Application of planar helix slow-wave structure in backward-wave oscillators

• Main Author: Mookkannoor Muraleedharan Nair Ajith Kumar
• Other Authors: Lee Yee Hui
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering::Antennas, wave guides, microwaves, radar, radio
• Online Access: https://hdl.handle.net/10356/141481
• Institution: Nanyang Technological University

Travelling-wave tubes (TWTs) are one of the most popular device among vacuum electron devices. TWTs can be used as amplifiers, constituting travelling-wave tube amplifiers (TWTAs), or as oscillators, constituting backward-wave oscillators (BWOs). A BWO is one of the most reliable and spectrally pure voltage-tuneable high frequency oscillator. A slow-wave structure (SWS) is a very important component in TWTs. It becomes challenging to build and operate TWTs at millimetre-wave frequencies (30 - 300 GHz) since the dimensions of the SWSs and the beam tunnel that accommodates the electron beam reduce as the frequency of operation increases. Microfabrication techniques are a possible solution to achieve the required small sizes. Hence research on microfabrication-compatible SWSs is very important. Planar helix slow-wave structure with straight-edge connections (PH-SEC) is a planar counterpart of circular helix SWS which is one of the most popular SWS for TWTs. The PH-SEC is readily amenable to microfabrication. The PH-SEC has been studied for TWTA applications in the past few years. In this thesis we investigate its potential for application in BWOs First, the Fourier decomposition method is used to evaluate, more accurately than before, the interaction impedance for the fundamental and non-fundamental space harmonics of the SWSs such as the circular helix. Accurate evaluation of the interaction impedance for different space harmonics in a SWS is an important step for the design of TWTs. Results are presented for the variation of interaction impedance inside the SWS with frequency and position. Field-theory based analysis is the fastest method to determine the dispersion characteristics of SWSs. The results from such an analysis can be used for the initial selection of dimensions of the PH-SEC for a TWT with given target specifications. In the second investigation, the dispersion characteristics as well as the interaction impedance of the PH-SEC are obtained using the tape-helix approximation. The analysis is simplified by combining the tape-helix analysis and the effective dielectric constant (EDC) method. Moreover, a PH-SEC immersed in a homogeneous dielectric medium has been fabricated and tested. The measured phase velocity compares very well with that obtained from the analytical results. The third investigation presents simulation results for the design and performance of a BWO that operates at W-band and uses a microfabrication-compatible PH-SEC as the SWS. The oscillator is designed to operate with a beam voltage varying from 7 KV to 11 KV and a beam current of 20 mA. The particle-in-cell (PIC) simulation results show that the oscillator frequency tunes from 86.9 GHz to 100 GHz with a tuneable bandwidth of 14%. Further, a scaled version of the PH-SEC operating at X-band is also fabricated and measured. The measured S-parameters and the phase velocity match very well with the simulation results. Finally, a new technique is proposed to improve the efficiency of the conventional BWOs. The technique uses an additional electron beam inside the conventional BWO; the additional beam is synchronized with the fundamental forward-wave space harmonic and provides amplification like a TWTA. The technique is illustrated using a circular helix SWS at Ku-band. It is shown that the proposed technique improves the DC-to-RF conversion efficiency of the conventional BWO by a factor ranging from three to six depending on the beam configuration. The same technique for efficiency improvement is also illustrated using the PH-SEC. The studies reported in this thesis show that the microfabrication-compatible PHSEC SWS has an excellent potential to realize BWOs which can operate at millimetre wave frequencies.