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An effective longitudinal space-charge impedance model for beamswith non-uniform and non-axissymmetric transverse density

Description: Use of a one-dimensional model of longitudinal space-charge(SC) impedance has been proposed for studying the microbunchinginstability in single-pass delivery systems relevant for the nextgeneration of FELs. For beams with uniform transverse density andcircular cross-section of radius r_b the SC impedance can be expressed ina handy analytical form, making this model particularly convenient. Inthis report we show how with an appropriate choice of r_b one can usethis as an effective-beam model to approximate beams with non-axissymmetric and non-uniform transverse densities.
Date: June 29, 2007
Creator: Venturini, Marco
Partner: UNT Libraries Government Documents Department

Models of Longitudinal Space-Charge Impedance for the Study of theMicrobunching Instability

Description: A 1D model of space-charge impedance, assuming atransversely uniform beam with circular cross-section, has been proposedand is being extensively used in the modelling of the microbunchinginstability of relevance for the beam delivery systems of x-ray FELs. Inthis paper we investigate the limitation of the model when applied tostudying the effect of shot noise--one of the sources of themicrobunching instability. We make comparison witha fully 3D calculationand identify the upper end of the frequency spectrum for applicability ofthe 1D model. Relaxation of the assumptions regarding axis-symmetry anduniformity of the transverse density is also reviewed.
Date: March 10, 2008
Creator: Venturini, Marco
Partner: UNT Libraries Government Documents Department

Study of the microbunching instability in single-pass systemsusing a direct 2D Vlasov solver

Description: We apply a recently developed Vlasov solver to the study ofthemicrobunching instability generated by shot noise in the beamdeliverysystems of x-ray Free Electron Lasers (FELs). We discusstwo latticespresently under consideration for the FEL FERMI project at Elettra andshow that at least one of the two lattices appears capable of deliveringa beam with the desired quality in the longitudinal phasespace.
Date: June 30, 2007
Creator: Venturini, Marco
Partner: UNT Libraries Government Documents Department

Modeling Microbunching from Shot Noise Using Vlasov Solvers

Description: Unlike macroparticle simulations, which are sensitive to unphysical statistical fluctuations when the number of macroparticles is smaller than the bunch population, direct methods for solving the Vlasov equation are free from sampling noise and are ideally suited for studying microbunching instabilities evolving from shot noise. We review a 2D (longitudinal dynamics) Vlasov solver we have recently developed to study the microbunching instability in the beam delivery systems for x-ray FELs and present an application to FERMI{at}Elettra. We discuss, in particular, the impact of the spreader design on microbunching.
Date: May 1, 2008
Creator: Venturini, Marco; Venturini, Marco & Zholents, Alexander
Partner: UNT Libraries Government Documents Department

Longitudinal Single-Bunch Instability in the ILC Damping Rings: Estimate of Current Threshold

Description: Characterization of single-bunch instabilities in the International Linear Collider (ILC) damping rings (DRs) has been indicated as a high-priority activity toward completion of an engineering design. In this paper we report on a first estimate ofthe current thresholds for the instability using numerical and analytical models of the wake potentials associated with the various machine components. The numerical models were derived (upon appropriate scaling) from designs of the correspondingcomponents installed in existing machines. The current thresholds for instabilities were determined by numerical solution of the Vlasov equation for the longitudinal dynamics. For the DR baseline lattice as of Feb. 2007 we find the critical current forinstability to be safely above the design specifications leaving room for further optimization of the choice of the momentum compaction.
Date: June 25, 2008
Creator: Venturini, Marco & Venturini, Marco
Partner: UNT Libraries Government Documents Department

Evolving bunch and retardation in the impedance formalism

Description: The usual expression for the longitudinal wake field in terms of the impedance is exact only for the model in which the source of the field is a rigid bunch. To account for a deforming bunch one has to invoke the complete impedance, a function of both wave number and frequency. A computation of the corresponding wake field would be expensive, since it would involve integrals over frequency and time in addition to the usual sum over wave number. We treat the problem of approximating this field in an example of current interest, the case of coherent synchrotron radiation (CSR) in the presence of shielding by the vacuum chamber.
Date: May 22, 2003
Creator: Warnock, Robert & Venturini, Marco
Partner: UNT Libraries Government Documents Department

