ROPB  —  Two-Stream Instabilities and Collective Processes   (19-May-05   13:50—17:05)

Chair: G. Rumolo, CELLS, Bellaterra (Cerdanyola del Vallès)

Paper Title Page
ROPB001 Suppressing Electron Cloud in Future Linear Colliders 24
  • M.T.F. Pivi, R.E. Kirby, T.O. Raubenheimer
    SLAC, Menlo Park, California
  • F. Le Pimpec
    PSI, Villigen
  Funding: Work supported by the U.S. DOE under contract DE-AC02- 76SF00515.

Any accelerator circulating positively charged beams can suffer from a build-up of an electron cloud in the beam pipe. The cloud develops through ionization of residual gases, synchrotron radiation and secondary electron emission and, when severe, can cause instability, emittance blow-up or loss of the circulating beam. The electron cloud is potentially a limiting effect for both the Large Hadron Collider (LHC) and the International Linear Collider (ILC). For the ILC positron damping ring, the development of the electron cloud must be suppressed. This paper presents the various effects of the electron cloud and evaluates their significance. It also discusses the state-of-the-art of the ongoing international R&D program to study potential remedies to reduce the secondary electron yield to acceptably low levels.

ROPB002 Experiments Studying Desorbed Gas and Electron Clouds in Ion Accelerators 194
  • A.W. Molvik, J.J. Barnard, R.H. Cohen, A. Friedman, M. Kireeff Covo, S.M. Lund
    LLNL, Livermore, California
  • D. Baca, F.M. Bieniosek, C.M. Celata, P.A. Seidl, J.-L. Vay, W. Waldron
    LBNL, Berkeley, California
  • J.L. Vujic
    UCB, Berkeley, California
  Funding: This work was performed under the auspices of the U.S. Department of Energy by University of California, LLNL under contract No. W-7405-Eng-48, and by LBNL under Contract DE-AC03-76F00098.

Electron clouds and gas pressure rise limit the performance of many major accelerator rings. We are studying these issues experimentally with ~1 MeV heavy-ion beams, coordinated with significant efforts in self-consistent simulation and theory.* The experiments use multiple diagnostics, within and between quadrupole magnets, to measure the sources and accumulation of electrons and gas. In support of these studies, we have measured gas desorption and electron emission coefficients for potassium ions impinging on stainless steel targets at angles near grazing incidence.** Our goal is to measure the electron particle balance for each source – ionization of gas, emission from beam tubes, and emission from an end wall – determine the electron effects on the ion beam and apply the increased understanding to mitigation.

*J-L. Vay, Invited paper, session TICP; R. H. Cohen et al., PRST-AB 7, 124201 (2004). **M. Kireeff Covo, this conference; A. W. Molvik et al., PRST-AB 7, 093202 (2004).

ROPB003 Electron Cloud Dynamics in High-Intensity Rings 245
  • L. Wang, J. Wei
    BNL, Upton, Long Island, New York
  Funding: Work performed under the auspices of the U.S. Department of Energy. SNS is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U.S. Department of Energy.

Electron cloud due to beam induced multipacting is one of the main concerns for the high intensity rings because the electron multipacting becomes stronger with the increment of beam intensity. Electrons generated and accumulated inside the beam pipe form an "electron cloud" that interacts with the circulating charged particle beam. With sizeable amount of electrons, this interaction can cause beam instability, beam loss and emittance growth. At the same time, the vacuum pressure will rise due to electron desorption. This talk intends to provide an overview of the dynamics of the typical electron multipacting in various magnetic fields and mitigation measures in both long bunch and short bunch rings.

