Electromagnetic Design of High-Gradient THz Accelerators

Abstract

Dielectric Loaded Waveguide Linear Accelerators (DLW LINACs) and single-sided THz boosters are the main accelerator components of the AXSIS machine, powered by multi-cycle and single-cycle THz pulses, respectively. These components hold great promise for compact linear accelerators (LINACs) due to their ability to sustain higher breakdown fields at THz frequencies compared to conventional RF components. Additionally, they provide a platform for accelerators that are highly synchronized with electron guns. This thesis explores the optimization, design, and performance evaluation of these THz-driven accelerators from an electromagnetic perspective, addressing key aspects to maximize the efficiency of electron acceleration.

Initially, the study presents a comprehensive analytical and numerical guideline for optimizing cylindrical DLW LINACs, which is the main focus of this thesis, considering factors such as initial electron energy and THz pulse energy. The desired $TM_{01}$ mode of the DLW, which has its maximum electric field component in the acceleration direction, is analyzed, and useful guideline figures are introduced as a key starting point for designing a DLW LINAC.

Various methods for exciting the desired $TM_{01}$ mode are reviewed, and a waveguide-based coupler and mode converter is proposed to couple the linearly polarized beam in a rectangular waveguide to the desired mode of the DLW. Challenges posed by parasitic modes are addressed, and it is shown that a symmetric structure used in the design of the coupler can suppress unwanted modes and enhance overall performance.

The subsequent chapter focuses on experimental methods to evaluate the performance of the DLW and coupler, utilizing rectangular waveguides as input ports for ease of testing with a Vector Network Analyzer (VNA). The phase velocity and dispersion curve of the DLW are measured for different modes, including the desired mode.

The following chapter focuses on the design and simulation of a multi-layer, single-sided pumped THz booster, including the impact of fabrication imperfections and alignment on performance, as well as details of the design of its retro-reflector mirror. This booster, driven by a single-cycle THz pulse from a nonlinear crystal, demonstrates the capability to accelerate electrons from 55~keV to above 400~keV.

Finally, the coupling efficiency of THz beams into waveguides is examined. The study emphasizes the importance of aperture antennas, particularly different-shaped horn antennas, for effective coupling. The reciprocity theorem in electromagnetics, along with mode matching theory (overlap integral of the fields), is leveraged to analyze the horn antenna as both a Gaussian beam receiver and transmitter, evaluating its performance in the acceleration concept.