Interaction of Terahertz Resonant Backward Wave with Inclined Electron Beam in Grating Structure

Abstract

The development of high power wide tunable and compact radiation sources at the terahertz (THz) wave regime is urgent for the THz technologies which have widespread potential applications in physics, biology, chemistry, medicine, imaging, spectroscopy, active-denial systems, and high-data-rate communications for defense and commercial sectors, etc. Since THz frequency range lies in the gap between microwave and infrared bands, there is potential of sources available in those bands to operate in THz one. I theoretically and experimentally examined the beam-wave interaction in compact and tunable non-relativistic (2.5-5 kV, 100-280 mA) resonant backward wave oscillator (BWO) with inclined electron beam – Clinotron. Analysis were done for 0.1 THz Clinotron which is the beginning of THz wave range and also fabrication, parts alignment is more easier for this frequency. The multimode theory of this device has been developed for the first time. Single frequency generation regime taking place under overlapping of resonant modes bandwidths has been studied. It has been shown that mode interaction provides phase velocity variation in interaction space. When phase velocity profile along the interaction space is favorable, some increase of electron beam-wave interaction power occurs that manifests as power peaks in Clinotron zone and, vice versa, when it is non-favorable, there is power drop in the bandwidth. Multimode theory results are compared with theory of BWO with reflections, particle-in-cell simulation. For experimental verification the slow-wave structure (circuit) with 0.28 mm period was fabricated by wire cutting method. 6 kV, 280 mA sheet beam electron gun was design, fabricated and tested successfully. Clinotron body and other parts were fabricated and assembled, then tested in electromagnet. The possibility of efficient operation of Clinotron with thick electron beam and at low magnetic focusing field has been analyzed. I theoretically analyzed the beam-wave interaction and showed that inclination of electron beam to the grating surface decreases demands of Clinotron for magnetic field magnitude and beam velocity spread compared to conventional BWO. The theoretical results are compared with particle-in-cell simulations. The automodulation processes in BWO operating at low focusing magnetic field have been studied. Automodulation behavior has been analyzed for different electron beam inclination angles. It has been shown that at low magnetic field the radio frequency (RF) transverse electric field leads to steep change of beam-wave interaction power that causes automodulation. The theoretical results are compared with particle-in-cell simulations. It has been found that the output coupler position is not optimized for existing conventional BWO and Clinotrons. Due to wide anode hole, some parts of RF field entering gun region and reducing the capability of electron gun and full device itself. The modification of coupler position and new mechanism of output coupling is under consideration, and experiments will be done in future works.

Keywords: THz, BWO, Clinotron, e-gun, beam-wave interaction, sheet beam, single mode operation, multimode operation, resonant device, coupling.

Student number: 2005-31262