches with low emittance and spaced by as little as 4 nsec between bunches. We report on the most recent observations and results. 		 
 
MOPS085 Wakefield Calculations for the LCLS in Multibunch Operation* 802
 
  • K.L.F. Bane
    SLAC, Menlo Park, California, USA
  
  Funding: Work supported by the U.S. Department of Energy under contract DE-AC02-76SF00515.
Normally the Linac Coherent Light Source (LCLS) operates in single-bunch mode, sending a bunch of up to 250 pC charge at 120 Hz through the linac and the undulator, and the resulting FEL radiation into one of the experimental hutches. With two bunches per rf pulse, each pulse could feed either two experiments or one experiment in a pump-probe type configuration. Two-bunch FEL operation has already been briefly tested at the LCLS, and works reasonably well*, although not yet routinely. In this report we study the longitudinal and transverse long-range (bunch-to-bunch) wakefields of the linacs and their effects on LCLS performance in two-bunch mode. The longitudinal wake changes the average energy and chirp at the second bunch, and the transverse wake misaligns the second bunch (in transverse phase space) in the presence of e.g. transverse injection jitter or quad misalignments. Finally, we extend the study to consider the LCLS with trains of up to 20 bunches per rf pulse.
* F.-J. Decker et al, "A demonstration of multi-bunch operation in the LCLS," Proceedings of FEL2010, Malmoe, Sweden, p. 467. 		 
 
MOPS086 Beam Breakup Simulation for the PEP-X ERL 805
 
  • Y. Jiao, Y. Cai, A. Chao
    SLAC, Menlo Park, California, USA
  
  Funding: The work is supported by the U.S. Department of Energy under contract No. DE-AC02-76SF00515.
The transverse beam breakup (BBU) is one of the dominant factors in ERL for the available beam current. A tracking code built in Matlab is developed and benchmarked by comparing with the analytical solutions with the simple model. Study on the threshold current and emittance growth due to the transverse BBU for PEP-X ERL are presented in this paper. 		 
 
MOPS088 Simulation of Electron Cloud Beam Dynamics for CesrTA 808
 
  • K.G. Sonnad, G. Dugan, M.A. Palmer, G. Ramirez, H.A. Williams
    CLASSE, Ithaca, New York, USA
  • K.R. Butler
    Cornell University, Ithaca, New York, USA
  • M.T.F. Pivi
    SLAC, Menlo Park, California, USA
  
  This presentation provides a comprehensive set of results obtained using the simulation program CMAD. CMAD is being used for studying electron cloud induced beam dynamics issues for CesrTA, which is a test facility for studying physics associated with electron and positron damping rings. In particular, we take a closer look at electron cloud induced effects on positron beams, including head-tail motion, emittance growth and incoherent tune shifts for parameters specific to ongoing experimental studies at CesrTA. The correspondence between simulation and experimental results will also be discussed.
Work supported by US Department of Energy grant number DE-FC02-08ER41538
and the National Science Foundation grant number PHY-0734867 		 
 
MOPS089 Identification of Bunch Dynamics in the Presence of E-cloud and TMCI for the CERN SPS Ring 811
 
  • O. Turgut, J.D. Fox, C.H. Rivetta, S. Uemura
    SLAC, Menlo Park, California, USA
  • W. Höfle
    CERN, Geneva, Switzerland
  
  Funding: Work is supported by the U.S. Department of Energy under contract DE-AC02-76SF00515 and the US LHC Accelerator Research Program (LARP).
Measurements and multi-particle simulation codes (i.e. HEAD-TAIL, WARP, CMAD) indicate that bunched particle beams show unstable motions induced by electron-clouds and strong head-tail interactions. The bunch dynamics exhibits highly non-linear, complex and unstable behavior under certain operating conditions. Feedback control systems have been proposed to mitigate these instabilities in the CERN SPS ring. The design of feedback systems requires the knowledge of a reduced dynamic model of the bunch. It allows to include and quantify the effect of noise and signal perturbations, as well as system robustness to parameter variation. Identification techniques are used to estimate those models based on bunch motion measurements. In this work we present reduced mathematical models representing the transverse bunch dynamics and identification techniques to extract the model parameters based on measurements. These techniques are validated using time domain simulations of the bunch motion conducted using multi-particle simulation codes. For that, different sections of the bunch are driven by random signals, and the vertical motion of those areas is used to estimate the reduced model. 		 
 
