Future x-ray free electron lasers will probe matter at the atomic scale with femtosecond time resolution. Such x-ray sources require a high current electron beam with very low emittance and energy spread. Any density fluctuation in an intense beam can launch space charge waves that lead to energy modulation. The energy modulations may cause further density modulations in any dispersive element and can, for example, excite the microbunching instability in x-ray free electron lasers. Hence, it is important to understand and control the evolution of density modulations on an intense beam. This dissertation focuses on long path-length experimental study of intense beams with density perturbations. The experimental results are compared with theory and computer simulation.

         We took advantage of the multi-turn operation of the University of Maryland Electron Ring (UMER), to carry out long path-length (100 m) experimental studies of space-charge-dominated beams with density perturbations. First, a single density perturbation is introduced on a space-charge dominated electron beam using photoemission from a laser. The perturbation splits and propagates as a fast and a slow wave on the beam. The speed of the space charge waves is measured experimentally as a function of beam current and perturbation strength. The results are in good agreement with Particle-in-cell (PIC) simulation and 1-D cold fluid theory in the linear regime. We then show that linear space-charge waves can be used as non-interceptive transverse beam diagnostics in UMER. Using time-resolved imaging techniques, we report the transverse effects of a longitudinal perturbation in a circular machine.

           We introduce multiple perturbations on the beam and show that the fast and the slow waves superpose and cross each other. We then present experimental results on the beam response from introducing a controlled energy modulation on the density modulated beam and compare them with the theory.  In the non-linear regime, where the strength of the perturbation is large (>25% compared to the beam current), we report, for the first time, a wave train formation of the space charge waves. Finally, experimental observation of a photo-emitted beam pulse splitting into sub-pulses under high laser power is presented and compared with 1-D virtual cathode theory.

            From this work, we conclude that density modulations on an intense beam produce fast and slow waves, which, in the linear regime at least, can be controlled through energy modulation. Moreover, a large amplitude density modulation, when allowed to propagate, can break into sub-pulses, causing energy modulation. Hence, a density modulation should not be allowed to grow and must be controlled as soon as possible.