Design and simulation of a ∼390 GHz seventh harmonic gyrotron using a large orbit electron beam

Abstract

Abstract. A~390 GHz harmonic gyrotron based on a cusp electron gun has been designed and numerically modelled. The gyrotron operates at the 7 th harmonic of the electron cyclotron frequency with the beam interacting with a TE 71 waveguide mode. Theoretical as well as numerical simulation results using the 3D Particle-In-Cell (PIC) code MAGIC are presented. The cusp gun generated an axis-encircling, annular shaped electron beam of energy 40 keV, current 1.5 A with a velocity ratio of 3. Smooth cylindrical waveguides have been studied as the interaction cavities and their cavity Q optimized for 390 GHz operation. In the simulations 600 W of output power at the design frequency has been demonstrated. [5,6]. The gyrotron is well known as a coherent millimeter wave source capable of high power and high frequency output which is extendable to the Terahertz (THz) frequency region. Recently a gyrotron achieved kilowatts of output power operated at the fundamental electron cyclotron frequency of 1 THz [7] by using a pulsed solenoid. However, continuous wave (CW) operation at such a frequency is a formidable task because of the large magnetic field (~40 T) that is required. As the output frequency increases, both larger magnetic fields and reduced interaction region size are needed when operating at the fundamental cyclotron frequency. An alternative approach to generate CW high frequency radiation is to work at higher cyclotron harmonics [8,9,10,11] which allow the use of larger cavity sizes and smaller magnetic fields by a factor of s, where s is the harmonic number. A CW high harmonic gyrotron operating at a frequency of~390 GHz at relatively lower B-field was designed and is presented in this paper. In this gyrotron cyclotron resonance takes place between the seventh harmonic of the electron cyclotron frequency (s = 7) and the TE 71 waveguide mode. A large orbit electron beam from a cusp gun is used to reduce the possibility of parasitic interactions. The small-signal theory of the beam-wave interaction was used to calculate the growth rate and the starting current. A smooth-bore cavity was designed with a suitable Q value by optimizing the angle of the output taper and the length of the cavity so that the starting current requirement is met. Finally, the beam-wave interaction of the gyrotron was simulated using the 3D Particle-In-Cell (PIC) code MAGIC and the results are presented. Keywords: Harmonic gyrotron, terahertz, large orbit electron beam

Validation of 3-D Time Domain Particle-in-Cell Simulations for Cold Testing a <inline-formula> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula>-Band Gyrotron Cavity

“…Thus, PIC simulations are emerging as powerful tools for understanding the complex dynamics of gyrotron operation. The PIC codes that have been commercially available for performing gyrotron simulations are 2-D/3-D MAGIC [11]- [14] and 3-D CST MICROWAVE STUDIO [15], [16] and VSIM [17], [18].…”

“…MAGIC is a well-established PIC code for performing simulations of vacuum tubes, such as traveling-wave tubes, backward wave oscillators, and klystrons. The gyrotron cavity is thought to be a very challenging structure to simulate using PIC codes, and recently, MAGIC has been successful in simulating gyrotron cavities [11]- [14]. MAGIC is a Finite time-domain (FDTD) [19] based PIC code in which the fields are defined on a spatial temporal grid.…”

Numerical Simulation of a Gyro-BWO with a Helically Corrugated Interaction Region, Cusp Electron Gun and Depressed Collector

“…In each case the electron beam passed through the beam tube without scraping or mirroring and had tolerable spreads in axial velocity and α spreads. The optimized electron gun design produced an electron beam of high enough quality in order to drive the beam-wave interaction within the gyro-BWO (Donaldson et al, 2009;Li et al, 2010).…”

Optimization of a Cusp Electron Gun for Millimeter Wave Gyro-Devices

“…The electron gun was designed to drive multiple microwave sources including two gyro-devices that are currently being experimentally tested at the University of Strathclyde as well as a 390 GHz gyrotron [3]. The gyro-BWO [4,5] and gyro-TWA [6] both use a helically corrugated waveguide [7] interaction region and the cusp gun drives the beamwave interaction.…”