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O'Shea, P.G.

Paper Title Page
MOPC010 Longitudinal Dynamics in the University of Maryland Electron Ring 713
 
  • J.R. Harris, D.W. Feldman, R. Feldman, Y. Huo, J.G. Neumann, P.G. O'Shea, B. Quinn
    IREAP, College Park, Maryland
  • M. Reiser
    University Maryland, College Park, Maryland
 
  Funding: Work supported by the Department of Energy, the Office of Naval Research, the Joint Technology Office, and the Directed Energy Professional Society.

The University of Maryland Electron Ring (UMER) is a low energy electron recirculator for the study of space charge dominated beam transport. The system’s pulse length (100 ns) and large number of diagnostics make it ideal for investigating the longitudinal evolution of intense beams. Pulse shape flexibility is provided by the pulser system and the gridded gun, which has the ability to produce thermionic and photoemission beams simultaneously. In this paper, we report on the generation and evolution of novel line charge distributions in UMER.

 
MPPE067 Refined Calculation of Beam Dynamics During UMER Injection 3733
 
  • G. Bai, S. Bernal, T.F. Godlove, I. Haber, R.A. Kishek, P.G. O'Shea, B. Quinn, J.C. Tobin Thangaraj, M. Walter
    IREAP, College Park, Maryland
  • M. Reiser
    University Maryland, College Park, Maryland
 
  Funding: This work is funded by U.S. Dept. of Energy under grants DE-FG02-94ER40855 and DE-FG02-92ER54178.

The University of Maryland Electron Ring (UMER) is built as a low-cost testbed for intense beam physics for benefit of larger ion accelerators. The beam intensity is designed to be variable, spanning the entire range from low current operation to highly space-charge-dominated transport. The ring has recently been closed and multi-turn commissioning has begun. Although we have conducted many experiments at high space charge during UMER construction, lower-current beams have become quite useful in this commissioning stage for assisting us with beam steering, measurement of phase advance, etc. One of the biggest challenges of multi-turn operation of UMER is correctly operating the Y-shaped injection section, hence called the Y-section, which is specially designed for UMER multi-turn operation. It is a challenge because the system requires several quadrupoles and dipoles in a very stringent space, resulting in mechanical, electrical, and beam control complexities. This paper presents a simulation study of the beam centroid motion in the injection region.

 
MOPB010 Simulations and Experiments of Electron Beams Pre-Modulated at the Photocathode 704
 
  • J.G. Neumann, R.B. Fiorito, P.G. O'Shea
    IREAP, College Park, Maryland
  • G.L. Carr, T.V. Shaftan, B. Sheehy, Y. Shen, Z. Wu
    BNL, Upton, Long Island, New York
  • W. Graves
    MIT, Middleton, Massachusetts
  • H. Loos
    SLAC, Menlo Park, California
 
  Funding: Work is supported by the Office of Naval Research, the Joint Technology Office, and the Department of Energy.

The University of Maryland and the Source Development Laboratory at Brookhaven National Laboratory have been collaborating on a project that explores the use of electron beam pre-modulation at the cathode to control the longitudinal structure of the electron beam. This technique could be applied to creating deliberate modulations which can lead to the generation of terahertz radiation, or creating a smooth profile in order to supress radiation. This paper focuses on simulations that explore some of the pre-modulated cases achieved experimentally.

 
TPAT004 Strongly Asymmetric Beams at the University of Maryland Electron Ring (UMER) 892
 
  • S. Bernal, R.A. Kishek, P.G. O'Shea, B. Quinn, M. Walter
    IREAP, College Park, Maryland
  • M. Reiser
    University Maryland, College Park, Maryland
 
  Funding: This work is funded by U.S. Dept. of Energy under grants DE-FG02-94ER40855 and DE-FG02-92ER54178.

The standard operation of the University of Maryland electron ring employs symmetric strong focusing with magnetic quadrupoles, i.e., a FODO scheme whereby the zero-current betatron phase advances per period in the two transverse planes are equal or nearly so. Asymmetric focusing, on the other hand, employs quadrupoles with different strengths in a FODO cell. Typically, a small focusing asymmetry is implemented in most accelerators to set the operating point (horizontal and vertical zero-current tunes) in order to avoid resonances and/or compensate for edge focusing of bend magnets. Extreme asymmetry, however, is rarely, if at all, used. We review the motivation and theory of beam transport with general focusing asymmetry. We also present results of preliminary experiments and simulations with highly asymmetric focusing of a space-charge dominated electron beam in UMER.

 
TPAT035 Coherent Synchrotron Radiation from an Electron Beam in a Curved Waveguide 2390
 
  • D.R. Gillingham, T. M. Antonsen, P.G. O'Shea
    IREAP, College Park, Maryland
 
  Funding: Research supported by the office of Naval Research and the Joint Technology Office.

