Marco Venturini

Accurate computation of transfer maps from magnetic field data

Abstract Consider an arbitrary beamline magnet. Suppose one component (for example, the radial component) of the magnetic field is known on the surface of some imaginary cylinder coaxial to and contained within the magnet aperture. This information can be obtained either by direct measurement or by computation with the aid of some 3D electromagnetic code. Alternatively, suppose that the field harmonics have been measured by using a spinning coil. We describe how this information can be used to compute the exact transfer map for the beamline element. This transfer map takes into account all effects of real beamline elements including fringe-field, pseudo-multipole, and real multipole error effects. The method we describe automatically takes into account the smoothing properties of the Laplace–Green function. Consequently, it is robust against both measurement and electromagnetic code errors. As an illustration we apply the method to the field analysis of high-gradient interaction region quadrupoles in the Large Hadron Collider (LHC).

Design features of small electron ring for study of recirculating space-charge-dominated beams

Abstract Rapid-cycling rings and other recirculator systems operating with intense beams beyond the conventional space charge limit are of great interest for applications in heavy ion inertial fusion, high energy physics, spallation neutron sources, and other fields. There is very little theoretical or experimental knowledge that would allow us to make any predictions on the beam behavior in such novel systems. We are proposing to build at the University of Maryland a small electron ring with a circumference of about 11 m for studying the evolution of a space-charge-dominated beam in a circular lattice. The general features of the lattice design with printed-circuit quadrupoles, dipoles, and other components and special problems are presented. Further details can be found in two related papers.

Coherent synchrotron radiation and bunch stability in a compact storage ring

We examine the effect of the collective force due to coherent synchrotron radiation (CSR) in an electron storage ring with small bending radius. In a computation based on time-domain integration of the nonlinear Vlasov equation, we find the threshold current for a longitudinal microwave instability induced by CSR alone. Themodel accounts for suppression of radiation at long wave lengths due to shielding by the vacuum chamber. In a calculation just above threshold, small ripples in the charge distribution build up over a fraction of a synchrotron period, but then die out to yielda relatively smooth but altered distribution with eventual oscillations in bunch length. The instability evolves from small noise on an initial smooth bunch of r.m.s. length much greaterthan the shielding cutoff. The paper includes a derivation and extensive analysis of the complete impedance function Z for synchrotron radiation with parallel plate shielding. We find corrections to the lowest approximation to the coherent force which involve off-diagonal values of Z, that is, fields with phase velocity not equal to the particle velocity.

Matching section and inflector design for a model electron ring

Abstract A small electron ring is being designed at the University of Maryland for studies of space-charge-dominated beams in rapid-cycling rings and recirculating accelerator systems. The motivation and general features of the ring are given in an accompanying paper. In this paper we describe the electron gun, matching section and pulsed inflector. One solenoid and five quadrupoles will be used for the matching section. This section will also serve as an experimental test bed for the printed-circuit quadrupoles and, in part, the lattice design. Our preliminary design for the injector system is based on pulsed, single-turn injection of the full beam current located at one of 36 ring dipoles. A pulsed, Panofsky quadrupole replaces one of the ring quadrupoles to accomodate the injection line. The low energy and small fields enable reasonable voltage and current for these iron-free components, but require careful compensation for external fields, including the earths field.

Design Studies for a High-Repetition-Rate FEL Facility at LBNL

Lawrence Berkeley National Laboratory (LBNL) is working to address the needs of the primary scientific Grand Challenges now being considered by the U.S. Department of Energy, Office of Basic Energy Sciences: we are exploring scientific discovery opportunities, and new areas of science, to be unlocked with the use of advanced photon sources. A partnership of several divisions at LBNL is working to define the science and instruments needed in the future. To meet these needs, we propose a seeded, high-repetition-rate, free-electron laser (FEL) facility. Temporally and spatially coherent photon pulses, of controlled duration ranging from picosecond to sub-femtosecond, are within reach in the vacuum ultraviolet (VUV) to soft X-ray regime, and LBNL is developing critical accelerator physics and technologies toward this goal. We envision a facility with an array of FELs, each independently configurable and tunable, providing a range of photon-beam properties with high average and peak flux and brightness.

Wigglers and single-particle dynamics in the NLC damping rings

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.

STUDIES OF THE PHYSICS OF SPACE-CHARGE-DOMINATED BEAMS FOR HEAVY ION INERTIAL FUSION

Abstract At the University of Maryland our research on beam physics focuses on space-charge effects and collective phenomena in high-intensity, high-brightness beams for advanced accelerator applications such as drivers for heavy ion inertial fusion. In this paper, we present results of recent beam physics experiments including study of the resistive-wall instability and beam transport in a FODO lattice. We also report on the status of an electron ring project to study the recirculation of beam currents well beyond the normal tune-shift limit.

BEAM DYNAMICS SIMULATIONS OF THE UNIVERSITY OF MARYLAND ELECTRON-RING PROJECT

Abstract A 3-D particle-in-cell code, WARP, is being used to study the self-consistent nonlinear dynamics of the space-charge-dominated beam in the University of Maryland E-ring project. Early work was concentrated on general studies of beam propagation in the nominal ring lattice, as well as more detailed calculations in the injector transport line. Simulations of the ring lattice are being used to address fundamental questions such as emittance growth, which can be caused by nonlinearities in the focusing elements, as well as distortions in the beam equilibrium by the bends. A preliminary design of the injector matching section is presented. Simulations of the injector section emphasize the relationship between the predictions of numerical models and the experimental measurements, in order to understand the requirements for adequately modeling the specific focusing elements used.

The problem of dispersion matching in space charge dominated beams

We recently proposed a new analytical model that incorporates the dispersion function into the framework of the rms envelope equations. Here we show how it can be used to achieve proper matching of space charge dominated beams. Comparison is done against a PIC code (WARP) simulations.

Dispersion and space charge

The presence of space charge affects the value of the dispersion function. On the other hand dispersion has a role in shaping the beam distribution and therefore in determining the resulting forces due to space charge. In this paper we present a framework where the interplay between space charge and dispersion for a continuous beam can be simultaneously treated. We revise the derivation of a new set of rms envelope-dispersion equations we have recently proposed in [1]. The new equations generalize the standard rms envelope equations currently used for matching to the case where bends and a longitudinal momentum spread are present. We report a comparison between the solutions of the rms envelope-dispersion equations and the results obtained using WARP, a Particle in Cell (PIC) code, in the modeling of the Maryland Electron Ring.