NRL, Washington, DC
UMD, College Park, Maryland
LANL, Los Alamos, New Mexico
SAIC, Burlington, Massachusetts
Funding: We gratefully acknowledge funding provided by the Joint Technology Office and the Office of Naval Research
Laser switched photocathodes are now the electron source of choice for short wavelength Free Electron Lasers. The photocathode requirements are profound: ideally, capabilities such as high peak and average current, high quantum efficiency (QE) in the visible, long lifetime in an rf injector and the ability to be repaired in situ are desired. We are pursuing cathodes with self-rejuvenating surfaces based on cesium dispenser cathode technology*,**, in which the physics of recesiation, evaporation, diffusion, and evolution of the surface coating and the QE are the metrics of performance. Here, we present predictive theoretical models of surface evolution and QE in a manner appropriate for inclusion in beam simulation codes, wherein emission non-uniformity and dark current affect emittance, beam halo, and dynamic evolution of bunched electron beams***. The emission models focus on bulk transport issues (including scattering processes) and surface conditions (including diffusion in the presence of random, non-uniform sub-monolayer coverage), and relate these factors to recent experimental characterizations of the surface evolution.
*Jensen, et al., JAP{10}2, 074902 ; Moody, et al., APL90, 114108.
**E. Montgomery, et al., (this conference)
***Petillo, et al., Proc IEEE PAC (2007); Jensen, et al., PRST-AB 11, 081001.
UMD, College Park, Maryland
NRL, Washington, DC
LANL, Los Alamos, New Mexico
Funding: This project is funded by the Joint Technology Office and the Office of Naval Research.
High performance FELs require photocathodes with quantum efficiencies of several percent at green wavelengths, kHr lifetime, kA/cm2 peak and A/cm2 average current, and ps response. Such cathodes are challenged to maintain requisite high quantum efficiency while in harsh accelerator vacuum conditions. Delicate surface coatings are often cesium-based, and therefore are reactive with contaminant gases. The dispenser photocathode architecture resupplies the cesium coating from a subsurface reservoir through a porous substrate, thereby extending lifetime*. Recesiation has been shown to rejuvenate Cs:Ag cathodes from O2, CO2, and N2O contamination**, and theory of dispenser photocathodes is advancing***. We here investigate the fabrication, contamination, and external recesiation of alkali antimonides with high quantum efficiency, in support of the dispenser photocathode design.
*Moody et al., APL90, 114108.
**Montgomery et al., Proc. AACW 2008 (submitted for publication).
***K. Jensen et al., (this conference).
BNL, Upton, Long Island, New York
UMD, College Park, Maryland
JLAB, Newport News, Virginia
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office
In this paper we report on a proof of principle experiment for demonstrating the possibility of reconstructing the time resolved-phase-space distribution of a space-charge dominated beam by a tomographic technique which provides us with far more information than a time-sliced emittance. We emphasize that this work describes and demonstrates a new methodology which can be applicable to any beam pulse using imaging methods with the appropriate time resolution for the pulse duration. The combination of a high precision tomographic diagnostic with fast imaging screens and a gated camera are used to produce phase space maps of two beams: one with a parabolic current profile and another with a short perturbation atop a rectangular pulse. The correlations between longitudinal and transverse phase spaces are apparent and their impact on the dynamics is discussed.
LLNL, Livermore, California
UMD, College Park, Maryland
Funding: This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Mismatch oscillations resulting from the propagation of space charge waves in intense beams may lead to halo generation and possible beam loss, and modify longitudinal beam dynamics. These oscillations have amplitudes and frequencies different from that of the main beam and are particularly important in machines such as the University of Maryland Electron Ring (UMER), in which the beam dynamics scale to parameters associated with heavy ion fusion drivers. We use the particle-in-cell (PIC) code, LSP, to simulate space charge wave dynamics in an intense electron beam propagating in a smooth focusing channel with 2-D cylindrical symmetry. We examine the evolution of linear and nonlinear density perturbations in the UMER parameter range for both matched and mismatched beams. Comparisons between LSP simulations and numerical models are presented.
UMD, College Park, Maryland
ANL, Argonne
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office.
Single foil OTR and two foil OTR interferometry have been successfully used to measure the size and divergence of electron beams with a wide range of energies. To measure rms emittance, two cameras are employed: one focused on the foil to obtain the spatial distribution of the beam, the other focused to infinity to obtain the angular distribution. The beam is first magnetically focused to a minimum size in directions which are orthogonal to the propagation axis, using a pair of quadrupoles. Then simultaneous measurements of the rms size (x,y) and divergence (x’,y’) of the beam are made. However, in the process of a quadrupole scan, the beam can go through a spot size minimum, a divergence minimum and a waist, i.e. the position where the cross-correlation term is zero. In general, the beam size, divergence and focusing strength for each of these conditions are different. We present new algorithms that relate the beam and magnetic parameters to the rms emittance for each of these three cases. We also compare the emittances, obtained using our algorithms and measurements made at the ANL AWA facility, with those produced by computer simulation.
