Author: O'Shea, P.G.
Paper Title Page
MOOBS3 Bunch-End Interpenetration During Evolution to Longitudinal Uniformity in a Space-Charge-Dominated Storage Ring 22
 
  • T.W. Koeth, B.L. Beaudoin, S. Bernal, I. Haber, R.A. Kishek, P.G. O'Shea
    UMD, College Park, Maryland, USA
 
  The University of Maryland Electron Ring is a facility for study of the novel physics that occurs as intense space-charge-dominated beams that are transported over long distances. An example presented here is the role of space-charge longitudinal expansion and bunch-end interpenetration in the relaxation of a coasting bunch towards uniformly filling the ring. By comparing experiment to simplified longitudinal simulations the relaxation process is shown to be largely independent of details of the transverse dynamics. However, to get detailed agreement it was found necessary to include the consequences of transverse current loss. Since the AC coupled diagnostics lose information on any DC current loss, a novel beam knockout technique was developed to recover this information.  
slides icon Slides MOOBS3 [2.501 MB]  
 
MOODS1 Space-Charge Effects in Bunched and Debunched Beams 85
 
  • B.L. Beaudoin, S. Bernal, K. Fiuza, I. Haber, R.A. Kishek, T.W. Koeth, P.G. O'Shea, M. Reiser, D.F. Sutter
    UMD, College Park, Maryland, USA
 
  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 a machine designed to study high-intensity beam physics. With the application of axial fields to the bunch ends, we are able to keep a beam with an injected tune shift of 1.0, bunched over multiple turns. This is feasible with the application of tailored fields to optimally match the space-charge self-fields while minimizing the excitation of longitudinal space-charge waves. With this scheme, we have been able to extend the number of turns at the University of Maryland Electron Ring (UMER) by a factor of ten. Without the use of longitudinal focusing, head and tail effects begin to dominate, especially with the higher current beams. Time resolved measurements of the peak correlated energy spread have shown in some cases a change in the overall spread of 1.8% for the 0.6 mA beam, from the injected beam energy.
 
slides icon Slides MOODS1 [2.834 MB]  
 
MOP243 Design of a Compact, High-Resolution Analyzer for Longitudinal Energy Studies in the University of Maryland Electron Ring 571
 
  • E.C. Voorhies, S. Bernal, I. Haber, R.A. Kishek, T.W. Koeth, P.G. O'Shea
    UMD, College Park, Maryland, USA
 
  Funding: Work supported by US Dept. of Energy Offices of High Energy Physics and Fusion Energy Sciences, the Dept. of Defense Office of Naval Research, and the Joint Technology Office.
Retarding-potential energy analyzers have long been used for energy spread measurements in low-energy beams. In addition to energy spread and energy profile measurements, a high-resolution analyzer can be used to reconstruct the longitudinal phase space. This is useful for our experimental studies of longitudinal physics topics, such as dispersion, space charge waves, and longitudinal focusing. A previous energy analyzer designed at the University of Maryland demonstrated high-resolution measurements of a 5 keV electron beam.* Motivated by the need to characterize the 10 keV electron beam of the University of Maryland Electron Ring, we have improved on the design of the earlier analyzer, increasing its high voltage breakdown threshold and vacuum performance. Results of high-voltage testing and particle optics simulations of the new design are presented.
*Y. Cui, Y. Zou, et al., "Design and Operation of a Retarding Field Energy Analyzer with Variable Focusing for Space-Charge Dominated Electron Beams," Review of Scientific Instruments 75(8), 2736 (2004).
 
 
WEOCN5 Beam Halo Measurements at UMER and the JLAB FEL Using an Adaptive Masking Method 1449
 
  • H.D. Zhang, S. Bernal, R.B. Fiorito, R.A. Kishek, P.G. O'Shea, A.G. Shkvarunets
    UMD, College Park, Maryland, USA
  • S.V. Benson, D. Douglas, F.G. Wilson, S. Zhang
    JLAB, Newport News, Virginia, USA
 
  Funding: US Dept. of Energy Offices of High Energy Physics and Fusion Energy Sciences and by the Dept. of Defense Office of Naval Research and Joint Technology Office.
Beam halo is a challenging issue for intense beams since it can cause beam loss, emittance growth, nuclear activation and secondary electron emission. Because of the potentially low number of particles in the halo compared with beam core, traditional imaging methods may not have sufficient contrast to detect faint halos. We have developed a high dynamic range, adaptive masking method to measure halo using a digital micro-mirror array device and demonstrated its effectiveness experimentally on the University of Maryland Electron Ring (UMER). We also report on similar experiments currently in progress at the Jefferson Lab Free Electron Laser (FEL) using this method.
 
slides icon Slides WEOCN5 [1.287 MB]  
 
WEP021 The Effect of Initial Energy Spread on Longitudinal Beam Modulations in an Electron Gun 1537
 
  • C.P. Neuman
    CUNY, Bayside, New York, USA
  • P.G. O'Shea
    UMD, College Park, Maryland, USA
 
  Computer simulations are used to investigate the evolution of longitudinal density and energy modulations of an electron beam in a linear accelerator system. This study examines the effect of initial energy spread on the modulations as the beam is accelerated in the electron gun.  
 