MOPS090 Observation of Beam Ion Instability in SPEAR3 814
 
  • L. Wang, Y. Cai, W.J. Corbett, T.O. Raubenheimer, J.A. Safranek, J.F. Schmerge, J.J. Sebek
    SLAC, Menlo Park, California, USA
  • D. Teytelman
    Dimtel, San Jose, USA
  
  Weak vertical coupled bunch instability with oscillation amplitude at μm level has been observed in SPEAR3. The instability becomes stronger when there is a vacuum pressure rise by partially turning off vacuum pumps and it becomes weaker when the vertical beam emittance is increased by turning off the skew quadrupole magnets. These confirmed that the instability was driven by ions in the vacuum. The threshold of the beam ion instability when running with a single bunch train is just under 200 mA. This paper presents the comprehensive observations of the beam ion instability in SPEAR3. The effects of vacuum pressure, beam current, beam filling pattern, chromaticity, beam emittance and bunch-by-bunch feedback are investigated in great detail.pattern, chromaticity, beam emittance and bunch-by-bunch feedback are investigated in great detail.  
 
MOPS091 Study of Electron Cloud for MEIC 817
 
  • S. Ahmed, J.D. Dolph, G.A. Krafft, T. Satogata, B.C. Yunn
    JLAB, Newport News, Virginia, USA
  
  Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
The Medium Energy Electron Ion Collider (MEIC) at Jefferson Lab has been envisioned as a future high energy particle accelerator beyond the 12 GeV upgrade of the existing Continuous Electron Beam Accelerator Facility (CEBAF). Synchrotron radiation from the closely spaced proton bunches in MEIC can generate photoelectrons inside the vacuum chamber and cause secondary emission due to multipacting in the presence of beam's electric field. This phenomenon can lead to fast build up of electron density, known as electron cloud effect – resulting into beam instability coupled to multi-bunches in addition to a single bunch. For MEIC, the estimated threshold value of the electron-cloud density is approximately 5 x 1012 m-3. In this paper, we would like to report the self-consistent simulation studies of electron cloud formation for MEIC. The code has been benchmarked against the published data of electron cloud effects observed in LHC. Our first simulations predict increase of electron clouds with the increase of repetition rate. The detailed simulations are under progress and will be reported. 		 
 
THOBA02
 Experimental Demonstration of Suppression of Coherent Synchrotron Radiation Wake-field  
 
  • V. Yakimenko, A.V. Fedotov, M.G. Fedurin, D. Kayran, V. Litvinenko
    BNL, Upton, Long Island, New York, USA
  • P. Muggli
    MPI, Muenchen, Germany
  
  In this paper we report on a first experimental demonstration of coherent synchrotron radiation (CSR) wake-field suppression by a narrow-gap vacuum chamber. Increase in the beam energy spread and emittance due to emission of coherent synchrotron radiation (CSR) is considered to be a limiting factor a high-current high-brightness beams. At the Brookhaven National Laboratory Accelerator Test Facility (ATF) we experimentally demonstrated the suppression of CSR wake-field - both the average energy loss and he energy spread growth – using polished Al plates. Well characterized electron bunches were propagated through a bending magnet with two parallel Al plates with gap controlled between 1 mm to 12 mm. Contrary to some theoretic predictions, our experimental results show that closing the plates significantly reduces both the beam energy loss and CSR-induced beam energy spread. In this paper we present our experimental results and compare then with rigorous analytical theory. These results open a possibility to predict analytically CSR shielding of future high-current high-brightness beams.  
slides icon Slides THOBA02 [12.706 MB]