Shot-Noise Seeded Microbunching Instability: Second-Order Correction to the Gain Function

Description: We determine the second-order correction to the gain function of the microbunching instability in single-pass systems of interest for the next generation of light sources. The calculation applies to the case where the instability is seeded by shot noise. We examine an analytically treatable model of beam dynamics where collective forces are active only in non-dispersive sections of the linac. We find that the second order term can augment the linear gain significantly while affecting the spectrum of the overall gain only marginally. The weight of the second-order correction relative to the linear gain is found to scale quadratically with respect to R56. The qualitative behavior predicted by the model is consistent with exact numerical solutions of the Vlasov equations for realistic lattices.
Date: June 30, 2008
Creator: Venturini, Marco
Partner: UNT Libraries Government Documents Department

E-Cloud Build-up in Grooved Chambers

Description: We simulate electron cloud build-up in a grooved vacuumchamber including the effect of space charge from the electrons. Weidentify conditions for e-cloud suppression and make contact withprevious estimates of an effective secondary electron yield for groovedsurfaces.
Date: May 1, 2007
Creator: Venturini, Marco
Partner: UNT Libraries Government Documents Department

Space-Charge Effects in the Super B-Factory LER

Description: Space-charge effects in the low-energy ring of the proposedSuper-B Factory are studied using a weak-strong model of dynamics asimplemented in the code Marylie/Impact (MLI). The impact of space chargeappears noticeable but our results suggest the existence of workableregions of the tune space where the design emittance is minimallyaffected. However, additional studies are recommended to fullysubstantiate this conclusion.
Date: January 31, 2007
Creator: Venturini, Marco
Partner: UNT Libraries Government Documents Department

A Vlasov Solver for Longitudinal Dynamics in Beam Delivery Systems for X-Ray FELs

Description: Direct numerical methods for solving the Vlasov equationoffer some advantages over macroparticle simulations, as they do notsuffer from the consequences of the statistical fluctuations inherent inusing a number of macroparticles smaller than thebunch population.Unfortunately these methods are more time-consuming and generallyconsidered impractical in a full 6D phase space. However, in alower-dimension phase space they may become attractive if the beamdynamics is sensitive to the presence of small charge-densityfluctuations and a high resolution is needed. In this paper we present a2D Vlasov solverfor studying the longitudinal beam dynamics insingle-pass systems of interest for X-Ray FELs, where characterization ofthe microbunching instability stemming from self-field amplified noise isof particular relevance.
Date: May 9, 2007
Creator: Venturini, Marco; Warnock, Robert & Zholents, Alexander
Partner: UNT Libraries Government Documents Department

A FOUR-DIMENSIONAL VLASOV SOLVER FOR MICROBUNCHING INSTABILITY INTHE INJECTION SYSTEM FOR X-RAY FELS

Description: The micro-bunching instability (MBI) is seeded by small charge-density fluctuations in the electron bunches, and is successively amplified by the combined effect of space charge and coherent synchrotron radiation, as the beam travels through magnetic compressors. The quantitative understanding of this effect demands for accurate numerical simulations. Here we report on the progress of an upgrading of a 2D Vlasov solver code toward a 4D grid-based Vlasov solver, including also the transversedynamics. The goal is to provide an accurate characterization of the MBI seeded by random noise present in the bunch distribution. We also comment on the advantages of our procedure with respect to other approaches, e.g. macroparticle simulations.
Date: June 23, 2008
Creator: Venturini, Marco; Migliorati, Mauro; Schiavi, Angelo; Dattoli, Giuseppe & Venturini, Marco
Partner: UNT Libraries Government Documents Department

Dynamical map for combined function magnets with solenoid, dipole, and quadrupole fields

Description: The interaction regions of colliders invariably include strong solenoid fields. Where quadrupoles and dipoles are embedded in the solenoid, the beam dynamics in the combined fields can be complicated to model using the traditional approach of interleaving slices of the different fields. The complexity increases if the design trajectory is offset from the magnetic axis; this is the case, for example, in PEP-II. In this paper, we present maps for combined solenoid, dipole and quadrupole fields that provide a much simpler alternative to the traditional approach, and show that the deviation of the design trajectory from the magnetic axis can be handle in a straightforward manner. We illustrate the techniques presented by reference to the PEP-II interaction region.
Date: June 30, 2004
Creator: Venturini, Marco & Wolski, Andy
Partner: UNT Libraries Government Documents Department