JLAB, Newport News, Virginia
UMD, College Park, Maryland
BNL, Upton, Long Island, New York
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office
Longitudinal space-charge waves can introduce energy perturbations into charge particle beams and degrade the beam quality, which is critical to many modern applications of particle accelerators. Although many longitudinal phenomena arising from small perturbations can be explained by a one-dimensional cold fluid theory, nonlinear behavior of space-charge waves observed in experiments has not been well understood. In this paper, we summarize our recent investigation by means of more detailed measurements and self-consistent simulations. Combining the numerical capability of a PIC code, WARP, with the detailed initial conditions measured by our newly developed time resolved 6-D phase space mapping technique, we are able to construct a self consistent model for studying the complex physics of longitudinal dynamics of space-charge dominated beams. Results from simulation studies suggest that the unexplained nonlinear behavior of space-charge waves may be due to transverse mismatch or misalignment of beams.
UMD, College Park, Maryland
Funding: * This work is funded by US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office.
The containment of beams in the longitudinal direction is fundamental to the operation of accelerators that circulate high intensity beams for long distances such as the University of Maryland Electron Ring (UMER); a scaled accelerator using low-energy electrons to model space-charge dynamics. The longitudinal space-charge forces in the beam, responsible for the expansion of the beam ends, cause a change in energy at the beam head/tail with respect to the main injected energy or flat-top part of the beam. This paper presents the first experimental results on using an induction cell to longitudinally focus the circulating beam within the UMER lattice for multiple turns.
Keywords: electron ring, focusing, induction cell.
UMD, College Park, Maryland
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office
The University of Maryland Electron Ring (UMER) can operate over a broader range of beam intensities than other circular machines. Naturally, transverse and longitudinal space charge effects limit the ability to store beams. In UMER, the resonance properties of the machine in the two regimes of operation, emittance- and space charge-dominated transport, differ significantly. We report on studies of linear betatron resonances in UMER from 0.6 mA to 80 mA beam current, corresponding to theoretical space charge incoherent tune shifts well over the Lasslet limit. The observations are related to existing theories as well as to computer simulations. We also describe the instrumentation and techniques used for tune measurements.
UMD, College Park, Maryland
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office.
Understanding the dynamics of the acceleration of high-intensity space-charge-dominated electron and ion beam is very important. Accelerating by steps a space-charge-dominated beam can be fundamentally different from beams at lower intensities, because at sufficiently high beam intensities the beam response to acceleration can drive to some unknown instabilities leading to a significant beam losses. This work analyses the acceleration of the University of Maryland Electron Ring (UMER) beam, i.e., high current, low-energy and space-charge-dominated electron beam which is applicable, on a scale basis, to a large class of other beam systems. We use the WARP particle-in-cell code to perform simulations that are compared with theoretical predictions and preliminary experimental results.
UMD, College Park, Maryland
BNL, Upton, Long Island, New York
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office
Beam matching is critical for avoiding envelope mismatch oscillations that can lead to emittance growth and halo formation, especially if the beam has significant space charge. The University of Maryland Electron Ring (UMER) is a research storage ring that is designed for scaled studies that are applicable to many larger machines. Using 10 keV electron beams at relatively high current (0.6 100 mA), space charge forces are relatively strong. Matching of the UMER beam is rendered difficult by the space charge, the crowdedness of the lattice, and especially the unique injection scheme where an offset oversized quadrupole is shared between the ring and the injector. In this paper we discuss several schemes for optimizing the matching at injection, both analytical and beam-based, which we test using particle-in-cell simulations with the code, WARP. Comparison to UMER experimental data is provided where available.
UMD, College Park, Maryland
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept of Defense Office of Naval Research and Joint Technology Office.
Halo is one important limiting factor for the continuous and reliable operation of intense electron or ion beam facilities, such as FELs and spallation neutron sources. A halo population outside the core of the beam can lead to uncontrolled beam loss, electron cloud effects and activation of the beam pipe, as well as beam quality degradation. In this study, we focus on the issue of halo removal, by means of beam collimation, and subsequent halo regeneration. We compare the particle-core model of halo creation to accurate, self consistent particle-in-cell (PIC) simulations. We show that under certain conditions the halo is regenerated even after collimation. This can only be understood within the context of collective effects, particularly in the case of intense beams.
UMD, College Park, Maryland
Funding: *This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office
The University of Maryland Electron Ring (UMER) is designed for operation over a broad range of beam intensities, including those normally achieved only in linacs. This is possible thanks to a combination of low-energy (10 keV) electrons and a high density of magnetic quadrupoles (72 over an 11.5 m circumference) that allow operation from 0.5 mA to 100 mA; that is, from the emittance dominated to the highly space charge dominated regimes. We present results of basic centroid-motion characterization, including measurements of closed-orbit distortion, momentum compaction factor, and natural chromaticity and dispersion. These are compared with results from computer simulations employing the code ELEGANT. We discuss the techniques and challenges behind the measurements with fast beam-position and wall-current monitors, and also the special role of the background ambient magnetic field for beam steering.
UMD, College Park, Maryland
Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office.
Beam centroid control is an important method for optimizing the performance for accelerators, including the University of Maryland Electron Ring (UMER), which is a scaled low-energy (10KeV) storage ring. The conventional response matrix and singular value decomposition approach do not work well on the UMER because of the unique ring structure. One of the purposes of this work is to verify that the beam centroid could be controlled in the presence of very strong space charge. In this paper, we present a novel algorithm which is based on the singular value decomposition, but uses a different response matrix, which is computed from the closed equilibrium orbit and beam positions up to the first four turns in the multi-turn beam circulation. Other issues like strong coupling between the horizontal steering dipoles and vertical steering dipoles in the beam injection section will be addressed. Implementation of this algorithm leads to significant improvement on the beam positions and multi-turn operation.