The radiation emitted by a pulsed electron beam as it travels on a circular trajectory inside a waveguide is calculated using a 3D simulation. Forward-propagating wave equations for the fields in the waveguide are calculated by a perturbation of the Maxwell equations where the radius of curvature is large compared to the dimensions of the waveguide. These are integrated self-consistently with the distribution of charge in the beam to provide the complete fields (electric and magnetic) for all times during the passage of the beam through the waveguide and therefore are applicable to sections of any length or combinations thereof. The distribution of electrons and their momentum are also modified self-consistently so that the results may be used to estimate the effect of the radiation on the beam quality (emittance and energy spread).

 
TPAT066 Significance of Space Charge and the Earth Magnetic Field on the Dispersive Characteristics of a Low Energy Electron Beam 3691
 
  • R.A. Kishek, G. Bai, S. Bernal, T.F. Godlove, I. Haber, P.G. O'Shea, B. Quinn, C. Tobin, M. Walter
    IREAP, College Park, Maryland
  • M. Reiser
    University Maryland, College Park, Maryland
 
  Funding: This work is funded by U.S. Dept. of Energy grant numbers DE-FG02-94ER40855 and DE-FG02-92ER54178.

The combination of energy spread and space charge provides a rich domain for interesting beam dynamics that are currently not well understood. The University of Maryland Electron Ring (UMER) [1] is a small scaled ring designed to probe the little-known regions of higher beam intensities using low-energy electrons. As such, design, commissioning and operation of UMER present many challenges, some quite novel. For example the UMER beam energy of 10 keV makes the beam very sensitive to the Earth magnetic field, which we can fortunately use to assist in bending the beam. This paper presents a systematic simulation study of the interaction of space charge and energy spread, with and without the earth magnetic field.

*"Commissioning of the University of Maryland Electron Ring (UMER)," S. Bernal, et al., this conference.

 
TPAT067 Study of Longitudinal Space-Charge Wave Dynamics in Space-Charge Dominated Beams 3712
 
  • K. Tian, Y. Cui, I. Haber, Y. Huo, R.A. Kishek, P.G. O'Shea, Y. Zou
    IREAP, College Park, Maryland
  • M. Reiser
    University Maryland, College Park, Maryland
 
  Funding: Work supported by the U.S. Department of Energy, Office of Science.

Understanding the dynamics of longitudinal space- charge waves is very important for advanced accelerator research. Although analytical solutions of space-charge wave equations based on the cold fluid model exist in one dimension, there are few results for two-dimensional wave evolution. One-dimensional theory predicts two eigen solutions, given an initial perturbation. One is called the fast wave, which moves toward the beam head in the beam frame and the other is termed the slow wave, which moves backward in the beam frame. In this paper, we report experimental results of space charge wave studies conducted on a 2.3 meter long straight beam line at the University of Maryland. An energy analyzer is used to directly measure the energy of space-charge waves at the end of the transport line, which demonstrates the decomposition of an initial current perturbation into a slow wave and a fast wave. A PIC code, WARP [1], is used to simulate this experiment and the behavior of longitudinal waves in space-charge dominated beams in an R-Z geometry. Simulations shown here also demonstrate if the initial current and velocity perturbation strengths are chosen properly, only fast or slow waves could be selectively generated.

 
TPPE039 Development of Advanced Models for 3D Photocathode PIC Simulations 2583
 
  • D.A. Dimitrov, D.L. Bruhwiler, J.R. Cary, P. Messmer, P. Stoltz
    Tech-X, Boulder, Colorado
  • D.W. Feldman, P.G. O'Shea
    IREAP, College Park, Maryland
  • K. Jensen
    NRL, Washington, DC
 
  Funding: This work is supported by the U.S. DOE, use of NERSC supercomputer facilities, and the Joint Technology office (JTO).

Codes for simulating photocathode electron guns invariably assume the emission of an idealized electron distribution from the cathode, regardless of the particular particle emission model that is implemented. The output of such simulations, a relatively clean and smooth distribution with very little variation as a function of the azimuthal angle, is inconsistent with the highly irregular and asymmetric electron bunches seen in experimental diagnostics. To address this problem, we have implemented a recently proposed theoretical model* that takes into account detailed solid-state physics of photocathode materials in the VORPAL particle-in-cell code.** Initial results from 3D simulations with this model and future research directions will be presented and discussed.

*K.L. Jensen, D.W. Feldman, M. Virgo, and P.G. O'Shea, Phys. Rev. ST Accel. Beams, 6:083501, 2003. **C. Nieter and J.R. Cary, J. Comp. Phys. 196 (2004), p. 448.