Dynamic aperture study for the NLC main damping rings

Description: A sufficiently large acceptance is critical for the NLC Main Damping Rings (MDR) as the high power carried by the beams demands very high injection efficiency. Chromatic sextupoles and wiggler insertions (needed for rapid damping) are substantial sources of nonlinearities limiting the dynamic aperture. We report on the techniques we are using for analysis of single-particle beam dynamics in the presence of wiggler fields with significant nonlinear components. We demonstrate that our approach gives results in good agreement with experimental data when applied to the BL11 wiggler in SPEAR2, and discuss the present status of studies for the NLC MDR.
Date: July 5, 2004
Creator: Wolski, Andrzej; Venturini, Marco & Marks, Steve
Partner: UNT Libraries Government Documents Department

Effect of Wiggler insertions on the single-particle dynamics of the NLC main damping rings

Description: As they are expected to occupy a large portion of the lattice, wiggler insert ions will introduce significant linear and nonlinear perturbations to the single-particle dynamics in the NLC Main Damping Rings (MDR). The nonlinearities are of particular concern as a sufficiently large Dynamic Aperture (DA) is required for high injection efficiency. The main content of this report is a study of the wigglers impact on the DA of the NLC-MDR latest lattice design. The particle dynamics is modeled by transfer maps calculated by integration through the wiggler fields. For field representation we employed a 3D multipole expansion derived from the field data that were obtained with the aid of a magnet design code. Additional contents of this paper include an investigation of a simplified model of wiggler consisting of a sequence of standard magnet elements (where thin octupoles are used to represent the dominant nonlinearities) and the suggestion of a possible correction scheme to compensate the wiggler nonlinearities.
Date: July 30, 2003
Creator: Venturini, Marco
Partner: UNT Libraries Government Documents Department

Longitudinal single-bunch instabilities in the NLC main damping rings

Description: Because of tight requirements on beam quality longitudinal single-bunch instabilities are a serious concern for the damping rings of the next generation of linear colliders. Unlike multi-bunch instabilities they cannot be damped using feed-back systems and need to be avoided altogether. We present an analysis of these instabilities for the current Feb. 03 NLC main damping ring design, with attention paid to coherent synchrotron radiation and vacuum chamber effects, with the latter including the main components (RF cavities, BPM's, and resistive wall). The study is carried out by solving the Vlasov-Fokker-Planck equation for the longitudinal motion numerically. Comparison is made, whenever possible, with linear theory. We find that collective effects are dominated by coherent synchrotron radiation and estimate the instability threshold to be safely above 6 times the design current.
Date: May 10, 2004
Creator: Venturini, Marco
Partner: UNT Libraries Government Documents Department

Wigglers and single-particle dynamics in the NLC damping rings

Description: Wiggler insertions are expected to occupy a significant portion of the lattice of the Next Linear Collider (NLC) Main Damping Rings (MDR) and have a noticeable impact on the single-particle beam dynamics. Starting from a realistic 3D representation of the magnetic fields we calculate the transfer maps for the wigglers, accounting for linear and nonlinear effects, and we study the beam dynamics with particular attention paid to the Dynamic Aperture(DA). A DA reduction is observed but appears to remain within acceptable limits.
Date: May 6, 2003
Creator: Venturini, Marco; Wolski, Andrzej & Dragt, Alex
Partner: UNT Libraries Government Documents Department

Simulations of the microbunching instability in beam deliverysystems for free electron lasers

Description: In this paper, we examine the growth of the microbunchinginstability in the electron beam delivery system of a free electron laser(FEL). We present the results of two sets of simulations, one conductedusing a direct Vlasov solver, the other using a particle-in-cell codeImpact-Z with the number of simulation macroparticles ranging up to 100million. Discussion is focused on the details of longitudinal dynamicsand on numerical values of uncorrelated (slice) energy spread atdifferent points in the lattice. In particular, we assess the efficacy oflaser heater in suppression of the instability, and look at the interplaybetween physical and numerical noise in particle-basedsimulations.
Date: June 29, 2007
Creator: Pogorelov, Ilya; Qiang, Ji; Ryne, Rob; Venturini, Marco; Zholents, Alexander & Warnock, Robert
Partner: UNT Libraries Government Documents Department