ROPB004 Effect of Lattice and Electron Distribution in Electron-Cloud Instability Simulations for the CERN SPS and LHC 387
  • E. Benedetto, E. Benedetto
    Politecnico di Torino, Torino
  • G. Arduini, F. Roncarolo, F. Zimmermann
    CERN, Geneva
  • B. Feng, A.F. Ghalam, T.C. Katsouleas
    USC, Los Angeles, California
  • G. Franchetti
    GSI, Darmstadt
  • K. Ohmi
    KEK, Ibaraki
  • G. Rumolo
    CELLS, Bellaterra (Cerdanyola del Vallès)
  Several simulation codes have been adapted so as to model the single-bunch electron-cloud instability including a realistic variation of the optical functions with longitudinal position. In addition, the electron cloud is typically not uniformly distributed around the ring, as frequently assumed, but it is mainly concentrated in certain regions with specific features, e.g., regions which give rise to strong multipacting or suffer from large synchrotron radiation flux. Particularly, electrons in a dipole magnet are forced to follow the vertical field lines and, depending on the bunch intensity, they may populate two vertical stripes, symmetrically located on either side of the beam. In this paper, we present simulation results for the CERN SPS and LHC, which can be compared with measurements or analytical predictions.  
ROPB005 Recent Experiment Results on Fast Ion Instability at 2.5 GeV PLS 466
  • E.-S. Kim, Y.J. Han, J.Y. Huang, I.S. Ko, P.C.D. Park, S.J. Park
    PAL, Pohang, Kyungbuk
  • H. Hukuma, H. Ikeda
    KEK, Ibaraki
  We present recent experiment results on the fast ion instability that were performed at the PLS storage ring. With higher vacuum pressures of three orders of magnitude than nominal one by He gas injection into the ring, increases of a factor of around three in the vertical beam size were observed by interferometer system. From the various measurement results, we estimated growth times for the instability as a funcion of vacuum pressure and beam current. We also compared the results with those of the computer simulations and analytical calculations.  
ROPB006 Filling in the Roadmap for Self-Consistent Electron Cloud and Gas Modeling 525
  • J.-L. Vay, M.A. Furman, P.A. Seidl
    LBNL, Berkeley, California
  • R.H. Cohen, K. Covo, A. Friedman, D.P. Grote, A.W. Molvik
    LLNL, Livermore, California
  • P. Stoltz, S.A. Veitzer
    Tech-X, Boulder, Colorado
  • J. Verboncoeur
    UCB, Berkeley, California
  Funding: This work was performed under the auspices of the U.S. Department of Energy by University of California, LLNL and LBNL under contracts W-7405-Eng-48, and DE-AC03-76F00098.

Electron clouds and gas pressure rise limit the performance of many major accelerators. A multi-laboratory effort to understand the underlying physics via the combined application of experiment,* theory, and simulation is underway. We present here the status of the simulation capability development, based on a merge of the three-dimensional parallel Particle-In-Cell accelerator code WARP and the electron cloud code POSINST, with additional functionalities.** The development of the new capability follows a "roadmap" describing the different functional modules, and their inter-relationships, that are ultimately needed to reach self-consistency. Newly developed functionalities include a novel particle mover bridging the time scales between electrons and ions motion.*** Samples of applications of the new capability to the modeling of intense charge dominated beams**** and LHC beams***** will be shown as available.

*A.W. Molvik, these proceedings. **J.-L. Vay, Proc. "ECLOUD04," Napa (California), 2004. ***R.H. Cohen, these proceedings. ****P.A. Seidl, these proceedings. *****M.A. Furman, these proceedings.