 
TPPE046 Computer Simulation of the UMER Gridded Gun 2908
 
  • I. Haber, S. Bernal, R.A. Kishek, P.G. O'Shea, Y. Zou
    IREAP, College Park, Maryland
  • A. Friedman, D.P. Grote
    LLNL, Livermore, California
  • M. Reiser
    University Maryland, College Park, Maryland
  • J.-L. Vay
    LBNL, Berkeley, California
 
  Funding: This work is supported by the U.S. DOE under contract Nos. DE-FG02-02ER54672 and DE-FG02-94ER40855 at the UMD, and DE-AC03-76SF00098 at LBNL and W-7405-ENG-48 at LLNL.

The electron source in the University of Maryland Electron Ring (UMER) injector employs a grid 0.15 mm from the cathode to control the current waveform. Under nominal operating conditions, the grid voltage during the current pulse is sufficiently positive relative to the cathode potential to form a virtual cathode downstream of the grid. Three-dimensional computer simulations have been performed that use the mesh refinement capability of the WARP particle-in-cell code to examine a small region near the beam center in order to illustrate some of the complexity that can result from such a gridded structure. These simulations have been found to reproduce the hollowed velocity space that is observed experimentally. The simulations also predict a complicated time-dependent response to the waveform applied to the grid during the current turn-on. This complex temporal behavior appears to result directly from the dynamics of the virtual cathode formation and may therefore be representative of the expected behavior in other sources, such as some photoinjectors, that are characterized by a rapid turn-on of the beam current.

 
TPPE047 Fabrication and Measurement of Low Work Function Cesiated Dispenser Photocathodes 2953
 
  • N.A. Moody, D.W. Feldman, P.G. O'Shea
    IREAP, College Park, Maryland
  • K. Jensen
    NRL, Washington, DC
 
  Funding: We gratefully acknowledge our funding agencies, the Joint Technology Office and the Office of Naval Research for their support.

Photoinjector performance is a limiting factor in the continued development of high powered FELs and electron beam-based accelerators. Presently available photocathodes are plagued with limited efficiency and short lifetime in an RF-gun environment, due to contamination or evaporation of a photosensitive surface layer. An ideal photocathode should have high efficiency at long wavelengths, long lifetime in practical vacuum environments, and prompt emission. Cathodes with high efficiency typically have limited lifetime, and vice versa, and the needs of the photocathode are generally at odds with those of the drive laser. A potential solution is the low work function dispenser cathode, where lifetime issues are overcome by periodic in situ regeneration that restores the photosensitive surface layer, analogous to those used in the microwave power tube industry. This work reports on the fabrication techniques and performance of cesiated metal photocathodes and cesiated dispenser cathodes, with a focus on understanding and improving quantum efficiency and lifetime, and analyzing issues of emission uniformity. The efficiency versus coverage behavior of cesiated metals is discussed and closely matches that predicted by recent theory.*

*K. L. Jensen, et al., "Photoemission from Low Work Function Coated Metal Surfaces: A Comparison of Theory to Experiment" (this conference).

 
TPPE062 Photoemission from Low Work Function Coated Metal Surfaces: A Comparison of Theory to Experiment
 
  • K. Jensen
    NRL, Washington, DC
  • D.W. Feldman, N.A. Moody, P.G. O'Shea
    IREAP, College Park, Maryland
 
  Funding: We gratefully acknowledge support provided by the Joint Technology Office and the Office of Naval Research.

The development of rugged and/or self rejuvenating photocathodes with high quantum efficiency (QE) using the longest wavelength drive laser is of paramount importance for RF photo-injectors for high power FELs and accelerators. We report on our program to develop advanced photocathodes and to develop and validate models of photoemission from coated metals to analyze experimental data,* provide emission models usable by beam simulation codes,** and project performance. The model accounts for the effects of laser heating, thermal evolution, surface conditions, laser parameters, and material characteristics, and predicts current distribution and QE. The photoemission and QE from metals and dispenser photocathodes is evaluated: the later introduces complications such as coverage non-uniformity and field enhancement. The performance of the models is compared to our experimental results for dispenser photocathodes and cesiated surfaces (e.g., tungsten, silver, etc.) in which the time-dependent models are shown to agree very well with experimental findings, but also to results in the literature. Extrapolations to performance regimes of interest shall be given.

*N. Moody et al., "Fabrication and Measurement of Low Workfunction Cesiated Dispenser Photocathodes" (this conference). **D.A. Dimitrov et al., "Development of Advanced Models for 3D Photocathode PIC Simulations" (this conference).

 
RPPE075 Injector Electronics for Multi-Turn Operation of the University of Maryland Electron Ring (UMER) 3952
 
  • M. Holloway, T.F. Godlove, P.G. O'Shea, B. Quinn, M. Walter
    IREAP, College Park, Maryland
  • M. Reiser
    University Maryland, College Park, Maryland
 
  Funding: This work is funded by U.S. Department of Energy under grants DE-FG02-94ER40855 and DE-FG02-92ER54178.

