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First bandwidth measurements of a novel gyrotron amplifier are presented. The coupling between the second harmonic cyclotron mode of a gyrating electron beam and the radiation field occurred in the region of near infinite phase velocity over a broad bandwidth by using a cylindrical waveguide with a helical corrugation on its internal surface. With a beam energy of 185 keV, the amplifier achieved a maximum output power of 1.1 MW, saturated gain of 37 dB, linear gain of 47 dB, saturated bandwidth of 8.4 to 10.4 GHz ( 21% relative bandwidth), and an efficiency of 29%, in good agreement with theory.

Helically corrugated waveguide has been used in various applications such as gyro-backward wave oscillators, gyro-traveling wave amplifier and microwave pulse compressor. A fast prediction of the dispersion characteristic of the operating eigenwave is very important when designing a helically corrugated waveguide. In this paper, multi-mode coupling wave equations were developed based on the perturbation method. This method was then used to analyze a five-fold helically corrugated waveguide used for X-band microwave compression. The calculated result from this analysis was found to be in excellent agreement with the results from numerical simulation using CST Microwave Studio and vector network analyzer measurements.

Results are presented from a numerical investigation of radiation emission from an electron beam with a horseshoe-shaped velocity distribution. This process is relevant to the phenomenon of auroral kilometric radiation (AKR) which occurs in the polar regions of the Earth's magnetosphere. In these regions of the auroral zone, particles accelerated into the increasing magnetic field of the Earth's dipole develop a horseshoe-shaped velocity distribution through conservation of magnetic moment. It has been shown theoretically that this distribution is unstable to a cyclotron maser instability. A 2D particle-in-cell (PIC) code model was constructed to simulate a scaled laboratory experiment in which an electron beam subject to significant magnetic compression may be studied and brought into resonance with TE modes of an interaction waveguide. Results were obtained for electron beam energies of 75-85 keV, magnetic compression factors of up to 30 and electron cyclotron frequencies of 4.42 and 11.7 GHz. At 11.7 GHz, beam-wave coupling was observed with the TE 03 mode and an RF output power of 20 kW was obtained corresponding to an RF conversion efficiency of 1.3%. At 4.42 GHz, excitation of the TE 01 mode was observed with an RF output power of 35 kW for a cyclotron-wave detuning of 2%. This corresponds to an RF conversion efficiency of 2.6%. In both cases PiC particle velocity distributions show the clear formation of a horseshoe-shaped velocity distribution and subsequent action of a cyclotron maser instability. The RF conversion efficiencies obtained are also comparable with estimates for the AKR generation efficiency.

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