The terahertz region of the electromagnetic spectrum (0.1 – 10 THz) is relatively unexplored compared to the other regions of the spectrum including microwave, X-ray, and infrared. Potential applications for terahertz abound. These include detecting hazardous chemicals, cancer cells, and hidden weapons. However the critical roadblock to full exploitation of the terahertz band is the lack of compact, powerful coherent terahertz radiation sources [1 – 4]. In order to address the above needs, we propose to investigate a novel terahertz radiation source based on a high-order harmonic gyrotron interaction. The device circuit dimension that makes use of the gyrotron interaction can be larger compared to the current state-of-the-art terahertz devices. Therefore, it can provide significantly higher power and efficiency. The device takes advantage of the concept that in harmonic devices the magnetic field requirement is reduced by a factor of s (harmonic number) so that the magnetic field can be supplied by a lightweight periodic permanent magnet (PPM) instead of a bulky solenoid magnet. This offers a new solution and significant promise for lighter and more practical terahertz systems for various applications. A large orbit electron gun and beam forming system for the terahertz device have been designed. Analytical calculations along with simulations were carried out to determine the axial velocity spread, the velocity ratio, the Larmor radius, and the guiding center radius. Based on the adiabatic theory and angular momentum conservation, the analytical analysis provides a basis for initial beam performance prediction. Beam parameters and electron gun configuration for the terahertz device is presented.
II. Large-Orbit Electron Gun
A large-orbit axis encircling electron gun was adopted for the terahertz gyro-oscillator. Produced by a cusp magnetic field, an axis-encircling large-orbit electron beam offers the advantage that it can be placed in the region of high electric field of the device. For effective interaction, high electron beam quality which is mainly characterized by low velocity spread, is essential for practical high power terahertz sources. The input electron beam parameters were derived and further optimized from a gun simulation code and the output beam parameters were sent to a 3D particle-in-cell code for accurate prediction of the device performance. Figure 1 shows the dispersion diagram of the proposed compact microfabricated terahertz high-order harmonic gyrodevice. The device operates at where beam and wave dispersion lines intersect each other. The specification of the high-order harmonic gyro-oscillator is listed in Table 1. The beam placement position in the waveguide can be determined by factors such as interaction strength and wall loading of the waveguide wall. The interaction strength can be maximized when beam is placed at the position where the electric field amplitude of each mode is maximum. All modes require certain beam position in the waveguide for maximum interaction. Figure 2 shows the relative electric field amplitude (H-fuction) versus rc/rw for the operating TE10,1 mode, where rc represents the guiding center radius and rw is the radius of the waveguide wall. For the operating TE10,1 mode, several peaks exist where beam-wave interaction will be maximized. Figure 3 shows a cross sectional drawing of periodic permanent magnet for the terahertz system.
References  D. Woolard, et. al, “Terahertz electronics for chemical and biological warfare agent detection,” IEEE MTT-S Dig., 1999.  Ives, R. L, “Microfabrication of high-frequency vacuum electron devices,” IEEE Trans. Plasma Science, vol.32, no. 3, 2004.  U. S. News and World Report, “Catching T-Waves,” 2003.  ScienceDirect vol. 19, issue 6, pp. 20-23 August 2006. * Acknowledgements: This work has been supported by the University of Colorado at Colorado Springs and National Institute of Science, Space, and Security Center (NISSSC).