Simulation and Theory
Patrick O'Shea, David Gillingham

The current design for a high average power Free Electron Laser (FEL) relies on pulse compression to achieve the large peak current required. The technique for compression of the high energy (ultra-relativistic) electrons is to apply an energy chirp to the pulse then send it through a dispersive element made up of alternating dipole bending magnets (chicane). During these bends, the pulse may emit synchrotron radiation onto itself and modulate its own energy, disturbing the electron trajectories in the chicane and possibly degrading the output beam quality. Since the FEL mechanism works under tight restrictions on beam parameters, this may limit the amount of beam current, and subsequently the output power of the laser. Previous analysis of this effect used the Coherent Synchrotron Radiation (CSR) wakefield applicable under steady-state conditions and without boundary surfaces (i.e., in vacuo). Under certain approximations, the CSR wakefield can be described by a paraxial wave equation. This has been solved numerically and illustrates some features of the problem applicable to a curved rectangular waveguide under transient conditions for which no analytical solution exists.

This research is supported by the Office of Naval Research and the Department of Defense Joint Technology Office.

Schematic of University of Maryland Electron Ring (UMER) showing cathode (left) and beam diagnostics (right)

Photoemission pulse superimposed on the main UMER beam to study the propagation of space charge waves

The dynamics of charged particle beams are governed by the particles' thermal velocities, external focusing forces, and Coulomb forces. Beams in which coulomb forces play the dominant role are known as space charge dominated, or intense. Intense beams are of great interest for heavy ion fusion, spallation neutron sources, free electron lasers, and other applications. In addition, all beams of interest are dominated by space charge forces when they are first created, so an understanding of space charge effects is critical to explain the later evolution of any beam. Historically, more attention has been paid to the transverse dynamics of beams. However, many interesting and important effects in beams occur along their length. These longitudinal effects can be limiting factors in many systems. For example, modulation or structure applied to the beam at low energy will evolve under space charge forces. Depending on the intended use of the beam and the nature of the modulation, this may result in improved or degraded performance.

To study longitudinal dynamics in intense beams, experiments are conducted using the University of Maryland Electron Ring, a 10 keV, 100 mA electron transport system. These experiments concentrate on space charge driven changes in beam length in parabolic and rectangular beams, beam density and velocity modulation, and space charge wave propagation. Coupling between the transverse and longitudinal dynamics are also investigated. These experiments involve operating the UMER gun in space charge limited, temperature limited, triode amplification, photon limited, and hybrid modes.

J. Harris, P. O'Shea, "Modulation of Intense Beams in the University of Maryland Electron Ring," Proc. FEL2005, Stanford Univ., 21-26 August, 2005.

This research is supported by the Department of Energy, Office of Naval Research, and the Department of Defense Joint Technology Office.

Transition Radiation Window Located after the UMER Electron Gun

OTR Radiation Provides Image of Electron Beam

We are engaged in an experimental and computational program to develop advanced diagnostics for high power electron beam sources such as Energy Recovery Linacs and Free Electron Lasers. We have performed proof-of-principle experiments at Brookhaven National Laboratory's Advanced Test Facility (40-70 MeV) accelerator and the Naval Postgraduate School 95 MeV linac to show that Optical Diffraction-Transition Radiation Interferometry can succfessfully measure the rms divergence and emittance of relativisitc electron beams. This technology will be refined to measure the divergence and emittance of the Thomas Jefferson National Laboratory's ERL and to perform Optical Phase Space Mapping of their high power electron beam accelerator operating at 100 MeV, as well as at much lower energies to measure the divergence of the Advanced Wakefield Accelerator 8 MeV injector at Argonne National Laboratory. We are also engaged in a collaborative effort with the Paul Scherrer Institut's Swiss Light Source to develop a single shot (i.e., a single pulse) non-interceptive Diffraction Radiation bunch length monitor. Experiments are now being designed to prove our computational designs and performance of this new diagnostic technique. We are also investigating other non-interceptive diagnostic concepts such as magnetic edge radiation interferometry for very high power beam applications.