WEP050 Advances in Modeling the University of Maryland Electron Ring 1585
 
  • R.A. Kishek, B.L. Beaudoin, S. Bernal, M. Cornacchia, K. Fiuza, I. Haber, T.W. Koeth, P.G. O'Shea, D.F. Sutter, H.D. Zhang
    UMD, College Park, Maryland, USA
 
  Funding: Work funded by the US Dept. of Energy Offices of Fusion Energy Sciences and High Energy Physics, and by the Dept. of Defense Office of Naval Research and the Joint Technology Office.
The University of Maryland Electron Ring (UMER) is a research accelerator designed to operate with extreme space charge. The existence of high-precision experimental measurements of tune, dispersion, chromaticity, response matrix elements, and other parameters*, **, *** has prompted a revision of the models used to describe the machine. Due to the low energy (10 keV) of the electrons, the dipole and quadrupole magnets used are air-core printed-circuit coils whose fields we calculate using a Biot-Savart solver. Different levels of approximations for the magnetic fields have been developed. We present simulation results from the particle-in-cell code WARP, and from the accelerator code, ELEGANT. These are compared both against simpler models as well as experimental results. The improved modeling has significantly reduced the discrepancies between simulation and experiment.
* D.F. Sutter, et al., Proc. PAC 2009
** C. Wu, et al., Proc. PAC 2009
*** S. Bernal, et al., Proc. AAC 2010
 
 
WEP101 Smooth Approximation of Dispersion with Strong Space Charge 1665
 
  • S. Bernal, B.L. Beaudoin, T.W. Koeth, P.G. O'Shea
    UMD, College Park, Maryland, USA
 
  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.
We apply the Venturini-Reiser envelope-dispersion equations* to a continuous beam in a uniform focusing/bending lattice to study the combined effects of linear dispersion and space charge. Within this simple model we investigate the scaling of average dispersion and the effects on beam dimensions; we also introduce a generalization of the space-charge intensity parameter and apply it to the University of Maryland Electron Ring (UMER) and other machines. In addition, we present results of calculations to test the smooth approximation by solving the Venturini-Reiser original equations and also through simulations with the code ELEGANT.
*M. Venturini and M. Reiser, Phys. Rev. Lett. 81, 1, p. 96, 6 July 1998
 
 
WEP102 Current Dependent Tune Shifts in the University of Maryland Electron Ring UMER 1668
 
  • D.F. Sutter, B.L. Beaudoin, S. Bernal, M. Cornacchia, R.A. Kishek, T.W. Koeth, P.G. O'Shea
    UMD, College Park, Maryland, USA
 
  Funding: Work supported by the U.S. DOE Offices of High Energy Physics and Fusion Energy Sciences and by the U.S. DOD Office of Naval Research and Joint Technology Office.
The shift in betatron tunes as a function of space charge has been studied in many accelerators and storage rings. Because of its low operating energy (10 keV, γ = 1.02) and wide range of beam currents (0.6 to 100 mA, corresponding respectively to predicted incoherent tune shifts of 1.2 to 5.2), the University of Maryland electron ring (UMER) provides a unique opportunity to study space charge driven tune shifts over a wide parameter space. Comparisons of predictions and measurements are presented, including a discussion of special factors such as the magnetic penetration of the vacuum chamber walls.
 
 
THP204 Corrections to Quantum Efficiency Predictions for Low Work Function Electron Sources 2504
 
  • K. L. Jensen
    NRL, Washington, DC, USA
  • D.W. Feldman, E.J. Montgomery, P.G. O'Shea
    UMD, College Park, Maryland, USA
  • J.J. Petillo
    SAIC, Billerica, Massachusetts, USA
 
  Funding: Funding by the Joint Technology Office and the Office of Naval Research.
The Three-Step Model of Spicer, or the analogous Moments-based models, can be used to predict photoemission from metals and cesiated metals. In either, it is a convenient approximation to neglect electrons that have undergone scattering. Using Monte Carlo to follow scattered electrons, we assess the utility of the approximation particularly for low work function (cesiated) surfaces.
 
 
THP208 Development of Alkali-Based High Quantum Efficiency Semiconductors for Dispenser Photocathodes 2510
 
  • E.J. Montgomery, D.W. Feldman, S.A. Khan, P.G. O'Shea, P.Z. Pan, B.C. Riddick
    UMD, College Park, Maryland, USA
  • K. L. Jensen
    NRL, Washington, DC, USA
 
  Funding: This work is supported by the Office of Naval Research.
Photocathodes as electron beam sources can meet the stringent requirements of high performance FELs, but exhibit a lifetime-efficiency tradeoff. High quantum efficiency (QE) cathodes are typically semiconductors, well described by recently enhanced theory*. Cesium dispenser technology, proven to extend lifetime of tungsten cathodes**, can be extended to high QE via the development of semiconductor coatings which are suitable for rejuvenation. Rejuvenation occurs via controlled cesium diffusion through a sintered substrate to resupply the surface (as described by models of pore*** and surface**** diffusion). Compatible coatings must be thermally stable materials with a cesium-based surface layer. Following standard fabrication processes*****, we discuss alkali antimonides and alkali aurides as cesium dispenser photocathode coatings and analyze future prospects. We also describe improvements to experimental techniques.
*K.L. Jensen et al., (this conference)
**Moody et al., J. Appl. Phys. 102(10), 2010
***B.C. Riddick et al., (this conference)
****P.Z. Pan et al., (this conference)
*****S.A. Khan et al., (this conference)