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Generation of narrow-band terahertz coherent Cherenkov radiation in a dielectric wakefield structure.

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Overview

This study explores the use of a dielectric-lined waveguide structure as a means of producing narrow-band terahertz radiation in the form of electron-beam-driven coherent Cherenkov radiation wakefields. This concept builds on previously studied scenarios such as the Cherenkov maser and the Cherenkov free-electron laser. It is distinct in that it relies solely on coherent wakefield excitation instead of a microbunching instability gain process, in analogy to the superradiant regime of FEL operation. The narrow bandwidth is due to the single-mode nature of the excitation, enabled by the exclusion (due to coherence) of discrete waveguide modes with wavelengths shorter than the driving electron bunch length. This allows an inherently broadband beam current profile to radiate power into a single frequency, which is selectable by appropriate choice of design parameters.;The theoretical component of this dissertation is aimed at making predictions for comparison with experimental results. The functional form and propagating mode frequencies of the electromagnetic fields in the waveguide structure are found by eigenmode solution in the source-free case beginning from Maxwell's equations; the response of the structure to a driving electron bunch is then found using a wakefield formalism. Predictions for the frequencies and radiated energy levels obtained from this analysis are corroborated computationally using the commercial particle-in-cell simulation code OOPIC PRO.;The experiment is designed to be a proof-of-principle demonstration of the effectiveness of this scenario in converting the energy in an electron beam into electromagnetic radiation. We present detailed measurements showing a narrow emission spectrum peaked at 367 +/- 3 GHz from a 1 cm long fused silica capillary tube with sub-mm transverse dimensions, matching the predicted (analytical and computational) TM01 mode resonance to within 1% error. This measurement confirms the expected preferential coherent excitation of the TM01 mode over the HEM11 mode, which lies nearby in frequency but still decisively outside the error estimate established over multiple measurements. The measured 3 dB bandwidth is on the order of ≲ 10% and is seen to be transform-limited. We observe a 100 GHz shift in the emitted central frequency when the tube wall thickness is changed by 50 mum, demonstrating the modular tunability of the source. Calibrated measurements of the radiated energy register up to 10 muJ per 60-80 ps pulse for an incident sub-picosecond electron beam carrying 200 pC of charge, corresponding to a peak power of approximately 150 kW. A case study considering the implementation of this scenario using a 10-cm-long structure with smaller transverse dimensions indicates a possible yield of 50 MW peak power at 1.8 THz and 0.1% bandwidth.;This dissertation reports the first direct measurements of narrow-band THz coherent Cherenkov radiation driven by a sub-picosecond electron beam in a dielectric wakefield structure, representing a successful adaptation of the previously proven Cherenkov FEL concept to the realm of ultra-short electron beams such as are available in state-of-the-art user facilities around the world. These results prove the potential of this method to produce tunable, narrow-band, pulse-length-variable, multi-megawatt peak-power radiation at f > 1 THz in existing modern electron accelerators.
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Product Details

  • BN ID: 2940032610243
  • Publisher: ProQuest LLC
  • Sold by: Barnes & Noble
  • Format: eTextbook
  • Pages: 137

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