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Bohn, C. L.

Paper Title Page
THPMS010 Polarized Pulsed Beam Source for Electron Microscopy 3011
  • N. Vinogradov, C. L. Bohn, P. Piot
    Northern Illinois University, DeKalb, Illinois
  • J. W. Lewellen, J. Noonan
    ANL, Argonne, Illinois
  A novel source of polarized pulsed electron beam is discussed. Unlike conventional devices based either on a thermionic cathodes or field-emission needle cathodes, in this source the electrons are produced by a laser beam hitting the cathode surface. Using a combination of gallium arsenide (GaAs) planar cathode and a suitable laser one can obtain a polarized picosecond electron bunch. Numerical simulations of the electron dynamics in the optimized cathode-anode geometry have shown that the beam with initial transverse size of a few mm can be focused down to 1 mm RMS at a distance of about 4 cm from the cathode. The suggested source can be installed instead of a tungsten filament source in an existing electron microscope with no modification of any column elements. The main advantages of this approach are that the beam can be easily pulsed, the beam is polarized which makes it an effective probe of some magnetic phenomena, and the laser can be used to provide larger beam intensity. The design of the source and subsequent fabrication has been completed. The paper presents numerical studies, conceptual design of the device, and results of beam measurements.  
FRPMS035 Vector Diffraction Theory and Coherent Transition Radiation Interferometry in Electron Linacs 4015
  • T. J. Maxwell, C. L. Bohn, D. Mihalcea, P. Piot
    Northern Illinois University, DeKalb, Illinois
  Funding: Work supported by US. Department of Energy, under Contract No. DE-FG02-06ER41435 with Northern Illinois University

Electrons impinging on a thin metallic foil are seen to deliver small bursts of transition radiation (TR) as they cross the boundary from one medium to the next. A popular diagnostic application is found for compact electron bunches. In this case they will emit radiation more or less coherently with an N-squared enhancement of the intensity on wavelengths comparable to the bunch size, generating coherent transition radiation (CTR). Several detailed analytical descriptions have been proposed for describing the resulting spectral distribution, often making different simplifying assumptions. Given that bunches tenths of millimeters long can generate measurable spectra into the millimeter range, concern may arise as to weak diffraction effects produced by optical interference devices containing elements with dimensions in the centimeter range. The work presented here is a report on an upcoming graduate thesis exploring these effects as they apply to the Fermilab/NICADD photoinjector laboratory using a minimal C++ code that implements the methods of virtual quanta and vector diffraction theory.

FRPMS036 Influence of Chaos on Resonance Crossings 4021
  • C. L. Bohn, E. W. Nissen
    Northern Illinois University, DeKalb, Illinois
  Funding: This work is supported by DOE grant DE-FG02-04ER41323.

We undertake a study of particle dynamics in a model fixed-field alternating-gradient (FFAG) synchrotron in which space-charge plays a central role. The space-charge force corresponds to a Gaussian charge distribution in both transverse dimensions. The betatron-tune is linearly ramped through resonance. This ramping alone can cause particles to enter orbits that have chaotic motion.. We found that space-charge can lead to spreading of the available tunes which can either increase or decrease the effects of resonance. By applying recently developed techniques to measure complexity in the orbital dynamics, we also determine whether chaoticity can arise in particle trajectories and subsequently influence resonance crossings. Furthermore, we can see that the chaoticity changes drastically in the area around a resonance crossing.

FRPMS037 Impact of Transverse Irregularities at the Photocathode on High-Charge Electron Bunches 4027
  • M. M. Rihaoui, C. L. Bohn, P. Piot
    Northern Illinois University, DeKalb, Illinois
  • J. G. Power
    ANL, Argonne, Illinois
  Electron beam properties in photoinjectors are strongly dependent on the initial conditions, e.g., non-uniformities in the drive-laser pulse and/or the photocathode surface. We explore the impact of well-defined transverse perturbation modes on the evolution of the electron beam phase space, and paying special attention to how certain types of perturbations mix. Numerical simulations performed with IMPACT-T (both the standard version and a new wavelet-based version) are presented along with experimental results aimed at validating the simulation codes. The experiments are conducted at the Argonne Wakefield Accelerator facility.