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Feldman, D.W.

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.

 
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.

 
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).

 
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.