Direct space-charge effects on the ILC damping rings: Task ForceReport

Description: In 2005 a global effort was initiated to conduct studies for a baseline recommendation for the various components of the International Linear Collider (ILC). Work for the damping rings was subdivided in a number of tasks. This Report contains the contribution to this effort by the Authors as Coordinators of the Task Force on space charge. (A slightly reduced version of this document can also be found as part of the ''Configuration Studies and Recommendations for the ILC Damping Rings'', Edts. A. Wolski, et al., LBNL-59449.) The studies documented in this Report were carried out for several of the reference lattices considered for the baseline recommendation. Space charge effects were found to be quite noticeable in the lattices with the longest circumference. Although it does not appear that they could prevent operation of any machine having such lattices they do favor a choice of a ring design with shorter ({approx}6km) circumference at 5 GeV.
Date: February 28, 2006
Creator: Venturini, Marco & Oide, Katsunobu
Partner: UNT Libraries Government Documents Department

Development of a 2D Vlasov Solver for Single-Pass Systems

Description: Direct numerical methods for solving the Vlasov equationoffer some advantages over macroparticle simulations, as they do notsuffer from the numerical noise inherent in using a number ofmacroparticles smaller than the bunch population. Unfortunately thesemethods are more time-consuming and generally considered impractical in afull 6D phase space. However, in a lower-dimension phase space they maybecome attractive if the beam dynamics is sensitive to the presence ofsmall charge-density fluctuations and a high resolution is needed. Inthis paper we present a 2D Vlasov solver for studying the longitudinalbeam dynamics in single-pass systems of interest for X-FEL's, wherecharacterization of the microbunching instability is of particularrelevance. The solver includes a model to account for the smearing effectof a finite horizontal emittance on microbuncing. We explore the effectof space charge and coherent synchrotron radiation (CSR). The numericalsolutions are compared with results from linear theory and good agreementis found in the regime where linear theory applies.
Date: July 31, 2006
Creator: Venturini, Marco; Warnock, Robert & Zholents, Alexander
Partner: UNT Libraries Government Documents Department

A MODEL FOR PRODUCING STABLE, BROADBAND TERAHERTZ COHERENT SYNCHROTRONRADIATION IN STORAGE RINGS

Description: We present a model for producing stable broadband coherent synchrotron radiation (CSR) in the terahertz frequency region in an electron storage ring. The model includes distortion of bunch shape from the synchrotron radiation (SR), enhancing higher frequency coherent emission and limits to stable emission due to a microbunching instability excited by the SR. We use this model to optimize the performance of a source for CSR emission.
Date: June 13, 2003
Creator: Sannibale, Fernando; Byrd, John M.; Loftsdottir, Agusta; Martin, MichaelC. & Venturini, Marco
Partner: UNT Libraries Government Documents Department

Space Charge and Equilibrium Emittances in Damping Rings

Description: We present a model of dynamics to account for the possible impact of space charge on the equilibrium emittances in storage rings and apply the model to study the current design of the International Linear Collider (ILC) damping rings.
Date: June 21, 2006
Creator: Venturini, Marco; Oide, Katsunobu & Wolski, Andy
Partner: UNT Libraries Government Documents Department

Modelling of e-cloud build-up in grooved vacuum chambers usingPOSINST

Description: Use of grooved vacuum chambers have been suggested as a wayto limitelectron cloud accumulation in the ILC-DR. We report onsimulations carried out using an augmented version of POSINST, accountingfor e-cloud dynamics in the presence of grooves, and make contact withprevious estimates of an effective secondary electron yield for groovedsurfaces.
Date: June 29, 2007
Creator: Venturini, Marco; Celata, C.; Furman, Miguel; Vay, Jean-Luc & Pivi, Mauro
Partner: UNT Libraries Government Documents Department