Modeling of a Compact Terahertz Source based on the Two-Stream Instability

Abstract

THz radiation straddles the microwave and infrared bands of the electromagnetic spectrum, thus combining the penetrating power of lower-frequency waves and imaging capabilities of higher-energy infrared radiation. THz radiation is employed in various fields such as cancer research, biology, agriculture, homeland security, and environmental monitoring. Conventional vacuum electronic sources of THz radiation (e.g., fast-and slow-wave devices) either require very small structures or are bulky and expensive to operate. Optical sources necessitate cryogenic cooling and are presently capable of producing milliwatt levels of power at THz frequencies. We propose a millimeter and sub-millimeter wave source based on a well-known phenomenon called the two-stream instability. The two-beam source relies on lowenergy and low-current electron beams for operation. Also, it is compact, simple in vi design, and does not contain expensive parts that require complex machining and precise alignment. In this dissertation, we perform 2-D particle-in-cell (PIC) simulations of the interaction region of the two-beam source. The interaction region consists of a beam pipe of radius r a and two electron beams of radius r b co-propagating and interacting inside the pipe. The simulations involve the interaction of unmodulated (no initial energy modulation) and modulated (energy-modulated, seeded at a given frequency) electron beams. In addition, both cold (monoenergetic) and warm (Gaussian) beams are treated. Using PIC simulations electromagnetic radiation is demonstrated over the frequency range 0.03 ≤ f ≤ 1 THz. The two-beam source is found to possess an extremely wide gain bandwidth (over a decade in frequency) from the microwave to the far infrared region of the electromagnetic spectrum. Moreover, the gain obtained is impressive. For example, the interaction of two 0.7-mm and 0.5-A electron beams with energies 20 keV and 16.95 keV (typical in this dissertation) yields the value of gain of 0.35 dB/mm, which is over a factor of 10 greater than that reported in the literature for a proposed two-stream relativistic klystron amplifier involving 1.0-and 5.0-kA annular relativistic electron beams. Hence, the two-beam amplifier promises to be a reliable and inexpensive source of millimeter and sub-millimeter wave radiation and has the potential to generate watts of power at THz frequencies. vii Contents List of Figures xiii List of Tables xxvii 2 Single particle dynamics 10