Progress in microwave to sub-THz sources at Strathclyde

Abstract

Research progress during recent years at the University of Strathclyde is presented. The main research themes include gyro-devices with particular emphasis on gyro-TWAs [1,2] and gyro-BWOs, and natural sources of radiation such as auroral kilometric radiation (AKR) [3][4][5][6], novel Cherenkov mm-wave sources [7,8] and pseudospark-driven sub-THz sources [9][10][11].While there has been some theoretical work, the majority of the research has involved either laboratory experiments, or numerical simulations. Gyro-devicesMost of the work on gyro-devices has involved helical waveguide interaction structures [1,2]. This interest began in the 1990's stimulated by collaborative research with IAP. The early experiments were in the 10GHz frequency range, whereas our more recent experiments have been in the 90 to 100 GHz range. The applications for these mm-wave amplifiers include DNP-NMR, cloud radar and communications, etc. Fig. 1. Gyro-TWA, 90 -100 GHzThe gyro-TWA shown in Fig. 1 has produced 3.4kW output power. The limit on the output power is presently the maximum power that is available at the input to the amplifier. Natural sources of EM radiationPrevious research on laboratory gyro-devices and electron cyclotron masers has provided the background for investigations of the mechanism of some naturally occurring sources of electromagnetic radiation. A research programme stretching over more than 10 years has explored the detailed mechanism of auroral kilometric radiation (AKR) and similar emissions emanating from other astrophysical bodies. Laboratory experiments to simulate the natural phenomena, combined with theoretical analysis and numerical modelling have been compared with natural observations. Using all four approaches it has been possible to understand how AKR arises and the mechanism by which the energy in electron streams can be converted with 1% or 2% efficiency into AKR. The mechanism depends upon the existence of an anisotropy in velocity space. This anisotropy is created when electrons are spiralling down the magnetospheric field lines. The magnetic moment is an adiabatic invariant of the electron motion and as the electrons penetrate further into the increasing magnetic field the electrons increase the perpendicular component of their velocity and decrease the component of their velocity parallel to the magnetic field lines. This results in a horseshoe shaped particle distribution in velocity space. The positive value of the gradient with respect to the perpendicular velocity component of the distribution function drives this kinetic instability that is now known as the "horseshoe instability". The theoretical analysis, the PiC simulations, the laboratory experiment and the space observations all agree. Fig. 2. Cylindrical over-sized Cherenkov structure Cherenkov sourcesOne of the aims of the research on Cherenkov sources is to retain reasonably high output power levels as the frequency increases. Traditionally Cherenkov sources have followed the trend that as the frequency increases and the wavelength...

A high-power Schottky diode frequency multiplier chain at 360 GHz for Gyro-TWA applications

“…Gyro-travelling wave amplifiers (gyro-TWAs) offer an attractive solution to generate hundreds of watts of CW and pulsed power for future applications in communications, plasma diagnostics and spectroscopy [1,2]. These devices are currently being developed to generate 40 dB of gain at 360 GHz and thus require RF power of the order of tens of milliwatts to adequately drive their input.…”