P. MacInnes

High-current oversized annular electron beam formation for high-power microwave research

The authors report the study of a high-current accelerator, capable of producing thin (2mm) annular, mildly relativistic (450keV) electron beams (up to 1.5kA) of diameter of 70mm. Propagation of an electron beam through a 2m long, coaxial beam channel, with inner and outer conductor radii of 30 and 40mm, respectively, was demonstrated with the electron beam current at the beam-channel output measured using a Rogowski coil. The results obtained are compared with those predicted via numerical simulation. The electron beam obtained was used for proof-of-concept work demonstrating the operation of a high-power maser operating at millimeter wavelengths, using two-dimensional distributed feedback.

Wave propagation and tunneling through periodic structures

The phenomenon of tunneling manifests itself in nearly every field of physics. The ability to distinguish a wave tunneling through a barrier from one propagating is important for a number of applications. Here we explore the properties of the wave traveling through the band gap created by a lattice, either as a consequence of tunneling through the barrier or due to the presence of a pass band inside the gap. To observe the pass band for studying tunneling and propagating waves simultaneously, a localized lattice defect was introduced. The differences between the two phenomena are highlighted via waves’ dispersion characteristics.

Experimental results on microwave pulse compression using helically corrugated waveguide

The paper presents new results on the development of a method to generate ultrahigh-power short-microwave pulses by using a known principle of compression (reduction in pulse duration accompanying with increase in pulse amplitude) of a frequency-swept wave packet propagating through a dispersive medium. An oversized circular waveguide with helical-corrugations of its inner surface ensures an eigenwave with strongly frequency dependent group velocity far from cutoff. These dispersive properties in conjunction with high rf breakdown strength and low Ohmic losses make a helically corrugated waveguide attractive for increasing microwave peak power. The experiments performed at kilowatt power levels, demonstrate that an X-band microwave pulse of 80 ns duration with a 5% frequency sweep can be compressed into a 1.5 ns pulse having 25 times higher peak power by optimizing the frequency modulation of the input wave packet.

Study of one-dimensional Bragg structures with localized defect

The results of studies of one-dimensional Bragg structures (one-dimensional periodic lattice) with localized defects are presented. The defects considered are localized, step changes (shifts) of the lattice phase (corrugation). The influence of the defects on the periodic lattice band-gap parameters has been analyzed. The presence of the defect resulted in the appearance of a pass band, associated with the defect eigenmode, inside the lattice band gap and it was demonstrated that the position of the pass band depended strongly on the parameters of the defect as well as the field structure. The experimental and theoretical results obtained are compared and discussed.

Free-electron maser based on a cavity with two- and one-dimensional distributed feedback

The study of a coaxial free-electron maser (FEM) based on two-dimensional (2D) and one-dimensional (1D) distributed feedback, driven by a 70mm diameter, annular electron beam is presented. A new cavity formed with 2D and 1D periodic lattices, positioned at the input and output of the interaction region, respectively, was used. It has been demonstrated that 2D distributed feedback in the input mirror allowed 8mm radiation emitted from different parts of the electron beam to be synchronized. The FEM operating in the 35.9–38.9GHz frequency region generated 65MW, 150ns duration millimeter wave pulses which contained ∼10J of energy in the pulse.

High-Current Electron Beams for High-Power Free-Electron Masers Based on Two-Dimensional Periodic Lattices

High-power gigawatt-level radiation can be generated by the interaction of an electromagnetic wave and an annular electron beam with a transverse dimension much larger than the operating wavelength. The use of such a large-circumference annular beam allows the generation of high beam currents while also maintaining low space charge and RF power densities inside the interaction region. This circumvents the problems associated with potential depression in the beam channel and RF breakdown inside the oscillator. In this paper, we present the studies of high-current magnetically confined annular electron beams and discuss their production and transportation through a coaxial beam channel which formed the interaction region of a free-electron maser (FEM). The results from numerical simulations, using the software packages KARAT and MAGIC, are compared with the experimental measurements. The operation of a FEM, driven by a high-current annular electron beam, is presented, and the tunability of the maser, inside a frequency range defined by an input 2-D Bragg mirror, is demonstrated.

5-fold helically corrugated waveguide dispersion measurements

The results of 3-fold helically corrugated waveguide microwave pulse compression experiments are presented. A pulse of 80 ns duration having a power of 5.6 kW is compressed into a 140 kW peak power pulse of 1.5 ns duration. To enhance the power capabilities of the microwave pulse compressor a larger diameter 5fold helical waveguide structure was studied; the eigenwave dispersion is discussed using numerical, analytical, and experimental techniques.

Ka-band gyro-TWA waveguide severs for circularly polarized waves

This paper presents the latest results from a second harmonic gyro-TWA experiment at Strathclyde University. The experiment uses a helically corrugated interaction region and operates at Ka-band. Simulations have been performed to predict the amplifier gain and bandwidth using linear theory. Amplifier stability margin can be improved with the addition of a sever. A sever has been designed and manufactured, results of simulations and measurement are shown.

The dispersion characteristics of helically corrugated waveguide

This paper gives a summary of the methods used to study the dispersion characteristics of helically corrugated waveguide. Analytical, numerical and experimental results for a five-fold helical waveguide are studied and compared.

A high-power Ka-band Free-Electron Maser, defined by a 2D – 1D Bragg lasing cavity

Summary form only given. One of the on-going research programs, at the University of Strathclyde, involves the development of high-power, pulsed, Free-Electron Masers (FEMs) with the lasing cavity defined using periodic corrugations on the drift-tube walls1–4. These corrugations form 1D and 2D Bragg resonators, whose reflection bands determine the dominant resonance of the maser5. Proper selection of the FEM undulator magnetic field strength, allows for efficient extraction of energy from a mildly relativistic (400 – 500 keV) electron beam at the resonant frequency of the lasing cavity, leading to monochromatic output at power levels of several tens of megawatts and pulse durations of ∼150ns (determined primarily by the pulse duration of the driving power supply of ∼250ns).