Research On The Terahertz Wakefield Radiation Based On The Electron Bunch Train

Terahertz(THz)radiation is located in the electromagnetic spectrum range between the microwave and the infrared.THz technology has broad applications in crowds of fields such as biological sciences,materials imaging and radar communications.High-power,frequency-adjustable and compact THz radiation sources are desired,and the scheme of wakefield radiation excited by the relativistic electron bunch train from accelerator is a promising way to generate THz radiation.Because the wakefield structure has the property of high coupling efficiency,compact size and high spectrum brightness.Besides,the electron bunch train with high peak current which is created nowadays with the advanced beam manipulation technics,can be applied as the drive beams to coherently enhance the THz wakefield,as well as to adjust the radiation frequency.In order to promote the development of accelerator-based high power adjustable compact terahertz radiation sources,this thesis systematically investigates the electron bunch train driven THz wakefield radiation by going through two aspects:the simulation optimization of beam manipulation and the experimental studies of THz radiation by several wakefield structures.Regarding the beam manipulation study,we propose a new scheme of velocity bunching based on the Tsinghua Thomson X-ray linear accelerator(TTX).The new scheme combines the velocity bunching with under-compression mode in a 3-m travelling wave structure(TWS)with a long drift after,which is different from the commonly used critical-compression bunching.The new scheme is proved in simulation that could attain much better bunching which leads to an ultra-high brightness beam with peak current?10kA.The form factor of such beam covers higher frequency range therefore it's suitable to drive the high-frequency high-power THz radiation source.The experimental demonstration of the adjustable narrow band THz radiation source is performed at the US Accelerator Test Facility(ATF).We design a dielectric wakefield structure which supports multiple wakefield modes,then selectively excite an individual wakefield mode by injecting an electron bunch train with specific separation into the structure.The spectrum of each selected modes is measured to verify our design.The results show that the measured central frequency for each mode varies from 0.6 THz to 1.6 THz,thus mark our attempt a success.The high power(MW-scale)THz wake field radiation experiment is performed at the US Argonne Wakefield Accelerator facility(AWA).We design a high-impedance W-band(?0.1 THz)metallic periodical wakefield structure,excited by the high current bunch train at the AWA.We experimentally obtain a high peak power radiation of 5 MW,corresponding to a wakefield gradient of 85 MV/mm,which are new world records in this frequency range.Furthermore,with the attempt to gain higher accuracy in the THz radiation envelope and phase detection,we proposed and demonstrated a "long-range wakefield interferometry method" to detect and control the wakefield phase.This new method is based on that the total wakefield energy varies as a function of the delay between the two drives bunches.Because it's very sensitive to detect the THz radiation energy,the long-range wakefield interferometry method is a high-accuracy mapping method for the wakefield phase,which is suited for the high-frequency wakefield accelerator and precise phase control of the wakefield.The study on the interactions between electron bunch and the wakefield is carried out on the same W-band structure.We develop a P2P code in MATLAB to simulate the process of a beam passing through the dipole-mode wakefield.We monitor the beam shape on the screen after the W-band structure in the experiment and compare it with the simulation.The P2P code predictions are in good agreements with the experiment observations.