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Cohen, R.H.

Paper Title Page
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).

 
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.

 
ROPA009 Bridging Timescales for Simulating Electron Clouds
 
  • R.H. Cohen, A. Friedman
    LLNL, Livermore, California
  • J.-L. Vay
    LBNL, Berkeley, California
 
  Funding: Work performed under the auspices of the U.S. Department of Energy by U. C. LLNL under contract No. W-7405-Eng-48 and by U.C. LBNL under Contract DE-AC03-76F00098.

For a range of acclerator applications, accurate simulation of electron-cloud effects requires self-consistent simulation of electrons and the main positively-charged accelerated species. For systems with multipole focusing magnets, the simulation must span timescales from electron cyclotron periods to the beam duration. We have devised a scheme to bridge over the shortest (electron cyclotron) timescale based on interpolation between full dynamics and drift kinetics.* We describe the method and results for heavy-ion beam simulations. We also discuss options for bridging to ion-transit timescales.

*R. H. Cohen, A. Friedman, S. M. Lund, A. W. Molvik, E. P. Lee, T. Azevedo, L.-L. Vay, P. Stoltz and S. Veitzer, PRST-AB 7, 124201 (2004)

 
FPAP015 Electron and Gas Effects on Intense, Space-Charge Dominated Ion Beams in Magnetic Quadrupoles: Comparison of Experiments and Simulations
 
  • P.A. Seidl, D. Baca, F.M. Bieniosek, J.-L. Vay
    LBNL, Berkeley, California
  • R.H. Cohen, A. Friedman, D.P. Grote, M. Kireeff Covo, S.M. Lund, A.W. Molvik
    LLNL, Livermore, 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.

Accelerators for inertial fusion energy, high-energy density physics and other high intensity applications have an economic incentive to minimize the clearance between the beam edge and the aperture wall. This increases the risk from electron clouds and gas desorbed from walls. Using the High Current Experiment at LBNL, we have measured the beam (0.18 A, 1 MeV K+ ) distribution upstream and downstream of a short lattice of magnetic quadrupoles where the 2·rms beam size is =50% of the quadrupole aperture, and the generalized perveance is ˜10-3. Between magnets, the transverse beam distribution is also imaged. The beam potential is 1-2 kV, large enough to trap electrons produced by, for example, K+ - gas collisions. Gas and electron effects are intentionally induced by varying gas pressure and the bias of e- controlling electrodes.* The measurements are compared to WARP PIC simulations that include the self-consistent tracking of electrons and ions.**

*A. W. Molvik et al., this conference. **J-L Vay et al., this conference.

 
FPAP016 Initial Self-Consistent 3-D Electron-Cloud Simulations of LHC Beam with the Code WARP+POSINST 1479
 
  • J.-L. Vay, M.A. Furman
    LBNL, Berkeley, California
  • R.H. Cohen, A. Friedman, D.P. Grote
    LLNL, Livermore, 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.

We present initial results from the self-consistent beam-cloud dynamics simulations of a sample LHC beam, using a newly developed set of modeling capability based on a merger of the three-dimensional parallel Particle-In-Cell accelerator code WARP and the electron cloud code POSINST.*,** Although the storage ring model we use as a test bed to contain the beam is much simpler and shorter than the LHC, its lattice elements are realistically modeled, as is the beam and the electron cloud dynamics. The simulated mechanisms for generation and absorption of the electrons at the walls are based on previously validated models available in POSINST.***

*J.-L. Vay, these proceedings. **J.-L. Vay, Proc. "ECLOUD04," Napa (California), 2004. ***M.T.F. Pivi and M.A. Furman, Phys. Rev. STAB, PRSTAB/v6/i3/e034201.

 
FPAP021 A Cross-Platform Numerical Model of Ion-Wall Collisions 1707
 
  • S.A. Veitzer, P. Stoltz
    Tech-X, Boulder, Colorado
  • R.H. Cohen, A.W. Molvik
    LLNL, Livermore, California
  • J.-L. Vay
    LBNL, Berkeley, California
 
  Ion collisions with beam-pipe walls is a significant source of secondary electron clouds and desorbed neutral gasses in particle accelerators. Ions may reflect from beam-pipe walls and undergo further collisions downstream. These effects can cause beam degradation and are expected to be problematic in the design of heavy ion accelerators. The well-known SRIM code provides physically-based monte carlo simulations of ion-wall collisions. However, it is difficult to interface SRIM with high-performance simulation codes. We present details on the development of a package of Python modules which integrate the simulation of ion-wall interactions at grazing incidences with the high-performance particle-in-cell and electron cloud codes WARP and POSINST. This software package, called GriPY, calculates reflected angles and energies of ions which strike beam-pipe walls at grazing incidences, based upon interpolation of monte carlo statistics generated by benchmark simulations run in SRIM for a variety of relevant incident angles and energies. We present here solutions for 1.8 MeV K+ ions and 1 Gev protons incident on stainless steel.  
FPAP033 Beam Energy Scaling of Ion-Induced Electron Yield from K+ Ions Impact on Stainless Steel Surfaces 2287
 
  • M. Kireeff Covo, J.J. Barnard, R.H. Cohen, A. Friedman, D.P. Grote, S.M. Lund, A.W. Molvik, G.A. Westenskow
    LLNL, Livermore, California
  • D. Baca, F.M. Bieniosek, C.M. Celata, J.W. Kwan, P.A. Seidl, J.-L. Vay
    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.

The cost of accelerators for heavy-ion inertial fusion energy (HIF) can be reduced by using the smallest possible clearance between the beam and the wall from the beamline. This increases beam loss to the walls, generating ion-induced electrons that could be trapped by beam space charge potential into an "electron cloud," which can cause degradation or loss of the ion beam. In order to understand the physical mechanism of production of ion-induced electrons we have measured impact of K+ ions with energies up to 400 KeV on stainless steel surfaces near grazing incidence, using the ion source test stand (STS-500) at LLNL. The electron yield will be discussed and compared with experimental measurements from 1 MeV K+ ions in the High-Current Experiment at LBNL.*

*A.W. Molvik et al., PRST-AB 7, 093202 (2004).