Interactions of pre-bunched relativistic electron beams and electromagnetic waves in strongly tapered undulators

Strongly tapered undulator interactions serve to dramatically increase the efficiency of energy exchange between an electromagnetic field and a relativistic electron beam. By extending this energy exchange beyond the energy bandwidth of the untapered interaction, a significant fraction of the electron beam power can be converted into coherent radiation or vice versa. Configured as a radiator, these interactions offer the scientific community and industry with a potential source of high peak and average power coherent radiation. The inverse interaction serves as a unique advanced accelerator known as the Inverse Free Electron Laser (IFEL), capable of achieving acceleration gradients far exceeding current RF technology.In this dissertation, a theoretical framework is first provided, describing the dynamics of strongly tapered undulator interactions configured both as a radiator and accelerator. Here also the concept of modulator chicane pre-bunching is introduced as a means to greatly increase the efficiency by better matching the incoming longitudinal phase space to the phase space acceptance of the interaction. Experimentally, high energy extraction efficiency was first demonstrated at mid-IR wavelengths in the Nocibur experiment performed at the Brookhaven National Laboratory's Accelerator Test Facility (BNL ATF). Reversing the orientation of the 11 period Rubicon helical undulator, strongly tapered in both period and field, up to 30\% of a pre-bunched 65 MeV electron beam's energy was converted to coherent radiation. The interaction was driven by 200 GW of seed power from the ATF's high power $CO_2$ laser with a wavelength of 10.3 $\mu$m. With the Rubicon undulator in IFEL configuration, the Double Buncher experiment demonstrated the ability to inject up to 96\% of a 52 MeV electron beam's charge into the phase space acceptance of the interaction, making use of the cascaded modulator-chicane pre-bunching scheme. This experiment was also performed at BNL ATF, driven by 75 GW of 10.3 $\mu$m seed power. The experimental design, components, measurements, diagnostics and experimental methods pertaining to both experiments are discussed as well as the results from both experiments