Study of Soliton Space Charge Waves in Intense Electron Beams

Abstract

High brightness electron beams have wide applications in accelerator-driven light sources, X-ray, free-electron lasers (FELs), spallation neutron sources and intense proton drivers. Advanced accelerators demand superior beam quality for such intense beams, where the non-linear space charge force will introduce collective effects and limit the maximum beam current and quality. Near the cathode, all beams of interest begin as space-charge dominated beams. Density fluctuations can naturally occur and lead to space charge waves. Therefore, it is crucial to understand and control how these beam modulations develop in an intense beam.

This dissertation addresses the longitudinal beam dynamics of large-amplitude perturbations on electron beams. I report on the first systematic characterization of solitons in electron beams. Solitons are localized persistent waves that behave like particles, preserving their properties (shape, velocity, etc.) over long distances and through collisions with other solitons. They have practical applications and are of interest to many disciplines such as condensed matter physics, plasma physics, beam physics, optics, biology and medicine. Whereas solitons in electron beams have been predicted on theoretical grounds decades ago in the form of longitudinal space charge waves, they were never experimentally observed until recently in the University of Maryland Electron Ring (UMER).

By introducing a pulsed laser beam on a thermionic cathode, an electron beam with a narrow density perturbation from photoemission is generated. The perturbation then evolves into longitudinal space charge waves that propagate along the beam. For large-amplitude initial perturbations, a soliton wave train is observed. The soliton's properties are confirmed experimentally. The results are compared with cold fluid model in theory and the WARP particle-in-cell (PIC) code in simulation. Reasonable agreement is achieved.

This reproducible nonlinear process provides an alternative for a tunable, coherent radiation sources without wigglers/undulators. The soliton pulse spacing is therefore investigated, which is found dependent on the pipe radius (g-factor) and beam plasma frequency.