LASER SWITCHED ELECTRON BEAM MODULATION WITH TERAHERTZ APPLICATIONS
O'Shea, Patrick G
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This dissertation describes the exploration of relativistic electron beams modulated at terahertz frequencies using laser driven photoemission. It is divided into three distinct areas: laser beam modulation; electron beam dynamics; and an application of electron beam modulation, the generation of terahertz radiation. The laser modulation portion covers the development of an interferometer system used to control the 266 nm drive laser modulation and the experimental results. The laser pulse is delivered to the photocathode of the accelerator, and is used as a switch that induces an initial electron beam modulation at frequencies between 0.5 and 1.6 terahertz. The electron beam dynamics section includes measurements of the electron beam longitudinal distribution after acceleration to relativistic energy as well as the results obtained from a numerical simulation using the code PARMELA. Both the experimental and numerical results indicate that some of the initial density modulation imposed by the drive laser modulation is retained on the electron beam, although the density modulation that remains, and the frequency of the modulation, falls as a function of increasing charge. Electron beam modulation is achieved between 0.712 and 1.66 terahertz. One application of the deliberate modulation of an electron beam is the generation of coherent radiation, as is seen in many devices ranging from the klystron to the free electron laser. The third section of this work discusses terahertz light generated by transition radiation when a mirror intercepts the modulated electron beam. In this section, transition radiation measured by a bolometric detector is compared to expected results based on the longitudinal electron beam distributions predicted by the PARMELA simulation as well as the measurements from the accelerator system. This dissertation demonstrates that it is possible for an electron beam pre-modulated at the cathode on a subpicosecond time scale to be accelerated to relativistic energy and used for the production of tunable terahertz radiation.