ROPB007 3-D Parallel Simulation Model of Continuous Beam-Electron Cloud Interactions 549
  • A.F. Ghalam, T.C. Katsouleas
    USC, Los Angeles, California
  • E. Benedetto, F. Zimmermann
    CERN, Geneva
  • V.K. Decyk, C. Huang, W.B. Mori
    UCLA, Los Angeles, California
  • G. Rumolo
    GSI, Darmstadt
  A 3D Particle-In-Cell model for continuous modeling of beam and electron cloud interaction in a circular accelerator is presented. A simple model for lattice structure, mainly the Quadruple and dipole magnets and chromaticity have been added to a plasma PIC code, QuickPIC, used extensively to model plasma wakefield acceleration concept. The code utilizes parallel processing techniques with domain decomposition in both longitudinal and transverse domains to overcome the massive computational costs of continuously modeling the beam-cloud interaction. Through parallel modeling, we have been able to simulate long-term beam propagation in the presence of electron cloud in many existing and future circular machines around the world. The exact dipole lattice structure has been added to the code and the simulation results for CERN-SPS and LHC with the new lattice structure have been studied. Also the simulation results are compared to the results from the two macro-particle modeling for strong head-tail instability. It is shown that the simple two macro-particle model can capture some of the physics involved in the beam- electron cloud interaction qualitatively.  
ROPB008 Halo Mitigation Using Nonlinear Lattices 620
  • K.G. Sonnad, J.R. Cary
    CIPS, Boulder, Colorado
  This work shows that halos in beams with space charge effects can be controlled by combining nonlinear focusing and collimation. The study relies on Particle-in-Cell (PIC) simulations for a one dimensional, continuous focusing model. The PIC simulation results show that nonlinear focusing leads to damping of the beam oscillations thereby reducing the mismatch. It is well established that reduced mismatch leads to reduced halo formation. However, the nonlinear damping is accompanied by emittance growth causing the beam to spread in phase space. As a result, inducing nonlinear damping alone cannot help mitigate the halo. To compensate for this expansion in phase space, the beam is collimated in the simulation and further evolution of the beam shows that the halo is not regenerated. The focusing model used in the PIC is analysed using the Lie Transform perturbation theory showing that by averaging over a lattice period, one can reuduce the focusing force to a form that is identical to that used in the PIC simulation.  
ROPB009 Betatron Sidebands Due to Electron Clouds Under Colliding Beam Conditions 680
  • J.W. Flanagan, H. Fukuma, Y. Funakoshi, S. Hiramatsu, H. Ikeda, K. Ohmi, M. Tobiyama, S. Uehara, S. Uno
    KEK, Ibaraki
  • E. Perevedentsev
    BINP SB RAS, Novosibirsk
  Recently, we have observed vertical betatron sidebands in the transverse beam spectra of positron bunches at the KEKB LER which are associated with the presence of electron clouds. When the LER is operating in single-beam mode (no colliding bunches in the HER), these sidebands are sharply peaked. When the bunches are in collision for physics running, the sidebands are still present but are found to be smeared out. The bunch-by-bunch specific luminosity is lower for bunches with sidebands than for those without sidebands. In this paper, the behavior of the sidebands in collision and the effects on luminosity are discussed.  
ROPB010 Self-Consistent Electron-Cloud Simulation for Long Proton Bunches 722
  • A.P. Shishlo, S.M. Cousineau, V.V. Danilov, S. Henderson, J.A. Holmes, Y. Sato
    ORNL, Oak Ridge, Tennessee
  • S.-Y. Lee
    IUCF, Bloomington, Indiana
  • R.J. Macek
    LANL, Los Alamos, New Mexico
  Funding: SNS is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U.S. Department of Energy. SNS is a partnership of six national laboratories: Argonne, Brookhaven, Jefferson, Lawrence Berkeley, Los Alamos, and Oak Ridge.

The results of numerical electron-cloud simulations for long-bunch proton beams in accumulator rings are presented and compared with data from the Proton Storage Ring at LANL. The frequency spectra and growth rate of proton-bunch transverse instabilities are studied as functions of the RF cavity voltage, external magnetic fields, beam pipe surface properties, and other factors. We used the recently developed electron-cloud module in the ORBIT code. The model includes a fully self-consistent coupled treatment of the "proton bunch – electron-cloud" dynamics and the multipacting process with a realistic secondary emission surface model. Realistic lattices and proton bunch distributions are used. The efficiency of electron-cloud instability suppression has also been studied using a new ORBIT model.