Progress is described toward the development of pulse generators required for injection and extraction of the University of Maryland Electron Ring (UMER). The geometry, described elsewhere, employs a fast ironless dipole at the junction of a Y-shaped section of the ring. The dipole as developed has an inductance of 600 nH. The required +21 A, long pulse generator for multi-turn operation is installed. A pulser providing -42 A for deflection in the opposite sense during injection is under development. It must have a fall time of ~100 ns in view of the 200 ns circulation time for the beam. A similar pulser, having a 100 ns risetime is required for beam extraction. The fast pulsers employ MOSFET switches.

 
RPPE076 Overview of Electrical Systems for the University of Maryland Electron Ring (UMER) 3988
 
  • B. Quinn, G. Bai, S. Bernal, T.F. Godlove, I. Haber, J.R. Harris, M. Holloway, H. Li, J.G. Neumann, P.G. O'Shea, K. Tian, M. Walter
    IREAP, College Park, Maryland
  • M. Reiser
    University Maryland, College Park, Maryland
 
  Funding: This work is funded by the United States Department of Energy under grants DE-FG02-94ER40855 and DE-FG02-92ER54178.

Commissioning of the University of Maryland Electron Ring (UMER) is underway (see general abstract on UMER). We discuss the various electrical systems of UMER. The power system includes 114 supplies for 70 air-core magnetic quadrupoles, 36 bending dipoles and 30+ steering dipoles as well as earth's field compensating coils. Systems for data collection comprise multiplexers and fast digitizers for diagnostics including 15 fast beam position monitors (BPMs)and video capture from fluorescent screen monitors. Several pulsers have been built in-house for injection and extraction magnets. The stringent timing schemes are also presented.

 
FPAE021 Alignment and Steering for Injection and Multi-Turn Operation of the University of Maryland Electron Ring (UMER) 1709
 
  • M. Walter, G. Bai, S. Bernal, I. Haber, M. Holloway, R.A. Kishek, P.G. O'Shea, B. Quinn
    IREAP, College Park, Maryland
  • M. Reiser
    University Maryland, College Park, Maryland
 
  Funding: This work is funded by US Dept. of Energy grant numbers DE-FG02-94ER40855 and DE-FG02-92ER54178.

The injection line and main lattice for the University of Maryland Electron Ring (UMER) has been completed. The electron beam has been guided around the full 360 degrees of the ring. Beam steering and matching in the injection line is achieved with six quadrupole magnets and several small steering dipole magnets. The dipole component of an offset quadrupole and a pulsed dipole are used to achieve the 10 degree bend required from the injection line into the ring. The pulsed dipole is designed to operate with a short pulse (2 kV, -30 A, 100 ns flat top duration) for injection superimposed on a long pulse (300 V, 15 A, 20·10-6 s duration) for multiple beam passes. The beam is controlled in the recirculating ring with a regular lattice of 36 dipole and 72 quadrupole magnets. Initial experimental results of the beam transport and control will be presented.

 
FOAD005 Commissioning of the University of Maryland Electron Ring (UMER) 469
 
  • S. Bernal, G. Bai, D.W. Feldman, R. Feldman, T.F. Godlove, I. Haber, J.R. Harris, M. Holloway, R.A. Kishek, J.G. Neumann, P.G. O'Shea, C. Papadopoulos, B. Quinn, D. Stratakis, K. Tian, J.C. Tobin Thangaraj, M. Walter, M. Wilson
    IREAP, College Park, Maryland
  • M. Reiser
    University Maryland, College Park, Maryland
 
  Funding: This work is funded by the U.S. Department of Energy under grants DE-FG02-94ER40855 and DE-FG02-92ER54178, and the office of Naval Research under grant N00014-02-1-0914.

The University of Maryland electron ring (UMER) is a low-energy, high current recirculator for beam physics research. The ring is completed for multi-turn operation of beams over a broad range of intensities and initial conditions. UMER is addressing issues in beam physics with relevance to many applications that rely on intense beams of high quality. Examples are advanced accelerators, FEL’s, spallation neutron sources and future heavy-ion drivers for inertial fusion. We review the motivation, ring layout and operating conditions of UMER. Further, we present a summary of beam physics areas that UMER is currently investigating and others that are part of the commissioning plan: from transverse beam dynamics (matching, halo formation, strongly asymmetric beams, space-charge waves, etc), longitudinal dynamics (bunch capture/shaping, evolution of energy spread, longitudinal space-charge waves, etc.) to future upgrades and planned research (acceleration and resonance traversal, modeling of galactic dynamics, etc.) We also emphasize the computer simulation work that is an integral part of the UMER project.